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Issue
A&A
Volume 689, September 2024
Article Number A148
Number of page(s) 19
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/202450584
Published online 26 September 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1 Introduction

Despite large-scale optical surveys over many decades, the internal structure of the inner regions of the Milky Way (MW) and the details of its formation and evolution were poorly understood. The main reason is the severe and non-uniform interstellar extinction and crowding towards the MW bulge and inner disk, which complicates observations, especially at the optical wavelengths. These inner regions are the most complex of our Galaxy to study, with a mixture of stellar populations from the inner disk, bulge, and halo, which exhibit a variety of physical properties.

This situation has improved in recent years, with several projects studying the inner regions of the MW (e.g. Barbuy, Chiappini, & Gerhard 2018; Saviane et al. 2020, and references therein). The VISTA Variables in the Vía Láctea (VVV) survey (Minniti et al. 2010) was designed to resolve the 3D structure of the MW by searching, precisely parameterising, and studying the distributions of known distance indicators such as RR Lyrae, Cepheids, and red clump stars in the inner Galaxy. By using observations at near-infrared wavelengths, VVV observations minimise the problems of extinction and crowding. Among many results, the VVV data have enabled the construction of highresolution extinction and photometric metallicity maps (e.g. Gonzalez et al. 2012, 2013), the discovery of stellar clusters (e.g. Borissova et al. 2014), and the production of 3D spatial structure maps based on red clump and RR Lyrae stars (e.g. Dékány et al. 2013; Wegg & Gerhard 2013).

In 2016, the VVV eXtended (VVVX) survey started operating. The VVVX survey was designed to ensure the long-term legacy of the VVV survey, characterising the structure and time domain properties of the inner Galaxy. The project is one of seven large public surveys (Arnaboldi et al. 2019) commissioned by the European Southern Observatory (ESO). The VVV+VVVX surveys were awarded about 4200 h of observing time over a timespan of ~ 13 years at the 4-metre Visible and Infrared Survey Telescope for Astronomy (VISTA; Emerson, McPherson, & Sutherland 2006) telescope at ESO Paranal Observatory, and were finally completed in March 2023, before the VISTA InfraRed CAMera (VIRCAM; Dalton et al. 2006; Emerson & Sutherland 2010) instrument was decommissioned from the VISTA telescope. Both surveys combined cover the Galactic bulge, as well as the adjacent disk towards the Galactic quadrants I and IV.

The VVV survey pioneered the discovery of variable stars, transients, and a select number of new clusters across a significant region around the Galactic centre and plane. VVVX bridges the gap between VVV findings and other surveys, extending into obscured regions. This aids in estimating survey completeness and mapping distributions of various tracers from the halo to the Galactic centre. The VVVX survey was designed to connect the VVV survey area with the VISTA Hemisphere Survey (VHS; McMahon et al. 2013) and the UKIDSS Galactic Plane Survey (Lucas et al. 2008). VVVX overlaps with the VST Photometric Hα Survey (VPHAS+; Drew et al. 2014) and the DECam Plane Survey (DECaPS; Schlafly et al. 2018), providing complementary near-IR imaging for those regions (see Fig. 1) as well as variability information. In addition, by re-observing the area of the original VVV, VVVX extends both the time baseline as well as reaches fainter flux limits, complementing other public optical and far-IR surveys. In particular, the mapped regions are located between Galactic longitudes l = −130 degrees and l = +20 degrees, detecting ≳1.5 × 109 point sources in an area of around 1,700 square degrees, including more than 50 known globular clusters and 1,000 open clusters. The specific goals of the VVVX survey stated in the original proposal are:

  • To map the structure of the optically obscured populations in position and velocity.

  • To find pulsating variable stars (RR Lyrae, Classical Cepheids, Type 2 Cepheids, Miras) as distance indicators probing the 3D structure of the inner MW.

  • To physically characterise known and newly detected star clusters (open star clusters as well as globular clusters), measuring their distances, extinctions, reddenings, sizes, and estimating their metallicities and ages.

  • To explore the stellar populations and variable stars of the Sagittarius dwarf galaxy located beyond the Galactic bulge.

  • To find rare variable sources such as transients and WIT (‘What Is This?’) objects, and also to identify the near-IR counterparts of benchmark high-energy sources discovered by recent X-ray missions.

  • To build a catalogue with the classification of dwarf stars beyond the peak of the luminosity function and their companions.

  • To detect heretoforth unknown background galaxies and QSOs in the Galactic Zone of Avoidance (ZoA).

  • To probe the Galactic structure close to the Galactic centre using near-IR microlensing.

VVVX observations provide essential input targets for spectroscopic surveys based on multiplexing spectrographs such as the SDSS-IV/V (Sloan Digital Sky Survey-IV/V; Blanton et al. 2017; Kollmeier et al. 2019), the Galactic 4MOST surveys (4-metre Multi-Object Spectroscopic Telescope; de Jong et al. 2019)1 and MOONS Galactic survey (Multi-Object Optical and Near-infrared Spectrograph; Gonzalez et al. 2020). In addition, our database complements measurements from important current and future space missions such as the Hubble Space Telescope (HST), Gaia, James Webb Space Telescope (JWST), Euclid, and Nancy Grace Roman Space Telescope.

There is a variety of final products, including deep JHKs images, multi-band JHKs and multi-epoch Ks band time series catalogues, and proper motions for ≳1.5 billion sources. Moreover, VVVX catalogues millions of variable stars, extend the VVV extinction and reddening maps, and increase the completeness and source density maps, thus presenting a treasure trove for the whole astronomical community. The VVVX public database will offer the possibility to explore a wide variety of scientific objectives, from those we listed previously to new ones, including serendipitous discoveries.

Due to its larger survey area VVVX provides a more complete picture of the inner MW than its predecessor VVV: a deep bulge map to establish structure differences between the oldest and younger populations, a map of the Sagittarius dwarf from its core across the whole bulge, and a much more extended disk map that probes star formation (SF) activity, disk stellar populations, and spiral arms structure. VVVX provides a public multicolour time domain database within the optically hidden MW regions, including 3D extinction maps that trace the non-stellar baryonic matter. Additionally, the VVVX survey provides observational constraints for the present-day MW structure as presently known (e.g., thin and thick disk structure, number of spiral arms and their locations), and even more importantly, provides insights into the assembly history of the MW.

The aim of this article is to describe the VVVX survey design, observations, data processing, and final status of the VVVX survey, emphasising the observation strategy and describing the observed data available to the astronomical community through the VISTA Science Archive (VSA) and ESO Science Archive. We also describe some usage examples of VVVX data within the MW and beyond. In the ESO Science Archive the VVV and VVVX data are published in the data collections identified by doi.eso.org/10.18727/archive/67 and doi.eso.org/10.18727/archive/68.

thumbnail Fig. 1

VVV+VVVX survey coverage of the MW bulge and southern plane. Top: the surveyed area shown Galactic coordinates. Our near-IR survey covers ~1700 sq. deg. in total, and the different regions are colour-coded according to the location, baseline, and number of observations (see Sec. 4). In red is the original VVV Bulge and VVV Disk, with observations from years 2010 and 2016 using the ZY JHKs filters, and VVVX observations between years 2016 and 2022 using the JHKs filters. Other colours mark the VVVX areas, observed with JHKs filters between years 2016 to 2022. Yellow is the Disk to Longitude +230, dark blue is the Low and High Extended Disk, green is the Disk to Longitude +20, and in light blue is the Low and High Extended Bulge. A zoomed view of the image with the tile names is presented in Appendix A. Bottom: Schematic representation of the areal coverage compared with the other selected complementary surveys mentioned in Sect. 1.

2 Telescope and instrument

The VVVX observations were carried out with Visible and Infrared Survey Telescope for Astronomy (VISTA), an ESO telescope located at the Cerro Paranal Observatory in the Atacama Desert, northern Chile. For all VISTA photometric surveys, the VIRCAM camera was used (Dalton et al. 2006; Emerson & Sutherland 2010). VIRCAM was equipped with sixteen 2048 × 2048 science detectors, arranged in a 4 × 4 array, with a large spacing of 90% and 42.5% of the detector size along the X and Y axes. Each individual detector covered ~ 694 × 694 arcsec2 on the sky, with average pixel scale. Its filter wheel was equipped with five broad-band filters (Z, Y, J, H, and Ks) and two narrow-band filters centred at 0.98 and 1.18 μm. For the VVVX observations only JHKs were used. Table 1 shows the centre wavelengths of each filter.

In all VIRCAM observations, each pointing of the telescope is a so-called a pawprint, which covers 0.59 sq. deg. and provides partial, coverage of the field of view. By combining six pawprint exposures with appropriate offsets, the contiguous coverage of a field is achieved with at least two exposures on separate pixels, except at the edges. That is called a tile and covers a field of view of 1.109 × 1.475 = 1.646 sq. deg., the largest unvignetted field of view in the near-IR regime on 4-m class telescopes.

In 2023, VIRCAM was decommissioned and will subsequently be replaced by the fibre-fed spectrograph 4MOST (de Jong et al. 2019). In fact, various 4MOST surveys will collect complementary spectroscopic data to VVV and VVVX. For details about the telescope and instrument, we refer the interested reader to Sutherland et al. (2015) and the VIRCAM instrument web-pages2, and the VISTA/VIRCAM user manual (Ivanov et al. 2021).

Table 1

VISTA filters used in the VVVX observations.

3 Survey area

The VVV survey observed ~562 sq deg in the MW bulge and the adjacent southern Galactic plane. The area was divided in bulge, with ~300 sq. deg. within −10.0° ≲ l ≲ +10.4° and −10.3° ≲ b ≲ +5.1°, hereafter called ‘VVV Bulge’, and ~ 220 sq. deg. within 294.7° ≲ l ≲ 350° and −2.25° ≲ b ≲ +2.25°, hereafter ‘VVV Disk’ (see Fig. 1). For the VVV Disk there is a total of 38 × 4 = 152 tiles while the VVV Bulge is filled up by 14 × 14 = 196 tiles. The tile sides are aligned with the Galactic coordinates for coverage optimisation.

The VVVX survey expanded the area of the original VVV footprint in both Galactic longitude and latitude, with an area of ~480 deg2 in the Galactic bulge plus ~1170 deg2 in the inner plane (including the original VVV), from l = −130 deg to l = +20 deg (7 h <RA< 19 h). For contiguous observations of large areas, the covering process was carried out using the ‘Survey Area Definition Tool’ (SADT3), which maximises the efficiency of VISTA observations by minimising the number of tiles needed to cover a given area, providing the tile centres and the guide and active optics stars necessary for the execution of the survey Observing Blocks (OBs). The new areas – with their respective tiles – were labelled as following:

  • Low extended disk (disk-low, for short): area with the lowest latitudes along the disk, located within +230° ≲ l ≲ +350° and −4.5° ≲ b ≲ −2.25°, totalling ~266 sq. deg. To cover the area, 83 × 2 = 166 tiles were needed, with the tile names ranging from e0601 to e0766.

  • High Extended Disk (Disk-high): symmetrical area to the Low Extended Disk at higher latitudes, +230° ~ l ≲ +350° and +2.25° ≲ b ≲ +4.5°, comprising ~ 266 sq. deg. Tile names range from e0767 to e0932.

  • Disk to longitude +20 (Disk20): extended the disk coverage to the north within +10° < l < +20° and |b| ≲ 4.50°. A total of 7 × 8 = 56 tiles were used to fill the area of ~90 sq. deg. The area has ~ 90 sq. deg. Tile names in this region are e0933 to e0988.

  • Disk to longitude +230 (Disk230): extended area to the southern disk coverage within +230° < l < +295° and |b| ≲ 2.25°. This is the largest new area, with ~292 sq. deg. A total of 45 × 4 = 180 tiles are in this region and labelled from d1001 to d1180.

  • Low extended bulge (bulge-low): extended the bulge area within −15° < b < −10° and |l| ≲ 10° with ~ 90 sq. deg. in size. There is a total of 14 × 4 = 56 tiles, labelled from b0401 to b0456.

  • High Extended bulge (bulge-high): extended the bulge within +5° < b < +10° and |l| ≲ 10o. The area has also ~90 sq. deg. A total of 14 × 4 = 56 tiles filled the area and are named from b0457 to b0512.

The VVVX footprint along with the VVV original coverage is shown in Fig. 1, along with a comparison with other complementary surveys, while the list of the tile names with the central coordinates are presented in Appendix A (first ten rows) and in electronic form4. Both VVV and VVVX have a total of 1028 tiles. There are no tiles with names d0153–d0200, b0397–b0400, b0513–b0600 and d0989–d1000. The absence of tile numbers is attributed to the naming conventions, which are based on the regions covered by the tiles. The new VVVX areas added up to ~2000 h of observations, split between ~450 h for JHKs and ~1550 h for the multi-epoch Ks.

4 Observing strategy

The VVVX survey was carried out in service mode, which is the standard for all VISTA observations. The observational blocks (OBs) were prepared by the VVVX team using the P2PP/P2 tools5 and sent to ESO for validation and observation at the site.

Observing blocks for each tile were defined for: a single Ks band observation (used in the variability campaign), multicolour JHKs observations (in a quasi-simultaneous schema: ~10 min for the sequence H → Ks → J), and additional J band observations to be combined for the multicolour observations. Due to scheduling constraints the multicolour observations had to be split into two separate observing blocks, including JHKs and J band observations, respectively. We note that depending on the region, between 2 and 8 adjacent tiles were combined in a concatenation and observed back-to-back. Minimum concatenation of 2 tiles was necessary to achieve a satisfactory background sky subtraction, and at the same time it reduced observational overheads.

All observations were carried out using a 2-point dither due to the variable and high sky background in the near-IR and to enable cosmic rays rejection and pixel defect correction. As a consequence, the total time per OB for an individual source (e.g., star) and filter is: DIT (detector integration time) × NDIT (number of detector integration time) × Njitter=2 (number of offsets executed at each of the six pawprint positions) × Npaw=6 (number of pawprints that together make a tile). These OB setups lead to a total time on the source in each OB of 120 s (J), 48 s (H) and 16 s (Ks), as presented in Table 2.

The number of epochs observed during the VVVX campaign in each of the new areas ranged between 23 and 50 epochs (see Appendix A). In the original VVV region, typically 3–12 new epochs were acquired by the VVVX campaign, except in the inner Galactic bulge, where in a few tiles up to 100 additional epochs have been secured. In the region around the Baade’s window, eight tiles covering ~13 sq. deg. in the VVV bulge reached up to 352 epochs, summing up the VVV and VVVX variability campaigns. Figure 2 shows the number of epochs observed for each tile while Fig. 3 illustrates the cadences for the Ks band variability campaign across the original VVV bulge area. Due to our flexible observing strategy, the observational sequence for all tiles is different, and the ranges of time separations between observations are also variable from tile to tile. Therefore, for variability studies, we recommend that each tile should be treated separately, as, for example, aliasing in the periods are tile dependent.

J filter only OBs were observed in order to double the exposure time in that filter. The deep J imaging corresponds to a time on source of ~ 4 min, and due to the 1 h restriction on OB duration, these additional J filter observations had to be separate from the JHKs observations, for a given tile. The two J images for each tile can be coadded to increase the useful depth of Ks vs. J − Ks colour magnitude diagrams. For instance, the combined Ks epochs and the deep J will reach J = 20.8 mag, Ks = 20 mag at 5σ, which is three magnitudes fainter than the unreddened bulge main-sequence turnoff. The densest fields will be confusion-limited, but applying both point-spread function (PSF) fitting techniques and differential photometry (DIA), it is possible to recover most objects down to J = 19.5 mag, H = 18.5 mag and Ks = 18 mag, even in fairly crowded fields. This is more than 3 mag fainter than the unreddened RR Lyrae in the Galactic bulge.

Contemporaneous JHKs epochs give the colours for variable stars (essential for precise dereddening of RR Lyrae and Cepheids) while the deep J observations allow the construction of deep colour-magnitude diagrams reaching past the age-sensitive main sequence turn-off. Because denser fields are confusion-limited, the limiting magnitudes vary along the area, especially in the innermost bulge area. Figure 4 shows the limiting magnitudes as a function of position for all the filters.

The J and H observations were ≳85% and ≳95% complete by the end of the 2018 and 2019 season, respectively. The median image quality in J and H are 0.88″ and 0.84″ for a median airmass of 1.19 and 1.17, respectively, measured on combined tile images (see Fig. 5). The image quality is measured from the average FWHM of sources classified as bona fide stars with high signal-to-noise. This value includes atmospheric, telescope and instrument related aberrations and is not the same as seeing, which is an inherent property of the atmosphere independent of the telescope (Martinez et al. 2010). We refer to Sutherland et al. (2015) for a detailed description of the image quality design and performance of the VIRCAM@VISTA. In Ks, the observations exceeded ≳90% completeness only in 2022, with median image quality of 0.93″ for a median airmass of 1.21. In most cases the observations satisfied the atmospheric turbulence and photometric image quality parameters and were classified as completed. Remaining JH observations were taken during 2020–2022 and Ks in 2020–2023 to complete the planned observations within the photometric and image quality parameters.

We summarise in Table 3 the observed areas along the years of the VVVX campaign. Regarding the multicolour data, the JHKs observations were ≳94% complete by the end of the 2019 season. The remaining observations were secured during 2020–2023 and comprised repeated observations of the tiles that did not pass the quality control in the first instance. The cumulative distributions for the JHKs are presented in Fig. 6.

The VVVX observations were pipeline-processed at the Cambridge Astronomical Survey Unit (CASU), using the VISTA data flow system (VDFS) pipeline (Lewis et al. 2010). CASU also produces the photometric calibration of stacked pawprint images and tile images (González-Fernández et al. 2018). Multiband catalogues have also been generated by the VISTA Science Archive (VSA). All tiles, their confidence maps and extracted source lists, in addition to the corresponding pawprints, were processed with version 1.5 of the CASU pipeline. In the catalogues provided by CASU, a flag is used to indicate the most probable morphological classification. These flags were derived from the curve-of-growth analysis of the flux from different aperture sizes (Irwin et al. 2004). The flags are:

  • ‘−1’ to denote stellar objects,

  • ‘−2’ to denote borderline stellar,

  • ‘−7’ denoting sources containing bad pixels,

  • ‘−9’ is used for saturated stars,

  • ‘+1’ is used for non-stellar objects and

  • ‘0’ is used to denote a noise measurement.

The entire reduction and calibration process is same as that applied to the VVV data. For more details on data processing and catalogue generation, we refer the reader to Sects. 2 and 3 of the VVV DR1 paper (Saito et al. 2012) and the references therein.

Table 2

Total exposure time for the VVVX observations.

thumbnail Fig. 2

Number of epochs in the Ks band observed by VVV+VVVX. The mean baseline for the original area is ~12 yr, with up 352 epochs for selected tiles in the inner bulge. For the outer bulge and disk the number of epochs are in the range 23–106 for the VVVX observations solely, varying according with the observational strategy.
thumbnail Fig. 3

Cadence of the VVV+VVVX bulge observations. Top: density plot showing the cadence of the bulge observations in both VVV and VVVX campaigns. Bottom: light curve example for the source VVV-VIVACE ID 533558 (Ks − 14.9 mag; Molnar et al. 2022), observed by VVV+VVVX in the five VISTA broad band filters in the bulge field b307. There are a total of 363 observations in the five filters along years 2010 to 2019. The coordinates for the target are RA, Dec (J2000) = 18:00:11.48, −28:25:13 (corresponding to l, b = 2.0688, −2.4904 deg.).
thumbnail Fig. 4

5σ magnitude limits of the catalogues in the J, H, and Ks, respectively, from top to bottom. The colour scale is the same in the three panels as shown in the vertical bars at the right. For the J and H bands, VVV and VVVX are combined because of the lack of VVVX observation in these bands in the original VVV area.
Table 3

Summary of the VVVX observational campaign.

thumbnail Fig. 5

Image quality and airmass cumulative distributions for the JHKs VVVX observations, in the top and bottom panels, respectively. The median values of image quality and airmass for each filter are presented in the legends.
thumbnail Fig. 6

Cumulative distributions for the JHKs observations during the VVVX campaign.

5 Previous data releases

The first VVVX data release (VVVX DR16) was published in February 2019 and contains data acquired between July 2016 and August 2017. The first release contains observations of about 590 sq. deg., typically covered in the three filters (J, H and Ks). DR1 released a total of 1660 tile images that passed quality control, plus their associated weight maps and single-band source catalogues. Moreover, it contains 9960 pawprint images and their associated weight maps that were used to create the tile images. The total data volume is about 1.7 TB in compressed format (CFITSIO RICE compression7).

In June 2021, we publicly released the VVVX DR28 with the data taken between April 2018 and October 2019. They consist of both multicolour JHKs observations and Ks band variability data. The DR2 has 10776 tile images, plus their associated single-band source catalogues and weight maps. For the paw-prints, there is a total of 64716 images, plus the single-band source catalogues and weight maps. The total data volume is eight times larger than DR1, reaching 13.4 TB in compressed format. The uncompressed data volume of the VVVX DR1 plus VVVX DR2 is ≳27 TB.

The Ks band taken prior to April 2018 was not released with the VVVX DR1, which contained data only from the multicolour JHKs OBs and J-only OBs. JHKs and J-only OBs taken during 2020–2022, as well as Ks from 2020 to 2023, represent a smaller part of the total volume. These data had been gradually processed by CASU and VSA, and ingested to the ESO Archive, with no announcement of a public data release. These remaining data complete the release to the astronomical community of all the observations planned by VVVX within the established quality criteria. In total, the VVV+VVVX survey observations comprise >350000 images, which including images, catalogs and confidence maps, they make a total of more than 106 files.

The VVVX data, as described above, can be downloaded directly via the ESO Archive9. Various options are available for how the data can be obtained, from simple cone searches to more complex options. In the ESO Archive it is also possible to browse the VVVX data using an Aladin Sky Atlas tool, and by clicking and selecting, determine which data among images and catalogues are of interest for direct download.

6 Advanced data products

The images and catalogues described in the section above, already available via the VSA and ESO Archive, are being complemented by advanced products, currently under preparation by VVVX science team members. As for VVV, a key product is the extraction of PSF photometry, which is more robust and accurate than aperture photometry data for the regions of high crowding of stars, such as in the innermost MW bulge and plane, and for the study of star clusters (e.g. Alonso-García et al. 2018).

PSF-based catalogues are compiled for each VVVX tile, however measured from the stacked pawprint images, where codes such as DAOPHOT (Stetson 1987), DOPHOT (Schechter et al. 1993), and SExtractor (Bertin & Arnouts 1996) work with better precision.

As well as for the VVV Survey, various authors will provide independently produced PSF based source catalogues of the multicolour data (e.g. Alonso-García et al. 2018; Surot et al. 2019), and the variability campaign (e.g. Contreras Ramos et al. 2017; Smith et al. 2018). Specific details will be published elsewhere. The catalogues will be available through the VSA10, using the same schema as for aperture and image data.

The Ks band variability data allows for a variety of uses, from constructing light curves to proper motion measurements. For the VVV area, our team published a number of catalogues containing hundreds of thousands candidate of variable stars, for example: the VVV Near-IR Variability Catalogue (VIVA-I; Ferreira Lopes et al. 2020), the Near-IR Catalogue of known variable stars (Herpich et al. 2021), and the VIRAC Variable Classification Ensemble (VIVACE; Molnar et al. 2022).

The extinction in Ks filter of VIRCAM is an order of magnitude lower than in optical. Hence, the VVVX proper motion data are complementary to those of Gaia in highly extincted Galactic disk and bulge regions, where the optical Gaia data result in significantly reduced depth. Over the area of the original VVV region, the Ks band data can be combined to increase the time baseline to more than a decade (≳2010–2020, or even 2022 for selected bulge fields), thus increasing the accuracy of the proper motion measurements. For the new extended area, the time base is approximately 4–5 yr.

Proper-motion catalogues will be incorporated into the VISTA Infrared Catalogue 2 (VIRAC2, in preparation), the latest version of the original VISTA Infrared Catalogue (VIRAC; Smith et al. 2018). Luna et al. (2023) compared the VIRAC2, Gaia DR3 and HST proper motions in a few fields towards the Galactic bulge, with different stellar crowding levels. The test showed that VIRAC2 proper motions have more reliable uncertainties than Gaia DR3 and are comparable to HST in dense fields, such as globular clusters and the Galactic bulge. The shorter time base of the new VVVX areas should influence the data quality, so we should expect larger fractional uncertainties for more distant and slower sources. VIRAC2 has been uploaded to ESO archive where it will be available for Virtual Observatory (VO) TAP queries.

Aside from the aperture and PSF photometry for the point sources, the images also contain extended sources. The optical detection of extragalactic sources beyond the MW is hampered in the ZoA, where the stellar crowding and Galactic absorption are rather extreme (see Sec. 8.3).

7 Examples of data usage: Tile e1084, Carina Nebula, and NGC 3324

7.1 Multicolour images and catalogues

One of the first images released by the JWST is of the Carina Nebula11. The nebula is located within the Disk to Longitude +230 area, in tile e1084, which has central coordinates RA, Dec (J2000) = 10:30:32.86, −58:39:41.8, corresponding to l, b = −74.524725, −0.649149 deg. The JHKs observations of tile e1084 were carried out on March 19, 2018. Table 4 summarises the observational log. A composite false colour image combining the JHKs images can be produced with the Aladin Sky Atlas12 for e1084 using the WCS data in the reader of each image to determine their location, rotation and image scale.

In Fig. 7, we compare the three colour JHKs image obtained with VISTA/VIRCAM with its JWST/NIRcam counterpart. Both images have ~ 9 × 7 arcmin size, with a different orientation than equatorial or Galactic coordinates. JWST composite image comprises separate exposures containing F090W, F187N, F200W, F335M, F444W, F470N NIRCam narrow and wide filters. These filters are part of both the short and long wavelength channels, covering wavelengths from 0.901 to 4.707 microns, corresponding to pixel scales of 0.031″ pixel−1 and 0.063″ pixel−1 for the short and long wavelength channels, respectively. Although the JWST images were taken at a broader range of longer wavelengths (0.90–4.70 μm compared to 1.25–2.15 μm for VISTA), the VVVX images are comparable in quality for relatively bright point sources, while the nebular structure definition is clearly superior in the JWST. For faint sources, the higher resolution of JWST compared to VISTA allows the first to reach several magnitudes deeper at the same wavelengths.

Near the Carina Nebula (Fig. 7) we find NGC 3324, a stellar cluster first described by Kharchenko et al. (2005), based on PPMLX and 2MASS data. As described before VVVX offers NIR data not only with a much better resolution, but also reaches several magnitudes deeper. In Fig. 8 we present the J, H and Ks band images for the cluster. It is possible to note the difference in the gas transparency towards longer wavelengths, especially in the lower part of the images where the gas concentration is higher, increasing the number of background sources that can be observed.

We created a multicolour catalogue for tile e1084 by cross-matching the J, H and Ks band catalogues provided by CASU (aperture photometry, see Table 4). For the cross-match procedure we made use of STILTS13 (Taylor 2006), allowing a tolerance of 1″ between the sky coordinates of the detected sources, resulting in a JHKs band catalogue with 568k sources, which is limited by the shorter exposure time in the Ks band compared with the other filters (see Table 2). Multiepoch Ks band images can be coadded to provide a deeper catalogue; however, that has not been applied here. In using the photometric flags and selecting only stellar sources in all 3 bands (flag ‘−1’, see previous section), the number of sources in the e1084 catalogue dropped from 568k to 262k sources. The J × (J − Ks) CMD of stellar sources in tile e1084 is presented in Fig. 9. We have also similarly prepared CMDs for the region of Carina and NGC 3324, for the same areas presented in Figs. 7 and 8.

In order to demonstrate the importance of the PSF photometry, we also present CMDs using PSF data for the same regions mentioned above: tile e1084, the Carina Nebula region, and NGC 3324. We are currently finishing the PSF atlas for the whole VVVX area (Alonso-García et al. in prep.), building on our previous experience extracting the PSF photometry from the original VVV footprint (Alonso-García et al. 2018). A preliminary version of the PSF catalogues was used to build the CMDs of these areas of interest in Fig. 9.

Although there is excellent agreement between the aperture and PSF photometries, a larger number of sources in the PSF catalogues is evident since our algorithm to extract the PSF photometry uses a less conservative limit than the CASU aperture photometry. This difference becomes larger for the more crowded fields over the Galactic plane where the PSF is far more efficient. The VVVX PSF catalogues for the entire area will be described in Alonso-Garcia et al. (in prep.) and publicly released to the community through VSA.

Table 4

Multicolour observations of tile e1084.

thumbnail Fig. 7

VVVX composite image of the Carina Nebula compared with the JWST image of the same field. The VVVX image (top) is in false colour and based on the JHKs observations while the JWST image (bottom) comprises separate exposures containing F090W, F187N, F200W, F335M, F444W and F470N NIRCam filters. The Carina Nebula is located towards tile e1084 in the Disk to Longitude +230 region of VVVX, with the JHKs observations secured on March 19, 2018. The images have ~ 9 × 7 arcmin size and are oriented with the north to the right and east to the top. Credits (JWST image): NASA, ESA, CSA, and STScI, J. DePasquale (STScI).
thumbnail Fig. 8

Individual J (left), H (centre) and Ks band images (right) of the star cluster NGC 3324. The image is centrered at RA/DEC (J2000)=10:37:21.8, −58:36:54, with 9 arcmin side and oriented in Galactic coordinates.

7.2 Variability data: Light curves and proper motions

Dékány et al. (2013) present the effects of the time-domain sampling on the detection efficiency of variable stars. They showed that even a random cadence in the observations will allow us to detect RR Lyrae and δ-Cepheids at Ks=14 mag unambiguously, provided a minimum number of 25 epochs. Therefore, between 25–40 individual Ks band observations were allocated to the new extended survey area. More frequent observations (40 epochs) were assigned to more highly reddened regions in the new VVVX disk area at 230° < l <295° (see Fig. 2), given that fainter variable stars require a larger number of observing epochs, and distant parts of the Galactic disk lie mainly in reddened, low latitude regions. Moreover, the number of epochs also depends on the type of variable star we expect to find in each region; for short-period variables, a greater number of epochs is required. Similarly, we have simulated the gain in proper motion precision of VVVX, relative to VVV. We find that 10 epochs offers the best compromise between precision and time spent, yielding an uncertainty of 300 μas yr−1 thanks to the 9.5 yr baseline. This very high precision is essential to enable high quality decontamination of star clusters to Ks = 15–16 mag in very crowded inner Galaxy fields and our goal of 5D mapping of Galactic structure. VVVX astrometry in VIRAC2 is placed on the Gaia absolute astrometric reference frame, which is very precise even in the plane due to the Gaia method of simultaneously observing two widely separated fields.

With the long time-baseline, the high precision proper motion measurements can disentangle the bulk bulge/disk stellar motions, producing a global pure bulge/disk colour-magnitude diagram (Libralato et al. 2015). As the innermost regions of the MW bulge and central disk remain out of reach for Gaia observations due to their high dust extinction, it will be up to VVV and VVVX to provide input catalogues for spectroscopic surveys such as MOONS, the Milky Way Mapper (MWM; Kollmeier et al. 2017) and others.

The observing strategy for e1084 consists in 41 Ks band epochs over a baseline of 4-years, which allows the search and study of various classes of variable and transient stars, including pulsating variable stars used as distance indicators, such as RR Lyrae and Cepheids. The complete set of Ks band source catalogues for tile e1084 were matched following the same procedure as for the multicolour catalogue in order to construct the light curves for the tile. Figure 10 shows the light curves derived and subsequently phase folded for two previously known RR Lyrae stars discovered by Gaia in the region of tile e1084 (Rimoldini et al. 2023).

By applying a Lomb–Scargle (LS) algorithm (Lomb 1976; Scargle 1982; Zechmeister & Kürster 2009) we detected the periodic signal, albeit in the case of Gaia DR3 5255393833932660992 with the double period, which is a common feature in this type of analysis (e.g. Catelan et al. 2013; Ferreira Lopes et al. 2020; Botan et al. 2021). This factor-of-two error in the period determination is a well-known issue that affects the LS method (and its variants) when faced with nearly sinusoidal light curves, both in the near-IR as in the optical (e.g. Graham et al. 2013, 2017; VanderPlas 2018). In applying the string length minimisation method (Burke et al. 1970; Dworetsky 1983; Clarke 2002), periods of 0.4763 days and 0.2380 days were obtained for Gaia DR3 5255393833932660992 and Gaia DR3 5255378372049581568, respectively, in agreement with those reported in the literature.

We emphasise that aperture data is not ideal for studying variability in crowded fields, and PSF data is more desirable, or even differential imaging analysis (DIA), in order to obtain more accurate photometry and thus reduce the scattering on the light curves. We also note that there is a potential problem due to the different epochs having slightly different zero points, so it is necessary to recalibrate the different epochs to a reference zero point. This preliminary result using CASU aperture data shows that VVVX data will find variables even with a smaller number of epochs and a shorter time baseline compared to VVV, especially in the innermost disk (|b| < 1 deg), where surveys carried out in the optical, for instance by Gaia or the upcoming Vera C. Rubin Observatory with its 3200 megapixel camera, are highly affected by extinction.

As described above, the VIRAC catalogue with the proper motion measurements based on the PSF data should be available soon for the VVVX area. While VIRAC2 covers the original VVV area using VVV+VVVX observations from 2010–2019, VIRAC3 will cover the whole VVV+VVVX area. VIRAC3 is currently being generated.

Regardless, we applied a different method to the CASU aperture data, by subtracting the differences of the RA/DEC between the latest and the first epochs observed in Ks for e1084 for Ks < 16 mag stellar sources (‘−1’ flag) in both epochs, without any other type of calibration or selection criteria. Even with this naive PM approach, the bulk motion is in agreement with that expected motion for stars in the Galactic disk (e.g. Gaia Collaboration 2023), with <PMra cos Dec> = −6.40 ± 5.46 mas yr−1 and <PMDec> = −1.53 ± 5.02 mas yr−1 (see Fig. 11). Given that Gaia is severely limited in the inner disk areas, only about 40% of stellar sources as faint as Ks > 16.0 mag are detected, compared to the CASU source catalogues. These objects are likely the closest to us, that is, those with bluer colour in the CMD. Since Gaia does not reach the stars closer to the Galactic centre, VVVX and VVV are invaluable for PM studies in those areas. This is even more evident when comparing Gaia with PSF based source catalogues.

thumbnail Fig. 9

Comparative Ks × (J − Ks) colour-magnitude diagrams. Top: CASU aperture photometry for stellar sources (flag −1) for the entire e1084 tile (left), for the region of the Carina Nebula shown in Fig. 7 (centre) and for the stellar cluster NGC 3324 (right panel, 6 arcmin radius area). The sharp end at Ks = 17.9 is caused by the stellar flag criterion applied. Bottom: the CMD for the same areas in the top using PSF photometry. In the CMDs for the entire tile, the narrow and vertical structure seen at (J − Ks) ~ 0.9 is the sequence of unreddened nearby M dwarfs (e.g. Alonso-García et al. 2018; Mejías et al. 2022).

8 The VVVX science returns

A large public survey such as the VVVX is expected to have a broad impact in different areas of astrophysics. As useful references for the reader interested in using the VVVX database, in this section we briefly summarise the published works on planetary, stellar, Galactic, and extragalactic astronomy, starting with those that include large-scale photometry and astrometry.

thumbnail Fig. 10

VVVX Ks band light curves for two Gaia RR Lyrae found in tile e1084. Gaia DR3 5255393833932660992 is a fundamental mode RR Lyrae (RRab) with a period of P ~ 0.476 days while Gaia DR3 5255378372049581568 is a first-overtone RR Lyrae (RRc) with P ~ 0.238 days. In the right panel, the phase folded light curve of Gaia DR3 5255393833932660992 is arbitrarily shifted by +1.3 mag for visualisation purposes.
thumbnail Fig. 11

Proper motion measurements for tile e1084. Top: histogram in Ks band for stellar sources in both the first and latest Ks band epochs observed for e1084. About 60% of the sources have counterparts in Gaia DR3. This number dropped to ≲40% for Ks > 16.0 mag. Bottom: Naive PM measurements based on the differences of the RA/Dec between the latest and the first epochs. The grid pattern is an artefact of the binning algorithm used for plotting, and is not a real feature. Only stellar sources with Ks < 16.0 have been used and the timespan is ~4.2 yr (1530 days). The mean values are PMRA cos Dec = −6.40 ± 5.46 mas yr−1 and PMDec = −1.53 ± 5.02 mas yr−1.

8.1 Photometry and astrometry

As discussed above, Alonso-García et al. (in prep.) will extend the near-IR atlas from the VVV footprint (Alonso-García et al. 2018) to the VVVX new surveyed regions in the Galactic plane, using PSF-fitting techniques to extract the photometry. Smith et al. (in prep.) will also extend the astrometric and multi-epoch photometric dataset from the original VIRAC catalogue (Smith et al. 2018) using PSF photometry and adding the new epochs provided by the VVVX.

8.2 Variable stars and Galactic astronomy

Regarding variable stars, Alonso-García (2021) described the potential of classical pulsators such as RR Lyrae, Type 2 Cepheids, and Classical Cepheids for tracing the structure of the inner Galactic bulge and Disk in the VVVX region. In addition, Daza-Perilla et al. (2023a) performed the automated classification of eclipsing binary systems discovered in the extended database, that are also useful tracers across the Galactic plane.

Molnar et al. (2022) performed a massive classification of variable stars across the Galactic bulge and disk using the VVV and VVVX survey data, automatically classifying 1.4 million point sources. In a follow-up study, Sanders et al. (2022) studied the Mira variable stars in the innermost regions of the MW, using these luminous variables as tracers of the nuclear stellar disk. This sample was also used to estimate the epoch of the Milky Way’s bar formation using detailed dynamical modelling (Sanders et al. 2024).

More recently, Lucas et al. (2024) carried out a massive search for the highest amplitude infrared variables, discovering a number of eruptive protostars, a new class of variables named dipping giants, as well as a variety of other interesting sources (transients, LPVs, microlensing events, etc.), thus enabling a variety of follow up studies. For example, Guo et al. (2024) presented a detailed, unique multi-wavelength study of an ongoing FUor-type outburst. VVVX will enable to develop a unified picture of eruptive Young Stellar Objects (YSOs) by longer term monitoring of slow variables and long-lasting YSO eruptions, to bridge the eruption timescale gap between EXors (weeks-months) and FUors (>10 yr).

Large surveys also open the door for serendipitous discoveries, which often lead to new science (e.g. Wolf et al. 2024). In this context, Saito et al. (2023) reported the discovery of VVV-WIT-12, which appears to be a 4-yr-long period YSO that induces variability in its surrounding nebula, and discuss the different possible scenarios that include a light echo or a precessing circumstellar disk.

8.3 Background galaxies

The VVVX images are deep enough to see through the MW, also enabling studies of background galaxies in the ZoA. As an example, the Circinus galaxy is the nearest known Seyfert II galaxy, located at a distance D = 4 Mpc. Obasi et al. (2024) carried out a search for globular clusters (GCs) in the halo of the Circinus galaxy, discovering dozens of bonafide GC candidates.

More distant galaxies can also be traced. Baravalle et al. (2021) and Duplancic et al. (2024) discovered thousands of galaxies in the ZoA behind the MW disk and bulge, respectively, that are useful to complete the picture of large-scale structure in the region. Daza-Perilla et al. (2023b) published an extensive near-IR galaxy catalogue in the northern part of the Galactic disk using machine learning techniques for the first time in these regions. Also, Marchant Cortés et al. (2024) explored the classification (and misclassification) of galaxies performed by Zhang, Zhao, & Wu (2021) using machine learning tools in the 4XMM-DR9 database.

Clusters of galaxies were also discovered in the near-IR database. Galdeano et al. (2022) published a deep near-IR view of the Ophiuchus galaxy cluster, the second-brightest X-ray cluster of galaxies in the sky, after the Coma cluster. In addition, Galdeano et al. (2023) unveiled a new such structure behind the MW.

8.4 Low-mass stars and exoplanets

The VVVX near-IR database is also useful for studies in the area of low luminosity stars and sub-stellar objects (M dwarfs, white dwarfs, brown dwarfs, and giant planets). Mejías et al. (2022) presented deep VVVX near-IR photometry for 99 low-mass stars in the Gaia EDR3 Catalogue of Nearby Stars, very useful for the characterisation of individual objects and for the identification of new faint objects in our vicinity. Cáceres et al. (2024) reported the discovery of a dozen planetary-mass binaries in the Lower Centaurus-Crux association. Also, Ferreira et al. (2024) presented the study of a benchmark White Dwarf – Ultracool Dwarf wide field binary in the Galactic plane.

8.5 Star clusters

The VVVX depth and resolution permits to see deep into our own Galaxy (Minniti et al. 2024). Therefore, the database also enabled the discovery of numerous star clusters in the reddened and crowded regions of the Galactic disk and bulge.

The works of Borissova et al. (2018, 2019, 2020); Peña Ramírez et al. (2021, 2022) identified dozens of open star clusters, either new or known, using VVV(X) data, enriching their near-infrared cluster sequences. This allowed the authors to characterise the clusters at different mass ranges, probing the small scale star formation in the Galactic disk.

Numerous old globular cluster were also identified and characterised in the VVVX areas of the disk and bulge (Bica et al. 2018; Obasi et al. 2021; Gran et al. 2019, 2022; Garro et al. 2020, 2021a, 2022, 2023; Fernández-Trincado et al. 2021, Saroon et al. 2024) and in the Sagittarius dwarf galaxy located behind the bulge (Garro et al. 2021b; Minniti et al. 2021a). The near-IR photometry allows the determination of some important cluster parameters, such as reddening, distance, luminosity, metallicity, mass, structure, and age, contributing to the understanding of the Milky Way GC system as a whole (Minniti et al. 2021b, Garro et al. 2024).

8.6 Long timescale microlensing events

VVV has demonstrated its ability to detect microlensing events, particularly in highly obscured and crowded fields, where such events are more frequent (Navarro et al. 2017, 2018, 2020a,b). Long-duration microlensing events exceeding one year are excellent candidates for stellar mass black holes. The extended time baseline of VVVX observations will enable the discovery of such events around the Galactic centre and plane, where models predict the presence of stellar mass black holes (e.g. Bahcall & Wolf 1976; Freitag et al. 2006; Alexander & Hopman 2009).

9 Survey legacy

The VVV and VVVX surveys are the result of more than 4000 h of observation with the most advanced ground-based near-IR facility. Although VVV+VVVX had observed ~4% of the celestial sphere, the region contains the majority of the Milky Way’s stars, as well as the largest concentration of gas and dust in the Galaxy.

There are currently no other near-IR projects with similar characteristics to VVV+VVVX, such as time-baseline, wavelength range, photometric depth, and most importantly, the large projected area. However, the legacy of both surveys is much enhanced by complementary data from DECaPS and Pan-STARRS (Chambers et al. 2016), that also cover the MW bulge and southern plane. Here we must also mention the complementary survey VISIONS (Meingast et al. 2023) that targets individual star forming regions in the MW.

In the future projects, such as the Vera C. Rubin Observatory which will use optical wavelengths for massive variability, it will also be very complementary. Also complementary are the JWST observations, with instruments covering a small field of view but with much higher resolution and depth (e.g., Sec. 7). However, it would be unfeasible for the JWST to cover large areas with the same efficiency as our survey.

Clearly, the VVVX survey also serves as a vast source of targets for spectroscopic follow-up with future infrared spectroscopic surveys such as MOONS, 4MOST, MWM and the ESO Wide-field Spectroscopic Telescope (WST; Mainieri et al. 2024). Our survey is also a source of targets for the various next-generation extremely large telescopes; and may only be surpassed in performance in the next decade by projects such as the Nancy Grace Roman Space Telescope, which will be able to produce deep infrared images of large regions, including the Galactic centre, with higher resolution from space (e.g. Paladini et al. 2023).

The VVVX (and VVV) legacy will last for many years to come. Despite the numerous results already obtained, the full exploitation of the data will take many years more, becoming more attractive with each new product that can be obtained, as detailed extinction (or even metallicity) maps, catalogues of variable sources and transient objects, and even studies of background galaxies, especially in the ZoA beyond the MW plane.

10 Summary

In March 2023, we finished successfully the VVVX survey observations that started in 2016, which represents a huge amount of data and processing, comprising ~200000 images that monitor >109 sources in our Galaxy and beyond. This is quite an accomplishment for VISTA, for ESO’s Paranal Observatory, for the CASU data reduction pipeline, for the teams extracting PSF photometry from the images, for the VISTA Science archive in Edinburgh and for the ESO archive: the successful VVVX observations are now 100% completed. We have described the survey, including the observations, areal and temporal coverage, reductions, photometry, and astrometry. We have also presented some specific scientific examples for database usage, providing some key useful references.

Clearly, there are many more applications of this ESO Public Survey for the community to exploit for future studies of Galactic structure, stellar populations, variable stars, star clusters of all ages, among other exciting research areas, from stellar and (exo)planetary astrophysics to extragalactic studies. The image processing, data analysis and scientific exploitation will continue for the next few years, with many discoveries yet to come. The VVVX Survey will also be combined with future facilities to boost its scientific outcome in unpredictable ways: we are sure that this survey will remain a goldmine for MW studies for a long time.

Acknowledgements

We gratefully acknowledge the use of data from the ESO Public Survey program IDs 179.B-2002 and 198.B2004 taken with the VISTA telescope and data products from the Cambridge Astronomical Survey Unit (CASU) and the VISTA Science Archive (VSA) and the ESO Science Archive. VVV and VVVX data are published in the ESO Science Archive in the data collections identified by the following DOIs: https://doi.eso.org/10.18727/archive/67 and https://doi.eso.org/10.18727/archive/68. R.K.S. acknowledges support from CNPq/Brazil through projects 308298/2022-5, 350104/2022-0 and 421034/2023-8. D.M. gratefully acknowledges support from the Center for Astrophysics and Associated Technologies CATA by the ANID BASAL projects ACE210002 and FB210003, by Fondecyt Project No. 1220724, and by CNPq/Brazil through project 350104/2022-0. J.A.-G. acknowledges support from Fondecyt Regular 1201490 and by ANID – Millennium Science Initiative Program – ICN12_009 awarded to the Millennium Institute of Astrophysics MAS. Support for J.B. and R.K. are provided by ANID’s FONDECYT Regular grant #1240249; ANID’s Millennium Science Initiative through grants ICN12_009 and AIM23-0001, awarded to the Millennium Institute of Astrophysics (MAS). C.C. acknowledges support by ANID BASAL project FB210003. N.J.G.C. acknowledges support from the UK Science and Technology Facilities Council. E.B.A. thanks Universidade Estadual de Feira de Santana for the support received by the Program FINAPESQ (project number 050/2021). L.R.B. acknowledges financial support by INAF under WFAP project, f.o.:1.05.23.05.05. J.I.A. acknowledges the financial support of DIDULS/ULS, through the project PR2324063. A.B. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC 2094 – 390783311. B.D. acknowledges support by ANID-FONDECYT iniciación grant No. 11221366 and from the ANID Basal project FB210003. A.C.G. acknowledges support from PRIN-MUR 2022 20228JPA3A “The path to star and planet formation in the JWST era (PATH)” funded by NextGeneration EU and by INAF-GoG 2022 “NIR-dark Accretion Outbursts in Massive Young stellar objects (NAOMY)” and Large Grant INAF 2022 “YSOs Outflows, Disks and Accretion: towards a global framework for the evolution of planet forming systems (YODA)”. J.A.C-B. acknowledges support from FONDECYT Regular N 1220083. Support for M.C. is provided by ANID’s FONDECYT Regular grant #1171273; ANID’s Millennium Science Initiative through grants ICN12_009 and AIM23-0001, awarded to the Millennium Institute of Astrophysics (MAS); and ANID’s Basal project FB210003. M.C.C. acknowledges financial support from the Universidad Complutense de Madrid (UCM) and the Agencia Estatal de Investigación (AEI/10.13039/501100011033) of the Ministerio de Ciencia e Innovación and the ERDF “A way of making Europe” through projects PID2019-109522GB-C5[4] and PID2022-137241NBC4[4]. P.C. and E.S. acknowledge financial support from the Spanish Virtual Observatory project funded by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 through grant PID2020-112949GB-I00. V. Motta acknowledges support from ANID FONDECYT Regular grant number 1231418. J.G.F.-T. gratefully acknowledges the grants support provided by ANID Fondecyt Iniciación No. 11220340, ANID Fondecyt Postdoc No. 3230001 (Sponsoring researcher), from the Joint Committee ESO-Government of Chile under the agreement 2021 ORP 023/2021 and 2023 ORP 062/2023. Support for C.E.F.L. is provided by the ANID/FONDECYT Regular grant 1231637. D.G. gratefully acknowledges the support provided by Fondecyt regular no. 1220264. D.G. also acknowledges financial support from the Dirección de Investigación y Desarrollo de la Universidad de La Serena through the Programa de Incentivo a la Investigación de Académicos (PIA-DIDULS). The work of F.N. is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. W.G. gratefully acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement 951549 (project UniverScale). F.G. gratefully acknowledges support from the French National Research Agency (ANR) – funded projects “MWDisc” (ANR-20-CE31-0004) and “Pristine” (ANR-18-CE31-0017). Z.G. is supported by the ANID FONDE-CYT Postdoctoral program No. 3220029. E.L.M. is supported by the European Research Council Advanced grant SUBSTELLAR, project number 101054354. V.M. acknowledges support from project DIDULS Regular No PR2353857. M.C. thanks the support of ANID BECAS/DOCTORADO NACIONAL 21110001. I.P. acknowledges support from ANID BECAS/DOCTORADO NACIONAL 21230761. G.P. acknowledges support from ANID through Millennium Science Initiative Programs ICN12 009. S.R.A. acknowledges support from Fondecyt Regular 1201490. E.S. acknowledges financial support from the Spanish Virtual Observatory project funded by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 through grant PID2020-112949GB-I00. D.S. acknowledged support from the Science and Technology Facilities Council (STFC, grant numbers ST/T007184/1, ST/T003103/1, ST/T000406/1 and ST/X001121/1). M.T. is supported by the JSPS Kakenhi No. 24H00242. P.B.T. gratefully acknowledge support by the ANID BASAL project FB210003 and Fondecyt 1240465. S.V. gratefully acknowledges the support provided by Fondcyt regular n. 1220264. SV gratefully acknowledges support by the ANID BASAL projects ACE210002 and FB210003. C.N.M. gratefully acknowledges support from the Research Department of the Austral University of Chile, Puerto Montt Campus (Project DIPM-CIB2303). Financial support for this work was also provided by the ANID BASAL Center for Astrophysics and Associated Technologies (CATA) through grants AFB170002, ACE210002 and FB210003, by the ANID Millennium Institute of Astrophysics (MAS) ICN12_009 and by ANID Fondecyt Regular 1230731 (PI: M.Z.). We are also deeply thankful to our dear colleague Rodolfo Barbá, who was a pillar for the survey but sadly passed away in late 2021, may he rest in peace.

Appendix A VVVX tile coordinates and observations

Here we list the tile centre coordinates for all VVV and VVVX pointing. There is a total of 1028 tiles, divided into 348 for the original VVV area and 680 tiles for VVVX. For the original and extended bulge area tiles, names start with ‘b’. Inner disk tiles in the original and extended area start with ‘d’, while for the low and high disk, as well as to disk to longitude +20 names start with ‘e’. Fig. A.1 shows the survey area with the tiles positions and respective names. For each tile we provide tile centre coordinates in Equatorial and Galactic systems. All tiles have been observed using an identical offsetting strategy, combining six pawprints to contiguously fill 1.5 × 1.1 sq. deg. area. Columns 6, 7 and 8 present the number of epochs taken in J, H and Ks during the VVV and VVVX campaigns. The first number is the total of epochs, and in parentheses the number of epochs observed in VVV and VVVX, respectively.

thumbnail Fig. A.1

Zoomed view of the VVVX survey area with the tiles positions and respective names. The original VVV area in shown in red, while the other colours mark the VVVX areas. For the original and extended bulge area tile, names start with ‘b’. Inner disk tiles in the original and extended area start with ‘d’, while for the low and high disk, as well to disk to longitude +20 names start with ‘e’.
Table A.1

List the VVV+VVVX tiles and observed number of epochs.

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4

The full table with the tile names and the central coordinates is available at https://zenodo.org/records/12587535

All Tables

Table 1

VISTA filters used in the VVVX observations.

In the text
Table 2

Total exposure time for the VVVX observations.

In the text
Table 3

Summary of the VVVX observational campaign.

In the text
Table 4

Multicolour observations of tile e1084.

In the text
Table A.1

List the VVV+VVVX tiles and observed number of epochs.

In the text

All Figures

thumbnail Fig. 1

VVV+VVVX survey coverage of the MW bulge and southern plane. Top: the surveyed area shown Galactic coordinates. Our near-IR survey covers ~1700 sq. deg. in total, and the different regions are colour-coded according to the location, baseline, and number of observations (see Sec. 4). In red is the original VVV Bulge and VVV Disk, with observations from years 2010 and 2016 using the ZY JHKs filters, and VVVX observations between years 2016 and 2022 using the JHKs filters. Other colours mark the VVVX areas, observed with JHKs filters between years 2016 to 2022. Yellow is the Disk to Longitude +230, dark blue is the Low and High Extended Disk, green is the Disk to Longitude +20, and in light blue is the Low and High Extended Bulge. A zoomed view of the image with the tile names is presented in Appendix A. Bottom: Schematic representation of the areal coverage compared with the other selected complementary surveys mentioned in Sect. 1.
In the text
thumbnail Fig. 2

Number of epochs in the Ks band observed by VVV+VVVX. The mean baseline for the original area is ~12 yr, with up 352 epochs for selected tiles in the inner bulge. For the outer bulge and disk the number of epochs are in the range 23–106 for the VVVX observations solely, varying according with the observational strategy.
In the text
thumbnail Fig. 3

Cadence of the VVV+VVVX bulge observations. Top: density plot showing the cadence of the bulge observations in both VVV and VVVX campaigns. Bottom: light curve example for the source VVV-VIVACE ID 533558 (Ks − 14.9 mag; Molnar et al. 2022), observed by VVV+VVVX in the five VISTA broad band filters in the bulge field b307. There are a total of 363 observations in the five filters along years 2010 to 2019. The coordinates for the target are RA, Dec (J2000) = 18:00:11.48, −28:25:13 (corresponding to l, b = 2.0688, −2.4904 deg.).
In the text
thumbnail Fig. 4

5σ magnitude limits of the catalogues in the J, H, and Ks, respectively, from top to bottom. The colour scale is the same in the three panels as shown in the vertical bars at the right. For the J and H bands, VVV and VVVX are combined because of the lack of VVVX observation in these bands in the original VVV area.
In the text
thumbnail Fig. 5

Image quality and airmass cumulative distributions for the JHKs VVVX observations, in the top and bottom panels, respectively. The median values of image quality and airmass for each filter are presented in the legends.
In the text
thumbnail Fig. 6

Cumulative distributions for the JHKs observations during the VVVX campaign.
In the text
thumbnail Fig. 7

VVVX composite image of the Carina Nebula compared with the JWST image of the same field. The VVVX image (top) is in false colour and based on the JHKs observations while the JWST image (bottom) comprises separate exposures containing F090W, F187N, F200W, F335M, F444W and F470N NIRCam filters. The Carina Nebula is located towards tile e1084 in the Disk to Longitude +230 region of VVVX, with the JHKs observations secured on March 19, 2018. The images have ~ 9 × 7 arcmin size and are oriented with the north to the right and east to the top. Credits (JWST image): NASA, ESA, CSA, and STScI, J. DePasquale (STScI).
In the text
thumbnail Fig. 8

Individual J (left), H (centre) and Ks band images (right) of the star cluster NGC 3324. The image is centrered at RA/DEC (J2000)=10:37:21.8, −58:36:54, with 9 arcmin side and oriented in Galactic coordinates.
In the text
thumbnail Fig. 9

Comparative Ks × (J − Ks) colour-magnitude diagrams. Top: CASU aperture photometry for stellar sources (flag −1) for the entire e1084 tile (left), for the region of the Carina Nebula shown in Fig. 7 (centre) and for the stellar cluster NGC 3324 (right panel, 6 arcmin radius area). The sharp end at Ks = 17.9 is caused by the stellar flag criterion applied. Bottom: the CMD for the same areas in the top using PSF photometry. In the CMDs for the entire tile, the narrow and vertical structure seen at (J − Ks) ~ 0.9 is the sequence of unreddened nearby M dwarfs (e.g. Alonso-García et al. 2018; Mejías et al. 2022).
In the text
thumbnail Fig. 10

VVVX Ks band light curves for two Gaia RR Lyrae found in tile e1084. Gaia DR3 5255393833932660992 is a fundamental mode RR Lyrae (RRab) with a period of P ~ 0.476 days while Gaia DR3 5255378372049581568 is a first-overtone RR Lyrae (RRc) with P ~ 0.238 days. In the right panel, the phase folded light curve of Gaia DR3 5255393833932660992 is arbitrarily shifted by +1.3 mag for visualisation purposes.
In the text
thumbnail Fig. 11

Proper motion measurements for tile e1084. Top: histogram in Ks band for stellar sources in both the first and latest Ks band epochs observed for e1084. About 60% of the sources have counterparts in Gaia DR3. This number dropped to ≲40% for Ks > 16.0 mag. Bottom: Naive PM measurements based on the differences of the RA/Dec between the latest and the first epochs. The grid pattern is an artefact of the binning algorithm used for plotting, and is not a real feature. Only stellar sources with Ks < 16.0 have been used and the timespan is ~4.2 yr (1530 days). The mean values are PMRA cos Dec = −6.40 ± 5.46 mas yr−1 and PMDec = −1.53 ± 5.02 mas yr−1.
In the text
thumbnail Fig. A.1

Zoomed view of the VVVX survey area with the tiles positions and respective names. The original VVV area in shown in red, while the other colours mark the VVVX areas. For the original and extended bulge area tile, names start with ‘b’. Inner disk tiles in the original and extended area start with ‘d’, while for the low and high disk, as well to disk to longitude +20 names start with ‘e’.
In the text

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