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A&A
Volume 686, June 2024
Article Number A171
Number of page(s) 19
Section Galactic structure, stellar clusters and populations
DOI https://doi.org/10.1051/0004-6361/202348769
Published online 14 June 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

Ultracool dwarfs (UCDs), including the lowest-mass stars, brown dwarfs (BDs), and planetary mass objects (PMOs), are the dimmest, coldest objects among our Galactic population. As the name suggests, these objects can have effective temperatures from 2700 K (Kirkpatrick et al. 1995) down to around 250 K, the lowest known so far (Luhman 2014). The established spectral classes for UCDs are the late-M type (M7 and later, Kirkpatrick et al. 1997), the L type (Martín et al. 1997, 1998, 1999; Kirkpatrick et al. 1999, 2000), the T type (Burgasser et al. 2002; Geballe et al. 2002; Burgasser et al. 2006), and recently the Y type (Delorme et al. 2008; Cushing et al. 2011; Kirkpatrick et al. 2011) with decreasing temperature.

Nowadays, we believe that the census of UCD is still incomplete beyond 20 pc. The coldest population with spectral types later than T8 has not been explored thoroughly even within 20 pc (Kirkpatrick et al. 2021). An appropriate method of discovering a UCD population is a deep, wide survey at the near-infrared (NIR) wavelength (Cushing 2014). This is because UCDs are faint and have their emission peaks in the NIR, and they are scattered all over the sky.

Space telescopes such as the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST), or space surveys like the Gaia mission (Gaia Collaboration 2016) and the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010), have the great advantage of being free of atmospheric influences compared with ground-based facilities. HST has discovered many UCDs in its deep fields (Pirzkal et al. 2005, 2009; Stanway et al. 2008; Ryan et al. 2005, 2011; Holwerda et al. 2014; van Vledder et al. 2016; Aganze et al. 2022a, 2022b). JWST has also discovered many UCDs in several fields of its Early Release Observations (Wang et al. 2023; Nonino et al. 2023; Holwerda et al. 2024) and galactic surveys (Hainline et al. 2024), including a discovery of the farthest BD to date at 4.8 kpc (Langeroodi & Hjorth 2023). JWST also demonstrated its power in characterising UCDs (Beiler et al. 2023; Burgasser et al. 2024; Holwerda et al. 2024; Manjavacas et al. 2024). Gaia not only discovers UCDs in the whole sky (Reylé 2018; Sarro et al. 2023) but also performs low-resolution spectroscopy on UCDs (Cooper et al. 2024); its precise astrometry reveals UCD companions of the other stars (Stevenson et al. 2023). WISE has also contributed to UCD discoveries (Kirkpatrick et al. 2011), especially the coldest population (Cushing et al. 2011, 2014), thanks to its specially designed passbands (Wright et al. 2010) and whole-sky coverage. Next-generation space telescopes like the Nancy Grace Roman Space Telescope will offer a great opportunity for UCD sciences as well (Holwerda et al. 2023).

Euclid1, a 1.2-m space telescope launched on July 1 2023, is a European Space Agency (ESA) mission aiming to conduct a deep survey covering ≈14 500 deg2 of the sky in the coming six years. Euclid is equipped with optical and infrared capabilities, with the Visible Instrument (VIS) and the Near Infrared Spectrometer and Photometer (NISP) on board. Compared with HST or JWST, a huge advantage of Euclid is its wide field of view (FoV). The VIS has an FoV of 0.787 deg × 0.709 deg in the IE band; the NISP covers the YE, JE, and HE bands and has an FoV of 0.763 deg × 0.722 deg. The Euclid Wide Surveys (EWSs) will have a 5-σ limit of 26.2 mag, 24.3 mag, 24.5 mag, and 24.4 mag in the IE, YE, JE, and HE bands, respectively (Euclid Collaboration 2022b). Compared with Gaia, Euclid is much more sensitive to UCDs. Euclid has a much better spatial resolution than WISE. The NISP can also perform low-resolution slitless spectroscopy (R ≈ 380 for point sources) from 1.25–1.85 μm by the red grism and 0.92–1.30 μm by the blue grism, but only in the EDFs (Euclid Collaboration 2023). The spectroscopy enables the spectral classification of UCDs. Euclid will be a powerful tool with which to hunt and characterise undiscovered UCDs (Solano et al. 2021; Martín et al. 2021).

Table 1.

Targets selected for spectroscopic observation.

thumbnail Fig. 1.

UCD candidates in the three EDFs (black borders). The circled objects are the ones for which we will get spectra.

Three EDFs2, North, Fornax, and South, are the patches of the sky that Euclid will repeatedly scan during its lifetime. They consist of a total of 53 deg2 of sky. After the co-adding, the depth in these regions will be about two magnitudes deeper than the EWSs. For point sources, it will reach an unprecedented 5-σ limit about 26.3, 26.5, 26.4 mag (AB system) in the YE, JE, and HE bands, respectively (Euclid Collaboration 2022b,a, 2023). Euclid will unveil UCDs far beyond the Galactic disc. In addition, monthly visits makes precise astrometry possible, which is key to measure parallaxes and proper motions of UCDs. The parallax is indispensable in determining the lumininosity of UCDs; the proper motion together with the radial velocity from a coarse spectroscopic measurement can classify the objects into different kinematic groups. The Large Synoptic Survey Telescope (LSST) of the Vera C. Rubin Observatory at Cerro Pachón, Chile will complement Euclid’s coverage, especially the two southern EDFs at optical wavelengths in depth. (Rhodes et al. 2017).

Euclid has YE, JE, HE-band filters and photometric systems that are different from the NIR systems of ground-based surveys such as the 2MASS, Mauna Kea Observatories (MKO), or the VHS (Euclid Collaboration 2022c). The Euclid optical filter is much broader than the i or z filters of the PS1 and the DES as well. In addition, the ground-based surveys suffer a lot from telluric attenuation. A correction must be applied to the photometric measurements, if we are to use the ground-based data, before they can serve towards the calibration of Euclid. Euclid Collaboration (2022c) presented linear transformations between the Euclid NISP photometric system and the other ground-based photometric system for M, L, and T-type UCDs. It fitted the simulated photometry of M, L, and T dwarfs from Euclid Collaboration (2019), who used observational templates to calculate corresponding colours. However, the transformations have great residuals. Therefore, having a list of UCDs characterised in the three EDFs prior to the start of the Euclid surveys is important. They serve as photometric and spectroscopic standard anchors before the very first nominal survey observations of Euclid and they will help Euclid to calibrate itself every time it goes back and points at the EDFs.

In this work, we selected UCD candidates as photometric and spectroscopic standards in the three EDFs: North, Fornax, and South. These photometric standards and reconnaissance spectral templates are ready for the first data release of Euclid. Section 2 explains the UCD selection. We spectroscopically characterised their representatives using the 10.4-m Gran Telescopio Canarias (GTC) on the Spanish island of La Palma (for EDFs North and Fornax) and the European Southern Observatory (ESO) 8.2-m Very Large Telescope (VLT) on Cerro Paranal, Chile (for EDF South). Section 3 describes the observation details. Section 4 explains the spectral classification method. Section 5 shows the results of spectral analysis for each object, and discusses the capability of Euclid in terms of UCD characterisation.

2. UCD samples in the EDFs

We used the Tool for OPerations on Catalogues And Tables (TOPCAT; Taylor 2005) to cross-match existing catalogues and select UCD candidates in specific areas. We used different criteria and techniques to make the selection in the three different regions.

2.1. Euclid Deep Field North

EDF North originally was a 10 deg2 circled area centered at the coordinate 17h58m55.9s +66°01′03.7#x2033; with a radius of 1.78 deg. In April 2022, a request to extend EDF North’s area to 20 deg2 (radius extended to 2.52 deg) was endorsed by ESA based on the need for a larger area of spectroscopic calibration.

We collected information on the i and z bands from the Panoramic Survey Telescope and Rapid Response System release 1 (Pan-STARRS, PS1; Chambers et al. 2016), J, H, and K bands from the Two Micron All Sky Survey (2MASS; Cutri et al. 2003; Skrutskie et al. 2006), and W1, 2, 3, 4 bands from the AllWISE programme (Cutri et al. 2021) of WISE. We cross-matched all the point sources in these three catalogues with a radius of 3 arcsec inside EDF North.

To select the UCDs, we applied photometric criteria from Carnero Rosell et al. (2019), but we generalised and slightly loosened the criteria according to the colours presented in Schmidt et al. (2015); Skrzypek et al. (2016). In practice, for late-M UCDs, we required

  • i − z >  0.65 mag;

  • 2.68 <  i − J <  4.0 mag;

  • 1.59 <  z − J <  2.55 mag;

  • z − W1 <  4.5 mag;

  • J − H >  0.47 mag;

  • J − W2 >  1.36 mag;

  • H − K >  0.17 mag;

  • 0.13 <  W1 − W2 <  0.25 mag; and

  • S/NW4 ≤ 3.

The colour criterion, W1 − W2 >  0.13 mag, already separates most of the red giants from the dwarfs (Li et al. 2016). To further ensure that we would not include any possible red giants, which could possess big and cooler envelopes, a non-detection criterion in the W4 was set by limiting the signal-to-noise ratio (S/N) of this band. We found 79 late-M UCDs. We note that in our search we did not classify the objects into subclasses.

Reylé (2018) selected UCDs from Gaia DR2 based on the locus of spectroscopically confirmed UCDs in the Hertzsprung–Russel (HR) diagram. There are in total 12 UCDs, all late-M type with tentative spectral classification (eight M7 dwarfs, three M7.5 dwarfs, and one M7 subdwarf) in EDF North. Our search recovered ten of these 12 M dwarfs in the catalogue of Reylé (2018). The other two have a slightly bluer J − W2 colour. We decided to include these two objects. Therefore, we have 81 late-M UCDs in EDF North (Table A.1).

For L-type UCDs, we required redder i − J, z − J, and z − W1 colours (Carnero Rosell et al. 2019):

  • i − z >  1.35 mag;

  • i − J ≥ 4.0 mag;

  • z − J ≥ 2.55 mag;

  • z − W1 ≥ 4.5 mag;

  • J − H >  0.47 mag;

  • J − W2 >  1.36 mag;

  • H − K >  0.17 mag;

  • W1 − W2 ≥ 0.25 mag;

  • S/NW4 ≤ 3.

We found eight L-type UCDs (Table A.2).

For T dwarfs, because methane absorption starts to play a role in the NIR, the J − H and H − K colours turn bluer when the type gets later (Leggett et al. 2003). Hence, we separated the criteria of the early-type and late-type T dwarfs. For early-T-type UCDs, we required

  • z − J >  1.59 mag;

  • 1 <  W1 − W2 <  2 mag;

  • S/NW4 ≤ 3,

and for late-T-type UCDs, we required

  • J − H <  −0.3 mag;

  • H − K <  0.2 mag;

  • W1 − W2 >  1.5 mag; and

  • S/NW4 ≤ 3.

Unfortunately our search found no T dwarf in EDF North. However, we noticed that there is a T7 dwarf WISE J174556.65+645933.8 in EDF North revealed by Mace et al. (2013) in WISE. This object is too faint to be detected by the PS1 or the 2MASS. The J-band spectrum was already obtained by Mace et al. (2013) using the Near-Infrared Spectrometer (NIRSPEC; McLean et al. 1998, 2000) of the 10-m Keck telescope on Mauna Kea, and was kindly provided to us by Dr. Gregory Mace.

We selected the M dwarf WISE1757 and the L dwarf WISE1754 for spectroscopic characterisation (Table 1). We note that WISE1754 could be an L-dwarf binary system consisting of Gaia DR3 1633622185772907392 (hereafter WISE1754 A) and Gaia DR3 1633622181475606528 (hereafter WISE1754 B), which were not resolved in WISE and the 2MASS. WISE1754 B is bluer than WISE1754 A. The left panel in Fig. 1 shows the UCD candidates in EDF North.

2.2. Euclid Deep Field Fornax

Euclid Deep Field Fornax is a 10 deg2 circled area centered on the coordinate 03h31m43.6s −28°05′18.6#x2033; with a radius of 1.78 deg. Carnero Rosell et al. (2019) did a UCD census in an area about 2400 deg2, using the i, z, and Y bands of the Dark Energy Survey (DES; Dark Energy Survey Collaboration 2016) Year 3 release, matched to the J, H, and Ks bands of the Vista Hemisphere Survey (VHS; McMahon et al. 2013) DR3 and the W1, W2 bands of WISE. We directly selected the UCD samples from this catalogue in the EDF Fornax field.

This catalogue returned 45 late-M dwarfs. We decided to include four extra late-M dwarfs from the Gaia UCD catalogue (Reylé 2018). At the end, we have 49 late-M dwarfs (Table B.1). It also returned 29 L dwarfs (Table B.2) and no T dwarf in EDF Fornax. All the L dwarfs are photometrically classified as early-L dwarfs (all of them have a spectral class ≤L2.0).

Despite no T dwarf being found in EDF Fornax, there is a T6 dwarf WISEA J033921.70−264906.5 (hereafter WISE0339) outside but close to the field (0.34 deg or 20 arcmin away from the northeast border). It is worth including it, considering the large and rectangular FoV of Euclid, and the possibility of changing of the area of EDFs in the future.

We selected an M dwarf, WISE0332, and the T dwarf WISE0339 for spectroscopic characterisation (Table 1). The middle panel in Fig. 1 shows the UCD candidates in and near EDF Fornax. The uneven UCD distribution is because the H-band VHS only covers about half of EDF Fornax.

2.3. Euclid Deep Field South

Euclid Deep Field South is a 23 deg2 octagon stadium-shaped area3 centered on the coordinate 04h04m57.84s −48°25′22.8#x2033;. We directly used the catalogues of Carnero Rosell et al. (2019) and Reylé (2018) as well.

We retrieved 213 late-M dwarfs (Table C.1). Only one late-M dwarf is in the 19 M dwarfs from the Gaia UCD catalogue of Reylé (2018). We end up with 231 late-M dwarfs in EDF South. We retrieved 115 L dwarfs (Table C.2) and two T dwarfs (Table C.3) in EDF South. Ten out of 115 L dwarfs are photometrically classified as mid-L dwarfs (L4.0-L6.0) and the rest are all early-L dwarfs. The two T dwarfs are late-T dwarfs: one T7.0 and the other T8.0. Figure 1 plots all the UCD candidates in EDF South.

2.4. Object for coherence check

It is essential to have a shared UCD target to evaluate the coherence between the two facilities. We thus chose a relatively bright L dwarf, WISEA J034209.37−290431.9 (Gagné et al. 2015), close to the EDF Fornax border. This object will still be covered by the EWSs and its position is shown in the middle panel of Fig. 1.

Table 2.

Summary of GTC/EMIR and VLT/X-shooter observations.

3. Observations and data reduction

We proposed a long-slit spectroscopic observation on a total of five UCD candidates in EDFs North and Fornax with their spectroscopic standards using the Espectrógrafo Multiobjeto Infra-Rojo (EMIR) on the GTC, programme GTC97-22B (PI J.-Y. Zhang). The Espectrógrafo Multiobjeto Infra-Rojo is a NIR wide-field imager and a multi-object spectrograph with a HAWAII-2 detector. The pixel size is 0.195#x2033;/pix. We used a 0.6 arcsec slit with YJ and HK grisms, covering 0.85–1.35 μm and 1.45–2.42 μm, respectively, yielding a low spectral resolution of R ≈ 987. We used a standard ABBA observing block sequence. Meanwhile, we asked for a dedicated imaging block of WISE1754 AB in the J band with a 7-point dithering pattern before its spectroscopic observation to confirm the companionship. During the spectroscopic observations, we put the slit directly on both objects to obtain their spectra simultaneously. We requested a seeing better than 0.9 arcsec, a clear sky, and no restriction on the moon phase for the GTC observations.

We also proposed a long-slit spectroscopic observation on a total of three UCD candidates in EDF South with spectroscopic standards using the X-shooter (Vernet et al. 2011) on the VLT, ESO programme 112.261S (PI J.-Y. Zhang). The X-shooter consists of three spectroscopic arms: UVB, VIS, and NIR. Each of the arms is an independent cross-dispersed echelle spectrograph. The UVB arm has an E2V CCD44-82 detector covering 0.30–0.56 μm with a pixel scale of 0.161#x2033;/pix; the VIS arm has a MIT/LL CCID 20 detector covering 0.56–1.02 μm with a pixel scale of 0.158#x2033;/pix; and the NIR arm has a Hawaii 2RG detector covering 1.02–2.48 μm with a pixel scale of 0.258#x2033;/pix. We asked for a standard ABBA observing block sequence and simultaneous observations in the three arms. For the UVB arm we used a 1.3 arcsec slit (R ≈ 4100). We used a 1.2 arcsec slit for both the VIS arm (R ≈ 6500) and the NIR arm (R ≈ 4300). We requested a seeing better than 1.5 arcsec, a sky condition of thin cirrus or better, and no restriction on the moon phase for the VLT observations. A summary of all the observations is provided in Table 2.

The 2D EMIR spectral frames were preliminarily reduced by the EMIR default pipeline PyEMIR4. The pipeline performed flat field correction, geometric distortion rectification, wavelength calibration (first using the empirical wavelength profile, and then using OH airglow lines in the images to refine the calibration), and ABBA stacking. The outputs are rectified and wavelength-calibrated 2D spectra. We also used the same pipeline to reduce the J-band image of WISE1754 AB. The pipeline performed flat field correction, sky subtraction, and stacking under the knowledge of the dithering pattern position. We then extracted 1D spectra in IRAF (Tody 1986).

The YJ and HK spectra of GTC/EMIR were not observed simultaneously. We convolved and normalised the spectra with the J and H filter profiles of full throughput of the 2MASS and the VHS5. We calculated the J − H colour according to the corresponding zero points of each filter. The difference between the observed J − H colour and the colour from the spectra is the coefficient that we should apply to adjust the YJ and HK spectra.

The 2D X-shooter data were reduced by the X-shooter pipeline inside the EsoReflex environment (Freudling et al. 2013). The pipeline produces flux-calibrated 1D spectra for each arm of X-shooter. Then the 1D spectra were corrected for the telluric contribution by Molecfit (Smette et al. 2015; Kausch et al. 2015) in EsoReflex. Because of the low S/N in UVB and VIS arm spectra, we only used NIR arm spectra starting from 1.02 μm in the future analysis.

4. Euclidisation and spectral classification

To ‘Euclidise’ the spectra, we used a Gaussian profile with a full width half maximum (FHWM) of 40 Å to convolve with our spectra (R ≈ 380 at 1.5 μm). We only consider the wavelength range of 0.92–1.86 μm for our spectra. The results mimic the spectra as if they were taken by the Euclid NISP with both blue and red grisms.

We collected the spectral templates from the L and T dwarf data archive6 of Chiu et al. (2006); Golimowski et al. (2004); Knapp et al. (2004) and the NIRSPEC Brown Dwarf Spectroscopic Survey7 (BDSS; McLean et al. 2003) to spectroscopically classify L and T dwarfs among our targets. We collected late-M dwarf templates and also early-L dwarf ones from the NASA Infrared Telescope Facility (IRTF) Spectral Library8 from Cushing et al. (2005); Rayner et al. (2009).

We normalised both the templates and our spectra. We used linearly distributed sampling points with a step of 0.1 Å in four wavelength ranges: 1.02–1.12 μm, 1.15–1.34 μm, 1.47–1.79 μm, and 1.97–2.30 μm to interpolate our spectra. Ultracool dwarf spectra in the EDFs will be covered by both red and blue grisms; hence, we interpolated the Euclidised spectra without the K band, which is not covered by Euclid. In particular, to Euclidise the J-band spectrum of WISE1745, we used the second wavelength range only. We emphasise that only the red grism will be used in the EWSs. We then interpolated the Euclidised spectra in two ranges: 1.25–1.34 μm and 1.47–1.79 μm. We applied the least squares method and found the best fit for the original spectrum and also the Euclidised spectrum of each object. The error was determined by the difference between the spectral subclass of the optimal solution and that of the two nearest points in the template grid.

5. Results

5.1. WISE1754 AB

With the J-band EMIR image and the images from the PS1 (Fig. 2), we excluded the possibility that WISE1754 AB is a co-moving binary system. WISE1754 A has a proper motion in a southeasterly direction, while WISE1754 B does not. The spectroscopy also proves that WISE1754 A is an L1 dwarf, while WISE1754 B is not a UCD at all.

thumbnail Fig. 2.

PS1 z-band image (left) and GTC/EMIR J-band image (right) of WISE1754 AB. The A component has a proper motion towards south-east direction, while the B component does not.

5.2. Common target WISE0342

thumbnail Fig. 3.

Normalised and Euclidised spectra of W0342 from both GTC/EMIR and the VLT/X-shooter. Strong telluric bands are masked. Features indicating the youth of the object are denoted. Both original spectra were classified as L0.0γ (upper panel). The spectral similarity underscores that GTC/EMIR and the VLT/X-shooter are coherent in characterising UCDs for Euclid. The two Euclidised spectra were all classified as L0.0γ (middle panel). The difference between the two Euclidised spectra is visualised in the lower panel.

The GTC/EMIR spectrum and the VLT/X-shooter spectrum exhibit strong agreement with each other, as is indicated in the upper panel of Fig. 3. The Euclidised spectra also demonstrate a high degree of similarity, as is illustrated in the middle panel of Fig. 3. The differences between two Euclidised spectra are within 0.1 unit of normalised flux, with some spikes of noise around 0.2 (the lower panels of Fig. 3). The coherence between the two instruments is good enough to characterise UCDs for Euclid in different EDFs.

In comparison with the field dwarf templates, both GTC/EMIR and VLT/X-shooter spectra have two similar local least squares minima at L4.5 and L7.0. The Euclidised GTC/EMIR spectrum has been classified as L4.5, and the Euclidised VLT/X-shooter spectrum has two similar local minima of the least squares at L5.5 and L7.0. The confusion of spectral type classification may be due to the youth and, hence, lower surface gravity of the object. For lower-gravity objects, in the J band, the FeH feature at 0.99 μm, and the potassium doublets are weaker than those of higher-gravity objects, while the VO absorption features at 1.06 and 1.18 μm are much more prominent. The H-band spectrum has a peaked, triangular shape for lower-gravity objects, due to stronger water absorption (Zapatero Osorio et al. 2000; Cushing et al. 2000; Lucas et al. 2001; Gorlova et al. 2003; McGovern et al. 2004; Lodieu et al. 2008, 2018; Scholz et al. 2012; Manjavacas et al. 2014; Mužić et al. 2015).

We decided to compare the W0342 spectra against young Lγ-type UCD templates in the Upper Scorpius association (5–10 Myr) (Lodieu et al. 2018). Our classification suggests a L0.0γ subclass, which agrees with the result of L0β from Gagné et al. (2015), but not with the photometric classification of L4.0 from Carnero Rosell et al. (2019). We demonstrate that Euclid slitless spectroscopy will be able to detect spectral indicators sensitive to the surface gravity or age, at least for late-M to early-L-type UCDs.

5.3. Spectral classes

Figure 4 depicts the original spectra and Euclidised spectra of the eight reconnaissance UCDs, except the common target WISE0342 (Fig. 3). The best-fit spectral templates are also overplotted. Saturated telluric wavelength regions were masked manually. The results and comparisons of spectral classification are summarised in Table 3. The accuracy of the spectral type depends on the density of the template grid.

thumbnail Fig. 4.

Normalised UCD spectra in the three EDFs. From the top to the bottom are UCDs from EDFs North, Fornax, and South, respectively. The VLT/X-shooter and GTC/EMIR spectra were smoothed with a 31 and 5-pixel-size window, respectively. All the spectra were classified into spectral subclasses, and the best-fit templates are plotted in grey (left panel). We Euclidised the spectra and the classifications remain consistent within 0.5 subclasses (right panel).

Table 3.

Summary of the spectral classification.

The spectral classifications of the reconnaissance UCDs are in line with their photometric spectral classifications from our search and from the selection of Carnero Rosell et al. (2019) and Reylé (2018), except for the young object WISE0342. The Euclidised spectra maintains the spectral classification results. We conclude that Euclid NISP slitless spectroscopy using two grisms in the EDFs can bring out an excellent spectral classification for UCDs, with a precision of 0.5 subclass without any major problems. With only the red grism in the EWSs, the spectral classification for mid-L to T dwarfs will be as precise as that in the EDFs; while for those field late-M to early-L dwarfs the precision is of two subclasses because those hotter UCDs have less characteristic features in the H band. For young early-L dwarfs like W0342, their peculiar H band improves the spectral classification precision of the red grism.

5.4. Sample purity

We selected eight UCD candidates from the whole sample pool for the spectroscopy and all of them were spectroscopically confirmed as UCDs. Assuming an overall purity of our selections in the three EDFs is p, the probability P(X = k, n) with k true UCD in n candidates follows a binomial distribution,

P ( X = k , n ) = n ! k ! ( n − k ) ! p k ( 1 − p ) n − k . $$ \begin{aligned} P(X=k, n)=\frac{n!}{k!(n-k)!}p^k(1-p)^{n-k}. \end{aligned} $$(1)

Hence, we estimated a 90% confidence interval (with only a lower tail) of the purity by P(X = 8, 8)=p8 ≥ 1 − 90%, yielding 75%≤p ≤ 100%. The lower limit is extremely conservative since it is completely based on our spectroscopic confirmation.

6. Conclusions

Euclid, a space-borne telescope dedicated to cosmology, will also be powerful for UCD science. It has advantages compared with other current space telescopes, especially the combination of its big FoV and its depth in the NIR range. It will start nominal surveys soon and will visit the three EDFs repeatedly. In this work, we provide a list of UCDs in the three EDFs for Euclid to characterise them and use them as references to identify new UCDs. We found 81 (49, 231) M, eight (29, 115) L, and one (0, 2) T dwarfs in EDFs North, Fornax, and South, respectively. The total number of these potential UCD standards is enough to provide a fairly good connection between Euclid and ground-based data, despite the possibility of photometric or spectroscopic variability of any single object.

From the candidate list in or next to all the EDFs, we selected in total eight UCD candidates of different spectral classes, for which we collected spectra with GTC/EMIR and the VLT/X-shooter, confirming their UCD nature. We used a common object, W0342, to demonstrate that these two telescopic instrument combinations provide a coherent UCD characterisation for Euclid. All of the eight selected candidates were spectroscopically confirmed as UCDs, suggesting a high UCD purity of the selection that we made. Together with a J-band spectrum of a T dwarf in EDF North from the literature, these nine UCD spectra will serve as reconnaissance spectra for UCD discoveries of Euclid. We estimated the spectral classification ability of Euclid by analysing Euclidised spectra and comparing them to the original spectra. We concluded that Euclid NISP spectroscopy will be able to deliver a precise spectral class for UCDs in the EDFs, and to determine their age groups.

At the end of the Euclid mission, with about 40–50 visits in each EDF, the co-added spectra will have an S/N at least six times better than the spectra from a single epoch. This may even permit a radial velocity measurement for many UCDs. Their parallaxes and proper motions will be also measured through multi-epoch astrometry. Multi-epoch and multi-band photometry will enable one to monitor the monthly photometric variability of UCDs, which could be caused by magnetic activity, inhomogeneous cloud distribution, rotation, or complex atmosphere dynamics. Combining the determination of spectral classes, distances, kinematics, long-term photometric variabilities, and the ages of thousands of new UCD discoveries, the Euclid deep surveys will bring us to a new era in UCD science, with unprecedented statistics for the understanding of the lowest-mass population in the Milky Way.

3

The envelope of EDF South is 63.25° −45.67°, 65.35° −46.10°, 66.40° −47.25°, 65.99° −48.72°, 59.25° −51.19°, 56.95° −50.82°, 55.90° −49.40°, 56.80° −47.99°.

Acknowledgments

We thank the referee for the useful and insightful report. NL acknowledges support from the Agencia Estatal de Investigación del Ministerio de Ciencia e Innovación (AEI-MCINN) under grant PID2019-109522GB-C53. Funding for this research was provided by the European Union (ERC, SUBSTELLAR, project number 101054354). Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. Based on observations collected at the European Southern Observatory under ESO programme 112.261S. This research has made use of the Spanish Virtual Observatory (https://svo.cab.inta-csic.es) project funded by MCIN/AEI/10.13039/501100011033/ through grant PID2020-112949GB-I00. This work has used the Pan-STARRS1 Surveys (PS1) and the PS1 public science archive that have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This project used public archival data from the Dark Energy Survey (DES). Funding for the DES Projects has been provided by the U.S. Department of Energy, the US National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo á Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Cientêfico e Tecnológico and the Ministério da Ciéncia, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy Survey.The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Enérgeticas, Medioambientales y Tecnoló-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciêncies de l’Espai (IEEC/CSIC), the Institut de Física d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität München and the associated Excellence Cluster Universe, the University of Michigan, the National Optical Astronomy Observatory, the University of Nottingham, The Ohio State University, the OzDES Membership Consortium, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University. This work made use of Astropy: (http://www.astropy.org) a community-developed core Python package and an ecosystem of tools and resources for astronomy (Astropy Collaboration 2013, 2018, 2022).

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Appendix A: Lists of ultracool dwarf candidates in Euclid Deep Field North

Table A.1.

81 late-M-type UCD candidates as photometric standards in EDF North and their photometry measurements.

Table A.2.

continued.

Table A.3.

Eight L-type UCD candidates as photometric standards in the EDF North and their photometry measurements.

Appendix B: Lists of ultracool dwarf candidates in Euclid Deep Field Fornax

Table B.1.

49 M-type UCD candidates as photometric standards in EDF Fornax and their photometry measurements.

Table B.2.

29 L-type UCD candidates as photometric standards in the EDF Fornax and their photometry measurements.

Appendix C: Lists of ultracool dwarf candidates in Euclid Deep Field South

Table C.1.

231 late-M-type UCD candidates as photometric standards in EDF South and their photometry measurements.

Table C.2.

continued.

Table C.3.

continued.

Table C.4.

continued.

Table C.5.

115 L-type UCD candidates as photometric standards in EDF South and their photometry measurements.

Table C.6.

continued.

Table C.7.

Two T-type UCD candidates as photometric standards in the EDF South and their photometry measurements.

All Tables

Table 1.

Targets selected for spectroscopic observation.

In the text
Table 2.

Summary of GTC/EMIR and VLT/X-shooter observations.

In the text
Table 3.

Summary of the spectral classification.

In the text
Table A.1.

81 late-M-type UCD candidates as photometric standards in EDF North and their photometry measurements.

In the text
Table A.2.

continued.

In the text
Table A.3.

Eight L-type UCD candidates as photometric standards in the EDF North and their photometry measurements.

In the text
Table B.1.

49 M-type UCD candidates as photometric standards in EDF Fornax and their photometry measurements.

In the text
Table B.2.

29 L-type UCD candidates as photometric standards in the EDF Fornax and their photometry measurements.

In the text
Table C.1.

231 late-M-type UCD candidates as photometric standards in EDF South and their photometry measurements.

In the text
Table C.2.

continued.

In the text
Table C.3.

continued.

In the text
Table C.4.

continued.

In the text
Table C.5.

115 L-type UCD candidates as photometric standards in EDF South and their photometry measurements.

In the text
Table C.6.

continued.

In the text
Table C.7.

Two T-type UCD candidates as photometric standards in the EDF South and their photometry measurements.

In the text

All Figures

thumbnail Fig. 1.

UCD candidates in the three EDFs (black borders). The circled objects are the ones for which we will get spectra.
In the text
thumbnail Fig. 2.

PS1 z-band image (left) and GTC/EMIR J-band image (right) of WISE1754 AB. The A component has a proper motion towards south-east direction, while the B component does not.
In the text
thumbnail Fig. 3.

Normalised and Euclidised spectra of W0342 from both GTC/EMIR and the VLT/X-shooter. Strong telluric bands are masked. Features indicating the youth of the object are denoted. Both original spectra were classified as L0.0γ (upper panel). The spectral similarity underscores that GTC/EMIR and the VLT/X-shooter are coherent in characterising UCDs for Euclid. The two Euclidised spectra were all classified as L0.0γ (middle panel). The difference between the two Euclidised spectra is visualised in the lower panel.
In the text
thumbnail Fig. 4.

Normalised UCD spectra in the three EDFs. From the top to the bottom are UCDs from EDFs North, Fornax, and South, respectively. The VLT/X-shooter and GTC/EMIR spectra were smoothed with a 31 and 5-pixel-size window, respectively. All the spectra were classified into spectral subclasses, and the best-fit templates are plotted in grey (left panel). We Euclidised the spectra and the classifications remain consistent within 0.5 subclasses (right panel).
In the text

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