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Issue
A&A
Volume 614, June 2018
Article Number A144
Number of page(s) 5
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201731840
Published online 03 July 2018

© ESO 2018

1. Introduction

Dwarf irregular galaxies are assumed to be simple gas-rich stellar systems, presumably the “building blocks” of large galaxies (Tosi 2003). An analysis of these systems, which are likely built up solely through self-enrichment, could provide insight into the early star formation history in the Universe.

Leo A (Fig. 1) is an isolated dwarf irregular galaxy in the Local Group and could serve as an example of such a building block. It is a gas-rich (Young & Lo 1996; Hunter et al. 2012) stellar system dominated by dark matter (Brown et al. 2007; Kirby et al. 2017) of low stellar mass (Cole et al. 2007) and low metallicity (van Zee et al. 2006; Kirby et al. 2017). It consists of multiple stellar populations that are between ~10 Myr to ~10 Gyr old. The present-day star formation activity is traced by H II regions, while the existence of an old stellar population is proved by the detection of RR Lyr stars (Dolphin et al. 2002; Bernard et al. 2013). Detailed studies of stellar content in Leo A were performed with the Hubble Space Telescope (HST), which was equipped with the Wide Field and Planetary Camera 2 (WFPC2; Tolstoy et al. 1998; Schulte-Ladbeck et al. 2002) and with the Advanced Camera for Surveys (ACS; Cole et al. 2007), by imaging of the central part.

thumbnail Fig. 1.

Subaru Suprime-Cam B-passband image of the Leo A galaxy (Stonkutė et al. 2014). The ellipses (semi-minor to semimajor axis ratio: b/a = 0.6; PA = 114°) of 8′ and 10′ along the semi-major axis, centred at α = 9h59m24s, δ = +30°44′47″ (J2000), are shown. The HST fields, marked white (ACS) and black (WFC3), are taken from the HST Proposal 12273 by Roeland van der Marel (https://archive.stsci.edu/proposal_search.php?mission=hst&id=12273). The RR Lyr stars discovered by Dolphin et al. (2002) and Bernard et al. (2013) are marked by white open circles. North is up, east is left.

The importance of stellar populations in the galaxy outskirts for studying galaxy build-up and star formation processes is well recognized and has been widely discussed recently (Gil de Paz et al. 2017; Knapen et al. 2017). An especially important point to address in the discussion of gas-rich dwarf galaxy evolution scenarios is the ratio of the sizes of their stellar and H I components. Wide photometric mapping of the stellar component was performed with the Subaru Telescope, which was equipped with the Suprime-Cam mosaic camera (Stonkutė et al. 2014). We analysed the distribution of red giant branch (RGB) stars, but they are not numerous at the very outskirts of the galaxy, and in the colour-magnitude diagram (CMD), they overlap with the Milky Way (MW) foreground stars. Therefore, a correct determination of the Leo A size based solely on RGB stars is a rather difficult task. To overcome this problem, we decided to study stellar populations below the horizontal branch by employing available archive HST Wide Field Camera 3 (WFC3/UVIS) observations.

The following basic parameters of the Leo A galaxy as derived from the RGB star distribution (Vansevičius et al. 2004) are adopted in this study: (1) centre coordinates of the galaxy, α = 9h59m24s, δ = +30°44′47″ (J2000); (2) ellipticity – ratio of the semi-minor to the semi-major axis, b/a = 0.6; (3) position angle of the major axis, PA = 114°; and (4) the semi-major axis, a = 7.5′. We also assumed the Holmberg semi-major axis, a = 3.5′ (Mateo 1998) and the semi-major axis of the H I envelope, a = 7.5′ (Young & Lo 1996). The distance to Leo A is assumed to be 800 kpc (1′ ≈ 230 pc; Dolphin et al. 2002). The foreground extinction towards Leo A is derived from the extinction maps (Schlafly & Finkbeiner 2011), A(F475W) = 0.067 and A(F814W) = 0.032, assuming the standard extinction law of the Milky Way with RV = 3.1 (Fitzpatrick 1999).

The structure of the paper is the following: Sect. 2 presents details of the archive observation data and reductions. Sect. 3 presents results and a brief discussion. Conclusions are presented in Sect. 4.

2. Observations and data reductions

We used HST WFC3 F475W and F814W passband observation data of the Leo A galaxy obtained for the HST Proper Motion Collaboration (HSTPMC) program – Mass of the Local Group from Proper Motions of Distant Dwarf Galaxies (the HST proposal GO 12273, the principal investigator Roeland P. van der Marel) during the HST cycle 18 on December 28, 2011, in parallel field WFC3/UVIS.

The WFC3/UVIS camera was pointed ~6′ away (N-W) from the Leo A galaxy centre in the outer halo (Fig. 1). A total exposure time of 5408 and 5282 seconds was obtained in passbands F475W and F814W, respectively. The archival data were downloaded from the Mikulski Archive for Space Telescopes (MAST). We retrieved bias-subtracted, flat-fielded, charge transfer efficiency (CTE) corrected WFC3/UVIS flc images produced by the STScI On-The-Fly-Reprocessing (OTFR) pipeline OPUS versions 2016_2, which used CALWF3 version 3.4.

To perform stellar photometry, we used the software package DOLPHOT 2.0 (Dolphin 2000, and many unpublished updates). We followed the recommended preprocessing steps and the photometry recipe provided in the manual for the WFC3 module (version April 17, 2016). We used DrizzlePac 2.1.6 (default parameter values) to create clean, deep-drizzled reference frames for object detection and coordinate transformations from four sub-exposures in each of the F475W and F814W passbands. This also allowed us to flag the cosmic rays in the individual flc images and update data quality images.

The HST WFC3/UVIS field is located at the uncrowded outskirts of the Leo A galaxy. Therefore, we used values of DOLPHOT parameters recommended in the WFC3 manual for uncrowded fields: the FitSky parameter was set to 1, which means the sky fitting in an annulus around each star (Rinner = 15, Router = 35 pixels) and point-spread function (PSF) fitting inside a radius of Rapert = 4 pixels.

DOLPHOT determines magnitudes, magnitude errors, object fit, and shape parameters in individual flc frames, and then combines them for each filter. To combine the magnitudes, we set a parameter FlagMask = 7, which means that only measurements with error flags equal to zero were used.

Numerous extended objects in the WFC3 field can hamper the photometry of nearby stars, therefore setting the DOLPHOT parameter Force1 = 0 could result in a false cleaning of actual stars from the photometric catalogue. Since the CMD is needed to be as complete as possible for our study, we set the parameter Force1 = 1, which forced all detected sources to be fitted as stars. This resulted in a strongly contaminated CMD, as hot pixels and extended objects were also measured as single stars.

The output photometry file had measurements of more than 60 000 objects. In order to discard non-point sources and measurements of artefacts, the photometry catalogue was cleaned. We rejected objects that had measurements in only one filter or had a measured DOLPHOT photometry magnitude error (in any filter) larger than 0.3 mag. Then we visually inspected the measured objects in the images and rejected those that lay on the obvious background galaxies, bright MW stars, or large artefacts. This left us with N = 756 objects. Sharpness and photometric errors of these objects in the F475W and F814W HST WFC3 passbands are shown in Fig. 2a–d.

thumbnail Fig. 2.

AST results in the F475W and F814W HST WFC3 passbands. Panels a and b: grey dots show the sharpness of the AST stars; filled (star-like objects) and open (extended objects) circles show the sharpness of the real measured objects. Panels c and d: grey dots indicate the difference of recovered and input magnitudes of the AST stars; black lines indicate average photometric errors provided by DOLPHOT for real objects. Results in panels a–d are shown vs. recovered and measured magnitudes. Derived photometry completeness estimates in the F475W and F814W passbands are shown in panels e and f vs. input magnitudes, respectively.

Star detection and photometric accuracy depend on image resolution, PSF quality, the object signal-to-noise ratio (S/N), and field crowding. A proper assessment of the photometric completeness and biases is crucial for a reliable CMD interpretation even of a single stellar population objects; see, e.g. Stonkutė et al. (2008). An artificial star test (AST) is the best method to evaluate photometric errors and observation completeness. We used the DOLPHOT fake star program to generate a list of 35 000 artificial stars, which were distributed evenly in the original images. The input colours and magnitudes of the artificial stars covered the complete range of the observed colours and magnitudes. The artificial stars sampled the colour −1 < (F475W–F814W) < 5 and magnitude 22 < F475W < 32 ranges.

In order to optimise parameters for the photometry, we performed numerous tests with various source detection thresholds (SigFind and SigFinal) and a minimum allowed separation for two stars (RCombine) values. The best quality of photometry was achieved with SigFind = 4, SigFinal = 4, and RCombine = 1.4. These parameter values were applied for the final photometry and AST procedures. Completeness (Nr/Ni × 100%, where Ni and Nr are numbers of input and recovered artificial stars, respectively) estimates as a function of the F475W and F814W passband input magnitudes are shown in Fig. 2e and f.

3. Results and discussion

The first step of the stellar population analysis within the observed field was to pre-select probable stars out of the 756 measured objects. In order to clean out the stellar population from extended objects, we used the sharpnessF814W vs. sharpnessF457W diagram (Fig. 3) that was constructed for the AST and real stars. Following the DOLPHOT prescription (Dolphin 2000, see Eq. (14)), we rejected diffuse sources that have overly negative sharpness in both passbands and are therefore likely background galaxies or unresolved blends of stars. We determined limiting criteria for star selection based on sharpness vs. magnitude diagrams (Fig. 2a and b) of real and AST stars by taking a 3σ criterion of sharpness scatter of the measured objects at F475W = 27.0 and F814W = 25.5, where the scatter of the AST star sharpness starts to overlap with the sharpness of objects that can be recognized by eye (sharpnessF457W > −0.099; sharpnessF814W > −0.078). Additionally, we cut out a corner with a straight line connecting the points sharpnessF457W = −0.099, sharpnessF814W = 0.0 and sharpnessF457W = 0.0, and sharpnessF814W = −0.078 (Fig. 3). As a result of this selection, we ended up with 128 probable star-like objects. However, some of these objects could still be unresolved galaxies, mainly the so-called “faint blue galaxies” that are abundant below F814W ~ 25 (Ellis 1997), Leo A blue stragglers (Momany 2015), or MW white dwarfs.

thumbnail Fig. 3.

Sharpness in the F475W and F814W HST WFC3 passbands of the measured point-like objects (filled and open circles) and AST stars (grey dots). Filled circles mark 128 star-like objects selected for further analysis, and open circles denote extended objects that were omitted from the analysis.

The colour-magnitude diagrams of the 128 star-like objects selected in the studied field are shown in Fig. 4. The PARSEC isochrone (Bressan et al. 2012) of 7 Gyr age and Z = 0.0001 metallicity (the blue line) is plotted in all panels. The isochrones in Figs. 4 and 5 are shifted assuming the Leo A distance modulus of 24.51 (Dolphin et al. 2002) and reddened taking into account only the foreground MW extinction, A(F475W) = 0.067 and A(F814W) = 0.032. In Fig. 4a we show photometric scatter limits with cyan and red lines, which are determined from the AST star distributions (Figs. 2c and d). In order to demonstrate age and metallicity effects, we plot the isochrones of 5 and 10 Gyr (Z = 0.0001) shown with cyan and red lines, respectively (Fig. 4b), and an isochrone of 7 Gyr and Z = 0.0006 shown with the red line (Fig. 4c).

thumbnail Fig. 4.

CMDs of star-like objects in the studied field. An isochrone of 7 Gyr age and Z = 0.0001 metallicity (the blue line) is plotted in all panels. Additionally, we plot a) photometric AST star scatter limits (based on Fig. 2c and d), shown by cyan and red lines, as well as the lower limit of the MW foreground star distribution derived from the Besançon models, shown with the grey dashed line; b) isochrones of 5 and 10 Gyr (Z = 0.0001) shown with cyan and red lines, respectively; and c) an isochrone of 7 Gyr and Z = 0.0006 shown with the red line.

thumbnail Fig. 5.

CMDs of star-like objects in the studied field. The isochrone of 7 Gyr age and Z = 0.0001 metallicity (the blue line), the limit of the MW stars as in Fig. 4a (the grey dashed line), and photometric scatter limits as in Fig. 4a (cyan and red lines) are plotted in all panels. The objects within radial distances of a) r < 8′, b) 8′ < r < 10′, and c) r > 10′ from the galaxy centre are shown with circles in the corresponding panels. Filled colour circles mark the objects: in the vicinity of the isochrone lie probable Leo A members (blue); far from the isochrone and inside the CMD area occupied by MW stars (red); far from the isochrone lie probable compact faint blue galaxies, Leo A blue stragglers, or MW white dwarfs (cyan). Open grey circles mark faint star-like objects that were not used for the analysis.

To account for CMD contamination with MW stars, we produced a synthetic CMD in the direction of the Leo A galaxy by using the Besançon models (Robin et al. 2003). The grey dashed line (Fig. 4a) shows the lower limit of the MW foreground star distribution assuming the MW halo extension (extremely large) of up to a 100 kpc distance. Based on the synthetic CMD of MW stars, we assume that the star-like objects in Fig. 4 that are redder than F475W–F814W ~ 2.0 mainly belong to the MW galaxy.

For the analysis we selected only star-like objects with a high photometric accuracy, F814W < 27. In order to trace stellar populations in the Leo A outer regions, we decided to employ RGB as well as horizontal and subgiant branch stars. From the isochrone of 7 Gyr age and Z = 0.0001 metallicity, we estimated an expected number ratio of stars along these sequences in the magnitude ranges of 24.5 < F814W < 27.0 and 20.5 < F814W < 24.5 and found a very good agreement with the observed star number ratio. This test suggested that the overdensity of stars along the isochrone seen in the CMD (Fig. 4) can be attributed to the Leo A galaxy stellar population.

In Fig. 5 the filled circles mark the objects that are probable Leo A members (blue), probable MW stars (red), probable compact faint blue galaxies, Leo A blue stragglers, or MW white dwarfs (cyan). Open grey circles mark faint star-like objects that we did not use for the analysis. An isochrone of 7 Gyr age and Z = 0.0001 metallicity (the blue line), photometric scatter limits as in Fig. 4a (cyan and red lines), and the limit of the MW stars as in Fig. 4a (the grey dashed line) are plotted in all panels of Fig. 5. The panels in Fig. 5 show objects located at various distances from the galaxy centre: (a) r < 8′, (b) 8′ < r < 10′, and (c) r > 10′. These objects are marked in Fig. 6 with the same colours.

thumbnail Fig. 6.

Distribution of star-like objects in the studied field. The colour-coding is the same as in Fig. 5. Segments of the ellipses centred on Leo A of a = 8′ and 10′ (b/a = 0.6; PA = 114°) are overplotted. Northeast directions are shown by white arrows.

In order to roughly estimate the age of stellar populations, we used the PARSEC isochrones, release v1.2S (Bressan et al. 2012) of Z = 0.0001–0.001 metallicity. We found the best match of the isochrones with the distribution of star-like objects in the CMD in the case of the lowest available metallicity, Z = 0.0001. Based on the CMD fitting with isochrones, we find old (>5 Gyr) stellar populations of very low metallicity (Z ~ 0.0001). However, the application of lower-metallicity isochrones would lead to an older age of stellar populations. On the other hand, the absence of stars on the blue horizontal branch would contradict this conclusion.

A distribution of star-like objects in the studied field is shown in Fig. 6. Segments of the ellipses (green) centred on Leo A of a = 8′ and 10′ (b/a = 0.6; PA = 114°) are overplotted. Therefore, at least up to a = 10′, we see probable members of the Leo A galaxy. This conclusion fits the finding by Dolphin et al. (2002) well that RR Lyr C1-V01 is residing close to the ellipse of a = 10′ (Fig. 1). It is also important to note that we found that the stellar populations of Leo A are distributed more widely than the envelope of H I (Young & Lo 1996; Hunter et al. 2012). This finding is important in the context of the discussion of an extremely gas-rich dwarf galaxy evolution scenario: inside-out, or outside-in.

4. Conclusions

We have performed photometry in the HST WFC3 field, which is located at the very outskirts of the Leo A galaxy and discovered that its stellar halo extends far beyond the previously known limits (Vansevičius et al. 2004). We stress, however, that the presence of a candidate RR Lyr star located at a distance of 5.9′ from the galaxy centre (Dolphin et al. 2002) also provides evidence for a large halo of Leo A.

The detection of the outer halo stellar populations is based on the CMD analysis below the horizontal branch. We can currently assume that the semi-major axis of the stellar halo is a ~ 2.3 kpc (a ~ 10′, b/a = 0.6). In order to establish the edge of the Leo A halo more clearly, we need to study a much larger field or to obtain accurate photometry for objects of F814W > 27. However, even a small single HST WFC3 field that includes subgiant stars suggests the galaxy size (stellar component) to be larger by about one-third than previously estimated. Based on the CMD fit with isochrones (Bressan et al. 2012), we find old (>5 Gyr) stellar populations of very low metallicity (Z ~ 0.0001). However, to strengthen this conclusion, extremely deep, currently not feasible spectroscopic observations are needed.

Acknowledgments

The research has made use of the SAOImage DS9, developed by Smithsonian Astrophysical Observatory. The data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute. This research was funded by a grant (No. LAT-09/2016) from the Research Council of Lithuania.

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All Figures

thumbnail Fig. 1.

Subaru Suprime-Cam B-passband image of the Leo A galaxy (Stonkutė et al. 2014). The ellipses (semi-minor to semimajor axis ratio: b/a = 0.6; PA = 114°) of 8′ and 10′ along the semi-major axis, centred at α = 9h59m24s, δ = +30°44′47″ (J2000), are shown. The HST fields, marked white (ACS) and black (WFC3), are taken from the HST Proposal 12273 by Roeland van der Marel (https://archive.stsci.edu/proposal_search.php?mission=hst&id=12273). The RR Lyr stars discovered by Dolphin et al. (2002) and Bernard et al. (2013) are marked by white open circles. North is up, east is left.
In the text
thumbnail Fig. 2.

AST results in the F475W and F814W HST WFC3 passbands. Panels a and b: grey dots show the sharpness of the AST stars; filled (star-like objects) and open (extended objects) circles show the sharpness of the real measured objects. Panels c and d: grey dots indicate the difference of recovered and input magnitudes of the AST stars; black lines indicate average photometric errors provided by DOLPHOT for real objects. Results in panels a–d are shown vs. recovered and measured magnitudes. Derived photometry completeness estimates in the F475W and F814W passbands are shown in panels e and f vs. input magnitudes, respectively.
In the text
thumbnail Fig. 3.

Sharpness in the F475W and F814W HST WFC3 passbands of the measured point-like objects (filled and open circles) and AST stars (grey dots). Filled circles mark 128 star-like objects selected for further analysis, and open circles denote extended objects that were omitted from the analysis.
In the text
thumbnail Fig. 4.

CMDs of star-like objects in the studied field. An isochrone of 7 Gyr age and Z = 0.0001 metallicity (the blue line) is plotted in all panels. Additionally, we plot a) photometric AST star scatter limits (based on Fig. 2c and d), shown by cyan and red lines, as well as the lower limit of the MW foreground star distribution derived from the Besançon models, shown with the grey dashed line; b) isochrones of 5 and 10 Gyr (Z = 0.0001) shown with cyan and red lines, respectively; and c) an isochrone of 7 Gyr and Z = 0.0006 shown with the red line.
In the text
thumbnail Fig. 5.

CMDs of star-like objects in the studied field. The isochrone of 7 Gyr age and Z = 0.0001 metallicity (the blue line), the limit of the MW stars as in Fig. 4a (the grey dashed line), and photometric scatter limits as in Fig. 4a (cyan and red lines) are plotted in all panels. The objects within radial distances of a) r < 8′, b) 8′ < r < 10′, and c) r > 10′ from the galaxy centre are shown with circles in the corresponding panels. Filled colour circles mark the objects: in the vicinity of the isochrone lie probable Leo A members (blue); far from the isochrone and inside the CMD area occupied by MW stars (red); far from the isochrone lie probable compact faint blue galaxies, Leo A blue stragglers, or MW white dwarfs (cyan). Open grey circles mark faint star-like objects that were not used for the analysis.
In the text
thumbnail Fig. 6.

Distribution of star-like objects in the studied field. The colour-coding is the same as in Fig. 5. Segments of the ellipses centred on Leo A of a = 8′ and 10′ (b/a = 0.6; PA = 114°) are overplotted. Northeast directions are shown by white arrows.
In the text

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