Free Access
Issue
A&A
Volume 660, April 2022
Article Number A79
Number of page(s) 10
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202142743
Published online 13 April 2022

© ESO 2022

1. Introduction

As stars form and evolve, intense stellar feedback (e.g. stellar winds, supernovae explosions, and ultraviolet radiation) affects the morphology and dynamics of the ambient interstellar medium. High energy feedback from multiple massive stars can disperse or sweep up the surrounding gas leading to the formation of H I holes and shells (Weaver et al. 1977; McCray & Kafatos 1987; Tenorio-Tagle & Bodenheimer 1988; Weisz et al. 2009a,b; Cannon et al. 2011; Warren et al. 2011; Pokhrel et al. 2020). Over time, a significant amount of gas can build up in some regions at the periphery of such structures, possibly leading to new star formation processes being triggered. Hence, given a favourable environment, self-propagating star formation can drastically change the morphology of stellar distribution and the interstellar medium over a period of a few hundred million years (Weisz et al. 2009a; Bastian et al. 2011; Cannon et al. 2011). However, the precise age determination of young stellar populations in combination with the study of neutral and ionised gas allows one to trace the recent star formation history of a galaxy back and understand how stellar populations evolve.

Age determination is of particular importance when studying the morphology of stellar structures. However, estimating stellar ages by comparing their photometric parameters to theoretical models can be tricky. When it comes to young populations, ages for only the brightest, most massive main sequence (MS) stars can be assigned with some certainty when comparing their positions in colour-magnitude diagrams (CMDs) to the stellar isochrones. For the main sequence stars of lower mass and older age, the isochrones are too closely spaced to indicate unambiguous ages for these stars, especially when photometric errors are also taken into account. Fortunately, young, intermediate-mass stars on a post-MS evolutionary stage of core He-burning (HeB) have a one-to-one age-luminosity relation and occupy a distinct part of the CMDs, which allows unambiguous ages for these stars to be determined. Stars in this evolutionary stage tend to become significantly bluer for a brief period of time (∼1 Myr) before moving back to the redder regions of CMDs, thus forming a so called blue loop. HeB stars spend ∼10 times longer near or at the extrema of the blue loop than in other parts of the loop, thus forming a prominent sequence of blue supergiant stars in CMDs (hereafter we refer to these blue stars as BHeB). The BHeB stage is relatively short lived, thus stars undergoing this evolutionary phase are fewer in number than MS stars; however, a reliable age determination makes them more viable when tracing the patterns of young stellar structures. The method was developed and successfully applied to study star formation histories of dwarf irregular galaxies in a series of papers by Dohm-Palmer et al. (1997, 1998, 2002).

In this paper, we study the youngest MS and BHeB stars in the dwarf irregular galaxy Leo A (DDO 69), located in the Local Group at a distance of ∼820 kpc; readers can refer to Leščinskaitė et al. (2021) and references therein for more information. It is a gas-rich (Young & Lo 1996; Hunter et al. 2012), metal-poor (van Zee et al. 2006; Kirby et al. 2017) galaxy of low stellar mass (Kirby et al. 2017) that is dominated by dark matter (∼80%; Brown et al. 2007; Kirby et al. 2017). The main parameters of the Leo A galaxy adopted in this study are provided in Table 1.

Table 1.

Parameters of the Leo A galaxy.

Leo A is a distinct galaxy with a considerable delay in its star formation history (Cole et al. 2007). Such an unusual delay in star formation is recognised in only one other isolated dwarf irregular galaxy in our proximate vicinity, namely Aquarius (Cole et al. 2014). Despite the postponed onset of active star formation, stars have been forming in Leo A since the early stages of its evolution, as it is evident from the identified ancient (≳10 Gyr) populations of RR Lyrae stars (Dolphin et al. 2002; Bernard et al. 2013). However, it is a predominantly blue galaxy with prominent young stellar populations and H II regions that indicate ongoing star formation to this day. Therefore, to better understand the evolution of stellar distribution within a galaxy dominated by self-propagating star formation, we studied populations of MS (< 30 Myr) and BHeB (< 300 Myr) stars in Leo A using multi-colour photometry data.

The paper is organised as follows. In Sect. 2 we introduce the stellar photometry catalogues and isochrones used in this study. The selection process of the brightest MS and BHeB stars is described in Sect. 3. In Sect. 4 we define the age determination method of the BHeB stars and derive the star formation history of Leo A. The spatial distribution of the brightest MS and BHeB stars is discussed in Sect. 5. The connection of the H I hole to the young stellar populations is discussed in Sect. 6. The identified Hα emission stars are presented in Sect. 7. Finally, the summary and conclusions are given in Sect. 8.

2. Data

Multi-colour stellar photometry data obtained with the Subaru telescope (Vansevičius et al. 2004) were used to cover an entire field of interest in Leo A, while the available high-resolution photometry obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) was additionally used in the crowded central part of the galaxy. Stellar photometry catalogues based on Suprime-Cam observations were taken from Stonkutė et al. (2014) for the B, V, and I passbands and from Stonkutė & Vansevičius (2022) for the broadband R (the Johnson-Cousins system) and the narrow non-standard passband NA6561. The central wavelength of this passband is 6566 Å, which approximately coincides with Hα; the full width at half maximum (FWHM) of the filter’s transmission curve is 140 Å (hereinafter for brevity, we mark the NA656 passband as Hα).

The catalogue by Stonkutė et al. (2014) is decontaminated of extended objects based on point spread function (PSF) fitting results and visual Suprime-Cam image inspection. However, we additionally decontaminated a subset of objects selected for the current study by visually inspecting ACS images alongside those obtained with ground-based observations. High-resolution ACS images enabled us to identify and remove, from the further analysis, 34 stellar blends and distant background galaxies (I <  23) in the catalogue, which were not recognisable in Suprime-Cam images.

We limited our analysis to the region defined by an ellipse with a semi-major axis, a, equal to 4′, centred on the galaxy. Parameters of the ellipse were determined by Vansevičius et al. (2004) based on the distribution of red giant branch (RGB) stars; however, it is important to note that the position of the galaxy’s centre was shifted in right ascension by +0.5s (Leščinskaitė et al. 2021). The limit of a = 4′ was chosen considering that no star-forming regions are visible outside of this ellipse and that there are no MS and BHeB stars (I <  23), with CMD positions implying ages of less than 300 Myr, located beyond this radius.

Throughout the paper we use a set of the PAdova and tRieste Stellar Evolutionary Code (PARSEC) release v1.2S isochrones (Bressan et al. 2012) of 10−320 Myr ages and of various metallicities. The isochrones are corrected for the distance modulus of 24.58 (Leščinskaitė et al. 2021) and reddened, taking into account the Milky Way foreground extinction: AB = 0.075, AV = 0.057, AI = 0.031, AF475W = 0.068, and AF814W = 0.032 (Schlafly & Finkbeiner 2011).

3. MS and BHeB stars

The MS and BHeB stars occupy different parts of a CMD, which allows the two to be analysed independently. The CMDs portraying the separation criteria for MS and BHeB stars, as well as their respective spatial distributions, are shown in Fig. 1. We define the separation limit (green lines in Figs. 1a and d) between MS (red) and BHeB (blue) stars to approximately align with the isochrones’ bluest points of the blue loops assuming a metallicity of Z = 0.0001. The BHeB stars were additionally limited to V − I <  0.5 in order to avoid contamination by Milky Way foreground stars. Photometric errors cause a considerably larger scatter in the CMDs of ground-based Suprime-Cam photometry when compared to that in the CMDs of ACS; therefore, the separation limit between MS and BHeB stars is somewhat less apparent for the former.

thumbnail Fig. 1.

MS (red) and BHeB (blue) stars of the Leo A galaxy. Panel a: I versus V − I diagram of Suprime-Cam data with isochrones of 15, 30, 55, 100, 160, and 220 Myr ages plotted as black lines (Z = 0.0001); the red line marks the isochrone of 320 Myr age (Z = 0.0001), which was used as a selection limit for the BHeB stars. The diagonal green line, I = 23.64 + 16.00 ⋅ (V − I), marks the assumed separation limit between the MS and BHeB stars. Panels b and c: spatial distributions of the MS and BHeB stars selected based on Suprime-Cam photometry data, respectively. Panel d: F814W versus F475W − F814W diagram of ACS data. The ages and metallicity of isochrones are the same as in panel a. The diagonal green line separating MS and BHeB stars is given by F814W = 24.00 + 11.11 ⋅ (F475W − F814W). Panels e and f: spatial distributions of the MS and BHeB stars selected based on ACS photometry data, respectively. An ellipse of a = 4′ (the black line) and the HST/ACS field (the blue line) are shown in each spatial distribution diagram. H I column density contours (Hunter et al. 2012) of the following density levels (atoms cm−2): 4 × 1020, 8 × 1020, 1.3 × 1021, and 2 × 1021 are plotted in each spatial distribution diagram (from grey to black; darker contours correspond to higher column density).

Since we combined Suprime-Cam and ACS data when analysing young populations, slight adjustments to the Suprime-Cam’s subsets of MS and BHeB stars were made based on the available counterparts in the ACS data. These subject adjustments were made by analysing ACS images of individual stars, as well as their locations in the F814W versus F475W − F814W diagram. If stars, according to the ACS images, have nearby objects that could alter their Suprime-Cam colours and thus their position in the CMD, ACS photometry was used to decide whether the star should be classified as MS or BHeB. There are two stars from the ACS subset of MS stars that fall to the right of the dividing line in the CMD of Suprime-Cam data and one BHeB star that falls to the left of this line (Fig. 1a).

Figures 1b and c show the spatial distributions of the MS and BHeB stars from the Suprime-Cam catalogue, respectively. Likewise, Figs. 1e and f show the spatial distributions of the selected stars from the ACS catalogue. All these panels also contain H I column density contours (Hunter et al. 2012) – from the thinnest to the thickest ones: 4 × 1020, 8 × 1020, 1.3 × 1021, and 2 × 1021 atoms cm−2. The ellipse, within which the selection of Suprime-Cam data was performed, is marked by the black line. The ellipse parameters are those from Table 1. The rectangle indicating the ACS field is marked by the blue line.

4. Star formation history

Accurate ages of individual stars are of paramount importance when studying the morphology of stellar distributions. Luminous BHeB stars have a one-to-one age-luminosity relation (Bertelli et al. 1994), which allows for an unambiguous age determination by comparing the positions of these stars in CMDs to theoretical models. The method was developed in detail by Dohm-Palmer et al. (1997, 1998, 2002); in this paper, we use a modified version of it. Figure 2 shows two CMDs used to determine ages of the BHeB stars (violet dots mark the MS stars). Figure 2a shows the F814W versus F475W − F814W diagram (ACS) and Fig. 2b shows the I versus B − I diagram (Suprime-Cam; the photometric passbands were chosen to correspond roughly to the ones of ACS).

thumbnail Fig. 2.

CMDs of the bright MS and BHeB stars in Leo A. Panel a: F814W versus F475W − F814W diagram (ACS). Panel b: I versus B − I diagram (Suprime-Cam). The two parallel blue dashed lines confine BHeB stars (colour-coded by age) for which individual ages were determined by interpolation between isochrones of various ages and metallicities. The BHeB stars located out of the valid age determination region have ages estimated only approximately and are marked by open (colour-coded by age) circles. MS stars are marked by violet dots. The isochrones of the same ages as in Fig. 1a and of two metallicities, that is Z = 0.0001 (dashed black lines) and Z = 0.0015 (solid black lines), are plotted for reference. The isochrone of 320 Myr age and of Z = 0.0001 metallicity (thick solid blue lines in both panels) marks the lower limit for BHeB star selection.

To assign ages (indicated by the colour scale, Fig. 2) to the BHeB stars, we interpolated between the isochrone blue loops. For this purpose, we performed quintic spline interpolation (interp2d2). We used a set of isochrones of metallicities (Z) ranging from 0.0001 to 0.0021 (with a step of 0.0002) and ages (log t) from 6.95 to 8.6 (with a step of 0.05). Several isochrones of ages 15, 30, 55, 100, 160, and 220 Myr and of Z = 0.0001 (dashed black lines) and Z = 0.0015 (solid black lines) metallicities are plotted in the CMDs (Fig. 2) for reference. The isochrones of 320 Myr age and of Z = 0.0001 metallicity are plotted (solid blue lines) to show selection limits of the oldest BHeB stars. The shortening of the blue loop with increasing metallicity is well shown by these isochrones (Fig. 2). However, there is no regular shortening of the blue loop at larger metallicities (Z >  0.0015), as its bluest point starts to shift to bluer colours for some ages, thus demonstrating some age-metallicity degeneracy. Therefore, we safely applied our age determination method only for the BHeB stars located between the two parallel (blue) dashed lines. Also, in Fig. 2 there are several BHeB stars marked by open (colour-coded by age) circles; however, their ages were estimated less accurately, as our interpolation method does not work reliably in this part of the diagram.

In Fig. 3 we illustrate the idea of the performed isochrone interpolation. The isochrones interpolated in the Suprime-Cam CMD diagram I versus B − V assure reasonably accurate individual ages of the BHeB stars located between the dashed lines. The same performance is demonstrated by the ACS CMD diagram F814W versus F475W − F814W; however, the brightest BHeB stars in the ACS frames are overexposed and limit the coverage of the youngest ages.

thumbnail Fig. 3.

Results of age interpolation between isochrones of various ages and metallicities. The two parallel blue dashed lines confine the location of BHeB stars (colour-coded by age). The isochrone of 320 Myr age and Z = 0.0001 metallicity (the thick solid blue line) marks the upper age limit used for interpolation.

In Fig. 4 we compare individual ages of the 94 BHeB stars derived based on the ACS and Suprime-Cam photometry data. The Suprime-Cam-based ages are systematically older by 6.9 Myr with a root-mean-square (rms) scatter of 6.4 Myr. A good match of both data sets suggests that the determined ages are reliable. Therefore, to derive a recent (≲300 Myr) star formation history of the Leo A galaxy, we used the Suprime-Cam-based ages of 115 BHeB stars because they consistently cover the entire region of interest. The list of these stars is provided in the electronic version of Table 2.

thumbnail Fig. 4.

Individual ages of the BHeB stars determined based on the ACS and Suprime-Cam photometry data. The red line was fitted by assuming a slope in a linear equation equal to 1.0.

Table 2.

BHeB stars in the Leo A galaxy.

In Fig. 5 we show the derived recent evolution of the star formation rate (SFR; in solar masses per million years) based on the individual ages of BHeB stars. We have assumed the initial mass function (0.1−100 M) by Kroupa (2002) and calculated SFRs within the following age intervals: 30−50−90−150−210−300 Myr. The photometric completeness estimates provided by Stonkutė et al. (2014) and a number of stellar image blends, removed in the present study, were carefully taken into account. The most recent (< 30 Myr) star formation rate was estimated considering only the brightest (I <  22.5) MS stars based on the isochrone of 15 Myr age.

thumbnail Fig. 5.

A recent star formation history of the Leo A galaxy based on the BHeB stars. The SFR for the youngest ages (< 30 Myr) was derived based on the bright (F814W <  22.5) main sequence stars.

During the period of ∼100−300 Myr, the SFR in Leo A was fairly constant. However, a noticeably lower SFR was derived during the period of ∼70−90 Myr. We also see three short (a duration of ∼10−20 Myr) drops in the SFR during the period of ∼100−300 Myr; however, a small number of the available BHeB stars prevented us from reliably deriving the SFR at such a high age resolution.

The average SFR during the period of 30−300 Myr is (561 ± 60) M/Myr. The most recent (< 30 Myr) SFR is (616 ± 56) M/Myr. This result is in a perfect agreement with the estimate by Karachentsev & Kaisina (2013) of 603 M/Myr based on far-ultraviolet (FUV) flux measurements. However, it is worth mentioning that their SFR estimate, based on the Hα flux, is much lower at only 93 M/Myr. This kind of discrepancy observed in dwarf galaxies was discussed in detail by Lee et al. (2009), and they concluded that a deficit of high-mass stars in a stellar initial mass function could be responsible for this effect. However, Weisz et al. (2012) suggest that for low-mass dwarf galaxies, such as Leo A, which possess a stellar mass of 3.3 × 106M (Kirby et al. 2017), this effect could be explained entirely by a variable SFR.

5. Spatial distribution of MS and BHeB stars

Dwarf irregular galaxies have an advantage over the massive star-forming galaxies, as they tend to show either solid-body rotation or even no conspicuous rotation at all (Young & Lo 1997). A lack of shear within a galaxy prolongs the time window during which the morphology of young stellar structures does not change much from its original distribution. Therefore, the simpler internal dynamics of such galaxies together with the age determination of individual stars allows for a better understanding of the inherent evolution of stellar populations.

Determining the age of the BHeB stars enabled us to study patterns of star formation in Leo A over the last ∼300 Myr. In Fig. 6 we present the distributions of MS and BHeB stars from Fig. 2 divided into four age bins (indicated in the lower right corner of each panel). Figure 6a shows the distribution of bright MS stars representing the youngest populations of less than 30 Myr, while Figs. 6b–d demonstrate the BHeB stars divided into three colour-coded age bins: 30−80 Myr, 80−180 Myr, and 180−300 Myr. We note that the sample of BHeB stars was collected by combining the ACS and Suprime-Cam catalogues. The ACS observations provide some stars which were removed from the Suprime-Cam catalogue due to blending effects. The Subaru observations supply stars distributed over a larger extent, as well as several objects that are located within the ACS field, yet they do not have available ACS photometry results (due to various defects affecting the images of those stars). Among the bright objects (I <  23 and F814W <  23) plotted in Fig. 6, there are 151 stars: 43 with only ACS photometry and 44 with only Suprime-Cam photometry. The H I column density contours (same as in Fig. 1) are plotted in each panel for a comparison of stellar population morphology in relation to the current distribution of neutral gas in Leo A.

thumbnail Fig. 6.

Spatial distribution of the MS (panel a) and BHeB (panels b–d) stars from the combined ACS and Suprime-Cam photometry. The symbols and colour-coding of MS and BHeB star ages are the same as in Fig. 2; corresponding age ranges are indicated in the bottom right corner of each panel. Parameters of the ellipse and contours of H I column density are the same as in Fig. 1.

The distributions of BHeB and MS stars (Fig. 6) clearly show significant spatial evolution of star formation regions within Leo A during the last ∼300 Myr. Even disregarding the fact that BHeB stars of ages 180−300 Myr (Fig. 6d) had more time (compared to the younger ones) to migrate away from the places of origin, we see a pronounced concentration of these stars in the south-eastern part of the galaxy. At this location, younger stars are currently not available and the H I column density is lower. Moreover, two suspected holes in the H I envelope were detected at this position by Pokhrel et al. (2020); they are marked in Fig. 7. The BHeB stars of 80−180 Myr ages project mainly onto the regions of present-day high H I column density (Fig. 6c). The distribution of these stars resembles the distribution of MS stars that are younger than 30 Myr shown in Fig. 6a, and this seems to coincide with the main regions of ongoing star formation. These results confirm the findings by Dohm-Palmer et al. (1997, 1998, 2002) that large-scale H I gas structures remain coherent for much longer periods than episodes of star formation (∼100 Myr) repeatedly ongoing at the same position in dwarf irregular galaxies.

thumbnail Fig. 7.

Composite image of the Leo A galaxy: Subaru/Suprime-Cam Hα (red) and B (green) passbands and the H I column density map (blue) with cyan contours, indicating the range of column density levels (atoms cm−2) from 4 × 1020 to 7 × 1021 with a constant step of 1020 (Hunter et al. 2012). The straight cyan line divides the largest H I hole into two regions with apparently different stellar populations. Positions and sizes of three small holes in the H I envelope, identified by Pokhrel et al. (2020), are indicated with cyan circles.

It is difficult to reveal the pattern during the relatively quiescent star formation period of ∼50−80 Myr because of a small number of BHeB stars of this age; however, in Fig. 6b we see the pattern differing from the older and younger periods of star formation. This suggests a rather rapid evolution of the pattern of star-forming regions in Leo A; three significant changes during ∼300 Myr suggest a characteristic time of ∼100 Myr. This estimate for the star-forming region structure survival time is in a good agreement with the studies of other dwarf irregular galaxies performed by Bastian et al. (2011), who found the lower limit for the characteristic structure evolution time to be ∼100 Myr (in the galaxy NGC 2366). However, an even more rapid change (∼75 Myr) of structures was determined in the Small Magellanic Cloud by Gieles et al. (2008).

6. H I hole

We would like to note that H I holes and shells are common structures within gas-rich, star-forming dwarf galaxies (Warren et al. 2011; Pokhrel et al. 2020). Since such structures are possibly a result of intense stellar feedback, an analysis of a conspicuous H I hole in Leo A (Fig. 7) and its connection to young stellar populations is essential when trying to understand the patterns of recent star formation.

A composite image of the Leo A galaxy produced from the Suprime-Cam Hα (red) and B (green) passband frames and the H I column density map (blue) from Hunter et al. (2012) is shown in Fig. 7. Cyan contours of the H I column density (atoms cm−2) are plotted from 4 × 1020 to 7 × 1021 with a constant step of 1020.

In the Leo A galaxy, four H I holes (one large prominent hole and three small, barely recognisable ones) were identified, and their parameters were determined by Pokhrel et al. (2020). Positions of three smaller holes in the H I envelope are marked with cyan circles in Fig. 7; their radii correspond to the estimated sizes of these holes. The straight cyan line divides the largest H I hole into two regions with apparently different stellar populations.

An inspection of the integrated H I column density map (Hunter et al. 2012) revealed that the column density inside the largest hole, rimmed with the contour of 4 × 1020 atoms cm−2, is up to ∼10 times lower than that of the bordering regions. The size of the hole in the H I envelope at the column density level of 4 × 1020 atoms cm−2 measures 428 ± 11 pc along its major axis. Taking into account that the average gas velocity dispersion in Leo A is ∼7 km s−1 (Hunter et al. 2012) and assuming that it is equal to the expansion velocity of the hole, it would take ∼30 Myr to create a hole of this size from a single centre. Therefore, we can safely assume this age as an upper limit of the H I hole in Leo A. Pokhrel et al. (2020) determined an effective diameter of this hole to be 331 pc (it transforms to the size of 523 pc along the major axis) and the age to be 23.1 Myr.

In Fig. 8 we show the Hα map produced by subtracting a reference frame obtained in the R passband from the one obtained in the Hα passband. The map shows darker areas of enhanced Hα emission revealing the H II zones, as well as our identified BHeB emission stars (marked by cyan open circles; see Sect. 7). Yellow open circles mark five star clusters (C1–C5) from Stonkutė et al. (2015, 2019) and one (C6) newly identified in the present study. Three of them (C1, C2, and C6) are still embedded within H II zones.

thumbnail Fig. 8.

Subaru/Suprime-Cam Hα map of the Leo A galaxy constructed by subtracting a frame in the R passband from the one in the Hα passband. Darker areas correspond to enhanced Hα emission regions (e.g. H II zones and emission stars). Cyan open circles mark 16 BHeB Hα emission stars. Yellow open circles mark star clusters (C1–C6). The red open circle marks planetary nebula (PN). Green arrows indicate an approximate size of the Hα emission ring. White contours correspond to the H I column density of 4 × 1020 atoms cm−2. Inside the closed white contour, the H I column density is lower than outside of it. North is up, and east is to the left.

In the Hα map (Fig. 8), we see a shock front (in the form of a semi-ring) of 20−30 pc in width and of more than 400 pc in length. Green arrows indicate an approximate size of the full Hα emission ring. To reveal this ring more clearly, we additionally used Hα images obtained using a narrower passband from Hunter et al. (2012). In Fig. 8 white contours show the H I column density of 4 × 1020 atoms cm−2. They correspond to the lowest density contours in Figs. 1 and 7. Inside the closed contour, the H I column density is up to ∼10 times lower compared to the surrounding regions. Therefore, it is a real ‘hole’ in the H I envelope of Leo A, which is framed from the west with an Hα emission front. It is obvious that the form of the closed H I column density contour of 4 × 1020 atoms cm−2 in the western part of the hole resembles the morphology of the Hα shock front remarkably well (Fig. 8). This feature suggests that the H I hole and the Hα shock front could be of the same origin. Furthermore, it is plausible that the feedback from the young stellar population, born within the area of the present-day H I hole, also triggered star formation ahead of the shock front where prominent H II regions and young star clusters (C1, C2, and C6) are seen as clear indicators of very recent star formation events.

It is worth noting that there are virtually no BHeB stars that fall within the prominent H I hole over the period of ∼30−300 Myr (Figs. 6b–d). The western part of this hole is especially clean of those stars. Therefore, it appears that this region has been quiescent for a long period of time since no evidence of star formation activity is traced even from the distribution of old BHeB stars (Fig. 6d). However, the young, less than 30 Myr, MS stars reveal a prominent population in this region of Leo A (Fig. 6a). A loose group of nine bright MS stars located in the western part of the H I hole is present. Their age, which was estimated based on the brightest member of this group, is less than 20 Myr. Therefore, it is feasible to assume that stellar feedback from such a young population could sweep up gas in the western part of the H I hole and create a shock front seen in the Hα map (Fig. 8). However, these stars are located strongly asymmetrically within the H I hole. They are not concentrated at a single centre, but are distributed over the entire western part of the hole. Therefore, they could hardly produce a rather symmetric shape compared to the H I hole.

Moreover, Fig. 7 reveals that there is a remarkable difference in the surface number density of stars in the eastern and western parts of the H I hole (separated by a cyan line). Noticeably more numerous and older stellar populations are seen in the eastern part of the hole. Both parts of the hole are equally filled with the old, ∼400−500 Myr, stellar populations. On the other hand, the western part of the hole is filled with a prominent, young (less than 20 Myr) stellar population. Therefore, the progenitor of the H I hole could have been a Type II supernova, which exploded ∼15 Myr ago and was located close to the middle of the indicated separation line (Fig. 7). The estimated energy required to produce this H I hole is ∼1.2 × 1051 erg, which is comparable to the kinetic energy produced by a single Type II supernova (for an exhaustive discussion of such scenarios, data, and references, see Pokhrel et al. 2020). Also, a hypothesis of a single burst is supported by the sharp shock front observed in the western part of the H I hole (Fig. 8).

7. Hα emission objects

Incorporating Suprime-Cam photometry in Hα and R passbands (Stonkutė & Vansevičius 2022) enabled us to identify stars with an enhanced Hα emission, which indicates the presence of spectral B-A-type emission-line stars in the Leo A galaxy. The CMD and the two-colour diagram of MS and BHeB stars are shown in Fig. 9 (red dots mark MS stars and blue ones are for BHeB stars according to the definition in Fig. 1).

thumbnail Fig. 9.

Diagrams of the bright (I <  23) Leo A stars (MS – red, BHeB – blue). Panel a: V versus B − V diagram; Hα emission stars are marked by cyan open circles. Panel b: Hα − R versus B − V diagram; an area occupied by prominent emission stars is marked by cyan dashed lines. Spectral type ranges, corresponding to the colour index B − V (corrected for the colour excess, E(B − V)=0.018) of normal Milky Way MS stars, are indicated for a reference.

Figure 9b shows a region used to select emission stars (outlined with cyan dashed lines). Just for guiding purposes, we indicate approximate spectral types, assigned according to the B − V colour index of normal Milky Way MS stars (Pecaut & Mamajek 2013) at the limits of the selection region. Taking a large metallicity difference between Leo A and Milky Way into account, the colour-based spectral B-types (Fig. 9b) would shift to the A-types. Therefore, there are 16 BHeB stars (blue dots with cyan circles) with a noticeably enhanced Hα emission (Hα − R ≤ −0.06). We note, however, that five of these stars are located within or near to the large H II zones (Fig. 8, Table 2) and their Hα photometry results could be contaminated by the diffuse emission.

The isochrones plotted in Fig. 9a indicate that prominent BHeB emission stars are found almost throughout the entire age range covered in this study (∼30−200 Myr). The emission-line stars make up to ∼15% of the BHeB star population within the studied region of the blue loop. However, spectroscopic observations are needed to reveal their true nature: Be, B[e], or A-shell stars (Slettebak 1982). It is worth noting that emission or circumstellar shell features are observed in 15% of objects among the Milky Way’s fast-rotating (v sin i >  150 km s−1) A-type stars (Bohlender 2016). Therefore, surveys to estimate the frequency of the emission-line BHeB stars by combining the narrowband Hα imaging with the deep R passband photometry are feasible in numerous dwarf irregular galaxies.

The red open circle in Fig. 8 marks a planetary nebula (PN), which is the most extreme Hα emitter in the Leo A galaxy. This PN was used to estimate the metallicity of older populations (van Zee et al. 2006) prior to the study of the RGB star metallicity by Kirby et al. (2017). The list of 16 BHeB Hα emission stars is provided in Table 2 (the full table is available at the CDS).

8. Summary and conclusions

We have presented a study of young stellar populations in the dwarf irregular galaxy Leo A based on multi-colour (B, V, R, I, and Hα) photometry data obtained with the Subaru/Suprime-Cam and two-colour (F475W and F814) photometry from HST/ACS. The bright MS stars revealed the morphology of the current star-forming regions, while BHeB stars enabled us to determine their individual ages. Therefore, we were able to analyse changes in patterns of various age star groups over the last ∼300 Myr. Based on this analysis, we binned the BHeB stars into the three most representative age groups of 30−80 Myr, 80−180 Myr, and 180−300 Myr (Figs. 6b–d). The differing morphology of these BHeB star groups indicates a significant spatial evolution of star-forming regions in Leo A. A noticeably different distribution of bright MS stars younger than ∼30 Myr (Fig. 6a) also supports this conclusion.

The analysis of the H I column density map (Hunter et al. 2012) provided additional insights into the connection between stellar populations and the interstellar medium in Leo A. The prominent H I hole (Fig. 7) in this galaxy measures 428 ± 11 pc in diameter and was initiated less than ∼20 Myr ago. By assuming that it was imploded from a single centre, an estimated average gas sweep-up velocity is more than 10 km s−1. Also, there is an obvious difference in the surface number density of stars in the western and eastern parts of the H I hole (Fig. 7). More numerous and older stellar populations are seen in the eastern part of the hole. On the other hand, the western part of the hole is filled with a young, less than 20 Myr, stellar population, which seems to be the first star formation event in this region over the last ∼300 Myr. Therefore, we hypothesise that a Type II supernova, which exploded ∼15 Myr ago in the middle of the present-day HI hole, could serve as its progenitor. Also, this hypothesis is supported by the sharp shock front seen in Hα, which shows a remarkable resemblance to the shape of the western edge of the H I hole.

By using Subaru/Suprime-Cam photometry in B, V, R, and Hα passbands, we identified 16 stars with an enhanced Hα emission (Hα − R ≤ −0.06). The prominent emission in these objects indicates the presence of B-A-type emission-line stars in the Leo A galaxy (Fig. 9, Table 2).


Acknowledgments

We are grateful to the anonymous referee for numerous constructive suggestions which improved the paper considerably. The Leo A stellar photometry data are based on Suprime-Cam images, collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. The HST data used in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute. This research has made use of: the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology; the SAOImage DS9, developed by the Smithsonian Astrophysical Observatory. This research was funded by a grant (No. LAT-09/2016) from the Research Council of Lithuania.

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All Tables

Table 1.

Parameters of the Leo A galaxy.

Table 2.

BHeB stars in the Leo A galaxy.

All Figures

thumbnail Fig. 1.

MS (red) and BHeB (blue) stars of the Leo A galaxy. Panel a: I versus V − I diagram of Suprime-Cam data with isochrones of 15, 30, 55, 100, 160, and 220 Myr ages plotted as black lines (Z = 0.0001); the red line marks the isochrone of 320 Myr age (Z = 0.0001), which was used as a selection limit for the BHeB stars. The diagonal green line, I = 23.64 + 16.00 ⋅ (V − I), marks the assumed separation limit between the MS and BHeB stars. Panels b and c: spatial distributions of the MS and BHeB stars selected based on Suprime-Cam photometry data, respectively. Panel d: F814W versus F475W − F814W diagram of ACS data. The ages and metallicity of isochrones are the same as in panel a. The diagonal green line separating MS and BHeB stars is given by F814W = 24.00 + 11.11 ⋅ (F475W − F814W). Panels e and f: spatial distributions of the MS and BHeB stars selected based on ACS photometry data, respectively. An ellipse of a = 4′ (the black line) and the HST/ACS field (the blue line) are shown in each spatial distribution diagram. H I column density contours (Hunter et al. 2012) of the following density levels (atoms cm−2): 4 × 1020, 8 × 1020, 1.3 × 1021, and 2 × 1021 are plotted in each spatial distribution diagram (from grey to black; darker contours correspond to higher column density).

In the text
thumbnail Fig. 2.

CMDs of the bright MS and BHeB stars in Leo A. Panel a: F814W versus F475W − F814W diagram (ACS). Panel b: I versus B − I diagram (Suprime-Cam). The two parallel blue dashed lines confine BHeB stars (colour-coded by age) for which individual ages were determined by interpolation between isochrones of various ages and metallicities. The BHeB stars located out of the valid age determination region have ages estimated only approximately and are marked by open (colour-coded by age) circles. MS stars are marked by violet dots. The isochrones of the same ages as in Fig. 1a and of two metallicities, that is Z = 0.0001 (dashed black lines) and Z = 0.0015 (solid black lines), are plotted for reference. The isochrone of 320 Myr age and of Z = 0.0001 metallicity (thick solid blue lines in both panels) marks the lower limit for BHeB star selection.

In the text
thumbnail Fig. 3.

Results of age interpolation between isochrones of various ages and metallicities. The two parallel blue dashed lines confine the location of BHeB stars (colour-coded by age). The isochrone of 320 Myr age and Z = 0.0001 metallicity (the thick solid blue line) marks the upper age limit used for interpolation.

In the text
thumbnail Fig. 4.

Individual ages of the BHeB stars determined based on the ACS and Suprime-Cam photometry data. The red line was fitted by assuming a slope in a linear equation equal to 1.0.

In the text
thumbnail Fig. 5.

A recent star formation history of the Leo A galaxy based on the BHeB stars. The SFR for the youngest ages (< 30 Myr) was derived based on the bright (F814W <  22.5) main sequence stars.

In the text
thumbnail Fig. 6.

Spatial distribution of the MS (panel a) and BHeB (panels b–d) stars from the combined ACS and Suprime-Cam photometry. The symbols and colour-coding of MS and BHeB star ages are the same as in Fig. 2; corresponding age ranges are indicated in the bottom right corner of each panel. Parameters of the ellipse and contours of H I column density are the same as in Fig. 1.

In the text
thumbnail Fig. 7.

Composite image of the Leo A galaxy: Subaru/Suprime-Cam Hα (red) and B (green) passbands and the H I column density map (blue) with cyan contours, indicating the range of column density levels (atoms cm−2) from 4 × 1020 to 7 × 1021 with a constant step of 1020 (Hunter et al. 2012). The straight cyan line divides the largest H I hole into two regions with apparently different stellar populations. Positions and sizes of three small holes in the H I envelope, identified by Pokhrel et al. (2020), are indicated with cyan circles.

In the text
thumbnail Fig. 8.

Subaru/Suprime-Cam Hα map of the Leo A galaxy constructed by subtracting a frame in the R passband from the one in the Hα passband. Darker areas correspond to enhanced Hα emission regions (e.g. H II zones and emission stars). Cyan open circles mark 16 BHeB Hα emission stars. Yellow open circles mark star clusters (C1–C6). The red open circle marks planetary nebula (PN). Green arrows indicate an approximate size of the Hα emission ring. White contours correspond to the H I column density of 4 × 1020 atoms cm−2. Inside the closed white contour, the H I column density is lower than outside of it. North is up, and east is to the left.

In the text
thumbnail Fig. 9.

Diagrams of the bright (I <  23) Leo A stars (MS – red, BHeB – blue). Panel a: V versus B − V diagram; Hα emission stars are marked by cyan open circles. Panel b: Hα − R versus B − V diagram; an area occupied by prominent emission stars is marked by cyan dashed lines. Spectral type ranges, corresponding to the colour index B − V (corrected for the colour excess, E(B − V)=0.018) of normal Milky Way MS stars, are indicated for a reference.

In the text

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