AGB and RGB stars in the dwarf irregular galaxy Leo A

Abstract

Context. Leo A is a gas-rich dwarf irregular galaxy of low stellar mass located in the outskirts of the Local Group. It has an extended star formation history with stellar populations spanning a wide age range (∼0.01−10 Gyr). As Leo A is a well-isolated dwarf galaxy, it is a perfect target to study a galactic structure formed entirely by processes of self-induced star formation. Aims. Our aim is to study populations of the brightest asymptotic giant branch (AGB) stars and red giant branch (RGB) stars over the entire extent of the Leo A galaxy. Methods. We analysed populations of AGB and RGB stars in the Leo A galaxy using multicolour photometry data obtained with the Subaru Suprime-Cam (B, V, R, I, Hα) and HST ACS (F475W, F814W) cameras. In order to separate the Milky Way and Leo A populations of red stars, we developed a photometric method that enabled us to study the spatial distribution of AGB and RGB stars within the Leo A galaxy. Results. We found a previously unknown sequence of 26 peculiar RGB stars which probably have a strong CN band in their spectra (∼380−390 nm). This conclusion is supported by the infrared CN spectral features observed in four of these stars with available spectra from the literature. Additionally, we present a catalogue of 32 luminous AGB stars and 3 candidate AGB stars. Twelve AGB stars (three of them might have dusty envelopes) from this sample are newly identified; the remaining 20 AGB stars were already presented in the literature based on near-infrared observations. By splitting the RGB sequence into blue and red parts, we revealed different spatial distributions of the two subsets, with the former being more centrally concentrated than the latter. Cross-identification with spectroscopic data available in the literature suggests that the bulk of blue and red RGB stars are, on average, similar in metallicity; however, the red RGB stars might have an excess of metal-deficient stars of [Fe/H] < −1.8. We also found that the distributions of luminous AGB and blue RGB stars have nearly equal scale lengths (0.′87 ± 0.′06 and 0.′89 ± 0.′09, respectively), indicating that they could belong to the same generation. This conclusion is strengthened by the similarities of the cumulative distributions of AGB and blue RGB stars, both showing more centrally concentrated populations compared to red RGB stars. There is also a prominent decline in the ratio of AGB to RGB stars with an increasing radius. These results suggest that the star-forming disk of Leo A is shrinking, which is in agreement with the outside-in star formation scenario of dwarf galaxy evolution.

Recent star formation history of the dwarf irregular galaxy Leo A

“…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 +0.5 s (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: A B = 0.075, A V = 0.057, A I = 0.031, A F475W = 0.068, and A F814W = 0.032 (Schlafly & Finkbeiner 2011).…”

“…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 2006Kirby 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).…”

Subaru Suprime-Cam Wide-field BVI Stellar Photometry of the M33 Galaxy

“…The CMDs (Figure 5) clearly show prominent main-sequence (MS) and blue-loop stars, as well as numerous RGB and asymptotic giant branch stars. They suggest that data are well suited for star formation history studies as was performed in the Leo A galaxy by Leščinskaitė et al (2021Leščinskaitė et al ( , 2022 based on the Suprime-Cam photometry data from the same set of observations as the present survey.…”