The role of mass loss in constraining quenching time in dwarf galaxies from AGB and RGB star counts

Abstract

The capability of reconstructing the past star formation history of dwarf elliptical galaxies out of the Local Volume relies on modelling bright stellar populations currently evolving through the red giant branch (RGB) and the asymptotic giant branch (AGB) phases. Recent studies proposed the use of the relative fractions of RGB and AGB stars populating specific boxes in the observational colour-magnitude plane to infer the epoch within which 90\% of the stellar population of the galaxy formed (T90). We aim at understanding the physical process of stellar evolution that are constrained by the relationship between the relative fraction of AGB and RGB stars of dwarf galaxies and the T90 epoch. We use updated stellar models that include the description of dust formation in the wind, to undertake a population synthesis approach, to allow monitoring the variation of the NAGB/NRGB ratio with time. The effects of some specific ingredients, such as the mass loss experienced by low-mass stars during the RGB phase, and the details of the time variation of the star formation rate, are extensively explored and tested against data. The mass lost by low-mass stars during the RGB evolution proves the most relevant ingredients affecting the time variation of NAGB/NRGB: at metallicities ~ 1/10 solar, a mass loss ~ 0.25Msun is required to reproduce the observations. This analysis allows to derive a relationship between NAGB/NRGB and T90, with a ~ 1 Gyr uncertainty on T90.

Figures (20)

• Figure 1: The updated correlation between the relative fraction of the number of AGB and RGB stars ($\rm N_{AGB}/N_{RGB}$) in nearby dwarf elliptical galaxies and the time before which $90\%$ of the stellar population of a given galaxy formed, $\rm T_{90}$harmsen23.
• Figure 2: Evolutionary tracks of model stars of metallicity $\rm [Fe/H]=-1$ and initial masses $\rm 0.8~M_{\odot}$ (blue line, until the helium-flash, then red line, from the start of the core helium burning phase) and $\rm 2~M_{\odot}$ (green), on the colour-magnitude $\rm (F606W-F814W, F814W)$ diagram. The regions inside the grey and cyan lines indicate the RGB and AGB boxes, respectively, introduced by H23. The evolutionary sequences are shown until the start of the post-AGB contraction, which is omitted for readability.
• Figure 3: Upper panels: Time variation of the F814W magnitude of the $\rm 0.8~M_{\odot}$ and $\rm 2~M_{\odot}$ model stars shown in Fig. \ref{['fcmd']}. The grey and yellow regions refer to the F814W magnitudes of the RGB and AGB boxes introduced by H23. The evolution of the $\rm 0.8~M_{\odot}$ model star is divided into the RGB (blue line) and the post-flash (red) parts. The magenta and green lines refer to the evolution of the $\rm 0.8~M_{\odot}$ model star, under the hypothesis that $\rm 0.1~M_{\odot}$ and $\rm 0.2~M_{\odot}$ were lost during the RGB phase. Lower panels: Time variation of the F814W magnitude of $\rm 1.5~M_{\odot}$ and $\rm 2.5~M_{\odot}$ stars.
• Figure 4: Duration of the phase during which the stars evolve into the RGB (black squares and lines) and the AGB (red dots and lines) boxes introduced by H23, as a function of the initial mass (left panel) and the formation epoch (right). Dotted lines and green triangles refer to the results obtained by assuming $\rm 0.2~M_{\odot}$ mass loss during the RGB evolution of $\rm M<1.5~M_{\odot}$ stars.
• Figure 5: The variation of the relative mass of the galaxy (solid line), of the fraction of AGB (dashed) and RGB (dotted) stars formed, for the $\rm \delta m (RGB)= 0.25~M_{\odot}$ case and the SFH by Weisz14 and savino25 for the galaxies Andromeda II (top, left panel) and Andromeda I (top, right), respectively. The bottom panels show the cumulative contribution of various star formation epochs to the present-day ratio of AGB to RGB stars of Andromeda II (bottom, left panel) and Andromeda I (bottom, right panel), based on different assumptions regarding the star formation history and the average mass lost by low-mass stars during the RGB phase. Solid lines indicate results found by adopting the SFH by Weisz14, for Andromeda II, and savino25, for Andromeda I; dashed tracks are based on the assumption of constant SFH; dotted lines refer to results obtained when assuming that the SFH decays with a time scale of 7.5 Gyr. Colour coding indicates the mass lost by stars during the RGB evolution: the cases $\rm \delta m (RGB)= 0.1~M_{\odot}$ (black lines), $\rm 0.2~M_{\odot}$ (magenta), $\rm 0.25~M_{\odot}$ (blue) are shown. The grey shaded region indicates the T90 epoch, the red line refers to the AGB to RGB ratio derived for Andromeda II and Andromeda I.