The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. IV. The Star Formation History of Andromeda VII Derived from Long-period Variable Stars

Abstract

We have examined the star formation history (SFH) of Andromeda VII (And VII), the brightest and most massive dwarf spheroidal (dSph) satellite of the Andromeda galaxy (M31). Although M31 is surrounded by several dSph companions with old stellar populations and low metallicity, it has a metal-rich stellar halo with an age of 6-8 Gyr. This indicates that any evolutionary association between the stellar halo of M31 and its dSph system is frail. Therefore, the question is whether And VII (a high-metallicity dSph located ∼220 kpc from M31) can be associated with M31ʼs young, metal-rich halo. Here we perform the first reconstruction of the SFH of And VII employing long-period variable (LPV) stars. As the most evolved asymptotic giant branch and red supergiant stars, the birth mass of LPVs can be determined by connecting their near-infrared photometry to theoretical evolutionary tracks. We found 55 LPV candidates within two half-light radii, using multiepoch imaging with the Isaac Newton Telescope in the i and V bands. Based on their birth mass function, the star formation rate (SFR) of And VII was obtained as a function of cosmic time. The main epoch of star formation occurred ; 6.2 Gyr ago with an SFR of 0.006 ± 0.002 M e yr −1 . Over the past 6 Gyr, we find slow star formation, which continued until 500 Myr ago with an SFR ∼ 0.0005 ± 0.0002 M e yr −1 . We determined And VII's stellar mass M = (13.3 ± 5.3) × 10 6 M e within a half-light radius = ¢  ¢ r 3.8 0.3 1 2 and metallicity Z = 0.0007, and we also derived its distance modulus of μ = 24.38 mag.

Satellites of MW/M31-like galaxies with TNG50: quenched fractions, gas content, and star formation histories

“…8, we compare the SFHs of the satellite populations of our TNG50 M31-like hosts to those of the observed dwarfs around M31, mostly obtained by colour-magnitude diagram analyses by Weisz et al (2014b). For And II and And I, we employ the more recently published SFHs from the Initial Star formation and Lifetimes of Andromeda Satellites (ISLAndS) project (Skillman et al 2017); for And VII, we include a second SFH based on long-period variable stars (dotted cyan curve, Navabi et al 2021). In both cases, we replicate the SFHs from observations in all columns, i.e.…”

“…9 illustrates 𝜏 90 (top panels) and 𝜏 50 (bottom panels) as a function of absolute 𝑉-band magnitude 𝑀 V and stellar mass for star-forming and quenched satellites (blue and empty circles, respectively), separately for MW-like (left panels) and M31-like hosts (right panels, see §2.2.1 for details). We compare the TNG50 satellites to the corresponding observed dwarfs of the MW and M31 (red and brown diamonds, respectively) with 𝜏 90 and 𝜏 50 extracted from the cumulative SFHs from Weisz et al 2013, 2014band Navabi et al 2021…”

“…Filled blue circles denote galaxies that are star-forming or in the green valley at 𝑧 = 0, according to their distance to the SFMS; empty circles represent quenched satellites. Furthermore, we compare our TNG50 satellites to the observed dwarfs of the MW and M31 (red and brown diamonds, respectively, Weisz et al 2013Weisz et al , 2014bNavabi et al 2021). to 6.9 +2.5 −3.3 Gyr when using the quenching times of Joshi et al (2021).…”

From evolved Long-Period-Variable stars to the evolution of M31

“…M31 is located at 785± 25 kpc, (m − M ) o = 24.47 mag (McConnachie et al 2004) and has low foreground reddening E(B-V)=0.06 mag. Our approach to investigating the star formation history (SFH) is based on employing long-period variable stars (LPVs), which we have successfully applied in other galaxies in the Local Group, such as M33 (Javadi et al 2011a;2011b;2017), Magellanic Clouds (Rezaeikh et al 2014), NGC147 and NGC185 (Golshan et al 2011), IC1613 (Hashemi et al 2019), Andromeda VII (Navabi et al 2020;2021) and Andromeda I (Saremi et al 2021). Asymptotic giant branch (AGB) and red supergiants (RSGs) are basic pillars in our research.…”

From evolved Long-Period-Variable stars to the evolution of M31

“…M31 is located at 785 ± 25 kpc, (m − M ) o = 24.47 mag (McConnachie et al 2004) and has low foreground reddening E(B-V)=0.06 mag. Our approach to investigating the star formation history (SFH) is based on employing long-period variable stars (LPVs) which we have successfully applied to other galaxies in the Local Group such as M33 (Javadi et al 2011a(Javadi et al ,b, 2017, Magellanic Clouds (Rezaeikh et al 2014), NGC147 andNGC185 (Golshan et al 2017), IC1613 (Hashemi et al 2019), Andromeda VII (Navabi et al 2020(Navabi et al , 2021 and Andromeda I (Saremi et al 2021). Asymptotic giant branch (AGB) and red super giants (RSGs) are basic pillars in our research.…”