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Register a new userThe Local Group Dwarf Galaxies: The Star Formation Histories derived using the Long Period Variable stars
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Dwarf galaxies in the Local Group (LG) represent a distinct as well as diverse family of tracers of the earliest phases of galaxy assembly and the processing resulting from galactic harassment. Their stellar populations can be resolved and used as probes of the evolution of their host galaxy. In this regard, we present the first reconstruction of the star formation history (SFH) of them using the most evolved AGB stars that are long period variable (LPV). For the nearby, relatively massive and interacting gas-rich dwarf galaxies, the Magellanic Clouds, we found that the bulk of the stars formed ~ 10 Gyr ago for the LMC, while the strongest episode of star formation in the SMC occurred a few Gyr later. A peak in star formation around 0.7 Gyr ago in both Clouds is likely linked to their recent interaction. The Andromeda satellite pair NGC147/185 show different histories; the main epoch of star formation for NGC 185 occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate (SFR). In the case of NGC 147, the SFR peaked only 6.9 Gyr ago, staying intense until ~ 3 Gyr ago. Star formation in the isolated gas-rich dwarf galaxy IC 1613 has proceeded at a steady rate over the past 5 Gyr, without any particular dominant epoch. Due to lack of sufficient data, we have conducted an optical monitoring survey at the Isaac Newton Telescope (INT) of 55 dwarf galaxies in the LG to reconstruct the SFH of them uniformly. The observations are made over ten epochs, spaced approximately three months apart, as the luminosity of LPV stars varies on timescales of months to years. The system of galactic satellites of the large Andromeda spiral galaxy (M31) forms one of the key targets of our monitoring survey. We present the first results in the And I dwarf galaxy, where we discovered 116 LPVs among over 10,000 stars.
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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 (M 31). Although M 31 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 M 31 and its dSph system is frail. Therefore, the question is whether And VII (a high-metallicity dSph located $sim$220 kpc from M 31), can be associated with M 31s 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 (AGB) and red supergiant (RSG) 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 multi-epoch 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 $simeq 6.2$ Gyr ago with a SFR of $0.006pm0.002$ M$_odot$ yr$^{-1}$. Over the past 6 Gyr, we find slow star formation, which continued until 500 Myr ago with a SFR $sim0.0005pm0.0002$ M$_odot$ yr$^{-1}$. We determined And VIIs stellar mass $M=(13.3pm5.3)times10^6$ M$_odot$ within a half-light radius $r_{frac{1}{2}}=3.8pm0.3$ arcmin and metallicity $Z=0.0007$, and also derived its distance modulus of $mu=24.38$ mag.
We present the first reconstruction of the star formation history (SFH) of the Large and Small Magellanic Clouds (LMC and SMC) using Long Period Variable stars. These cool evolved stars reach their peak luminosity in the near-infrared; thus, their K-band magnitudes can be used to derive their birth mass and age, and hence the SFH can be obtained. In the LMC, we found a 10-Gyr old single star formation epoch at a rate of $sim1.5$ M$_odot$ yr$^{-1}$, followed by a relatively continuous SFR of $sim0.2$ M$_odot$ yr$^{-1}$, globally. In the core of the LMC (LMC bar), a secondary, distinct episode is seen, starting 3 Gyr ago and lasting until $sim0.5$ Gyr ago. In the SMC, two formation epochs are seen, one $sim6$ Gyr ago at a rate of $sim0.28$ M$_odot$ yr$^{-1}$ and another only $sim0.7$ Gyr ago at a rate of $sim0.3$ M$_odot$ yr$^{-1}$. The latter is also discernible in the LMC and may thus be linked to the interaction between the Magellanic Clouds and/or Milky Way, while the formation of the LMC bar may have been an unrelated event. Star formation activity is concentrated in the central parts of the Magellanic Clouds now, and possibly has always been if stellar migration due to dynamical relaxation has been effective. The different initial formation epochs suggest that the LMC and SMC did not form as a pair, but at least the SMC formed in isolation.
We use the APOSTLE and Auriga cosmological simulations to study the star formation histories (SFHs) of field and satellite dwarf galaxies. Despite sizeable galaxy-to-galaxy scatter, the SFHs of APOSTLE and Auriga dwarfs exhibit robust average trends with galaxy stellar mass: faint field dwarfs ($10^5<M_{rm star}/M_odot<10^{6.5}$) have, on average, steadily declining SFHs, whereas brighter dwarfs ($10^{7.5}<M_{rm star}/M_odot<10^{9}$) show the opposite trend. Intermediate-mass dwarfs have roughly constant SFHs. Satellites exhibit similar average trends, but with substantially suppressed star formation in the most recent $sim 5$ Gyr, likely as a result of gas loss due to tidal and ram-pressure stripping after entering the haloes of their primaries. These simple mass and environmental trends are in good agreement with the derived SFHs of Local Group (LG) dwarfs whose photometry reaches the oldest main sequence turnoff. SFHs of galaxies with less deep data show deviations from these trends, but this may be explained, at least in part, by the large galaxy-to-galaxy scatter, the limited sample size, and the large uncertainties of the inferred SFHs. Confirming the predicted mass and environmental trends will require deeper photometric data than currently available, especially for isolated dwarfs.
Local Group (LG) galaxies have relatively accurate SFHs and metallicity evolution derived from resolved CMD modeling, and thus offer a unique opportunity to explore the efficacy of estimating stellar mass M$_{star}$ of real galaxies based on integrated stellar luminosities. Building on the SFHs and metallicity evolution of 40 LG dwarf galaxies, we carried out a comprehensive study of the influence of SFHs, metallicity evolution, and dust extinction on the UV-to-NIR color-$M/L$ (color-log$Upsilon_{star}$($lambda$)) relations and M$_{star}$ estimation of local universe galaxies. We find that: The LG galaxies follow color-log$Upsilon_{star}$($lambda$) relations that fall in between the ones calibrated by previous studies; Optical color-log$Upsilon_{star}$($lambda$) relations at higher metallicities ([M/H]) are generally broader and steeper; The SFH concentration does not significantly affect the color-log$Upsilon_{star}$($lambda$) relations; Light-weighted ages and [M/H] together constrain log$Upsilon_{star}$($lambda$) with uncertainties ranging from $lesssim$ 0.1 dex for the NIR up to 0.2 dex for the optical passbands; Metallicity evolution induces significant uncertainties to the optical but not NIR $Upsilon_{star}$($lambda$) at given light-weighted ages and [M/H]; The $V$ band is the ideal luminance passband for estimating $Upsilon_{star}$($lambda$) from single colors, because the combinations of $Upsilon_{star}$($V$) and optical colors such as $B-V$ and $g-r$ exhibit the weakest systematic dependence on SFHs, [M/H] and dust extinction; Without any prior assumption on SFHs, M$_{star}$ is constrained with biases $lesssim$ 0.3 dex by the optical-to-NIR SED fitting. Optical passbands alone constrain M$_{star}$ with biases $lesssim$ 0.4 dex (or $lesssim$ 0.6 dex) when dust extinction is fixed (or variable) in SED fitting. [abridged]
We explore the quenching of low-mass galaxies (10^4 < Mstar < 10^8 Msun) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived from analyzing color-magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) Lower mass galaxies quench earlier than higher mass galaxies; (2) Inside of virial radius there is no correlation between a satellites current proximity to a massive host and its quenching epoch; (3) There are hints of systematic differences in quenching times of M31 and Milky Way (MW) satellites, although the sample sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with literature results, we qualitatively consider the redshift evolution (z=0-1) of the quenched galaxy fraction over ~7 dex in stellar mass (10^4 < Mstar < 10^11.5 Msun). The quenched fraction of all galaxies generally increases toward the present, with both the lowest and highest mass systems exhibiting the largest quenched fractions at all redshifts. In contrast, galaxies between Mstar ~ 10^8-10^10 Msun have the lowest quenched fractions. We suggest that such intermediate-mass galaxies are the least efficient at quenching. Finally, we compare our quenching times with predictions for infall times of low-mass galaxies associated with the MW. We find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have been quenched before infall while higher mass satellites (e.g., Leo I, Fornax) typically quench ~1-4 Gyr after infall.
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