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Register a new userThe Isaac Newton Telescope monitoring survey of Local Group dwarf galaxies -- IV. The star formation history of Andromeda VII derived from long period variable 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.
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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.
An optical monitoring survey in the nearby dwarf galaxies was carried out with the 2.5-m Isaac Newton Telescope (INT). 55 dwarf galaxies and four isolated globular clusters in the Local Group (LG) were observed with the Wide Field Camera (WFC). The main aims of this survey are to identify the most evolved asymptotic giant branch (AGB) stars and red supergiants at the end-point of their evolution based on their pulsational instability, use their distribution over luminosity to reconstruct the star formation history, quantify the dust production and mass loss from modelling the multi-wavelength spectral energy distributions, and relate this to luminosity and radius variations. In this first of a series of papers, we present the methodology of the variability survey and describe the photometric catalogue of Andromeda I (And I) dwarf galaxy as an example of the survey, and discuss the identified long period variable (LPV) stars. We detected 5581 stars and identified 59 LPV candidates within two half-light radii of the centre of And I. The amplitudes of these candidates range from 0.2 to 3 mag in the $i$-band. 75 % of detected sources and 98 % of LPV candidates are detected at mid-infrared wavelengths. We show evidence for the presence of dust-producing AGB stars in this galaxy including five extreme AGB (x-AGB) stars, and model some of their spectral energy distributions. A distance modulus of 24.41 mag for And I was determined based on the tip of the red giant branch (RGB). Also, a half-light radius of 3.2 arcmin is calculated.
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.
The Local Group (LG) hosts many dwarf galaxies with diverse physical characteristics in terms of morphology, mass, star formation, and metallicity. To this end, LG can offer a unique site to tackle questions about the formation and evolution of galaxies by providing detailed information. While large telescopes are often the first choices for such studies, small telescope surveys that perform dedicated observations are still important, particularly in studying bright objects in the nearby universe. In this regard, we conducted a nine epoch survey of 55 dwarf galaxies called the Local Group dwarf galaxies survey using the 2.5m Isaac Newton Telescope (INT) in La Palma to identify Long-Period Variable (LPV) stars, namely Asymptotic Giant Branch (AGB) and Red Super Giant (RSG) stars. AGB stars formed at different times and studying their radial distribution and mass-loss rate can shed light on the structure formation in galaxies. To further investigate the evolutionary path of these galaxies, we construct their star formation history (SFH) using the LPV stars, which are at the final stages of their evolution and therefore experience brightness fluctuations on the timescales between hundred to thousand days. In this paper, we present some of the results of the Local Group dwarf galaxies survey.
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.
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