The Isaac Newton Telescope monitoring survey of Local Group dwarf galaxies--V. The star formation history of Sagittarius dwarf irregular galaxy derived from long period variable stars

Abstract

We conducted an optical monitoring survey of the Sagittarius dwarf irregular galaxy (SagDIG) during the period of June 2016 -- October 2017, using the 2.5-m Isaac Newton Telescope (INT) at La Palama. Our goal was to identify Long Period Variable stars (LPVs), namely asymptotic giant branch stars (AGBs) and red supergiant stars (RSGs), to obtain the Star Formation History (SFH) of isolated, metal-poor SagDIG. For our purpose, we used a method that relies on evaluating the relation between luminosity and the birth mass of these most evolved stars. We found $27$ LPV candidates within two half-light radii of SagDIG. $10$ LPV candidates were in common with previous studies, including one very dusty AGB (x-AGB). By adopting the metallicity $Z = 0.0002$ for older population and $Z=0.0004$ for younger ages, we estimated that the star formation rate changes from $0.0005\pm0.0002$ M$_{\odot}$yr$^{-1}$kpc$^{-2}$ ($13$ Gyr ago) to $0.0021 \pm 0.0010$ M$_{\odot}$yr$^{-1}$kpc$^{-2}$ ($0.06$ Gyr ago). Like many dwarf irregular galaxies, SagDIG has had continuous star formation activity across its lifetime, though with different rates, and experiences an enhancement of star formation since $z \simeq 1$. We also evaluated the total stellar mass within two half-light radii of SagDIG for three choices of metallicities. For metallicity $Z = 0.0002$ and $Z=0.0004$ we estimated the stellar mass M$_*$ = ($5.4 \pm 2.3$) $\times$ $10^ 6$ and ($3.0 \pm 1.3$) $\times$ $10^ 6$ M$_{\odot}$, respectively. Additionally, we determined a distance modulus $\mu$ = $25.27\pm0.05$ mag, using the tip of the red giant branch (TRGB).