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|Paper IPM / Astronomy / 17215||
We conducted an optical monitoring survey of the Sagittarius dwarf irregular galaxy (SagDIG) during the period of 2016 Juneâ2017 October, using the 2.5 m Isaac Newton Telescopeat La Palama. Our goal was to identify long-period variable stars (LPVs), namely, asymptotic giant branch stars (AGBs) and red supergiant stars, to obtain the star formation history 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 2 half-light radii of SagDIG. 10 LPV candidates were in common with previous studies, including one extreme-AGB (x-AGB). By adopting the metallicity Z = 0.0002 for older populations and Z = 0.0004 for younger ages, we estimated that the star formation rate changes from 0.0005 Â± 0.0002 Mâ yrâ1 kpcâ2 (13 Gyr ago) to 0.0021 Â± 0.0010 Mâ yrâ1 kpcâ2 (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 â 1. We also evaluated the total stellar mass within 2 half-light radii of SagDIG for three choices of metallicities. For metallicity Z = 0.0002 and 0.0004, we estimated the stellar mass M* = (5.4 Â± 2.3) Ã 106 and (3.0 Â± 1.3) Ã 106 Mâ, respectively. Additionally, we determined a distance modulus of Î¼ = 25.27 Â± 0.05 mag, using the tip of the red giant branch.
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