The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. VI. The Star Formation History and Dust Production in Andromeda IX

Abstract

We present a photometric study of the resolved stellar populations in Andromeda IX (And IX), the closest satellite to M31, a metal-poor and low-mass dwarf spheroidal galaxy. We estimate a distance modulus of ${24.56}_{-0.15}^{+0.05}$ mag based on the tip of the red giant branch. By probing the variability of asymptotic giant branch stars, we study the star formation history of And IX. We identified 50 long-period variables (LPVs) in And IX using the Isaac Newton Telescope in two filters, the Sloan $i^{\prime} $ and Harris V. In this study, we selected LPVs within two half-light radii with amplitudes in the range of 0.2–2.20 mag. It is found that the peak of star formation reaches ∼8.2 ± 3.1 × 10−4M⊙ yr−1 at ≈6 Gyr ago. Our findings suggest an outside-in galaxy formation scenario for And IX with a quenching occurring ${3.65}_{-0.13}^{+1.52}$ Gyr ago with a star formation rate (SFR) in the order of 2.0 × 10−4M⊙ yr−1 at redshift <0.5. We calculate the total stellar mass by integrating the SFR within two half-light radii ∼3.0 × 105M⊙. By employing spectral energy distribution fitting for the observed LPVs in And IX, we evaluate a mass-loss rate in the range of 10−7 ≤ $\dot{M}$ ≤ 10−5M⊙ yr−1. Finally, we show that the total mass deposition to the interstellar medium (ISM) is ∼2.4 × 10−4M⊙ yr−1 from the C- and O-rich types of dust-enshrouded LPVs. The ratio of the total mass returned to the ISM by LPVs to the total stellar mass is ∼8.0 × 10−10 yr−1, and so at this rate it would take ∼1 Gyr to reproduce this galaxy.