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 And IX, the closest satellite to the 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 (TRGB). By probing the variability of asymptotic giant branch stars (AGB), we study the star formation history of And IX. We identified 50 long period variables (LPVs) in And IX using the Isaac Newton Telescope (INT) in two filters, 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 $\sim$ $8.2\pm 3.1\times 10^{-4}$ M_sun yr${}^{-1}$ at $\approx 6$ Gyr ago. Our findings suggest an outside-in galaxy formation scenario for And IX with a quenching occurring $3.65_{-1.52}^{+0.13}$ Gyr ago with the SFR in the order of $2.0\times 10^{-4}$ M_sun yr${}^{-1}$ at redshift < $0.5$. We calculate the total stellar mass by integrating the star formation rate (SFR) within two half-light radii $\sim$ $3.0\times 10^5$ M_sun. By employing the spectral energy distribution (SED) fitting for observed LPVs in And IX, we evaluate the mass-loss rate in the range of $10^{-7}$ $\le$ $\dot{M}$ $\le$ $10^{-5}$ M_sun yr${}^{-1}$. Finally, we show that the total mass deposition to the interstellar medium (ISM) is $\sim$ $2.4\times 10^{-4}$ M_sun yr${}^{-1}$ from the C- and O-rich type of dust-enshrouded LPVs. The ratio of the total mass returned to the ISM by LPVs to the total stellar mass is $\sim 8.0\times 10^{-10}$ yr${}^{-1}$, and so at this rate, it would take $\sim$ 1 Gyr to reproduce this galaxy

Accepted for publication in ApJ

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