Star Formation History and Dust Production Rate in And IX, the Closest Satellite of the Andromeda Galaxy
Hedieh Abdollahi
Konkoly Observatory


We present a photometric study of the resolved stellar populations in And IX, the closest satellite to M31, a metal-poor and low-mass dwarf spheroidal galaxy. We estimate a distance modulus of 24.56 mag (∼ 816.58 kpc) 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' and Harris V. In this study, we selected LPVs within two half-light radii (∼ 5 arcmin) with amplitudes in the range of 0.20 to 2.20 mag. It is found that the peak of star formation reached ∼ 8.2 ± 3.1 × 10−4 M⊙/yr at ≈ 6 Gyr ago. Our findings suggest an outside-in galaxy formation scenario for And IX with a quenching occurring 3.65 Gyr ago with the SFR in the order of 2.0 × 10−4 M⊙/yr at redshift < 0.5. We calculate the total stellar mass by integrating the star formation rate (SFR) within two half-light radii to be ∼ 3.0 × 105 M⊙. 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 to 10−5 M⊙/yr. Finally, we show that the total mass deposition to the interstellar medium (ISM) is ∼ 2.4 × 10−4 M⊙/yr 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 ∼ 8.0 × 10−10 1/yr, and so at this rate, it would take ∼ 1 Gyr to reproduce this galaxy.