The Star Formation History of M32
Antonela Monachesi (1,2), Scott C. Trager (1), Tod R. Lauer (3),, Sebasti\'an L. Hidalgo (4), Wendy Freedman (5), Alan Dressler (5), Carl, Grillmair (6), Kenneth J. Mighell (3) ((1) Kapteyn Astronomical Institute,, University of Groningen, (2) Department of Astronomy

TL;DR
This study reconstructs the star formation history of M32 using deep HST observations, revealing a complex mix of old and intermediate-age stars, and compares it with M31's background population to infer galaxy formation processes.
Contribution
It provides the first detailed star formation history of M32 from resolved stellar populations, highlighting the presence of multiple stellar populations and challenging previous spectroscopic age gradient results.
Findings
40%+/-17% of M32's mass is from 2-5 Gyr old stars
55%+/-21% of M32's mass is from stars older than 5 Gyr
Majority of M31's background stars are old, formed 5-14 Gyr ago
Abstract
We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5Gyr old stars contribute \som40%+/- 17% of M32's mass, while 55%+/-21% of M32's mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are <Age>=6.8+/-1.5 Gyr and <[M/H]>=-0.01+/-0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H]\sim-0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining \sim3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1…
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