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arXiv · 2311.16397

The Hubble Space Telescope Survey of M31 Satellite Galaxies. III. Calibrating the Horizontal Branch as an Age Indicator for Nearby Galaxies

Connor Jennings·Alessandro Savino·Daniel Weisz·Nitya Kallivayalil·Andrew Cole·Michelle Collins·Andrew Dolphin·Annette Ferguson·Karoline Gilbert·Puragra Guhathakurta·Evan Kirby·Geraint Lewis·Nicolas Martin·Michael Rich·Evan Skillman·Roeland van der Marel·Jack Warfield
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Abstract

We present a new method for measuring the mean age of old/intermediate stellar populations in resolved, metal-poor ($\rm \langle[Fe/H]\rangle \lesssim -1.5$) galaxies using only the morphology of the horizontal branch (HB) and an estimate of the average metallicity. We calculate the ratio of blue-to-red HB stars and the mass-weighted mean ages of 27 M31 satellite galaxies that have star formation histories (SFHs) measured from Hubble Space Telescope-based color-magnitude diagrams (CMDs) that include the oldest Main Sequence Turn-off (MSTO) ages. We find a strong correlation between mean age, metallicity, and HB morphology, for stellar populations older than $\sim6$~Gyr. The correlation allows us to predict a galaxy's mean age from its HB morphology to a precision of $\lesssim 1$~Gyr. We validate our method by recovering the correct ages of Local Group galaxies that have robust MSTO-based ages and are not in our calibration sample. We also use our technique to measure the mean ages of isolated field galaxies KKR25 ($11.21^{+0.70}_{-0.65}$~Gyr) and VV124 ($11.03^{+0.73}_{-0.68}$~Gyr), which indicate that their main star formation episodes may have lasted several Gyr and support the picture that they achieved their early-type characteristics (e.g., low gas content, low star formation activity) in isolation and not through environment. Because the HB is $\sim80\times$ brighter than the oldest MSTO, our method can provide precise characteristic ages of predominantly old galaxies at distances $\sim 9$ times farther. We provide our calibrations in commonly used HST/ACS filters.

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Connor Jennings, Alessandro Savino, Daniel Weisz, Nitya Kallivayalil, Andrew Cole, Michelle Collins, Andrew Dolphin, Annette Ferguson, Karoline Gilbert, Puragra Guhathakurta, Evan Kirby, Geraint Lewis, Nicolas Martin, Michael Rich, Evan Skillman, Roeland van der Marel, Jack Warfield. 2023-11-28. The Hubble Space Telescope Survey of M31 Satellite Galaxies. III. Calibrating the Horizontal Branch as an Age Indicator for Nearby Galaxies. https://arxiv.org/abs/2311.16397

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