Abstract
Reconstructing the mass assembly history of the Milky Way relies on obtaining detailed measurements of the
properties of many stars in the Galaxy, especially in the stellar halo. One of the most constraining quan
tities is stellar age, as it can shed light on the accretion time and quenching of star formation in merging
satellites. However, obtaining reliable age estimates for large samples of halo stars is difficult. We report
published ages of 120 subgiant halo stars with highly-radial orbits that likely belong to the debris of the Gaia
Enceladus/Sausage (GES) galaxy. The majority of these halo stars are old, with an age distribution characterized
by a median of 11.6 Gyr and 16th(84th) percentile of 10.5 (12.7) Gyr. However, the distribution is skewed, with
a tail of younger stars that span ages down to ∼ 6–9 Gyr. All highly-radial halo stars have chemical and kine
matic/orbital quantities that associate them with the GES debris. Initial results suggest that these intermediate
age stars are not a product of mass transfer and/or stellar mergers, which can bias their age determination low. If
this conclusion is upheld by upcoming spectro-photometric studies, then the presence of these stars will pose an
important challenge for constraining the properties of the GES merger and the accretion history of the Galaxy
properties of many stars in the Galaxy, especially in the stellar halo. One of the most constraining quan
tities is stellar age, as it can shed light on the accretion time and quenching of star formation in merging
satellites. However, obtaining reliable age estimates for large samples of halo stars is difficult. We report
published ages of 120 subgiant halo stars with highly-radial orbits that likely belong to the debris of the Gaia
Enceladus/Sausage (GES) galaxy. The majority of these halo stars are old, with an age distribution characterized
by a median of 11.6 Gyr and 16th(84th) percentile of 10.5 (12.7) Gyr. However, the distribution is skewed, with
a tail of younger stars that span ages down to ∼ 6–9 Gyr. All highly-radial halo stars have chemical and kine
matic/orbital quantities that associate them with the GES debris. Initial results suggest that these intermediate
age stars are not a product of mass transfer and/or stellar mergers, which can bias their age determination low. If
this conclusion is upheld by upcoming spectro-photometric studies, then the presence of these stars will pose an
important challenge for constraining the properties of the GES merger and the accretion history of the Galaxy
| Original language | English |
|---|---|
| Article number | arXiv:2403.09777 |
| Number of pages | 18 |
| Journal | arXiv e-prints |
| DOIs | |
| Publication status | E-pub ahead of print - 11 Jun 2024 |