Kavli Affiliate: Luis C. Ho
| First 5 Authors: Cheqiu Lyu, Yingjie Peng, Yipeng Jing, Xiaohu Yang, Luis C. Ho
| Summary:
The properties of the galaxies are tightly connected to their host halo mass
and halo assembly history. Accurate measurement of the halo assembly history in
observation is challenging but crucial to the understanding of galaxy formation
and evolution. The stellar-to-halo mass ratio ($M_*/M_{mathrm{h}}$) for the
centrals has often been used to indicate the halo assembly time
$t_{mathrm{h,50}}$ of the group, where $t_{mathrm{h,50}}$ is the lookback
time at which a halo has assembled half of its present-day virial mass. Using
mock data from the semi-analytic models, we find that $M_*/M_{mathrm{h}}$
shows a significant scatter with $t_{mathrm{h,50}}$, with a strong systematic
difference between the group with a star-forming central (blue group) and
passive central (red group). To improve the accuracy, we develop
machine-learning models to estimate $t_{mathrm{h,50}}$ for galaxy groups using
only observable quantities in the mocks. Since star-formation quenching will
decouple the co-growth of the dark matter and baryon, we train our models
separately for blue and red groups. Our models have successfully recovered
$t_{mathrm{h,50}}$, within an accuracy of $sim$ 1.09 Gyr. With careful
calibrations of individual observable quantities in the mocks with SDSS
observations, we apply the trained models to the SDSS Yang et al. groups and
derive the $t_{mathrm{h,50}}$ for each group for the first time. The derived
SDSS $t_{mathrm{h,50}}$ distributions are in good agreement with that in the
mocks, in particular for blue groups. The derived halo assembly history,
together with the halo mass, make an important step forward in studying the
halo-galaxy connections in observation.
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