On detecting halo assembly bias with galaxy populations
Yen-Ting Lin1*, Rachel Mandelbaum2, Yun-Hsin Huang3, Hung-Jin Huang2, Neal Dalal4, Benedikt Diemer3, Hung-Yu Jian1, Andrey Kravtsov3
1Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
2Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
3Department of Astronomy and Astrophysics, The University of Chicago, Chicago, Illinois, USA
4Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
* presenting author:Yen-Ting Lin, email:ytl@asiaa.sinica.edu.tw
The fact that the clustering and concentration of dark matter halos depend not
only on their mass, but also the formation epoch, is a prominent, albeit
subtle, feature of the cold dark matter structure formation theory, and is
known as assembly bias. At low mass scales (~10¹² Msun/h), early-forming
halos are predicted to be more strongly clustered than the late-forming ones.
In this study we aim to robustly detect the signature of assembly bias
observationally, making use of formation time indicators of central galaxies in
low mass halos as a proxy for the halo formation history. Weak gravitational
lensing is employed to ensure our early- and late-forming halo samples have
similar masses, and are free of contamination of satellites from more massive
halos. For the two formation time indicators used (resolved star formation
history and current specific star formation rate), we do not find convincing
evidence of assembly bias. For a pair of early- and late-forming galaxy
samples with mean mass M₂₀₀~9x10¹¹ Msun/h, the relative bias is 1.00±0.12.
We attribute the lack of detection to the possibilities that either the current
measurements of these indicators are too noisy, or they do not correlate well
with the halo formation history. Alternative proxies for the halo formation
history that should perform better are suggested for future studies.

Keywords: cosmology, large scale structure, galaxy formation, dark matter halo