On the Prospect of Using the Maximum Circular Velocity of Halos to Encapsulate Assembly Bias in the Galaxy–Halo Connection
2019
We investigate a conceptual modification of the halo occupation distribution approach, using the halos’ present-day
maximal circular velocity, Vmax, as an alternative to halo mass. In particular, using a semianalytic galaxy formation
model applied to the Millennium WMAP7 simulation, we explore the extent that switching to Vmax as the primary
halo property incorporates the effects of assembly bias into the formalism. We consider fixed number density
galaxy samples ranked by stellar mass and examine the variations in the halo occupation functions with either halo
concentration or formation time. We find that using Vmax results in a significant reduction in the occupancy
variation of the central galaxies, particularly for concentration. The satellites’ occupancy variation on the other
hand increases in all cases. We find effectively no change in the halo clustering dependence on concentration, for
fixed bins of Vmax compared to fixed halo mass. Most crucially, we calculate the impact of assembly bias on galaxy
clustering by comparing the amplitude of clustering to that of a shuffled galaxy sample, finding that the level of
galaxy assembly bias remains largely unchanged. Our results suggest that while using Vmax as a proxy for halo
mass diminishes some of the occupancy variations exhibited in the galaxy–halo relation, it is not able to
encapsulate the effects of assembly bias potentially present in galaxy clustering. The use of other more complex
halo properties, such as Vpeak, the peak value of Vmax over the assembly history, provides some improvement and
warrants further investigation.
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