On the Prospect of Using the Maximum Circular Velocity of Halos to Encapsulate Assembly Bias in the Galaxy–Halo Connection

Abstract

We investigate a conceptual modification of the halo occupation distribution approach, using the halos' present-day maximal circular velocity, V max , as an alternative to halo mass. In particular, using a semi-analytic galaxy formation model applied to the Millennium WMAP7 simulation, we explore the extent that switching to V max 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 V max 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 V max 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 V max as a proxy for halo mass diminishes some of 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 V peak , the peak value of V max over the assembly history, provides some improvement and warrants further investigation.

Large-scale dark matter simulations

“…All these impact the expected spatial distribution of galaxies, thus recent HOD implementations have relaxed some of the assumptions of this model to enable a more realistic description of galaxies. In particular, attempts are made to incorporate secondary correlations with other halo properties beyond mass or environmental properties (Hadzhiyska et al 2020(Hadzhiyska et al , 2021a, velocity bias (Guo et al 2015), or flexibility in the spatial distribution to incorporate assembly bias (Hearin et al 2016;Zehavi et al 2019;Xu et al 2021).…”

The assembly bias of emission-line galaxies

“…Likewise, the manifestation of assembly bias in galaxy clustering, commonly referred to as galaxy assembly bias, has been found both in SAMs (e.g. Croton, Gao & White 2007;Zehavi et al 2018Zehavi et al , 2019Contreras et al 2019;Jiménez et al 2019;Xu, Zehavi & Contreras 2021) and hydrodynamical simulations (e.g. Artale et al 2018;Montero-Dorta et al 2020;Xu & Zheng 2020).…”

Predicting halo occupation and galaxy assembly bias with machine learning

“…Again, this is shown for the central galaxies only on the left-hand side and for the full samples, including satellites, on the right. These ratios have already been studied with this specific SAM sample (Zehavi et al 2018(Zehavi et al , 2019Xu et al 2021). The roughly 15% increase of clustering in the original SAM sample versus the shuffled one arises from the differentiated occupation of haloes with galaxies according to secondary halo properties which exhibit halo assembly bias.…”

“…In such a case, the halo occupation by galaxies will no longer depend solely on halo mass, but will vary with these secondary halo and environmental properties. These expected occupancy variations have recently been studied in SAM and hydrodynamical simulations (Zehavi et al 2018(Zehavi et al , 2019Artale et al 2018;Bose et al 2019;Xu et al 2021)).…”