This blog is based on arXiv:2304.07189.

Halo formation time

The assembly of dark matter halos proceeds hierarchically, where low-mass halos are formed early and they merge with each other to form larger halos. This process can be described by a halo merger tree, which grows from the descendant halos and splits to multiple progenitor halos recursively. This tree structure is too complicated to be understood, nor incorporated into theoretical modelling. Therefore, data compression is required. A conventional way is to extract the main branch by recursively selecting the most massive progenitor, and this main branch is also referred to as the mass accretion history. From the mass accretion history, one can define a characteristic formation time as the time that the main progenitor has accumulated half of its final mass, which is also referred to as the halo formation time.

Wang et al. 2023 found that the ratio between the stellar mass of the central galaxy and the halo mass is a good observational proxy of halo formation time, where early-formed halos tend to more dominant central galaxies. This relation is also found in hydrodynamical simulations and empirical models.

Observational evidence

First of all, there is no direct evidence of the statistical correlation between halo formation time and the quenching of central galaxies at given halo masses for two reasons:

• There is no reliable halo mass estimation for individual halo for a large and unbiased galaxy group sample at the current stage.
• Halo formation time is not a observable. But the central-to-total stellar mass ratio is a good proxy of halo formation time.

Secondly, there are several indirect observational evidences that central galaxies in early-formed halos are more star-forming and spiral-like. For example, Posti et al. 2019 found a population of star-forming spiral central galaxies with $M_*\sim 10^{11} M_\odot$ lives in $\sim 10^{12} M_\odot$ halos, where the halo mass is obtained through the modelling of the HI rotation curve. Their results indicate that the stellar conversion efficiency $\epsilon\equiv M_*/f_b/M_h$ achieves nearly $100\%$ in these halos. Similarly, Zhang et al. 2021 found that star-forming central galaxies with $M_*\sim 10^{11} M_\odot$ live in $\sim 10^{12} M_\odot$ halos with a stellar conversion efficiency of $\sim 60\%$, where the halo mass is obtained through weak gravitational lensing and satellite kinematics. Therefore, we are pretty sure that there is a population of star-forming spiral galaxies in $\sim 10^{12}M_\odot$ halos with a stellar conversion efficiency $\epsilon\gtrsim 60\%$, while a typical $10^{12}M_\odot$ halo has $\epsilon\sim 20\%$. Combined with the result that earlier-formed halos tend to host more massive central galaxies, we obtained that galaxies in early-formed halos tend to be more star-forming and spiral-like than galaxies in late-formed halos.

Wang et al. 2023 studied the dependence of the star formation activities and morphologies of central galaxies on the formation time of host halos, which is indicated by the central stellar mass to halo mass ratio. And they also found that early-formed halos tend to host star-forming and spiral-like central galaxies compared with those late-formed halos.

Physical origin

Currently, there is no satisfying mechanism to explain this relation. Here I introduce my own speculation. To begin with, early-formed halos tend to have more concentrated mass distribution, which is well-established two decades ago (see Wechsler et al. 2002), and concentrated halos drives the formation of galaxy disk (see Hopkins et al. 2023). Then, the AGN feedback is less efficient in disk galaxies, so that they are less quenched.