Critical Stellar Central Densities Drive Galaxy Quenching in the Nearby Universe

We study the structural and environmental dependence of star formation on the plane of stellar mass versus central core density (Σ1 kpc) in the nearby universe. We study the central galaxies in the sparse environment and find a characteristic population-averaged Σ1 kpc ∼ 109–109.2 M ⊙ kpc−2, above which quenching is operating. This Σ 1 kpc crit only weakly depends on the stellar mass, suggesting that the mass quenching of the central galaxies is closely related to the processes that operate in the central region rather than over the entire galaxies. For satellites, at a given stellar mass, environment quenching appears to operate in a similar fashion as mass quenching in centrals, also starting from galaxies with high Σ1 kpc to low Σ1 kpc, and Σ 1 kpc crit becomes strongly mass-dependent, in particular in dense regions. This is because (1) more low-mass satellites are quenched by the environmental effects in denser regions and (2) at fixed stellar mass and environment, the environment-quenched satellites have, on average, larger Σ1 kpc, M 1 kpc/M ⋆, and Sérsic index n, and as well as smaller size. These results imply that either some dynamical processes change the structure of the satellites during quenching or the satellites with higher Σ1 kpc are more susceptible to environmental effects.

Active Galactic Nuclei Abundance in Cosmic Voids

The abundance of active galactic nuclei (AGN) in cosmic voids is relatively unexplored in the literature, but can potentially provide new constraints on the environmental dependence of AGN activity and the AGN-host coevolution. We investigated AGN fractions in one of the largest samples of optically selected cosmic voids from Sloan Digital Sky Survey Data Release 12 for redshift range 0.2–0.7 for moderately bright and bright AGN. We separated inner and outer void regions based on the void size, given by its effective void radius. We classified galaxies at a distance <0.6 R eff as inner void members and galaxies in the interval 0.6 < R/R eff < 1.3 as outer void galaxies. We found higher average fractions in the inner voids (4.9 ± 0.7)% than for their outer counterparts (3.1 ± 0.1)% at z > 0.42, which clearly indicates an environmental dependence. This conclusion was confirmed upon further separating the data in narrower void-centric distance bins and measured a significant decrease in AGN activity from inner to outer voids for z > 0.42. At low redshifts (z < 0.42), we find very weak dependence on the environment for the inner and outer regions for two out of three bins. We argue that the higher fraction in low-density regions close to void centers relative to their outer counterparts observed in the two higher-redshift bins suggests that more efficient galaxy interactions may occur at a one-to-one level in voids that may be suppressed in denser environments due to higher velocity dispersions. It could also indicate less prominent ram pressure stripping in voids or some intrinsic host or void environment properties.

On the initial mass-radius relation of stellar clusters

Young stellar clusters across nearly five orders of magnitude in mass appear to follow a power-law mass-radius relationship (MRR), $R_{\star }\propto M_{\star }^{\alpha }$, with α ≈ 0.2 − 0.33. We develop a simple analytic model for the cluster mass-radius relation. We consider a galaxy disc in hydrostatic equilibrium, which hosts a population of molecular clouds that fragment into clumps undergoing cluster formation and feedback-driven expansion. The model predicts a mass-radius relation of $R_{\star }\propto M_{\star }^{1/2}$ and a dependence on the kpc-scale gas surface density $R_{\star }\propto \Sigma _{\rm g}^{-1/2}$, which results from the formation of more compact clouds (and cluster-forming clumps within) at higher gas surface densities. This environmental dependence implies that the high-pressure environments in which the most massive clusters can form also induce the formation of clusters with the smallest radii, thereby shallowing the observed MRR at high-masses towards the observed $R_{\star }\propto M_{\star }^{1/3}$. At low cluster masses, relaxation-driven expansion induces a similar shallowing of the MRR. We combine our predicted MRR with a simple population synthesis model and apply it to a variety of star-forming environments, finding good agreement. Our model predicts that the high-pressure formation environments of globular clusters at high redshift naturally led to the formation of clusters that are considerably more compact than those in the local Universe, thereby increasing their resilience to tidal shock-driven disruption and contributing to their survival until the present day.

Connections between galaxy properties and halo formation time in the cosmic web

ABSTRACT By linking galaxies in Sloan Digital Sky Survey to subhaloes in the ELUCID simulation, we investigate the relation between subhalo formation time and the galaxy properties, and the dependence of galaxy properties on the cosmic web environment. We find that central and satellite subhaloes have different formation time, where satellite subhaloes are older than central subhaloes at fixed mass. At fixed mass, the galaxy stellar-to-subhalo mass ratio is a good proxy of the subhalo formation time, and increases with the subhalo formation redshifts, especially for massive galaxies. The subhalo formation time is dependent on the cosmic web environment. For central subhaloes, there is a characteristic subhalo mass of ${\sim}10^{12} \, \mathrm{ h}^{-1}\,{\rm M}_\odot$, below which subhaloes in knots are older than subhaloes of the same mass in filaments, sheets, or voids, while above which it reverses. The cosmic web environmental dependence of stellar-to-subhalo mass ratio is similar to that of the subhalo formation time. For centrals, there is a characteristic subhalo mass of ${\sim}10^{12} \, \mathrm{ h}^{-1}\,{\rm M}_\odot$, below which the stellar-to-subhalo mass ratio is higher in knots than in filaments, sheets and voids, above which it reverses. Galaxies in knots have redder colours below $10^{12} \, \mathrm{ h}^{-1}\,{\rm M}_\odot$, while above $10^{12} \, \mathrm{ h}^{-1}\,{\rm M}_\odot$, the environmental dependence vanishes. Satellite fraction is strongly dependent on the cosmic web environment, and decreases from knots to filaments to sheets to voids, especially for low-mass galaxies.