We use the IllustrisTNG hydrodynamical simulations to show how the fractions of quenched galaxies vary across different environments and cosmic time, and to quantify the role AGN feedback and pre-processing play in quenching group and cluster satellites. At z = 0, we select galaxies with Mstars = 109 − 12 M⊙ residing within (≤R200c) massive groups and clusters of total host mass M200c = 1013 − 15.2 M⊙ in TNG100 and TNG300. The model predicts a quenched fraction of ∼70-90 per cent (on average) for centrals and satellites of mass ≳ 1010.5 M⊙, regardless of host mass, cosmic time (0 ≤ z ≤ 0.5), cluster-centric distance and time since infall in the z = 0 host. Low-mass central galaxies (≲ 1010 M⊙), on the other hand, are rarely quenched unless they become members of groups (1013 − 14 M⊙) or clusters (≥1014 M⊙), where the quenched fraction rises to ∼ 80 per cent. Typically, the fraction of low-mass passive galaxies is higher closer to the host center and for progressively more massive hosts. The population of low-mass satellites accreted more than ∼ 4-6 Gyr ago in massive hosts is almost entirely passive, thus suggesting an upper limit for the time needed for environmental quenching to occur. In fact, about 30 per cent of group and cluster satellites that are quenched at z = 0 were already quenched before falling into their current host, and the bulk of them quenched as early as 4 to 10 billion years ago. For low-mass galaxies (Mstars ≲ 1010 − 10.5 M⊙), this is due to pre-processing, whereby current satellites may have been members of other hosts, and hence have undergone environmental processes, before falling into their final host, this mechanism being more common and more effective for the purposes of quenching for satellites found today in more massive hosts. On the other hand, massive galaxies quench on their own and because of AGN feedback, regardless of whether they are centrals or satellites.

We use the IllustrisTNG simulations to study the demographics and properties of jellyfish galaxies in the full cosmological context. By jellyfish galaxies, we mean satellites orbiting in massive groups and clusters that exhibit highly asymmetric distributions of gas and gas tails. In particular, we select TNG100 galaxies at low redshifts (z ≤ 0.6) with stellar mass exceeding 10 9.5 M and with host halo masses in the range 10 13 ≤ M 200c / M ≤ 10 14.6 . Among more than about 6 000 (2 600) galaxies with stars (and some gas), we identify 800 jellyfish galaxies by visually inspecting their gas and stellar mass maps in random projections. Namely, about 31 per cent of cluster satellites are found with signatures of ram-pressure stripping and gaseous tails stemming from their main luminous bodies. This is a lower limit: the random orientation entails a loss of about 30 per cent of galaxies that in an optimal projection would otherwise be identified as jellyfish. Furthermore, jellyfish galaxies are more frequent at intermediate and large cluster-centric distances (r/R 200c 0.25), in more massive hosts and at smaller satellite masses, and they typically orbit supersonically. The gaseous tails usually extend in opposite directions to the galaxy trajectory, with no relation between tail orientation and position of the hosts center. Finally, jellyfish galaxies are late infallers (< 2.5 − 3 Gyrs ago, at z = 0) and the emergence of gaseous tails correlates well with the presence of bow shocks in the intra-cluster medium.

We compile a sample of SDSS galaxy clusters with high-quality Chandra X-ray data to directly study the influence of the dense intra-cluster medium (ICM) on the quenching of satellite galaxies. We study the quenched fractions of satellite galaxies as a function of ICM density for low-(10 9 M 10 10 M ), intermediate-(10 10 M 10 10.5 M ), and high-mass (M 10 10.5 M ) satellite galaxies with > 3000 satellite galaxies across 24 low-redshift (z < 0.1) clusters. For low-mass galaxies we find evidence for a broken powerlaw trend between satellite quenched fraction and local ICM density. The quenched fraction increases modestly at ICM densities below a threshold before increasing sharply beyond this threshold toward the cluster center. We show that this increase in quenched fraction at high ICM density is well matched by a simple, analytic model of ram pressure stripping. These results are consistent with a picture where low-mass cluster galaxies experience an initial, slow-quenching mode driven by steady gas depletion, followed by rapid quenching associated with ram pressure of cold-gas stripping near (one quarter of the virial radius, on average) the cluster center.

ABSTRACT We study the stellar morphological evolution of disc galaxies within clusters in the TNG50 and TNG100 runs from the IllustrisTNG simulation suite. We select satellites of masses 109.7 ≤ M*, z = 0/M⊙ ≤ 1011.6 residing in clusters of masses 1014 ≲ M200c, z = 0/M⊙ ≤ 1014.6 at z = 0 and that were discs at accretion according to a kinematic morphology indicator (the circularity fraction). These are traced from the time of accretion to z = 0 and compared to a control sample of central galaxies mass-matched at accretion. Most cluster discs become non-discy by z = 0, in stark contrast with the control discs, of which a significant fraction remains discy over the same timescales. Cluster discs become non-discy accompanied by gas removal and star formation quenching, loss of dark matter, and little growth or a loss of stellar mass. In contrast, control discs transform while also losing gas mass and quenching, but growing significantly in dark matter and stellar mass. Most cluster satellites change morphologies on similar timescales regardless of stellar mass, in ∼0.5–4 Gyr after accretion. Cluster discs that experienced more numerous and closer pericentric passages show the largest change in morphology. Morphological change in all cases requires the presence of a gravitational perturbation to drive stellar orbits to non-discy configurations, along with gas removal/heating to prevent replenishment of the disc through continued star formation. For cluster discs, the perturbation is impulsive tidal shocking at pericentres and not tidal stripping of outer disc stellar material, whereas for control discs, a combination of mergers and feedback from active galactic nuclei appears to be the key driving force behind morphological transformations.

We study the stellar-to-halo mass relation (SHMR) for central and satellite galaxies with total dynamical masses above $10^{10.5}~\rm {M}_\odot$ using the suite of cosmological magneto-hydrodynamical simulations IllustrisTNG. In particular, we quantify environmental effects on satellite populations from TNG50, TNG100, and TNG300 located within the virial radius of group- and cluster-like hosts with total masses of $10^{12-15.2}~\rm {M}_\odot$. At fixed stellar mass, the satellite SHMR exhibits a distinct shift towards lower dynamical mass compared to the SHMR of centrals. Conversely, at fixed dynamical mass, satellite galaxies appear to have larger stellar-to-total mass fractions than centrals by up to a factor of a few. The systematic deviation from the central SHMR is larger for satellites in more massive hosts, at smaller cluster-centric distances, with earlier infall times, and that inhabit higher local density environments; moreover, it is in place already at early times (z ≲ 2). Systematic environmental effects might contribute to the perceived galaxy-to-galaxy variation in the measured SHMR when galaxies cannot be separated into satellites and centrals. The SHMR of satellites exhibits a larger scatter than centrals (by up to ∼0.8 dex), over the whole range of dynamical mass. The shift of the satellite SHMR results mostly from tidal stripping of their dark matter, which affects satellites in an outside-in fashion: the departure of the satellite SHMR from the centrals’ relation diminishes for measurements of dynamical mass in progressively smaller apertures. Finally, we provide a family of fitting functions for the SHMR predicted by IllustrisTNG.