Probing Galaxy Evolution in Massive Clusters Using ACT and DES: Splashback as a Cosmic Clock

We measure the projected number density profiles of galaxies and the splashback feature in clusters selected by the Sunyaev–Zel’dovich effect from the Advanced Atacama Cosmology Telescope (AdvACT) survey using galaxies observed by the Dark Energy Survey (DES). The splashback radius is consistent with CDM-only simulations and is located at 2.4 − 0.4 + 0.3 Mpc h − 1 . We split the galaxies on color and find significant differences in their profile shapes. Red and green-valley galaxies show a splashback-like minimum in their slope profile consistent with theory, while the bluest galaxies show a weak feature at a smaller radius. We develop a mapping of galaxies to subhalos in simulations and assign colors based on infall time onto their hosts. We find that the shift in location of the steepest slope and different profile shapes can be mapped to the average time of infall of galaxies of different colors. The steepest slope traces a discontinuity in the phase space of dark matter halos. By relating spatial profiles to infall time, we can use splashback as a clock to understand galaxy quenching. We find that red galaxies have on average been in clusters over 3.2 Gyr, green galaxies about 2.2 Gyr, while blue galaxies have been accreted most recently and have not reached apocenter. Using the full radial profiles, we fit a simple quenching model and find that the onset of galaxy quenching occurs after a delay of about a gigayear and that galaxies quench rapidly thereafter with an exponential timescale of 0.6 Gyr.

Bright galaxy sample in the Kilo-Degree Survey Data Release 4

We present a bright galaxy sample with accurate and precise photometric redshifts (photo-zs), selected using ugriZYJHKs photometry from the Kilo-Degree Survey (KiDS) Data Release 4. The highly pure and complete dataset is flux-limited at r < 20 mag, covers ∼1000 deg2, and contains about 1 million galaxies after artifact masking. We exploit the overlap with Galaxy And Mass Assembly spectroscopy as calibration to determine photo-zs with the supervised machine learning neural network algorithm implemented in the ANNz2 software. The photo-zs have a mean error of |⟨δz⟩|∼5 × 10−4 and low scatter (scaled mean absolute deviation of ∼0.018(1 + z)); they are both practically independent of the r-band magnitude and photo-z at 0.05 < zphot < 0.5. Combined with the 9-band photometry, these allow us to estimate robust absolute magnitudes and stellar masses for the full sample. As a demonstration of the usefulness of these data, we split the dataset into red and blue galaxies, used them as lenses, and measured the weak gravitational lensing signal around them for five stellar mass bins. We fit a halo model to these high-precision measurements to constrain the stellar-mass–halo-mass relations for blue and red galaxies. We find that for high stellar mass (M⋆ > 5 × 1011 M⊙), the red galaxies occupy dark matter halos that are much more massive than those occupied by blue galaxies with the same stellar mass.

The Origin of the Colour Bimodality in the Scatter of the Stellar-to-Halo Mass Relation

Recent observations reveal that there is a strong bimodality in the scatter around the galaxy stellar-to-halo mass relation (SHMR): at a given halo mass, galaxies with a higher stellar mass tend to be blue indicating a higher specific star formation rate, while galaxies having a lower stellar mass tend to be red and quiescent; or at a given stellar mass, blue galaxies tend to live in halos with lower mass while red galaxies tend to have massive host halo. This has important implications for abundance matching and halo occupancy models commonly used in cosmological studies, but its physical origin remains debated. The SIMBA cosmological galaxy formation simulation successfully reproduces these observations, enabling us to investigate the physical driver behind this phenomenon. We show that the offset from the mean SHMR is strongly correlated with both halo formation time when half the halo mass assembled, as well as galaxy transition time defined as when the stellar doubling time becomes longer than 10 Gyr. Moreover, these two quantities are anti-correlated: early formed halos tend to host late transition galaxies corresponding to blue galaxies today, and vice versa, particularly for halo masses 11.5 ≲ log Mhalo ≲ 12.8M⊙ and galaxy stellar masses log M∗ ≥ 10M⊙. Prior to their transition time, galaxies lie on the SHMR for blue galaxies. Early transition galaxies, hosted by late formed halos, have their stellar mass growth almost ceased owing to AGN feedback even though their host halos continue to accrete mass, which moves these galaxies off the blue SHMR towards the red one creating the SHMR bimodality. We then investigate why early formed halos tend to host late transition galaxies. We find two key interconnected times: the gas-to-stellar domination time when the galaxy’s cold gas mass becomes smaller than its stellar mass, and the black hole (BH) jet ignition time governed by the BH Eddington ratio. Both show strong linear correlations with the galaxy transition time. Early formed halos have higher cold gas fractions (defined by cold gas mass in central galaxy with respect to the host halo mass) with a lower stellar-to-halo mass growth ratio before the transition time compared to the median or late forming halos; this allows them to sustain their stellar growth longer. Eventually, the continued growth fed by the cold gas reservoir allows them to surpass the galaxies with early transition times. Conversely, galaxies hosted by late formed halos have less cold gas with high stellar-to-halo mass growth ratios. Hence the Eddington rate be-comes low earlier on, which triggers AGN into an energetic jet mode that heats gas, rapidly truncates further accretion and also stops star formation. These processes thus conspire to create the SHMR bimodality. In SIMBA, the cold gas evolution occurs naturally owing to the interplay of accretion and star formation feedback, while the AGN feedback transitions from a radiative mode at high Eddington ratios that is ineffective at quenching, to a jet mode at low Eddington ratios that suppresses star formation. SIMBA further includes X-ray feedback that drives the last remaining cold gas out, completing the quenching and strengthening the SHMR bimodality.

AMICO galaxy clusters in KiDS-DR3: galaxy population properties and their redshift dependence

ABSTRACT A catalogue of galaxy clusters was obtained in an area of 414 $\deg ^2$ up to a redshift z ∼ 0.8 from the Data Release 3 of the Kilo-Degree Survey (KiDS-DR3), using the Adaptive Matched Identifier of Clustered Objects (amico) algorithm. The catalogue and the calibration of the richness–mass relation were presented in two companion papers. Here, we describe the selection of the cluster central galaxy and the classification of blue and red cluster members, and analyse the main cluster properties, such as the red/blue fraction, cluster mass, brightness, and stellar mass of the central galaxy, and their dependence on redshift and cluster richness. We use the Illustris-TNG simulation, which represents the state-of-the-art cosmological simulation of galaxy formation, as a benchmark for the interpretation of the results. A good agreement with simulations is found at low redshifts (z ≤ 0.4), while at higher redshifts the simulations indicate a lower fraction of blue galaxies than what found in the KiDS-amico catalogue: we argue that this may be due to an underestimate of star-forming galaxies in the simulations. The selection of clusters with a larger magnitude difference between the two brightest central galaxies, which may indicate a more relaxed cluster dynamical status, improves the agreement between the observed and simulated cluster mass and stellar mass of the central galaxy. We also find that at a given cluster mass the stellar mass of blue central galaxies is lower than that of the red ones.

A method for classification of red, blue, and green galaxies using fuzzy set theory

ABSTRACT Red and blue galaxies are traditionally classified using some specific cuts in colour or other galaxy properties, which are supported by empirical arguments. The vagueness associated with such cuts are likely to introduce a significant contamination in these samples. Fuzzy sets are vague boundary sets that can efficiently capture the classification uncertainty in the absence of any precise boundary. We propose a method for classification of galaxies according to their colours using fuzzy set theory. We use data from the Sloan Digital Sky Survey (SDSS) to construct a fuzzy set for red galaxies with its members having different degrees of ‘redness’. We show that the fuzzy sets for the blue and green galaxies can be obtained from it using different fuzzy operations. We also explore the possibility of using fuzzy relation to study the relationship between different galaxy properties and discuss its strengths and limitations.

Properties of the environment around active galactic nucleus / luminous galaxy pairs through the HSC wide survey

We investigated the properties of active galactic nucleus (AGN) environments, particularly environments where the association of a luminous galaxy (LG) is found within 4 Mpc from an AGN with redshift 0.8–1.1. For comparison, three additional AGN environments, (namely, AGNs of all types, type 1 AGNs with X-ray and/or radio detection, and type 2 AGNs) and an environment of blue M*, the characteristic luminosity of the Schechter function, galaxies were investigated. The cross-correlation function with the surrounding galaxies was measured and compared between the AGN and blue galaxy samples. We also compared the distributions of color, absolute magnitude, and stellar mass of the galaxies around such target objects. The properties of clusters detected using surrounding galaxies selected based on a photometric redshift were examined and compared for different samples. The target AGNs were drawn from the Million Quasars (MILLIQUAS) catalog, and the blue galaxies were drawn from six redshift survey catalogs (SDSS, WiggleZ, DEEP2, VVDS, VIPERS, and PRIMUS). The galaxies used as a measure of the environment around the targets were drawn from the S18a internal data released by the Hyper Suprime-Cam Subaru Strategic Program. We found that, among the five AGN and blue galaxy samples considered, the environment of AGN–LG pairs is the most enriched with luminous galaxies. We also found an enhancement in the number of mass-selected clusters in the AGN–LG pair sample against those in the other samples. The results obtained in this study indicate that existence of multiple clusters is the major driver in the association of AGNs and LGs, rather than a single large-mass dark matter halo hosting the AGN.