Projected Phase Space Research Articles

Unveiling the shape: A multi-wavelength analysis of the galaxy clusters Abell 76 and Abell 1307

We analyse the dynamical state of the galaxy clusters Abell 76 and Abell 1307 from the optical point of view, presenting a coherent scenario that responds to the X-ray emissions observed in these structures. Our study is based on 231 and 164 spectroscopic redshifts, for the clusters A76 and A1307, respectively, obtained mostly with the Telescopio Nazionale Galileo, and complemented with others collected from the SDSS DR16 spectroscopic database and the literature. We find that A76 and A1307 are two galaxy clusters at z = 0.0390 and 0.0815, respectively, with a velocity dispersion of 650 ± 56 km s−1 and 863 ± 85 km s−1, and they show velocity distributions following, in practice, Gaussian profiles. From our dynamical analysis, X-ray studies and SZ-Planck emission, we obtain a mean total mass M500 = 1.7 ± 0.6 ⋅ 1014 M⊙ and 3.5 ± 1.3 ⋅ 1014 M⊙ for A76 and A1307, respectively. Using the SDSS DR16 photometric database, we find that the spatial distribution of likely cluster members in the case of A76 is very anisotropic, while A1307 shows a compact distribution of galaxies, but it is double peaked and elongated in the south-north direction. Using XMM-Newton X-ray data, we compared the surface brightness maps with galaxy distributions and noticed that both distributions are correlated. We reconstructed the total mass profile and velocity anisotropy of both clusters by analysing the full projected phase space, through the MG-MAMPOSSt code. Our study reveals a slight indication of radial orbits for A76, while A1307 seems to prefer more isotropic orbits in the whole cluster range. In summary, A76 represents a typical young cluster, in an early stage of formation, with a very low X-ray surface brightness but a high temperature showing a very anisotropic galaxy distribution. A1307 is however more consolidated and massive showing in-homogeneous galaxy distribution and an asymmetric X-ray emission, which suggest a scenario characterised by recent minor mergers.

Exploring galaxy evolution time-scales in clusters: insights from the projected phase space

ABSTRACT Galaxies infalling into clusters undergo both star formation quenching and morphological transformation due to environmental effects. We investigate these processes and their time-scales using a local sample of 20 191 cluster and 11 674 field galaxies from Sloan Digital Sky Survey. By analysing morphology as a function of distance from the star formation main sequence, we show that environmental influence is especially pronounced for low-mass galaxies, which emerge from the green valley (GV) with early-type morphologies before their star formation is fully suppressed. Using the galaxies’ positions in the clusters’ projected phase space, we examine the evolution of blue cloud, GV, and red-sequence fractions as a function of time since infall. Interestingly, the GV fraction remains constant with time since infall, suggesting a balanced flow of galaxies in and out of this class. We estimate that galaxies less massive than $10^{10}\,\rm {\rm M}_{\odot }$ spend approximately 0.4 Gyr in the GV. By comparing quenched and early-type populations, we provide further evidence for the ‘slow-then-rapid’ quenching model and suggest that it can also be applied to morphological transitions. Our results indicate that morphological transformation occurs at larger radii than complete star formation quenching. About 75 per cent of galaxies undergoing morphological transition in clusters are spirals evolving into S0s, suggesting that infalling galaxies retain their discs, while massive ellipticals are relics of early merger events. Finally, we show it takes approximately 2.5 and 1.2 Gyr after the delay time ($\sim 3.8 {\,\rm Gyr}$) for the population of low-mass galaxies in clusters to reach a 50 per cent threshold in quenched and early-type fraction, respectively. These findings suggest morphological transition precedes full star formation quenching, with both processes possibly being causally linked.

The Radial Orbits of Ram-pressure-stripped Galaxies in Clusters from the GASP Survey

We analyze a sample of 244 ram-pressure-stripped candidate galaxy members within the virial radius of 62 nearby clusters to determine their velocity anisotropy profile β(r). We use previously determined mass profiles for the 62 clusters to build an ensemble cluster by stacking the 62 cluster samples in projected phase space. We solve the Jeans equation for dynamical equilibrium by two methods, MAMPOSSt and the Jeans inversion technique, and determine β(r) both in parametric form and nonparametrically. The two methods consistently indicate that the orbits of the ram-pressure-stripped candidates are increasingly radial with distance from the cluster center, from almost isotropic (β ≃ 0) at the center, to very radial at the virial radius (β ≃ 0.7). The orbits of cluster galaxies undergoing ram pressure stripping are similar to those of spiral cluster galaxies but more radially elongated at large radii.

Satellite quenching and morphological transformation of galaxies in groups and clusters

ABSTRACT We investigate the role that dense environments have on the quenching of star formation and the transformation of morphology for a sample of galaxies selected from the Sloan Digital Sky Survey. We make a distinction between galaxies falling into groups [13 ≤ log(Mhalo/M⊙) < 14] and clusters [log(Mhalo/M⊙) ≥ 14], and compare to a large sample of field galaxies. Using galaxy position in projected phase space as a proxy for time since infall, we study how galaxy specific star formation rate and morphology, parametrized by the bulge-to-total light ratio, change over time. After controlling for stellar mass, we find clear trends of increasing quenched and elliptical fractions as functions of infall time for galaxies falling into both groups and clusters. The trends are strongest for low-mass galaxies falling into clusters. By computing quenching and morphological transformation time-scales, we find evidence that star formation quenching occurs faster than morphological transformation in both environments. Comparing field galaxies to recently infalling galaxies, we determine that there is pre-processing of both star formation and morphology, with pre-processing affecting star formation rates more strongly. Our analysis favours quenching mechanisms that act quickly to suppress star formation, while other mechanisms that act on longer time-scales transform morphology through bulge growth and disc fading.

The Kormendy relation of cluster galaxies in PPS regions

ABSTRACT We study a sample of 936 early-type galaxies located in 48 low-z regular galaxy clusters with M200 ≥ 1014 M⊙ at z < 0.1. We examine variations in the Kormendy relation (KR) according to their location in the projected phase space (PPS) of the clusters. We have used a combination of Bayesian statistical methods to identify possible differences between the fitted relations. Our results indicate that the overall KR is better fitted when we take into account the information about PPS regions. We also find that objects with time since infall ≥6.5 Gyr have a significant statistical difference of the KR coefficients relative to objects that are more recent in the cluster environment. We show that giant central ellipticals are responsible for tilting the KR towards smaller slopes. These galaxies present a late growth probably due to cumulative preprocessing during infall, plus cannibalism and accretion of smaller stripped objects near the centre of the clusters.

A Gradual Decline of Star Formation since Cluster Infall: New Kinematic Insights into Environmental Quenching at 0.3 < z < 1.1

The environments where galaxies reside crucially shape their star formation histories. We investigate a large sample of 1626 cluster galaxies located within 105 galaxy clusters spanning a large range in redshift (0.26 < z < 1.13). The galaxy clusters are massive (M 500 ≳ 2 × 1014 M ⊙) and uniformly selected from the SPT and ACT Sunyaev–Zel’dovich surveys. With spectra in hand for thousands of cluster members, we use the galaxies’ position in projected phase space as a proxy for their infall times, which provides a more robust measurement of environment than quantities such as projected clustercentric radius. We find clear evidence for a gradual age increase of the galaxy’s mean stellar populations (∼0.71 ± 0.4 Gyr based on a 4000 Å break, Dn4000) with the time spent in the cluster environment. This environmental quenching effect is found regardless of galaxy luminosity (faint or bright) and redshift (low or high-z), although the exact stellar age of galaxies depends on both parameters at fixed environmental effects. Such a systematic increase of Dn4000 with infall proxy would suggest that galaxies that were accreted into hosts earlier were quenched earlier due to longer exposure to environmental effects such as ram pressure stripping and starvation. Compared to the typical dynamical timescales of 1–3 Gyr of cluster galaxies, the relatively small age increase (∼0.71 ± 0.4 Gyr) found in our sample galaxies seems to suggest that a slow environmental process such as starvation is the dominant quenching pathway. Our results provide new insights into environmental quenching effects spanning a large range in cosmic time (∼5.2 Gyr, z = 0.26–1.13) and demonstrate the power of using a kinematically derived infall time proxy.

C[formula omitted]-GaMe: Classification of cluster galaxy membership with machine learning

We present Classification of Cluster Galaxy Members (C2-GaMe), a classification algorithm based on a suite of machine learning models that differentiates galaxies into orbiting, infalling, and background (interloper) populations, using phase space information as input. We train and test C2-GaMe with the galaxies from UniverseMachine mock catalog based on Multi-Dark Planck 2 N-body simulations. We show that probabilistic classification is superior to deterministic classification in estimating the physical properties of clusters, including density profiles and velocity dispersion. We propose a set of estimators to get an unbiased estimation of cluster properties. We demonstrate that C2-GaMe can recover the distribution of orbiting and infalling galaxies’ position and velocity distribution with <1% statistical error when using probabilistic predictions in the presence of interlopers in the projected phase space. Additionally, we demonstrate the robustness of trained models by applying them to a different simulation. Finally, adding a specific star formation rate and the ratio of the galaxy’s halo mass to the cluster’s halo mass as additional features improves the classification performance. We discuss potential applications of this technique to enhance cluster cosmology and galaxy quenching.

Examining transitional galaxies to understand the role of clusters and their dynamical status in galaxy quenching

ABSTRACT In this work, we consider four different galaxy populations and two distinct global environments in the local Universe (z ≤ 0.11) to investigate the evolution of transitional galaxies (such as star-forming spheroids and passive discs) across different environments. Our sample is composed of 3899 galaxies within the R200 radius of 231 clusters and 11 460 field galaxies. We also investigate the impact of the cluster’s dynamic state, as well as the galaxy’s location in the projected phase space diagram. We found that although the cluster environment as a whole influences galaxy evolution, the cluster dynamical state does not. Furthermore, star-forming galaxies represent recent cluster arrivals in comparison to passive galaxies (especially in the case of early-types). Among the early-type galaxies, we find that the Dn(4000) and H δ parameters indicate a smooth transition between the subpopulations. In particular, for the star-forming early-type galaxies, we detect a significant difference between field and cluster galaxies, as a function of stellar mass, for objects with log M*/M⊙ &amp;gt; 10.5. Analysing the colour gradient, the results point towards a picture where field galaxies are more likely to follow the monolithic scenario, while the cluster galaxies the hierarchical scenario. In particular, if we split the early-type galaxies into lenticulars and ellipticals, we find that the steeper colour gradients are more common for the lenticulars. Finally, our results indicate the need for galaxy pre-processing in smaller groups, before entering clusters.

Constraining quenching time-scales in galaxy clusters by forward-modelling stellar ages and quiescent fractions in projected phase space

ABSTRACT We forward-model mass-weighted stellar ages (MWAs) and quiescent fractions (fQ) in projected phase space (PPS), using data from the Sloan Digital Sky Survey, to jointly constrain an infall quenching model for galaxies in log (Mvir/M⊙) &amp;gt; 14 galaxy clusters at z ∼ 0. We find the average deviation in MWA from the MWA–M⋆ relation depends on position in PPS, with a maximum difference between the inner cluster and infalling interloper galaxies of ∼1 Gyr. Our model employs infall information from N-body simulations and stochastic star-formation histories from the universemachine model. We find total quenching times of tQ = 3.7 ± 0.4 Gyr and tQ = 4.0 ± 0.2 Gyr after first pericentre, for 9 &amp;lt; log (M⋆/M⊙) &amp;lt; 10 and 10 &amp;lt; log (M⋆/M⊙) &amp;lt; 10.5 galaxies, respectively. By using MWAs, we break the degeneracy in time of quenching onset and time-scale of star formation rate (SFR) decline. We find that time of quenching onset relative to pericentre is $t_{\mathrm{delay}}=3.5^{+0.6}_{-0.9}$ Gyr and $t_{\mathrm{delay}}=-0.3^{+0.8}_{-1.0}$ Gyr for 9 &amp;lt; log (M⋆/M⊙) &amp;lt; 10 and 10 &amp;lt; log (M⋆/M⊙) &amp;lt; 10.5 galaxies, respectively, and exponential SFR suppression time-scales are τenv ≤ 1.0 Gyr for 9 &amp;lt; log (M⋆/M⊙) &amp;lt; 10 galaxies and τenv ∼ 2.3 Gyr for 10 &amp;lt; log (M⋆/M⊙) &amp;lt; 10.5 galaxies. Stochastic star formation histories remove the need for rapid infall quenching to maintain the bimodality in the SFR of cluster galaxies; the depth of the green valley prefers quenching onsets close to first pericentre and a longer quenching envelope, in slight tension with the MWA-driven results. Taken together these results suggest that quenching begins close to, or just after pericentre, but the time-scale for quenching to be fully complete is much longer and therefore ram-pressure stripping is not complete on first pericentric passage.

Late growth of early-type galaxies in low-z massive clusters

ABSTRACTWe study a sample of 936 early-type galaxies (ETGs) located in 48 low-z regular galaxy clusters with M200 ≥ 1014 M⊙ at z &amp;lt; 0.1. We examine variations in the concentration index, radius, and colour gradient of ETGs as a function of their stellar mass and loci in the projected phase space (PPS) of the clusters. We aim to understand the environmental influence on the growth of ETGs according to the time since infall into their host clusters. Our analysis indicates a significant change in the behaviour of the concentration index C and colour gradient around $M_{\ast } \approx 2\times 10^{11} ~M_\odot \equiv \tilde{M}_{\ast }$. Objects less massive than $\tilde{M}_{\ast }$ present a slight growth of C with M*, with negative and approximately constant colour gradients in all regions of the PPS. Objects more massive than $\tilde{M}_{\ast }$ present a slight decrease of C with M*, with colour gradients becoming less negative and approaching zero. We also find that objects more massive than $\tilde{M}_{\ast }$, in all PPS regions, have smaller R90 for a given R50, suggesting a smaller external growth in these objects or even a shrinkage possibly due to tidal stripping. Finally, we estimate different dark matter fractions for galaxies in different regions of the PPS, with the ancient satellites having the largest fractions, fDM ≈ 65 per cent. These results favour a scenario where cluster ETGs experience environmental influence the longer they remain and the deeper into the gravitational potential they lie, indicating a combination of tidal stripping + harassment, which predominate during infall, followed by mergers + feedback effects affecting the late growth of ancient satellites and BCGs.

Exploring the stellar populations of backsplash galaxies

Abstract Backsplash galaxies are those that traverse and overshoot cluster cores as they fall into these structures. They are affected by environment, and should stand out in contrast to the infalling population. We target galaxies in the vicinity of clusters (R≳R200) and select a sample in projected phase space (PPS), from the compilation of Sampaio et al. based on SDSS data. We present a statistical analysis, comparing two regions in PPS, with the same projected distance to the cluster but different velocity. The analysis relies on the presence of variations in the stellar population content of backsplash galaxies. We find a lower limit in the fractional contribution of ∼5 per cent with respect to the general sample of infalling galaxies at similar group-centric distance when using single line strength analysis, or ∼15-30 per cent when adopting bivariate distributions. The stellar populations show a subtle but significant difference towards older ages, and a higher fraction of quiescent galaxies. We also compare this set with a general field sample, where a substantially larger difference in galaxy properties is found, with the field sample being consistently younger, metal poorer and with a lower fraction of quiescent galaxies. Noting that our ‘cluster’ sample is located outside of the virial radius, we expect this difference to be caused by pre-processing of the infalling galaxies in the overall higher density regions.

Reconstructing orbits of galaxies in extreme regions (ROGER) III: Galaxy evolution patterns in projected phase space around massive X-ray clusters

ABSTRACT We use the roger code by de los Rios et al. to classify galaxies around a sample of X-ray clusters into five classes according to their positions in the projected phase space diagram: cluster galaxies, backsplash galaxies, recent infallers, infalling galaxies, and interlopers. To understand the effects of the cluster environment to the evolution of galaxies, we compare across the five classes: stellar mass, specific star formation rate, size, and morphology. Following the guidelines of Coenda et al., a separate analysis is carried out for red and blue galaxies. For red galaxies, cluster galaxies differ from the other classes, having a suppressed specific star formation rate, smaller sizes, and are more likely to be classified as ellipticals. Differences are smaller between the other classes, however backsplash galaxies have significantly lower specific star formation rates than early or recent infalling galaxies. For blue galaxies, we find evidence that recent infallers are smaller than infalling galaxies and interlopers, while the latter two are comparable in size. Our results provide evidence that, after a single passage, the cluster environment can diminish a galaxy’s star formation, modify its morphology, and can also reduce in size blue galaxies. We find evidence that quenching occurs faster than morphological transformation from spirals to ellipticals for all classes. While quenching is evidently enhanced as soon as galaxies get into clusters, significant morphological transformations require galaxies to experience the action of the physical mechanisms of the cluster for longer time-scales.

Accretion processes in the galaxy cluster Hydra A/Abell 780

Context. Clusters of galaxies evolve and accrete mass, mostly from small galaxy systems. Aims. Our aim is to study the velocity field of the galaxy cluster Abell 780, which is known for the powerful radio source Hydra A at its center and where a spectacular X-ray tail associated with the galaxy LEDA 87445 has been discovered. Methods. Our analysis is based on the new spectroscopic data for hundreds of galaxies obtained with the Italian Telescopio Nazionale Galileo and the Very Large Telescope. We have constructed a redshift catalog of 623 galaxies and selected a sample of 126 cluster members. We analyzed the internal structure of the cluster using a number of techniques. Results. We estimate the mean redshift z = 0.0545, the line-of-sight velocity dispersion σV ∼ 800 km s−1, and the dynamical mass M200 ∼ 5.4 × 1014 M⊙. The global properties of Abell 780 are typical of relaxed clusters. On a smaller scale, we can detect the presence of a galaxy group associated with LEDA 87445 in projected phase space. The mean velocity and position of the center of the group agree well with the velocity and position of LEDA 87445. We estimate the following parameters of the collision. The group is characterized by a higher velocity relative to the main system. It is infalling at a rest frame velocity of Vrf ∼ +870 km s−1 and lies at a projected distance of D ∼ 1.1 Mpc to the south, slightly southeast of the cluster center. The mass ratio of the group to the cluster is ∼1:5. We also find evidence of an asymmetry in the velocity distribution of galaxies in the inner cluster region, which might be related to a small low-velocity group detected as a substructure at Vrf ∼ −750 km s−1. Conclusions. We conclude that A780, although dynamically relaxed at first sight, contains small substructures that may have some impact on the energetics of the core region.

From blue cloud to red sequence: evidence of morphological transition prior to star formation quenching

ABSTRACT We present a study of a sample of 254 clusters from the SDSS-DR7 Yang Catalogue and an auxiliary sample of field galaxies to perform a detailed investigation on how galaxy quenching depends on both environment and galaxy stellar mass. Our samples are restricted to 0.03 ≤ z ≤ 0.1 and we only consider clusters with log (Mhalo/M⊙) ≥ 14. Comparing properties of field and cluster galaxies in the blue cloud, green valley, and red sequence, we find evidence that field galaxies in the red sequence hosted star formation events $\rm 2.1 \pm 0.7$ Gyr ago, on average, more recently than galaxies in cluster environments. Dissecting the star formation rate versus stellar mass diagram we show how morphology rapidly changes after reaching the green valley region, while the star formation rate keeps decreasing. In addition, we use the relation between location in the projected phase space and infall time to explore the time delay between morphological and specific star formation rate variations. We estimate that the transition from late- to early-type morphology happens in Δtinf ∼ 1 Gyr, whereas the quenching of star formation takes ∼3 Gyr. The time-scale we estimate for morphological transitions is similar to the expected for the delayed-then-rapid quenching model. Therefore, we suggest that the delay phase is characterized mostly by morphological transition, which then contributes morphological quenching as an additional ingredient in galaxy evolution.

WALLABY Pilot Survey: The Diversity of Ram Pressure Stripping of the Galactic H i Gas in the Hydra Cluster

Abstract This study uses H i image data from the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) pilot survey with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, covering the Hydra cluster out to 2.5r 200. We present the projected phase–space distribution of H i-detected galaxies in Hydra, and identify that nearly two-thirds of the galaxies within 1.25 r 200 may be in the early stages of ram pressure stripping. More than half of these may be only weakly stripped, with the ratio of strippable H i (i.e., where the galactic restoring force is lower than the ram pressure in the disk) mass fraction (over total H i mass) distributed uniformly below 90%. Consequently, the H i mass is expected to decrease by only a few 0.1 dex after the currently strippable portion of H i in these systems has been stripped. A more detailed look at the subset of galaxies that are spatially resolved by WALLABY observations shows that, while it typically takes less than 200 Myr for ram pressure stripping to remove the currently strippable portion of H i, it may take more than 600 Myr to significantly change the total H i mass. Our results provide new clues to understanding the different rates of H i depletion and star formation quenching in cluster galaxies.

Mg-mamposst: a code to test modifications of gravity with internal kinematics and lensing analyses of galaxy clusters

ABSTRACT We present an upgraded version of mg-mamposst, an extension of the mamposst (Modelling Anisotropy and Mass Profile of Spherical Observed Systems) algorithm that performs Bayesian fits of models of mass and velocity anisotropy profiles to the distribution of tracers in projected phase space, to handle modified gravity models and constrain their parameters. The new version implements two distinct types of gravity modifications, namely general chameleon and Vainshtein screening, and is further equipped with a Monte Carlo Markov chain module for an efficient parameter space exploration. The programme is complemented by the clustergen code, capable of producing mock galaxy clusters under the assumption of spherical symmetry, dynamical equilibrium, and Gaussian local velocity distribution functions as in mamposst. We demonstrate the potential of the method by analysing a set of synthetic, isolated spherically symmetric dark matter haloes, focusing on the statistical degeneracies between model parameters. Assuming the availability of additional lensing-like information, we forecast the constraints on the modified gravity parameters for the two models presented, as expected from joint lensing + internal kinematics analyses, in view of upcoming galaxy cluster surveys. In Vainshtein screening, we forecast the weak lensing effect through the estimation of the full convergence-shear profile. For chameleon screening, we constrain the allowed region in the space of the two free parameters of the model, further focusing on the $\displaystyle f(\mathcal {R})$ subclass to obtain realistic bounds on the background field $\displaystyle |f_{\mathcal {R}0}|$. Our analysis demonstrates the complementarity of internal kinematics and lensing probes for constraining modified gravity theories, and how the bounds on Vainshtein-screened theories improve through the combination of the two probes.

The Corona Borealis supercluster: connectivity, collapse, and evolution

Context. Rich superclusters of galaxies represent dynamically active environments in which galaxies and their systems form and evolve. Aims. We study the dynamical properties and connectivity of the richest galaxy clusters in the Corona Borealis (CB) supercluster and of the whole supercluster, and analyse star formation of galaxies in them with the aim to understand the evolution of the supercluster and the galaxies within it. We compare it with the supercluster SCl A2142. Methods. We used the luminosity-density field to determine the high-density cores of the CB. We identified the richest galaxy clusters in them and studied the dynamical state of the clusters, analysed their substructure, and studied the star formation properties of galaxies in them using normal mixture modelling and the projected phase space diagram. We determined filaments in the supercluster to analyse the connectivity of clusters. To understand the possible future evolution of the CB, we compared the mass distribution in it with predictions from the spherical collapse model and analysed the gravitational acceleration field in the CB. Results. The richest clusters in the high-density cores of the CB are the Abell clusters A2065, A2061 (together with A2067), A2089, and Gr2064. At a radius R30 around each cluster (corresponding to the density contrast Δρ ≈ 30), the galaxy distribution shows a minimum. The R30 values for individual clusters lie in the range of 3 − 6 h−1 Mpc. The radii of the clusters (splashback radii) lie in the range of Rcl ≈ 2 − 3 Rvir. The projected phase space diagrams and the comparison with the spherical collapse model suggest that R30 regions have passed turnaround and are collapsing, forming infall regions around each cluster. Galaxies in the richest cluster of the CB, A2065, and in its infall region have on average younger stellar populations than other clusters and their environment. The cluster A2061 has the highest fraction of galaxies with very old stellar populations, similar to those in A2142. The number of long filaments that begin near clusters vary from one near A2089 to five near A2061. The total connectivity of these clusters (the number of infalling groups and filaments) varies from two to nine. Conclusions. During the future evolution, the clusters in the main part of the CB may merge and form one of the largest bound systems in the nearby Universe. Another part, with the cluster Gr2064, will form a separate system. Our study suggests that structures with a current characteristic density contrast Δρ ≈ 30 have passed turnaround and started to collapse at redshifts z ≈ 0.3 − 0.4. The comparison of the number and properties of the most massive collapsing supercluster cores from observations and simulations may serve as a test for cosmological models.

The Colors of Bulges and Disks in the Core and Outskirts of Galaxy Clusters

Abstract The role of the environment on the formation of S0 galaxies is still not well understood, specifically in the outskirts of galaxy clusters. We study eight low-redshift clusters, analyzing galaxy members up to cluster-centric distances of ∼2.5 R 200. We perform 2D photometric bulge–disk decomposition in the g, r, and i bands from which we identify 469 double-component galaxies. We separately analyze the colors of the bulges and disks and their dependence on the projected cluster-centric distance and local galaxy density. For our sample of cluster S0 galaxies, we find that bulges are redder than their surrounding disks, show a significant color–magnitude trend, and have colors that do not correlate with environment metrics. On the other hand, the disks associated with our cluster S0s become significantly bluer with increasing cluster-centric radius but show no evidence for a color–magnitude relation. The disk color–radius relation is mainly driven by galaxies in the cluster core at 0 ≤ R/R 200 &lt; 0.5. No significant difference is found for the disk colors of backsplash and infalling galaxies in the projected phase space (PPS). Beyond R 200, the disk colors do not change with the local galaxy density, indicating that the colors of double-component galaxies are not affected by preprocessing. A significant color–density relation is observed for single-component disk-dominated galaxies beyond R 200. We conclude that the formation of cluster S0 galaxies is primarily driven by cluster core processes acting on the disks, while evidence of preprocessing is found for single-component disk-dominated galaxies. We publicly release the data from the bulge–disk decomposition.

Investigating the projected phase space of Gaussian and non-Gaussian clusters

ABSTRACT By way of the projected phase space (PPS), we investigate the relation between galaxy properties and cluster environment in a subsample of groups from the Yang catalogue. The sample is split according to the Gaussianity of the velocity distribution in the group into Gaussian (G) and non-Gaussian (NG). Our sample is limited to massive clusters with $\rm \mathit{ M}_{200} \ge 10^{14}\, M_{\odot }$ and $\rm 0.03\le \mathit{ z} \le 0.1$. NG clusters are more massive, less concentrated and have an excess of faint (F) galaxies compared to G clusters. NG clusters show mixed distributions of galaxy properties in the PPS compared to the G case. Using the relation between infall time and locus on the PPS, we find that, on average, NG clusters accreted ${\sim}\rm 10^{11}\, M_{\odot }$ more stellar mass in the last ∼5 Gyr than G clusters. The relation between galaxy properties and infall time is significantly different for galaxies in G and NG systems. The more mixed distribution in the PPS of NG clusters translates into shallower relations with infall time. F galaxies whose first crossing of the cluster virial radius happened 2–4 Gyr ago in NG clusters are older and more metal-rich than in G systems. All these results suggest that NG clusters experience a higher accretion of pre-processed galaxies, which characterizes G and NG clusters as different environments to study galaxy evolution.

Star Formation Activity of Galaxies Undergoing Ram Pressure Stripping in the Virgo Cluster

We study galaxies undergoing ram pressure stripping in the Virgo cluster to examine whether we can identify any discernible trend in their star formation activity. We first use 48 galaxies undergoing different stages of stripping based on HI morphology, HI deficiency, and relative extent to the stellar disk, from the VIVA survey. We then employ a new scheme for galaxy classification which combines HI mass fractions and locations in projected phase space, resulting in a new sample of 365 galaxies. We utilize a variety of star formation tracers, which include g - r, WISE [3.4] - [12] colors, and starburstiness that are defined by stellar mass and star formation rates to compare the star formation activity of galaxies at different stripping stages. We find no clear evidence for enhancement in the integrated star formation activity of galaxies undergoing early to active stripping. We are instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, in agreement with previous studies. Our results suggest that if there is any ram pressure stripping induced enhancement, it is at best locally modest, and galaxies undergoing enhancement make up a small fraction of the total sample. Our results also indicate that it is possible to trace galaxies at different stages of stripping with the combination of HI gas content and location in projected phase space, which can be extended to other galaxy clusters that lack high-resolution HI imaging.