Galaxy Group Scale Research Articles

The hot circumgalactic medium in the eROSITA All-Sky Survey II. Scaling relations between X-ray luminosity and galaxies' mass

Understanding how the properties of galaxies relate to the properties of the hot circum-galactic medium (CGM) around them can constrain galaxy evolution models. We aim to measure the scaling relations between the X-ray luminosity of the hot CGM and the fundamental properties (stellar mass and halo mass) of a galaxy. We measured the X-ray luminosity of the hot CGM based on the surface brightness profiles of central galaxy samples measured from Spectrum Roentgen Gamma (SRG)/eROSITA all-sky survey data. We related the X-ray luminosity to the galaxies' stellar and halo mass, and we compared the observed relations to the self-similar model and intrinsic (i.e., not forward-modeled) output of the IllustrisTNG, EAGLE, and SIMBA simulations. The average hot CGM X-ray luminosity ($L_ X,CGM $) correlates with the galaxy's stellar mass ($M_*$). It increases from $(1.6 $ to $(3.4 $, when $ increases from 10.0 to 11.5. A power law describes the correlation as $ X,CGM The hot CGM X-ray luminosity as a function of halo mass is measured within $ 500c )=11.3-13.7$, extending our knowledge of the scaling relation by more than two orders of magnitude. $L_ X,CGM $ increases with $M_ 500c $ from $(3.0 $ at $ 500c )=11.3$ to $(1.3 $ at $ 500c )=13.7$. The relation follows a power law of $ X,CGM 500c Our observations highlight the necessity of non-gravitational processes at the galaxy group scale while suggesting these processes are sub-dominant at the galaxy scale. We show that the outputs of current cosmological galaxy simulations generally align with the observational results uncovered here but with possibly important deviations in selected mass ranges. We explore, at the low mass end, the average scaling relations between the CGM X-ray luminosity and the galaxy's stellar mass or halo mass, which constitutes a new benchmark for galaxy evolution models and feedback processes.

KiDS+VIKING+GAMA: Halo occupation distributions and correlations of satellite numbers with a new halo model of the galaxy-matter bispectrum for galaxy-galaxy-galaxy lensing

Context. Halo models and halo occupation distributions (HODs) are important tools to model the distribution of galaxies and matter. Aims. We present and assess a new method for constraining the parameters of HODs using the mean gravitational lensing shear around galaxy pairs, so-called galaxy-galaxy-galaxy lensing (G3L). In contrast to galaxy-galaxy lensing, G3L is also sensitive to the correlations between the per-halo numbers of galaxies from different populations. We employed our G3L halo model to probe these correlations and test the default hypothesis that they are negligible. Methods. We derived a halo model for G3L and validated it with realistic mock data from the Millennium Simulation and a semi-analytic galaxy model. Then, we analysed public data from the Kilo-Degree Survey (KiDS), the VISTA Infrared Kilo-Degree Galaxy Survey (VIKING) and data from the Galaxy And Mass Assembly Survey (GAMA) to infer the HODs of galaxies at z < 0.5 in five different stellar mass bins between 108.5h−2 M⊙ and 1011.5h−2 M⊙ and two colours (red and blue), as well as correlations between satellite numbers. Results. The analysis accurately recovers the true HODs in the simulated data for all galaxy samples within the 68% credibility range. The model best fits agree with the observed G3L signal on the 95% confidence level. The inferred HODs vary significantly with colour and stellar mass. In particular, red galaxies prefer more massive halos ≳1012 M⊙, while blue galaxies are present in halos ≳1011 M⊙. There is strong evidence (> 3σ) for a high correlation, increasing with halo mass, between the numbers of red and blue satellites and between galaxies with stellar masses below 1010 M⊙. Conclusions. Our G3L halo model accurately constrains galaxy HODs for lensing surveys of up to 103 deg2 and redshift below 0.5 probed here. Analyses of future surveys may need to include non-Poisson variances of satellite numbers or a revised model for central galaxies. Correlations between satellite numbers are ubiquitous between various galaxy samples and are relevant for halos with masses ≳1013 M⊙, that is, of galaxy-group scale and more massive. Possible causes of these correlations are the selection of similar galaxies in different samples, the survey flux limit, or physical mechanisms such as a fixed ratio between the satellite numbers of distinct populations. The decorrelation for halos with smaller masses is probably an effect of shot noise by low-occupancy halos. The inferred HODs can be used to complement galaxy-galaxy lensing or galaxy-clustering HOD studies or as input to cosmological analyses and improved mock galaxy catalogues.

Properties of brightest group galaxies in cosmic web filaments

Context. The cosmic web, a complex network of galaxy groups and clusters connected by filaments, is a dynamical environment in which galaxies form and evolve. However, the impact of cosmic filaments on the properties of galaxies is difficult to study because of the much more influential local (galaxy-group scale) environment. Aims. The aim of this paper is to investigate the dependence of intrinsic galaxy properties on distance to the nearest cosmic web filament, using a sample of galaxies for which the local environment is easily assessable. Methods. Our study is based on a volume-limited galaxy sample with Mr ≤ −19 mag, drawn from the SDSS DR12. We chose brightest group galaxies (BGGs) in groups with two to six members as our probes of the impact of filamentary environment because their local environment can be determined more accurately. We use the Bisous marked point process method to detect cosmic-web filaments with radii of 0.5−1.0 Mpc and measure the perpendicular filament spine distance (Dfil) for the BGGs. We limit our study to Dfil values up to 4 Mpc. We use the luminosity density field as a tracer of the local environment. To achieve uniformity of the sample and to reduce potential biases we only consider filaments longer than 5 Mpc. Our final sample contains 1427 BGGs. Results. We note slight deviations between the galaxy populations inside and outside the filament radius in terms of stellar mass, colour, the 4000 Å break, specific star formation rates, and morphologies. However, all these differences remain below 95% confidence and are negligible compared to the effects arising from local environment density. Conclusions. Within a 4 Mpc radius of the filament axes, the effect of filaments on BGGs is marginal. The local environment is the main factor in determining BGG properties.

NGC 741—Mergers and AGN Feedback on a Galaxy-group Scale

Abstract Low-mass galaxy cluster systems and groups will play an essential role in upcoming cosmological studies, such as those to be carried out with eROSITA. Though the effects of active galactic nuclei (AGNs) and merging processes are of special importance to quantify biases like selection effects or deviations from hydrostatic equilibrium, they are poorly understood on the galaxy-group scale. We present an analysis of recent deep Chandra and XMM-Newton integrations of NGC 741 that provides an excellent example of a group with multiple concurrent phenomena: both an old central radio galaxy and a spectacular infalling head-tail source, strongly bent jets, a 100-kpc radio trail, intriguing narrow X-ray filaments, and gas-sloshing features. Supported principally by X-ray and radio continuum data, we address the merging history of the group, the nature of the X-ray filaments, the extent of gas-stripping from NGC 742, the character of cavities in the group, and the roles of the central AGN and infalling galaxy in heating the intra-group medium.

Reconstruction of small-scale galaxy cluster substructure with lensing flexion

We present a reconstructions of galaxy-cluster-scale mass distributions from simulated gravitational lensing data sets including strong lensing, weak lensing shear, and measurements of quadratic image distortions -- flexion. The lensing data is constructed to make a direct comparison between mass reconstructions with and without flexion. We show that in the absence of flexion measurements, significant galaxy-group scale substructure can remain undetected in the reconstructed mass profiles, and that the resulting profiles underestimate the aperture mass in the substructure regions by $\sim25-40\%$. When flexion is included, subhaloes down to a mass of $\sim3\times10^{12}$ M$_\odot$ can be detected at an angular resolution smaller than 10\arcsec. Aperture masses from profiles reconstructed with flexion match the input distribution values to within an error of $\sim13\%$, including both statistical error and scatter. This demonstrates the important constraint that flexion measurements place on substructure in galaxy clusters and its utility for producing high-fidelity mass reconstructions.

A BOUND VIOLATION ON THE GALAXY GROUP SCALE: THE TURN-AROUND RADIUS OF NGC 5353/4

The first observational evidence for the violation of the maximum turn-around radius on the galaxy group scale is presented. The NGC 5353/4 group is chosen as an ideal target for our investigation of the bound-violation because of its proximity, low-density environment, optimal mass scale, and existence of a nearby thin straight filament. Using the observational data on the line-of-sight velocities and three-dimensional distances of the filament galaxies located in the bound zone of the NGC 5353/4 group, we construct their radial velocity profile as a function of separation distance from the group center and then compare it to the analytic formula obtained empirically by Falco et al. (2014) to find the best-fit value of an adjustable parameter with the help of the maximum likelihood method. The turn-around radius of NGC 5353/4 is determined to be the separation distance where the adjusted analytic formula for the radial velocity profile yields zero. The estimated turn-around radius of NGC 5353/4 turns out to substantially exceed the upper limit predicted by the spherical model based on the LambdaCDM cosmology. Even when the restrictive condition of spherical symmetry is released, the estimated value is found to be only marginally consistent with the LambdaCDM expectation.

Dark energy in six nearby galaxy flows: Synthetic phase diagrams and self-similarity

Outward flows of galaxies are observed around groups of galaxies on spatial scales of about 1 Mpc, and around galaxy clusters on scales of 10 Mpc. Using recent data from the Hubble Space Telescope (HST), we have constructed two synthetic velocity-distance phase diagrams: one for four flows on galaxy-group scales and the other for two flows on cluster scales. It has been shown that, in both cases, the antigravity produced by the cosmic dark-energy background is stronger than the gravity produced by the matter in the outflow volume. The antigravity accelerates the flows and introduces a phase attractor that is common to all scales, corresponding to a linear velocity-distance relation (the local Hubble law). As a result, the bundle of outflow trajectories mostly follow the trajectory of the attractor. A comparison of the two diagrams reveals the universal self-similar nature of the outflows: their gross phase structure in dimensionless variables is essentially independent of their physical spatial scales, which differ by approximately a factor of 10 in the two diagrams.

The ATLAS3D project - XIII. Mass and morphology of H i in early-type galaxies as a function of environment

We present the Atlas3D HI survey of 166 nearby early-type galaxies (ETGs) down to M(HI)~10^7 M_sun. We detect HI in ~40% of all ETGs outside the Virgo cluster and in ~10% of all ETGs inside it. This demonstrates that it is common for non-cluster ETGs to host HI. The HI morphology varies from regular discs/rings (the majority of the detections) to unsettled gas distributions. The former are either small discs (M(HI)<10^8 M_sun) confined within the stellar body and sharing the same kinematics of the stars, or large discs/rings (M(HI) up to 5x10^9 M_sun) extending to tens of kpc from the host galaxy and frequently kinematically decoupled from the stars. Neutral hydrogen provides material for star formation in ETGs. Galaxies with central HI exhibit signatures of star formation in ~70% of the cases, ~5 times more frequently than galaxies without central HI. The central ISM is dominated by molecular gas. In ETGs with a small gas disc the conversion of HI into H_2 is as efficient as in spirals. The ETG HI mass function has M*~2x10^9 M_sun and slope=-0.7. ETGs host much less HI than spirals as a family. However, a significant fraction of them is as HI-rich as spirals. The main difference between ETGs and spirals is that the former lack the high-column-density HI typical of the bright stellar disc of the latter. We find an envelope of decreasing M(HI) with increasing environment density. The gas-richest ETGs live in the poorest environments (where star-formation is more common), galaxies in the centre of Virgo have the lowest HI content, and the cluster outskirts are a transition region. We find an HI morphology-density relation. At low environment density HI is mostly distributed on large discs/rings. More disturbed HI morphologies dominate environment densities typical of rich groups, confirming the importance of processes occurring on a galaxy-group scale for the evolution of ETGs.

A new window of exploration in the mass spectrum: strong lensing by galaxy groups in the SL2S

The existence of strong lensing systems with Einstein radii (Re) covering the full mass spectrum, from ~1-2" (produced by galaxy scale dark matter haloes) to >10" (produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Re around 1-2" and above 10" have been reported but very few in between. In this article, we present a sample of 13 strong lensing systems with Re in the range 3"- 8", i.e. systems produced by galaxy group scale dark matter haloes, spanning a redshift range from 0.3 to 0.8. This opens a new window of exploration in the mass spectrum, around 10^{13}- 10^{14} M_{sun}, which is a crucial range for understanding the transition between galaxies and galaxy clusters. Our analysis is based on multi-colour CFHTLS images complemented with HST imaging and ground based spectroscopy. Large scale properties are derived from both the light distribution of the elliptical galaxies group members and weak lensing of the faint background galaxy population. On small scales, the strong lensing analysis yields Einstein radii between 2.5" and 8". On larger scales, the strong lenses coincide with the peak of the light distribution, suggesting that mass is traced by light. Most of the luminosity maps have complicated shapes, indicating that these intermediate mass structures are dynamically young. Fitting the reduced shear with a Singular Isothermal Sphere, we find sigma ~ 500 km/s and an upper limit of ~900 km/s for the whole sample. The mass to light ratio for the sample is found to be M/L_i ~ 250 (solar units, corrected for evolution), with an upper limit of 500. This can be compared to mass to light ratios of small groups (with sigma ~ 300 km/s and galaxy clusters with sigma > 1000 km/s, thus bridging the gap between these mass scales.

Properties of galaxy groups in the Sloan Digital Sky Survey -- I. The dependence of colour, star formation and morphology on halo mass

Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey Data Release 2, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early-types, late-types and intermediate-types, based on their colour and specific star formation rate. At fixed luminosity, the late- (early-)type fraction of galaxies increases (decreases) with decreasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass-scale. We argue that the previous claim of a characteristic feature on galaxy group scales is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak, especially over the range 0.25? L/L*? 2.5: galaxy type depends more strongly on halo mass than on luminosity. In agreement with previous studies, the late- (early-)type fraction increases (decreases) with increasing halocentric radius. However, we find that this radial dependence is present in haloes of all masses probed (down to 10 12 h -1 M ⊙ ), while previous studies did not find any radial dependence in haloes with M? 10 13.5 h -1 M ⊙ . We argue that this discrepancy owes to the fact that we have excluded central galaxies from our analysis. We also find that the properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early-type fraction of satellites is significantly higher in a halo with an early-type central galaxy than in a halo of the same mass but with a late-type central galaxy. This phenomenon, which we call 'galactic conformity', is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate-type galaxies is always ∼20 per cent, independent of luminosity, independent of halo mass, independent of halocentric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implicat ns of all these findings for galaxy formation and evolution.

Dissipationless Merging and the Assembly of Central Galaxies

We reanalyze the galaxy-mass correlation function measured by the Sloan Digital Sky Survey to obtain host dark matter halo masses at galaxy and galaxy group scales. We extend the data to galaxy clusters in the 2MASS catalog and study the relation between central galaxy luminosity and halo mass. While the central galaxy luminosity scales as ~M^{0.7-0.8} at low masses, the relation flattens to ~M^{<0.3} above ~4x10^{13} M_sun. The total luminosity of galaxies in the halo, however, continues to grow as a power-law ~M^{0.8-0.9}. Starting from the hypothesis that the central galaxies grow by merging ("galactic cannibalism"), we develop a simple model for the evolution of their luminosities as a consequence of the accretion of satellite galaxies. The luminosity-mass relation flattens when the time scale on which dynamical friction induces orbital decay in the satellite galaxies exceeds the age of the dark matter halo. Then, the growth of the central galaxy is suppressed as it can cannibalize only the rare, massive satellite galaxies. The model takes the dependence of the total luminosity of galaxies in a halo on its mass and the global galaxy luminosity function as input, and reproduces the observed central galaxy luminosity-mass relation over three decades in halo mass, (10^{12}-10^{15}) M_sun. The success of the model suggests that gas cooling and subsequent star formation did not play an important role in the final assembly of central galaxies from sub-L_star precursors.