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Register a new userGalaxy groups in the low-redshift Universe
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We apply a halo-based group finder to four large redshift surveys, the 2MRS, 6dFGS, SDSS and 2dFGRS, to construct group catalogs in the low-redshift Universe. The group finder is based on that of Yang et al. but with an improved halo mass assignment so that it can be applied uniformly to various redshift surveys of galaxies. Halo masses are assigned to groups according to proxies based on the stellar mass/luminosity of member galaxies. The performances of the group finder in grouping galaxies according to common halos and in halo mass assignments are tested using realistic mock samples constructed from hydrodynamical simulations and empirical models of galaxy occupation in dark matter halos. Our group finder finds $sim 94%$ of the correct true member galaxies for $90-95%$ of the groups in the mock samples; the halo masses assigned by the group finder are un-biased with respect to the true halo masses, and have a typical uncertainty of $sim0.2,{rm dex}$. The properties of group catalogs constructed from the observational samples are described and compared with other similar catalogs in the literature.
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We present a new suite of mock galaxy catalogs mimicking the low-redshift Universe, based on an updated halo occupation distribution (HOD) model and a scaling relation between optical properties and the neutral hydrogen (HI) content of galaxies. Our algorithm is constrained by observations of the luminosity function and luminosity- and colour-dependent clustering of SDSS galaxies, as well as the HI mass function and HI-dependent clustering of massive HI-selected galaxies in the ALFALFA survey. Mock central and satellite galaxies with realistic values of $r$-band luminosity, $g-r$ and $u-r$ colour, stellar mass and HI mass are populated in an $N$-body simulation, inheriting a number of properties of the density and tidal environment of their host halos. The host halo of each central galaxy is also `baryonified with realistic spatial distributions of stars as well as hot and cold gas, along with the corresponding rotation curve. Our default HOD assumes that galaxy properties are a function of group halo mass alone, and can optionally include effects such as galactic conformity and colour-dependent galaxy assembly bias. The mocks predict the relation between the stellar mass and HI mass of massive HI galaxies, as well as the 2-point cross-correlation function of spatially co-located optical and HI-selected samples. They enable novel null tests for galaxy assembly bias, provide predictions for the HI velocity width function, and clarify the origin and universality of the radial acceleration relation in the $Lambda$CDM framework.
We present a study of the environment of barred galaxies using a volume-limited sample of over 30,000 galaxies drawn from the Sloan Digital Sky Survey. We use four different statistics to quantify the environment: the projected two-point cross-correlation function, the background-subtracted number count of neighbor galaxies, the overdensity of the local environment, and the membership of our galaxies to galaxy groups to segregate central and satellite systems. For barred galaxies as a whole, we find a very weak difference in all the quantities compared to unbarred galaxies of the control sample. When we split our sample into early- and late-type galaxies, we see a weak but significant trend for early-type galaxies with a bar to be more strongly clustered on scales from a few 100 kpc to 1 Mpc when compared to unbarred early-type galaxies. This indicates that the presence of a bar in early-type galaxies depends on the location within their host dark matter halos. This is confirmed by the group catalog in the sense that for early-types, the fraction of central galaxies is smaller if they have a bar. For late-type galaxies, we find fewer neighbors within $sim$50 kpc around the barred galaxies when compared to unbarred galaxies form the control sample, suggesting that tidal forces from close companions suppress the formation/growth of bars. Finally, we find no obvious correlation between overdensity and the bars in our sample, showing that galactic bars are not obviously linked to the large-scale structure of the universe.
(Abridged) This paper presents an absorption-line study of the multiphase circumgalactic medium (CGM) based on observations of Lya, CII, CIV, SiII, SiIII, and SiIV absorption transitions in the vicinities of 195 galaxies at redshift z<0.176. The galaxy sample is established based on a cross-comparison between public galaxy and QSO survey data and is characterized by a median redshift of <z>=0.041, a median projected distance of <d>=362 kpc to the sightline of the background QSO, and a median stellar mass of log(M_star/M_sun) = 9.7 pm 1.1. Comparing the absorber features identified in the QSO apectra with known galaxy properties has led to strong constraints for the CGM absorption properties at z<~0.176. First, abundant hydrogen gas is observed out to d~500 kpc, well beyond the dark matter halo radius Rh of individual galaxies, with a mean covering fraction of ~60%. In contrast, no heavy elements are detected at d>~0.7 Rh from either low-mass dwarfs or high-mass galaxies. The lack of detected heavy elements in low- and high-ionization states suggests that either there exists a chemical enrichment edge at d~0.7 Rh or gaseous clumps giving rise to the observed absorption lines cannot survive at these large distances. Considering all galaxies at d>Rh leads to a strict upper limit for the covering fraction of heavy elements of ~3% (at a 95% confidence level) over d=(1-9) Rh. At d<Rh, differential covering fraction between low- and high-ionization gas is observed, suggesting that the CGM becomes progressively more ionized from d<0.3 Rh to larger distances. Comparing CGM absorption observations at low and high redshifts shows that at a fixed-fraction of Rh the CGM exhibits stronger mean absorption at z=2.2 than at z~0. We discuss possible pseudo-evolution of the CGM as a result of misrepresentation of halo radius.
Galaxies that are being stripped of their gas can sometimes be recognized from their optical appearance. Extreme examples of stripped galaxies are the so-called ``jellyfish galaxies, that exhibit tentacles of debris material with a characteristic jellyfish morphology. We have conducted the first systematic search for galaxies that are being stripped of their gas at low-z (z=0.04-0.07) in different environments, selecting galaxies with varying degrees of morphological evidence for stripping. We have visually inspected B and V-band images and identified 344 candidates in 71 galaxy clusters of the OMEGAWINGS+WINGS sample and 75 candidates in groups and lower mass structures in the PM2GC sample. We present the atlas of stripping candidates and a first analysis of their environment and their basic properties, such as morphologies, star formation rates and galaxy stellar masses. Candidates are found in all clusters and at all clustercentric radii, and their number does not correlate with the cluster velocity dispersion sigma or X-ray luminosity L_X. Interestingly, convincing cases of candidates are also found in groups and lower mass haloes (10^{11}-10^{14} M_{sun}), although the physical mechanism at work needs to be securely identified. All the candidates are disky, have stellar masses ranging from log M/M_{sun} < 9 to > 11.5 and the majority of them form stars at a rate that is on average a factor of 2 higher (2.5 sigma) compared to non-stripped galaxies of similar mass. The few post-starburst and passive candidates have weak stripping evidence. We conclude that the stripping phenomenon is ubiquitous in clusters and could be present even in groups and low mass haloes. Further studies will reveal the physics of the gas stripping and clarify the mechanisms at work.
Galaxy redshift surveys are one of the pillars of the current standard cosmological model and remain a key tool in the experimental effort to understand the origin of cosmic acceleration. To this end, the next generation of surveys aim at achieving sub-percent precision in the measurement of the equation of state of dark energy $w(z)$ and the growth rate of structure $f(z)$. This however requires comparable control over systematic errors, stressing the need for improved modelling methods. In this contribution we review at the introductory level some highlights of the work done in this direction by the {it Darklight} project. Supported by an ERC Advanced Grant, {it Darklight} developed novel techniques for clustering analysis, which were tested through numerical simulations before being finally applied to galaxy data as in particular those of the recently completed VIPERS redshift survey. We focus in particular on: (a) advances on estimating the growth rate of structure from redshift-space distortions; (b) parameter estimation through global Bayesian reconstruction of the density field from survey data; (c) impact of massive neutrinos on large-scale structure measurements. Overall, {it Darklight} has contributed to paving the way for forthcoming high-precision experiments, such as {it Euclid}, the next ESA cosmological mission.
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