ABSTRACT
We present a sample of 329 low-to intermediate-redshift (0.05 <z< 0.3) brightest cluster galaxies (BCGs) in X-ray-selected clusters from the SPectroscopic IDentification of eRosita Sources ...survey, a spectroscopic survey within Sloan Digital Sky Survey-IV (SDSS-IV). We define our BCGs by simultaneous consideration of legacy X-ray data from ROSAT, maximum-likelihood outputs from an optical cluster-finder algorithm and visual inspection. Using SDSS imaging data, we fit Sérsic profiles to our BCGs in three bands (g, r, i) with sigma a galfit-based software wrapper. We examine the reliability of our fits by running our pipeline on ∼104 point spread function-convolved model profiles injected into eight random cluster fields; we then use the results of this analysis to create a robust subsample of 198 BCGs. We outline three cluster properties of interest: overall cluster X-ray luminosity (LX), cluster richness as estimated by redMaPPer (λ),and cluster halo mass (M200), which is estimated via velocity dispersion. In general, there are significant correlations with BCG stellar mass between all three environmental properties, but no significant trends arise with either Sérsic index or effective radius. There is no major environmental dependence on the strength of the relation between effective radius and BCG stellar mass. Stellar mass therefore arises as the most important factor governing BCG morphology. Our results indicate that our sample consists of a large number of relaxed, mature clusters containing broadly homogeneous BCGs up to z ∼ 0.3, suggesting that there is little evidence for much ongoing structural evolution for BCGs in these systems.
We study the distribution and evolution of the stellar mass and the star formation rate (SFR) of the brightest group galaxies (BGGs) over 0.04 < z < 1.3 using a large sample of 407 X-ray galaxy ...groups selected from the COSMOS, AEGIS, and XMM–LSS fields. We compare our results with predictions from the semi-analytic models based on the Millennium simulation. In contrast to model predictions, we find that, as the Universe evolves, the stellar mass distribution evolves towards a normal distribution. This distribution tends to skew to low-mass BGGs at all redshifts implying the presence of a star-forming population of the BGGs with MS
∼ 1010.5 M⊙ which results in the shape of the stellar mass distribution deviating from a normal distribution. In agreement with the models and previous studies, we find that the mean stellar mass of BGGs grows with time by a factor of ∼2 between z = 1.3 and z = 0.1, however, the significant growth occurs above z = 0.4. The BGGs are not entirely a dormant population of galaxies, as low-mass BGGs in low-mass haloes are more active in forming stars than the BGGs in more massive haloes, over the same redshift range. We find that the average SFR of the BGGs evolves steeply with redshift and fraction of the passive BGGs increases as a function of increasing stellar mass and halo mass. Finally, we show that the specific SFR of the BGGs within haloes with M
200 ≤ 1013.4 M⊙ decreases with increasing halo mass at z < 0.4.
One of the most fundamental correlations between the properties of galaxies in the local Universe is the so-called morphology-density relation (Dressler 1980). A plethora of studies utilizing ...multi-wavelength tracers of activity have shown that late type star forming galaxies favour low density regions in the local Universe (e.g. G´omez et al. 2003). In particular, the cores of massive galaxy clusters are galaxy graveyards full of massive spheroids that are dominated by old stellar populations. A variety of physical processes might be effective in suppressing star formation and affecting the morphology of cluster and group galaxies. Broadly speaking, these can be grouped in two big families: (i) interactions with other cluster members and/or with the cluster gravitational potential and (ii) interactions with the hot gas that permeates massive galaxy systems. Galaxy groups are the most common galaxy environment in our Universe, bridging the gap between the low density field and the crowded galaxy clusters. Indeed, as many as 50%-70% of galaxies reside in galaxy groups in the nearby Universe (Huchra & Geller 1982; Eke et al. 2004), while only a few percent are contained in the denser cluster cores. In addition, in the current bottom-up paradigm of structure formation, galaxy groups are the building blocks of more massive systems: they merge to form clusters. As structures grow, galaxies join more and more massive systems, spending most of their life in galaxy groups before entering the cluster environment. Thus, it is plausible to ask if group-related processes may drive the observed relations between galaxy properties and their environment.
To shed light on this topic we have built the largest X-ray selected samples of galaxy groups with secure spectroscopic identification on the major blank field surveys. For this purpose, we combine deep X-ray Chandra and XMM data of the four major blank fields (All-wavelength Extended Groth Strip International Survey (AEGIS), the COSMOS field, the Extended Chandra Deep Field South (ECDFS), and the Chandra Deep Field North (CDFN) ). The group catalog in each field is created by associating any X-ray extended emission to a galaxy overdensity in the 3D space. This is feasible given the extremely rich spectroscopic coverage of these fields. Our identification method and the dynamical analysis used to identify the galaxy group members and to estimate the group velocity dispersion is extensively tested on the AEGIS field and with mock catalogs extracted from the Millennium Simulation (Springel et al. 2005). The effect of dynamical complexity, substructure, shape of X-ray emission, different radial and redshift cuts have been explored on the LX −sigma relation. We also discover a high redshift group at z~1.54 in the AEGIS field. This detection illustrates that mega-second Chandra exposures are required for detecting such objects in the volume of deep fields. We provide an accurate measure of the Star Formation Rate (SFR) of galaxies by using the deepest available Herschel PACS and Spitzer MIPS data available for the considered fields. We also provide a well-calibrated estimate of the SFR derived by using the SED fitting technique for undetected sources in mid- and far-infrared observations.
Using this unique sample, we conduct a comprehensive analysis of the dependence of the total SFR , total stellar masses and halo occupation distribution (HOD) of massive galaxies (M*>10^10 M_sun) on the halo mass of the groups with rigorous consideration of uncertainties. We observe a clear evolution in the level of star formation (SF) activity in galaxy groups. Indeed, the total star formation activity in high redshift
(0.5<z<1.1) groups is higher with respect to the low redshift (0.15<z<0.5) sample at any mass by almost 0.8 ± 0.1 dex. A milder difference (0.35 ± 0.1 dex) is observed between the 0.15-0.5 redshift bin and the groups at z < 0.085. This evolution seems to be much faster than the one observed in the whole galaxy population dominated by lower mass halos. This would imply that the level of SF activity is declining more rapidly since z~1.1 in the more massive halos than in the more common lower mass halos, confirming a “halo downsizing” effect as discussed already in Popesso et al. (2012). The HOD and the total
stellar mass-M200 relation are consistent with a linear relation in any redshift bin in the M_200 range considered in our analysis. We do not observe any evolution in the HOD since z~1.1. Similarly we do not observe evolution in the relation between the total stellar mass of the groups and the total mass, in agreement with the results of Giodini et al (2012). The picture emerging from our findings is that massive groups at M_200~10^13−14 M_sun have already accreted the same amount of mass and have the same number of galaxies as the low redshift counterpart, as predicted by Stewart et al. (2008). This implies that the most evident evolution of the galaxy population of the most massive systems acts in terms of quenching their galaxy star formation activity. The analysis of the evolution of the fraction of SF galaxies as a function of halo mass or velocity dispersion show that high mass systems seem to be already evolved at z~1 by showing a fraction of star forming galaxies consistent with the low redshift counterpart at z < 0.085. Given the almost linear relation between the total SFR and M_200 in the high-z sample, this implies that most of the contribution to the total SFR of the most massive systems (M_200~ 10^14 M_sun) is given by few highly star forming galaxies, while in lower mass systems (M_200~10^13 M_sun) is given by many galaxies of average activity. This would be an additional sign of a faster evolution in the more massive systems in terms of star formation activity with respect to lower mass groups. Thus, it would confirm the “halo downsizing” effect. The comparison of our results with the prediction of the Millennium Simulation semi-analytical model confirms the known problem of the models. We confirm the strong bias due to the “satellite overquenching” problem in suppressing significantly the SF activity of group galaxies (more than an order of magnitude) at any redshift with respect to observations. The HOD predicted by the simulations is remarkably in agreement with the observations. But due to the low SF activity of galaxies in massive halos, the models predict also a lower total stellar mass in groups with respect to the observed one at any redshift.
In order to compare the SF activity level of galaxies in different environment, we also define a sample of field galaxies and “filament-like” galaxies. This is done by using the galaxy density field to find isolated galaxies (field) and galaxies in high density region but not associated to any group or more generically to an X-ray extended emission. These two classes of environment in addition to the galaxy group sample are used to study the location of galaxies in SFR-mass plane since z~1.1 as a function of the environment. Indeed, several studies have already shown there is a tight correlation between the SFR and the stellar masses of the bulk of the star forming galaxy population at least over the past 10 Gyr. Quiescent galaxies are mainly located under this main sequence (MS) and in a more scattered cloud. Our analysis shows that the Main Sequence of star forming galaxies in the two redshift bins considered (0.15 < z < 0.5 and 0.5 < z < 1.1) is not a linear relation but it shows a flattening towards higher masses (M* > 10^10.4−10.6 M_sun). Above this limit, the galaxy SFR has a very weak dependence on the stellar mass. This flattening, to different extent, is present in all environments. At low redshift, group galaxies tend to deviate more from the mean MS towards the region of quiescence
with respect to isolated and filament-like galaxies. This environment dependent location of low redshift group galaxies with respect to the mean MS causes the increase of the dispersion of the distribution of galaxies around the MS as a function of the stellar mass. At high redshift we do not find significant evidence for a differential location of galaxies with respect to the MS as a function of the environment. Indeed, in this case we do not observe a significant increase of the dispersion of the distribution of galaxies around the MS as a function of the stellar mass. We do not find evidence for a differential distribution in the morphological type of MS galaxies in different environments. Instead, we observe a much stronger dependence of the mean S´ersic index on the stellar mass. These results suggest that star formation quenching in group galaxies is not due to galaxy structural
transformations. It also suggests that while morphology of MS galaxies is more stellar mass dependent, star formation quenching is mostly environment dependent. We conclude that the membership to a massive halo is a key ingredient in the galaxy evolution and that this acts in terms of star formation quenching in group sized halos.
We present a sample of 329 low to intermediate redshift ($0.05 < z < 0.3$)
brightest cluster galaxies (BCGs) in X-ray selected clusters from the
SPectroscopic IDentification of eRosita Sources ...(SPIDERS) survey, a
spectroscopic survey within Sloan Digital Sky Survey-IV (SDSS-IV). We define
our BCGs by simultaneous consideration of legacy X-ray data from ROSAT, maximum
likelihood outputs from an optical cluster-finder algorithm and visual
inspection. Using SDSS imaging data, we fit S\'ersic profiles to our BCGs in
three bands (\textit{g}, \textit{r}, \textit{i}) with \textsc{SIGMA}, a
\textsc{GALFIT}-based software wrapper. We examine the reliability of our fits
by running our pipeline on ${\sim}10^{4}$ psf-convolved model profiles injected
into 8 random cluster fields, we then use the results of this analysis to
create a robust subsample of 198 BCGs. We outline three cluster properties of
interest: overall cluster X-ray luminosity ($L_{X}$), cluster richness as
estimated by \textsc{redMaPPer} ($ \lambda $) and cluster halo mass
($M_{200}$), which is estimated via velocity dispersion. In general, there are
significant correlations with BCG stellar mass between all three environmental
properties, but no significant trends arise with either S\'ersic index or
effective radius. There is no major environmental dependence on the strength of
the relation between effective radius and BCG stellar mass. Stellar mass
therefore arises as the most important factor governing BCG morphology. Our
results indicate that our sample consists of a large number of relaxed, mature
clusters containing broadly homogeneous BCGs up to $z \sim 0.3$, suggesting
that there is little evidence for much ongoing structural evolution for BCGs in
these systems.
We study the distribution and evolution of the stellar mass and the star formation rate (SFR) of the brightest group galaxies (BGGs) over \( 0.04<z<1.3 \) using a large sample of \( 407 \) X-ray ...galaxy groups selected from the COSMOS, AEGIS, and XMM-LSS fields. We compare our results with predictions from the semi-analytic models based on the Millennium simulation. In contrast to model predictions, we find that, as the universe evolves, the stellar mass distribution evolves towards a normal distribution. This distribution tends to skew to low mass BGGs at all redshifts implying the presence of a star-forming population of the BGGs with \( M_S\sim10^{10.5} M_{\odot} \) which results in the shape of the stellar mass distribution deviating from a normal distribution. In agreement with models and previous studies, we find that the mean stellar mass of BGGs grows with time by a factor of \(\sim2\) between \(z=1.3\) to \(z=0.1\), however, the significant growth occurs above \( z=0.4\). The BGGs are not entirely a dormant population of galaxies, as low mass BGGs in low mass halos are more active in forming stars than the BGGs in more massive halos, over the same redshift range. We find that the average SFR of the BGGs evolves steeply with redshift and fraction of the passive BGGs increases as a function of increasing stellar mass and halo mass. Finally, we show that the specific SFR of the BGGs within halos with \( M_{200} \leq 10^{13.4} M_{\odot} \) decreases with increasing halo mass at \( z<0.4 \).
We present the results of a search for galaxy clusters and groups in the \(\sim2\) square degree of the COSMOS field using all available X-ray observations from the XMM-Newton and Chandra ...observatories. We reach an X-ray flux limit of \(3\times10^{-16}\;ergs\;cm^{-2}\;s^{-1}\) in 0.5--2 keV range, and identify 247 X-ray groups with \(M_{200c}=8\times10^{12}-3\times10^{14}\;M_{\odot}\) at a redshift range of \(0.08\leq z<1.53\), using the multiband photometric redshift and the master spectroscopic redshift catalogues of the COSMOS. The X-ray centres of groups are determined using high-resolution Chandra imaging. We investigate the relations between the offset of the brightest group galaxies (BGGs) from halo X-ray centre and group properties and compare with predictions from semi-analytic models and hydrodynamical simulations. We find that BGG offset decreases with both increasing halo mass and decreasing redshift with no strong dependence on the X-ray flux and SNR. We show that the BGG offset decreases as a function of increasing magnitude gap with no considerable redshift dependent trend. The stellar mass of BGGs in observations extends over a wider dynamic range compared to model predictions. At \(z<0.5\), the central dominant BGGs become more massive than those with large offsets by up to 0.3dex, in agreement with model prediction. The observed and predicted lognormal scatter in the stellar mass of both low- and large-offset BGGs at fixed halo mass is \(\sim0.3\)dex.
We present a sample of 329 low to intermediate redshift (\(0.05 < z < 0.3\)) brightest cluster galaxies (BCGs) in X-ray selected clusters from the SPectroscopic IDentification of eRosita Sources ...(SPIDERS) survey, a spectroscopic survey within Sloan Digital Sky Survey-IV (SDSS-IV). We define our BCGs by simultaneous consideration of legacy X-ray data from ROSAT, maximum likelihood outputs from an optical cluster-finder algorithm and visual inspection. Using SDSS imaging data, we fit Sérsic profiles to our BCGs in three bands (\textit{g}, \textit{r}, \textit{i}) with \textsc{SIGMA}, a \textsc{GALFIT}-based software wrapper. We examine the reliability of our fits by running our pipeline on \({\sim}10^{4}\) psf-convolved model profiles injected into 8 random cluster fields, we then use the results of this analysis to create a robust subsample of 198 BCGs. We outline three cluster properties of interest: overall cluster X-ray luminosity (\(L_{X}\)), cluster richness as estimated by \textsc{redMaPPer} (\( \lambda \)) and cluster halo mass (\(M_{200}\)), which is estimated via velocity dispersion. In general, there are significant correlations with BCG stellar mass between all three environmental properties, but no significant trends arise with either Sérsic index or effective radius. There is no major environmental dependence on the strength of the relation between effective radius and BCG stellar mass. Stellar mass therefore arises as the most important factor governing BCG morphology. Our results indicate that our sample consists of a large number of relaxed, mature clusters containing broadly homogeneous BCGs up to \(z \sim 0.3\), suggesting that there is little evidence for much ongoing structural evolution for BCGs in these systems.
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