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Register a new userOngoing growth of the brightest cluster galaxies via major dry mergers in the last ~6 Gyr
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Brightest Cluster Galaxies (BCGs) might have been assembled relatively late (z<1) via mergers. By exploiting the high-resolution HST/ACS imaging, we find four BCGs (COSMOS-P 125516, 102810, 036694 and 089357) in major dry merging in 29 X-ray clusters at $0.3 le z le 0.6$ in the Cosmological Evolutionary Survey (COSMOS). These BCGs show prominent but quiescent double nuclei with a magnitude difference of $delta m<1.5$ and a projected separation of $r_p<$ 10 kpc. Clear signatures of interaction such as extended plumes and/or significant asymmetries are also observed in their residual images. We infer a major merger rate of $0.55pm0.27$ merger per Gyr at $zsim0.43$ assuming the merger time-scale estimate of Kitzbichler & White (2008). This inferred rate is significantly higher than the rate in the local Universe ($0.12pm0.03$ at $zsim0.07$) presented in Liu et al. (2009). We estimate that present-day BCGs increase their luminosity (mass) by $sim35pm15$ per cent $(f_{mass}/0.5)$ via major dry mergers since $z=0.6$, where $f_{mass}$ is the mean mass fraction of companion galaxies accreted onto the central ones. Although the statistical uncertainty due to our small sample size is relatively large, our finding is consistent with both recent observational and theoretical results. Furthermore, in conjunction with our previous findings in Liu et al. (2009), the discovery of these intermediate-redshift merging BCGs is clear evidence of ongoing assembly of BCGs via major dry mergers over the last $sim$6 Gyr.
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We search for ongoing major dry-mergers in a well selected sample of local Brightest Cluster Galaxies (BCGs) from the C4 cluster catalogue. 18 out of 515 early-type BCGs with redshift between 0.03 and 0.12 are found to be in major dry-mergers, which are selected as pairs (or triples) with $r$-band magnitude difference $dm<1.5$ and projected separation $rp<30$ kpc, and showing signatures of interaction in the form of significant asymmetry in residual images. We find that the fraction of BCGs in major dry-mergers increases with the richness of the clusters, consistent with the fact that richer clusters usually have more massive (or luminous) BCGs. We estimate that present-day early-type BCGs may have experienced on average $sim 0.6 (tmerge/0.3Gyr)^{-1}$ major dry-mergers and through this process increases their luminosity (mass) by $15% (tmerge/0.3Gyr)^{-1} (fmass/0.5)$ on average since $z=0.7$, where $tmerge$ is the merging timescale and $fmass$ is the mean mass fraction of companion galaxies added to the central ones. We also find that major dry-mergers do not seem to elevate radio activities in BCGs. Our study shows that major dry-mergers involving BCGs in clusters of galaxies are not rare in the local Universe, and they are an important channel for the formation and evolution of BCGs.
We study the dark matter (DM) assembly in the central regions of massive early-type galaxies up to $zsim 0.65$. We use a sample of $sim 3800$ massive ($log M_{rm star}/M_{rm odot} > 11.2$) galaxies with photometry and structural parameters from 156 sq. deg. of the Kilo Degree Survey, and spectroscopic redshifts and velocity dispersions from SDSS. We obtain central total-to-stellar mass ratios, $M_{rm dyn}/M_{rm star}$, and DM fractions, by determining dynamical masses, $M_{rm dyn}$, from Jeans modelling of SDSS aperture velocity dispersions and stellar masses, $M_{rm star}$, from KiDS galaxy colours. We first show how the central DM fraction correlates with structural parameters, mass and density proxies, and demonstrate that most of the local correlations are still observed up to $z sim 0.65$; at fixed $M_{rm star}$, local galaxies have larger DM fraction, on average, than their counterparts at larger redshift. We also interpret these trends with a non universal Initial Mass Function (IMF), finding a strong evolution with redshift, which contrast independent observations and is at odds with the effect of galaxy mergers. For a fixed IMF, the galaxy assembly can be explained, realistically, by mass and size accretion, which can be physically achieved by a series of minor mergers. We reproduce both the $R_{rm e}-M_{rm star}$ and $M_{rm dyn}/M_{rm star}-M_{rm star}$ evolution with stellar and dark mass changing at a different rate. This result suggests that the main progenitor galaxy is merging with less massive systems, characterized by a smaller $M_{rm dyn}/M_{rm star}$, consistently with results from halo abundance matching.
We present the results of a survey of the brightest UV-selected galaxies in protoclusters. These proto-brightest cluster galaxy (proto-BCG) candidates are drawn from 179 overdense regions of $g$-dropout galaxies at $zsim4$ from the Hyper Suprime-Cam Subaru Strategic Program identified previously as good protocluster candidates. This study is the first to extend the systematic study of the progenitors of BCGs from $zsim2$ to $zsim4$. We carefully remove possible contaminants from foreground galaxies and, for each structure, we select the brightest galaxy that is at least 1 mag brighter than the fifth brightest galaxy. We select 63 proto-BCG candidates and compare their properties with those of galaxies in the field and those of other galaxies in overdense structures. The proto-BCG candidates and their surrounding galaxies have different rest-UV color $(i - z)$ distributions to field galaxies and other galaxies in protoclusters that do not host proto-BCGs. In addition, galaxies surrounding proto-BCGs are brighter than those in protoclusters without proto-BCGs. The image stacking analysis reveals that the average effective radius of proto-BCGs is $sim28%$ larger than that of field galaxies. The $i-z$ color differences suggest that proto-BCGs and their surrounding galaxies are dustier than other galaxies at $zsim4$. These results suggest that specific environmental effects or assembly biasses have already emerged in some protoclusters as early as $z sim 4$, and we suggest that proto-BCGs have different star formation histories than other galaxies in the same epoch.
We constrain the evolution of the brightest cluster galaxy plus intracluster light (BCG+ICL) using an ensemble of 42 galaxy groups and clusters that span redshifts of z = 0.05-1.75 and masses of $M_{500,c}=2times10^{13}-10^{15}$ M$_odot$ Specifically, we measure the relationship between the BCG+ICL stellar mass $M_star$ and $M_{500,c}$ at projected radii 10 < r < 100 kpc for three different epochs. At intermediate redshift (z = 0.40), where we have the best data, we find $M_starpropto M_{500,c}^{0.48pm0.06}$. Fixing the exponent of this power law for all redshifts, we constrain the normalization of this relation to be $2.08pm0.21$ times higher at z = 0.40 than at high redshift (z = 1.55). We find no change in the relation from intermediate to low redshift (z = 0.10). In other words, for fixed $M_{500,c}$, $M_star$ at 10 < r < 100 kpc increases from z = 1.55 to z = 0.40 and not significantly thereafter. Theoretical models predict that the physical mass growth of the cluster from z = 1.5 to z = 0 within $r_{500,c}$ is a factor of 1.4, excluding evolution due to definition of $r_{500,c}$. We find that $M_star$ within the central 100 kpc increases by a factor of 3.8 over the same period. Thus, the growth of $M_star$ in this central region is more than a factor of two greater than the physical mass growth of the cluster as a whole. Furthermore, the concentration of the BCG+ICL stellar mass, defined by the ratio of stellar mass within 10 kpc to the total stellar mass within 100 kpc, decreases with increasing $M_{500,c}$ at all redshift. We interpret this result as evidence for inside-out growth of the BCG+ICL over the past ten Gyrs, with stellar mass assembly occuring at larger radii at later times.
We analyse maps of the spatially-resolved nebular emission of $approx$1500 star-forming galaxies at $zapprox0.6$-$2.2$ from deep KMOS and MUSE observations to measure the average shape of their rotation curves. We use these to test claims for declining rotation curves at large radii in galaxies at $zapprox1$-$2$ that have been interpreted as evidence for an absence of dark matter. We show that the shape of the average rotation curves, and the extent to which they decline beyond their peak velocities, depends upon the normalisation prescription used to construct the average curve. Normalising in size by the galaxy stellar disk-scale length after accounting for seeing effects ($R_{rm{d}}^{prime}$), we construct stacked position-velocity diagrams that trace the average galaxy rotation curve out to $6R_{rm{d}}^{prime}$ ($approx$13 kpc, on average). Combining these curves with average HI rotation curves for local systems, we investigate how the shapes of galaxy rotation curves evolve over $approx$10 Gyr. The average rotation curve for galaxies binned in stellar mass, stellar surface mass density and/or redshift is approximately flat, or continues to rise, out to at least $6R_{rm{d}}^{prime}$. We find a trend between the outer slopes of galaxies rotation curves and their stellar mass surface densities, with the higher surface density systems exhibiting flatter rotation curves. Drawing comparisons with hydrodynamical simulations, we show that the average shapes of the rotation curves for our sample of massive, star-forming galaxies at $zapprox0$-$2.2$ are consistent with those expected from $Lambda$CDM theory and imply dark matter fractions within $6R_{rm{d}}$ of at least $approx60$ percent.
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