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Register a new userMajor Dry-Mergers In Early-Type Brightest Cluster Galaxies
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F. S. Liu
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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.
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We identify a total of 120 early-type Brightest Cluster Galaxies (BCGs) at 0.1<z<0.4 in two recent large cluster catalogues selected from the Sloan Digital Sky Survey (SDSS). They are selected with strong emission lines in their optical spectra, with both H{alpha} and [O II]{lambda}3727 line emission, which indicates significant ongoing star formation. They constitute about ~ 0.5% of the largest, optically-selected, low-redshift BCG sample, and the fraction is a strong function of cluster richness. Their star formation history can be well described by a recent minor and short starburst superimposed on an old stellar component, with the recent episode of star formation contributing on average only less than 1 percent of the total stellar mass. We show that the more massive star-forming BCGs in richer clusters tend to have higher star formation rate (SFR) and specific SFR (SFR per unit galaxy stellar mass). We also compare their statistical properties with a control sample selected from X-ray luminous clusters, and show that the fraction of star-forming BCGs in X-ray luminous clusters is almost one order of magnitude larger than that in optically-selected clusters. BCGs with star formation in cooling flow clusters usually have very flat optical spectra and show the most active star formation, which may be connected with cooling flows.
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.
Dissipationless (gas-free or dry) mergers have been suggested to play a major role in the formation and evolution of early-type galaxies, particularly in growing their mass and size without altering their stellar populations. We perform a new test of the dry merger hypothesis by comparing N-body simulations of realistic systems to empirical constraints provided by recent studies of lens early-type galaxies. We find that major and minor dry mergers: i) preserve the nearly isothermal structure of early-type galaxies within the observed scatter; ii) do not change more than the observed scatter the ratio between total mass M and virial mass R_e*sigma/2G (where R_e is the half-light radius and sigma the projected velocity dispersion); iii) increase strongly galaxy sizes [as M^(0.85+/-0.17)] and weakly velocity dispersions [as M^(0.06+/-0.08)] with mass, thus moving galaxies away from the local observed M-R_e and M-sigma relations; iv) introduce substantial scatter in the M-R_e and M-sigma relations. Our findings imply that, unless there is a high degree of fine tuning of the mix of progenitors and types of interactions, present-day massive early-type galaxies cannot have assembled more than ~50% of their mass, and increased their size by more than a factor ~1.8, via dry merging.
ABRIDGED: We study the evolution since z~1 of the rest-frame B luminosity function of the early-type galaxies (ETGs) in ~0.7 deg^2 in the COSMOS field. In order to identify ALL progenitors of local ETGs we construct the sample of high-z galaxies using two complementary criteria: (i) A morphological selection based on the Zurich Estimator of Structural Types, and (ii) A photometric selection based on the galaxy properties in the (U-V)-M_V color-magnitude diagram. We furthermore constrain both samples so as to ensure that the selected progenitors of ETGs are compatible with evolving into systems which obey the mu_B-r_{hl} Kormendy relation. Assuming the luminosity evolution derived from studies of the fundamental plane for high-z ETGs, our analysis shows no evidence for a decrease in the number density of the most massive ETGs out to z~ 0.7: Both the morphologically- and the photometrically-selected sub-samples show no evolution in the number density of bright (~L>2.5L*) ETGs. Allowing for different star formation histories, and cosmic variance, we estimate a maximum decrease in the number density of massive galaxies at that redshift of ~30%. We observe, however, in both the photometrical and morphological samples, a deficit of up to ~2-3 of fainter ETGs over the same cosmic period. Our results argue against a significant contribution of recent dissipationless ``dry mergers to the formation of the most massive ETGs. We suggest that the mass growth in low luminosity ETGs can be explained with a conversion from z~0.7 to z=0 of blue, irregular and disk galaxies into low- and intermediate-mass ``red ETGs, possibly also through gas rich mergers.
Using three fiducial Nbody+SPH simulations, we follow the merging of two disk galaxies with a hot gaseous halo component each, and examine whether the merger remnant can be a spiral galaxy. The stellar progenitor disks are destroyed by violent relaxation during the merging and most of their stars form a classical bulge, while the remaining form a thick disk and its bar. A new stellar disk forms subsequently and gradually in the remnant from the gas accreted mainly from the halo. It is vertically thin and well extended in its equatorial plane. A bar starts forming before the disk is fully in place, contrary to what is assumed in idealised simulations of isolated bar-forming galaxies. It has morphological features such as ansae and boxy/peanut bulges. Stars of different ages populate different parts of the box/peanut. A disky pseudobulge forms also, so that by the end of the simulation, all three types of bulges coexist. The oldest stars are found in the classical bulge, followed by those of the thick disk, then by those in the thin disk. The youngest stars are in the spiral arms and the disky pseudobulge. The disk surface density profiles are of type II (exponential with downbending), and the circular velocity curves are flat and show that the disks are submaximum in these examples: two clearly so and one near-borderline between maximum and submaximum. On average, only roughly between 10 and 20% of the stellar mass is in the classical bulge of the final models, i.e. much less than in previous simulations.
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