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Register a new userBrightest Cluster Galaxies at the Present Epoch
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We have observed 433 z<=0.08 brightest cluster galaxies (BCGs) in a full-sky survey of Abell clusters. The BCG Hubble diagram is consistent to within 2% of a Omega_m=0.3, Lambda=0.7 Hubble relation. The L_m-alpha relation for BCGs, which uses alpha, the log-slope of the BCG photometric curve of growth, to predict metric luminosity, L_m, has 0.27 mag residuals. We measure central stellar velocity dispersions, sigma, of the BCGs, finding the Faber-Jackson relation to flatten as the metric aperture grows to include an increasing fraction of the total BCG luminosity. A 3-parameter metric plane relation using alpha and sigma together gives the best prediction of L_m, with 0.21 mag residuals. The projected spatial offset, r_x, of BCGs from the X-ray-defined cluster center is a gamma=-2.33 power-law over 1<r_x<10^3 kpc. The median offset is ~10 kpc, but ~15% of the BCGs have r_x>100 kpc. The absolute cluster-dispersion normalized BCG peculiar velocity |Delta V_1|/sigma_c follows an exponential distribution with scale length 0.39+/-0.03. Both L_m and alpha increase with sigma_c. The alpha parameter is further moderated by both the spatial and velocity offset from the cluster center, with larger alpha correlated with the proximity of the BCG to the cluster mean velocity or potential center. At the same time, position in the cluster has little effect on L_m. The luminosity difference between the BCG and second-ranked galaxy, M2, increases as the peculiar velocity of the BCG within the cluster decreases. Further, when M2 is a close luminosity rival of the BCG, the galaxy that is closest to either the velocity or X-ray center of the cluster is most likely to have the larger alpha. We conclude that the inner portions of the BCGs are formed outside the cluster, but interactions in the heart of the galaxy cluster grow and extend the envelopes of the BCGs.
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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.
Variability of a galaxys core radio source can be a significant consequence of AGN accretion. However, this variability has not been well studied, particularly at high radio frequencies. As such, we report on a campaign monitoring the high radio frequency variability of 20 nearby, cool-core brightest cluster galaxies. Our highest cadence observations are at 15 GHz and are from the Owens Valley Radio Observatory (OVRO). They have a median time interval of 7 days and mostly span between 8 and 13 years. We apply a range of variability detection techniques to the lightcurves of the sources to analyse changes in their flux density on week to decade long timescales. Over the full period in which each source was observed, $chi^{2}$ tests suggest that 13/20 are inconsistent with the flat lightcurve of a non-varying source. Variability amplitude tests suggest that 12/20 sources are variable on 300 day timescales, while 19/20 are variable on 3000 day timescales. At least half of the sources also show 20 per cent peak to trough variability on 3~year timescales, while at least a third vary by 60 per cent on 6~year timescales. Significant variability is therefore a common feature of these sources. We also show how the variability relates to spectral properties at frequencies of up to 353 GHz using data from the Korean VLBI network (KVN), the NIKA2 instrument of the IRAM 30m telescope, and the SCUBA-2 instrument of the James Clerk Maxwell Telescope.
Brightest cluster galaxies (BCGs) are excellent laboratories to study galaxy evolution in dense Mpc-scale environments. We have observed in CO(1-0), CO(2-1), CO(3-2), or CO(4-3), with the IRAM-30m, 18 BCGs at $zsim0.2-0.9$ that are drawn from the CLASH survey. Our sample includes RX1532, which is our primary target, being among the BCGs with the highest star formation rate (SFR$gtrsim100~M_odot$/yr) in the CLASH sample. We unambiguously detected both CO(1-0) and CO(3-2) in RX1532, yielding a large reservoir of molecular gas, $M_{H_2}=(8.7pm1.1)times10^{10}~M_odot$, and a high level of excitation $r_{31}=0.75pm0.12$. A morphological analysis of the HST I-band image of RX1532 reveals the presence of clumpy substructures both within and outside the half-light radius $r_e=(11.6pm0.3)$ kpc, similarly to those found independently both in ultraviolet and in H$_alpha$ in previous work. We tentatively detected CO(1-0) or CO(2-1) in four other BCGs, with molecular gas reservoirs in the range $M_{H_2}=2times10^{10-11} M_odot$. For the remaining 13 BCGs we set robust upper limits of $M_{H_2}/M_starlesssim0.1$, which are among the lowest molecular gas to stellar mass ratios found for distant ellipticals and BCGs. By comparison with distant cluster galaxies observed in CO our study shows that RX1532 ($M_{H_2}/M_star = 0.40pm0.05$) belongs to the rare population of star forming and gas-rich BCGs in the distant universe. By using available X-ray based estimates of the central intra-cluster medium entropy, we show that the detection of large reservoirs of molecular gas $M_{H_2}gtrsim10^{10}~M_odot$ in distant BCGs is possible when the two conditions are met: i) high SFR and ii) low central entropy, which favors the condensation and the inflow of gas onto the BCGs themselves, similarly to what has been previously found for some local BCGs.
Understanding the formation history of brightest cluster galaxies is an important topic in galaxy formation. Utilizing the Planck Sunyaev-Zeldovich cluster catalog, and applying the Ansatz that the most massive halos at one redshift remain among the most massive ones at a slightly later cosmic epoch, we have constructed cluster samples at redshift z~0.4 and z~0.2 that can be statistically regarded as progenitor-descendant pairs. This allows us to study the stellar mass assembly history of BCGs in these massive clusters at late times, finding the degree of growth between the two epochs is likely at only few percent level, which is far lower compared to the prediction from a state-of-the-art semi-analytic galaxy formation model.
We explore the persistence of the alignment of brightest cluster galaxies (BCGs) with their local environment. We find that a significant fraction of BCGs do not coincide with the centroid of the X-ray gas distribution and/or show peculiar velocities (they are not at rest with respect to the cluster mean). Despite this, we find that BCGs are generally aligned with the cluster mass distribution even when they have significant offsets from the X-ray centre and significant peculiar velocities. The large offsets are not consistent with simple theoretical models. To account for these observations BCGs must undergo mergers preferentially along their major axis, the main infall direction. Such BCGs may be oscillating within the cluster potential after having been displaced by mergers or collisions, or the dark matter halo itself may not yet be relaxed.
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