ALBERT

Description: We present the first pointed X-ray observations of 10 candidate fossil galaxy groups and clusters. With these Suzaku observations, we determine global temperatures and bolometric X-ray luminosities of the intracluster medium (ICM) out to r 500 for six systems in our sample. The remaining four systems show signs of significant contamination from non-ICM sources. For the six objects with successfully determined r 500 properties, we measure global temperatures in the range 2.8 ≤  T X  ≤ 5.3 keV, bolometric X-ray luminosities of 0.8  x  10 44  ≤  L X, bol  ≤ 7.7  x  10 44 erg s –1 , and estimate masses, as derived from T X , of M 500 10 14 M . Fossil cluster scaling relations are constructed for a sample that combines our Suzaku observed fossils with fossils in the literature. Using measurements of global X-ray luminosity, temperature, optical luminosity, and velocity dispersion, scaling relations for the fossil sample are then compared with a control sample of non-fossil systems. We find the fits of our fossil cluster scaling relations are consistent with the relations for normal groups and clusters, indicating fossil clusters have global ICM X-ray properties similar to those of comparable mass non-fossil systems.

Description: Galaxy clusters contain a large population of low-mass dwarf elliptical galaxies whose exact origin is unclear: their colours, structural properties and kinematics differ substantially from those of dwarf irregulars in the field. We use the Illustris cosmological simulation to study differences in the assembly histories of dwarf galaxies (3 x 10 8 〈 M * /M 〈 10 10 ) according to their environment. We find that cluster dwarfs achieve their maximum total and stellar mass on average ~8 and ~4.5 Gyr ago (or redshifts z  = 1.0 and 0.4, respectively), around the time of infall into the clusters. In contrast, field dwarfs not subjected to environmental stripping reach their maximum mass at z  = 0. These different assembly trajectories naturally produce a colour bimodality, with blue isolated dwarfs and redder cluster dwarfs exhibiting negligible star formation today. The cessation of star formation happens over median times 3.5–5 Gyr depending on stellar mass, and shows a large scatter (~1–8 Gyr), with the lower values associated with starburst events that occur at infall through the virial radius or pericentric passages. We argue that such starbursts together with the early assembly of cluster dwarfs can provide a natural explanation for the higher specific frequency of globular clusters (GCs) in cluster dwarfs, as found observationally. We present a simple model for the formation and stripping of GCs that supports this interpretation. The origin of dwarf ellipticals in clusters is, therefore, consistent with an environmentally driven evolution of field dwarf irregulars. However, the z  = 0 field analogues of cluster dwarf progenitors have today stellar masses a factor of ~3 larger – a difference arising from the early truncation of star formation in cluster dwarfs.

Description: We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed.

Description: We analyse the mass–size relation of ~400 quiescent massive ETGs ( M * /M  〉 3 10 10 ) hosted by massive clusters (M 200 ~ 2–7 x 10 14 M ) at 0.8 〈  z  〈 1.5, compared to those found in the field at the same epoch. Size is parametrized using the mass-normalized B -band rest-frame size, $\gamma =R_{\rm e}/M_{11}^{0.57}$ . We find that the distributions in both environments peak at the same position, but the distributions in clusters are more skewed towards larger sizes. This tail induces average sizes ~30–40 per cent larger for cluster galaxies than for field galaxies of similar stellar mass, while the median sizes are statistically the same with a difference of ~10 ± 10 per cent. Since this size difference is not observed in the local Universe, the evolution of average galaxy size at fixed stellar mass from z  ~ 1.5 for cluster galaxies is less steep at more than 3 ((1 +  z ) –0.53 ± 0.04 ) than the evolution of field galaxies ((1 +  z ) –0.92 ± 0.04 ). The difference in evolution is not measured when the median values of are considered: (1 +  z ) –0.84 ± 0.04 in the field versus (1 +  z ) –0.71 ± 0.05 in clusters. In our sample, the tail of large galaxies is dominated by galaxies with 3 10 10  〈 M * /M  〈 10 11 . At this low-mass end, the difference in the average size is better explained by the accretion of new galaxies that are quenched more efficiently in clusters and/or by different morphological mixing in the cluster and field environments. If part of the size evolution would be due to mergers, the difference that we see between cluster and field galaxies could be caused by higher merger rates in clusters at higher redshift, when galaxy velocities are lower.

Description: We present the results of a detailed analysis of galaxy properties along the red sequence in XMMU J1229+0151, an X-ray selected cluster at z  = 0.98 drawn from the High Acuity Wide field K -band Imager Cluster Survey. Taking advantage of the broad photometric coverage and the availability of 77 spectra in the cluster field, we fit synthetic spectral energy distributions, and estimate stellar masses and photometric redshifts, which we use to determine the cluster membership. We investigate morphological and structural properties of red sequence galaxies and find that elliptical galaxies populate the bright end, while S0 galaxies represent the predominant population at intermediate luminosities, with their fraction decreasing at fainter magnitudes. A comparison with the low-redshift sample of the Wide Field Nearby Galaxy-clusters Survey cluster survey reveals that at z  ~ 1 the bright end of the red sequence of XMMU J1229+0151 is richer in S0 galaxies. The faint end of the red sequence in XMMU J1229+0151 appears rich in disc-dominated galaxies, which are rarer in the low-redshift comparison sample at the same luminosities. Despite these differences between the morphological composition of the red sequence in XMMU J1229+0151 and in low-redshift samples, we find that to within the uncertainties, no such difference exists in the ratio of luminous to faint galaxies along the red sequence.

Description: We present a new deep determination of the spectroscopic luminosity function (LF) within the virial radius of the nearby and massive Abell 85 (A85) cluster down to the dwarf regime ( M * + 6) using Very Large Telescope/Visible Multi-Object Spectrograph (VLT/VIMOS) spectra for ~2000 galaxies with m r ≤ 21 mag and 〈μ e , r 〉 ≤ 24 mag arcsec –2 . The resulting LF from 438 cluster members is best modelled by a double Schechter function due to the presence of a statistically significant upturn at the faint end. The amplitude of this upturn ( $\alpha _{{\rm f}} = -1.58^{+0.19}_{-0.15}$ ), however, is much smaller than that of the Sloan Digital Sky Survey (SDSS) composite photometric cluster LF by Popesso et al., α f ~ –2. The faint-end slope of the LF in A85 is consistent, within the uncertainties, with that of the field. The red galaxy population dominates the LF at low luminosities, and is the main factor responsible for the upturn. The fact that the slopes of the spectroscopic LFs in the field and in a cluster as massive as A85 are similar suggests that the cluster environment does not play a major role in determining the abundance of low-mass galaxies.

Description: We present a new deep spectroscopic catalogue for Abell 85, within 3.0 x 2.6 Mpc 2 and down to $M_{r} \sim M_{r}^{\ast } +6$ . Using the Visible Multi-Object Spectrograph at the Very Large Telescope and the AutoFiber 2 at the William Herschel Telescope, we obtained almost 1430 new redshifts for galaxies with m r ≤ 21 mag and 〈μ e , r 〉 ≤ 24 mag arcsec –2 . These redshifts, together with Sloan Digital Sky Survey Data Release 6 and NASA/IPAC Extragaalctic Database spectroscopic information, result in 460 confirmed cluster members. This data set allows the study of the luminosity function (LF) of the cluster galaxies covering three orders of magnitudes in luminosities. The total and radial LFs are best modelled by a double Schechter function. The normalized LFs show that their bright ( M r ≤ –21.5) and faint ( M r ≥ –18.0) ends are independent of clustercentric distance and similar to the field LFs unlike the intermediate luminosity range (–21.5 ≤ M r ≤ –18.0). Similar results are found for the LFs of the dominant types of galaxies: red, passive, virialized and early-infall members. On the contrary, the LFs of blue, star forming, non-virialized and recent-infall galaxies are well described by a single Schechter function. These populations contribute to a small fraction of the galaxy density in the innermost cluster region. However, in the outskirts of the cluster, they have similar densities to red, passive, virialized and early-infall members at the LF faint end. These results confirm a clear dependence of the colour and star formation of Abell 85 members in the cluster centric distance.

Description: We analyse the evolution of the red sequence in a sample of galaxy clusters at redshifts 0.8 〈 z 〈 1.5 taken from the HAWK-I Cluster Survey (HCS). The comparison with the low-redshift (0.04 〈 z 〈 0.08) sample of the WIde-field Nearby Galaxy-cluster Survey (WINGS) and other literature results shows that the slope and intrinsic scatter of the cluster red sequence have undergone little evolution since z = 1.5. We find that the luminous-to-faint ratio and the slope of the faint end of the luminosity distribution of the HCS red sequence are consistent with those measured in WINGS, implying that there is no deficit of red galaxies at magnitudes fainter than $M_V^{\ast }$ at high redshifts. We find that the most massive HCS clusters host a population of bright red sequence galaxies at M V 〈 –22.0 mag, which are not observed in low-mass clusters. Interestingly, we also note the presence of a population of very bright ( M V 〈 –23.0 mag) and massive (log ( M * /M ) 〉 11.5) red sequence galaxies in the WINGS clusters, which do not include only the brightest cluster galaxies and which are not present in the HCS clusters, suggesting that they formed at epochs later than z = 0.8. The comparison with the luminosity distribution of a sample of passive red sequence galaxies drawn from the COSMOS/UltraVISTA field in the photometric redshift range 0.8 〈 z phot 〈 1.5 shows that the red sequence in clusters is more developed at the faint end, suggesting that halo mass plays an important role in setting the time-scales for the build-up of the red sequence.

Description: We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U - and I -band images obtained with Very Large Telescope/Visible Multi-Object Spectrograph. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies. We measure spheroid half-light radii in the range 0.7 r eff /kpc 1.5 with luminosities of –11.65 M U –9.42 and –12.79 M I –10.58 mag, corresponding to a colour range of ( U – I ) 0 ~= 1.1–2.2 mag and surface brightness levels of μ U ~= 28.1 mag arcsec –2 and μ I ~= 27.4 mag arcsec –2 . Their colours and luminosities are consistent with a diverse set of stellar population properties. Assuming solar and 0.02 Z metallicities we obtain stellar masses in the range M * | Z ~= 10 5.7 – 6.3 M and $M_{\ast }|_{0.02\,\mathrm{Z}_{\odot }} \simeq 10^{6.3-8}\,\mathrm{M}_{{\odot }}$ . Three dwarfs are older than 1 Gyr, while the other two significantly bluer dwarfs are younger than ~2 Gyr at any mass/metallicity combination. Altogether, the new LSB dwarf galaxy candidates share properties with dwarf galaxies found throughout the Local Volume and in nearby galaxy clusters such as Fornax. We find a pair of candidates with ~2 kpc projected separation, which may represent one of the closest dwarf galaxy pairs found. We also find a nucleated dwarf candidate, with a nucleus size of r eff ~= 46–63 pc and magnitude M U , 0 = –7.42 mag and ( U – I ) 0 = 1.51 mag, which is consistent with a nuclear stellar disc with a stellar mass in the range 10 4.9 – 6.5 M .

Description: Recent independent results from numerical simulations and observations have shown that brightest cluster galaxies (BCGs) have increased their stellar mass by a factor of almost 2 between z  ~ 0.9 and z  ~ 0.2. The numerical simulations further suggest that more than half this mass is accreted through major mergers. Using a sample of 18 distant galaxy clusters with over 600 spectroscopically confirmed cluster members between them, we search for observational evidence that major mergers do play a significant role. We find a major merger rate of 0.38 ± 0.14 mergers per Gyr at z  ~ 1. While the uncertainties, which stem from the small size of our sample, are relatively large, our rate is consistent with the results that are derived from numerical simulations. If we assume that this rate continues to the present day and that half of the mass of the companion is accreted on to the BCG during these mergers, then we find that this rate can explain the growth in the stellar mass of the BCGs that is observed and predicted by simulations. Major mergers therefore appear to be playing an important role, perhaps even the dominant one, in the build up of stellar mass in these extraordinary galaxies.