ALBERT

Description: We compare X-ray and caustic mass profiles for a sample of 16 massive galaxy clusters. We assume hydrostatic equilibrium in interpreting the X-ray data, and use large samples of cluster members with redshifts as a basis for applying the caustic technique. The hydrostatic and caustic masses agree to better than 20 per cent on average across the radial range covered by both techniques (~[0.2–1.25] R 500 ). The mass profiles were measured independently and do not assume a common functional form. Previous studies suggest that, at R 500 , the hydrostatic and caustic masses are biased low and high, respectively. We find that the ratio of hydrostatic to caustic mass at R 500 is $1.20^{+0.13}_{-0.11}$ ; thus it is larger than 0.9 at 3 and the combination of under- and overestimation of the mass by these two techniques is 10 per cent at most. There is no indication of any dependence of the mass ratio on the X-ray morphology of the clusters, indicating that the hydrostatic masses are not strongly systematically affected by the dynamical state of the clusters. Overall, our results favour a small value of the so-called hydrostatic bias due to non-thermal pressure sources.

Description: The Andromeda galaxy is observed to have a system of two large dwarf ellipticals and ~13 smaller satellite galaxies that are currently corotating in a thin plane, in addition to 2 counter-rotating satellite galaxies. We explored the consistency of those observations with a scenario where the majority of the corotating satellite galaxies originated from a subhalo group, where NGC 205 was the host and the satellite galaxies occupied dark matter sub-subhaloes. We ran N -body simulations of a close encounter between NGC 205 and M31. In the simulations, NGC 205 was surrounded by massless particles to statistically sample the distribution of the sub-subhaloes expected in a subhalo that has a mass similar to NGC 205. We made Monte Carlo samplings and found that, using a set of reference parameters, the probability of producing a thinner distribution of sub-subhaloes than the observed NGC 205 + 15 smaller satellites (thus including the two counter-rotators, but excluding M32) increased from 〈10 –8 for the initial distribution to ~10 –2 at pericentre. The probability of the simulated sub-subhaloes occupying the locations of the observed corotating satellites in the line-of-sight velocity versus projected on-sky distance plane is at most 2 x 10 –3 for 11 out of 13 satellites. Increasing the mass of M31 and the extent of the initial distribution of sub-subhaloes gives a maximum probability of 4 x 10 –3 for all 13 corotating satellites, but the probability of producing the thinness would drop to ~10 –3 .

Description: We present the study of the colour–magnitude diagram of the cluster Abell 2151 (A 2151), with a particular focus on the low-mass end. The deep spectroscopy with AF2/WYFFOS@WHT and the caustic method enable us to obtain 360 members within 1.3 R 200 and absolute magnitude $M_r \lesssim M_r^{\ast }+6$ . This nearby cluster shows a well defined red sequence up to M r  ~ –18.5; at fainter magnitudes only 36 per cent of the galaxies lie on the extrapolation of the red sequence. We compare the red sequences of A 2151 and Abell 85, which is another nearby cluster with similar spectroscopic data, but with different mass and dynamical state. Both clusters show similar red sequences at the bright end ( M r ≤ –19.5), whereas large differences appear at the faint end. This result suggests that the reddening of bright galaxies is independent of environment, unlike the dwarf population ( M r ≥ –18.0).

Description: This article explores the agreement between the predictions of modified Newtonian dynamics (MOND) and the rotation curves and stellar velocity dispersion profiles measured by the DiskMass Survey (DMS). A bulge–disk decomposition was made for each of the thirty published galaxies, and a MOND Poisson solver was used to simultaneously compute, from the baryonic mass distributions, model rotation curves and vertical velocity dispersion profiles, which were compared to the measured values. The two main free parameters, the stellar disk's mass-to-light ratio ( M / L ) and its exponential scaleheight ( h z ), were estimated by Markov Chain Monte Carlo modelling. The average best-fitting K -band stellar mass-to-light ratio was M / L ~= 0.55 ± 0.15. However, to match the DMS data, the vertical scaleheights would have to be in the range h z  = 200–400 pc which is a factor of 2 lower than those derived from observations of edge-on galaxies with a similar scalelength. The reason is that modified gravity versions of MOND characteristically require a larger M / L to fit the rotation curve in the absence of dark matter and therefore predict a stronger vertical gravitational field than Newtonian models. It was found that changing the MOND acceleration parameter, the shape of the velocity dispersion ellipsoid, the adopted vertical distribution of stars, as well as the galaxy inclination, within any realistic range, all had little impact on these results.

Description: The classical dwarf spheroidals (dSphs) provide a critical test for Modified Newtonian Dynamics (MOND) because they are observable satellite galactic systems with low internal accelerations and low, but periodically varying, external acceleration. This varying external gravitational field is not commonly found acting on systems with low internal acceleration. Using Jeans modelling, Carina in particular has been demonstrated to require a V -band mass-to-light ratio greater than 5, which is the nominal upper limit for an ancient stellar population. We run MOND N -body simulations of a Carina-like dSph orbiting the Milky Way to test if dSphs in MOND are stable to tidal forces over the Hubble time and if those same tidal forces artificially inflate their velocity dispersions and therefore their apparent mass-to-light ratio. We run many simulations with various initial total masses for Carina and Galactocentric orbits (consistent with proper motions), and compare the simulation line-of-sight velocity dispersions (losVDs) with the observed losVDs of Walker et al. We find that the dSphs are stable, but that the tidal forces are not conducive to artificially inflating the losVDs. Furthermore, the range of mass-to-light ratios that best reproduces the observed losVDs of Carina is 5.3 to 5.7 and circular orbits are preferred to plunging orbits. Therefore, some tension still exists between the required mass-to-light ratio for the Carina dSph in MOND and those expected from stellar population synthesis models. It remains to be seen whether a careful treatment of the binary population or triaxiality might reduce this tension.

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 use our Modified Newtonian Dynamics (MOND) cosmological particle-mesh N -body code to investigate the feasibility of structure formation in a framework involving MOND and light sterile neutrinos in the mass range 11–300 eV: always assuming that $\Omega _{\nu _s}=0.225$ for H 0  = 72 km s –1 Mpc –1 . We run a suite of simulations with variants on the expansion history, cosmological variation of the MOND acceleration constant, different normalizations of the power spectrum of the initial perturbations and interpolating functions. Using various box sizes, but typically with ones of length 256 Mpc h –1 , we compare our simulated halo mass functions with observed cluster mass functions and show that (i) the sterile neutrino mass must be larger than 30 eV to account for the low-mass ( M 200  〈 10 14.6 M ) clusters of galaxies in MOND and (ii) regardless of sterile neutrino mass or any of the variations we mentioned above, it is not possible to form the correct number of high-mass ( M 200  〉 10 15.1 M ) clusters of galaxies: there is always a considerable over production. This means that the ansatz of considering the weak-field limit of MOND together with a component of light sterile neutrinos to form structure from z  ~ 200 fails. If MOND is the correct description of weak-field gravitational dynamics, it could mean that subtle effects of the additional fields in covariant theories of MOND render the ansatz inaccurate, or that the gravity generated by light sterile neutrinos (or by similar hot dark matter particles) is different from that generated by the baryons.

Description: Weak gravitational lensing by galaxy clusters on faint higher redshift galaxies has been traditionally used to study the cluster mass distribution and as a tool to identify clusters as peaks in the shear maps. However, it becomes soon clear that peak statistics can also be used as a way to constrain the underlying cosmological model due to its dependence on both the cosmic expansion rate and the growth rate of structures. This feature makes peak statistics particularly interesting from the point of view of discriminating between General Relativity and modified gravity. Here we consider a general class of f ( R ) theories and compute the observable mass function based on the aperture mass statistics. We complement our theoretical analysis with a Fisher matrix forecast of the constraints that an Euclid-like survey can impose on the f ( R ) model parameters. We show that peak statistics alone can in principle discriminate between General Relativity and f ( R ) models and strongly constrain the f ( R ) parameters that are sensitive to the non-linear growth of structure. However, further analysis is needed in order to include possible selection function in the peaks redshift determination.

Description: This work is focused on the study of the distribution in the Cluster Projected Phase-Space (CPPS) of passive(ly-evolving) and star-forming galaxy populations and also, the intense and quiescent star-forming populations for a set of nine nearby z  〈 0.05 galaxy clusters. Furthermore, we compare the CPPS distribution of the passive galaxy population with the accreted halo population of a set of 28 simulated clusters and the star-forming population with the non-accreted population. We consider various cluster accretion epochs and accretion radii, where it is assumed that star formation in galaxies becomes quenched, in order to segregate the accreted population from the non-accreted population. Just applying this segregation in simulations, we get a qualitative agreement between the CPPS distributions of the passive and the accreted populations and also between the star-forming and the non-accreted populations. The uncertainty in cluster centring strongly affects the pronounced cuspy profiles of the projected density and also, it can explain the main difference (i.e. inner slope) between the CPPS distribution of passive and accreted populations. The CPPS density of star-forming galaxies and the intensity of ram-pressure stripping present an opposite trend throughout the CPPS. This implies that ram-pressure stripping significantly contributes to modulate the observed CPPS distribution of star-forming galaxies in cluster virial regions and their surroundings. The significant fraction of star-forming galaxies at the projected centre of clusters are mainly those galaxies with low line-of-sight velocities and they can be mainly identified as those galaxies with a remaining star formation activity (quiescent star-forming galaxies) inside the physical virial region or, in a lower degree, as galaxy interlopers, i.e. outside the physical virial region. This paper also includes a test of the effects caused by the Sloan fibre collision on the completeness of the Main Galaxy Sample as a function of cluster-centric radius.