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Register a new userDeep spectroscopy in nearby galaxy clusters: III Orbital structure of galaxies in Abell 85
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Jose Alfonso Lopez Aguerri
Publication date
2017
fields
Physics
and research's language is
English
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Galaxies in clusters are strongly affected by their environment. They evolve according to several physical mechanisms that are active in clusters. Their efficiency can strongly depend on the orbital configuration of the galaxies. Our aim is to analyse the orbits of the galaxies in the cluster Abell 85, based on the study of the galaxy velocity anisotropy parameter. We have solved the Jeans equation under the assumption that the galaxies in A85 are collisionless objects, within the spherically symmetric gravitational potential of the virialized cluster. The mass of the cluster was estimated with X-ray and caustic analyses. We find that the anisotropy profile of the full galaxy population in A85 is an increasing monotonic function of the distance from the cluster centre: on average, galaxies in the central region (r/r200 < 0.3) are on isotropic orbits, while galaxies in the outer regions are on radial orbits. We also find that the orbital properties of the galaxies strongly depend on their stellar colour. In particular, blue galaxies are on less radial orbits than red galaxies. The different families of cluster galaxies considered here have the pseudo phase-space density profiles Q(r) and Qr(r) consistent with the profiles expected in virialized dark matter halos in $N$-body simulations. This result suggests that the galaxies in A85 have reached dynamical equilibrium within the cluster potential. Our results indicate that the origin of the blue and red colour of the different galaxy populations is the different orbital shape rather than the accretion time.
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We present a new deep spectroscopic catalogue for Abell 85, within 3.0 $times$ 2.6 Mpc$^2$ and down to $M_{r} sim M_{r}^* +6$. Using the Visible Multi-Object Spectrograph at the Very Large Telescope (VIMOS@VLT) and the AutoFiber 2 at the William Herschel Telescope (AF2@WHT), we obtained almost 1,430 new redshifts for galaxies with $m_r leq 21$ mag and $langle mu_{e,r} rangle leq 24$ mag arcsec$^{-2}$. These redshifts, together with SDSS-DR6 and NED spectroscopic information, result in 460 confirmed cluster members. This dataset 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 ($M_{r} leq -21.5$) and faint ($M_{r}geq -18.0$) ends are independent of clustercentric distance and similar to the field LFs unlike the intermediate luminosity range ($-21.5 leq M_{r} leq -18.0$). 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.
Our aim is to understand the role of the environment in the quenching of star formation of galaxies located in the infall cluster region of Abell 85 (A85). This is achieved by studying the post-starburst galaxy population as tracer of recent quenching. By measuring the equivalent width (EW) of the [OII] and Hdelta spectral lines, we classify the galaxies in three groups: passive (PAS), emission line (EL), and post-starburst (PSB) galaxies. The PSB galaxy population represents about 4.5% of the full sample. Dwarf galaxies (Mr > -18.0) account for about 70 - 80% of PSBs, which indicates that most of the galaxies undergoing recent quenching are low-mass objects. Independently of the environment, PSB galaxies are disk-like objects with g - r colour between the blue ELs and the red PAS ones. The PSB and EL galaxies in low-density environments show similar luminosities and local galaxy densities. The dynamics and local galaxy density of the PSB population in high density environments are shared with PAS galaxies. However, PSB galaxies inside A85 are at shorter clustercentric radius than PAS and EL ones. The value of the EW(Hdelta) is larger for those PSBs closer to the cluster centre. We propose two different physical mechanisms producing PSB galaxies depending on the environment. In low density environments, gas-rich minor mergers or accretions could produce the PSB galaxies. For high density environments like A85, PSBs would be produced by the removal of the gas reservoirs of EL galaxies by ram-pressure stripping when they pass near to the cluster centre.
We carried out the deep spectroscopic observations of the nearby cluster A2151 with AF2/WYFFOS@WHT. The caustic technique enables us to identify 360 members brighter than $M_r = -16$ and within 1.3$R_{200}$. We separated the members into subsamples according to photometrical and dynamical properties such as colour, local environment and infall time. The completeness of the catalogue and our large sample allow us to analyse the velocity dispersion and the luminosity functions of the identified populations. We found evidence of a cluster still in its collapsing phase. The LF of the red population of A2151 shows a deficit of dwarf red galaxies. Moreover, the normalized LFs of the red and blue populations of A2151 are comparable to the red and blue LFs of the field, even if the blue galaxies start dominating one magnitude fainter and the red LF is well represented by a single Schechter function rather than a double Schechter function. We discuss how the evolution of cluster galaxies depends on their mass: bright and intermediate galaxies are mainly affected by dynamical friction and internal/mass quenching, while the evolution of dwarfs is driven by environmental processes which need time and a hostile cluster environment to remove the gas reservoirs and halt the star formation.
For the first time, we explore the dynamics of the central region of a galaxy cluster within $r_{500}sim 600h^{-1}$~kpc from its center by combining optical and X-ray spectroscopy. We use (1) the caustic technique that identifies the cluster substructures and their galaxy members with optical spectroscopic data, and (2) the X-ray redshift fitting procedure that estimates the redshift distribution of the intracluster medium (ICM). We use the spatial and redshift distributions of the galaxies and of the X-ray emitting gas to associate the optical substructures to the X-ray regions. When we apply this approach to Abell 85 (A85), a complex dynamical structure of A85 emerges from our analysis: a galaxy group, with redshift $z=0.0509 pm 0.0021$ is passing through the cluster center along the line of sight dragging part of the ICM present in the cluster core; two additional groups, at redshift $z=0.0547 pm 0.0022$ and $z=0.0570 pm 0.0020$, are going through the cluster in opposite directions, almost perpendicularly to the line of sight, and have substantially perturbed the dynamics of the ICM. An additional group in the outskirts of A85, at redshift $z=0.0561 pm 0.0023$, is associated to a secondary peak of the X-ray emission, at redshift $z=0.0583^{+0.0039}_{-0.0047}$. Although our analysis and results on A85 need to be confirmed by high-resolution spectroscopy, they demonstrate how our new approach can be a powerful tool to constrain the formation history of galaxy clusters by unveiling their central and surrounding structures.
We present the first results of a new spectroscopic survey of the cluster Abell 85 targeting 1466 candidate cluster members within the central $sim$1 deg$^2$ of the cluster and having magnitudes $m_r < 20.5$ using VIMOS/VLT and HYDRA/WIYN. A total of 520 galaxies are confirmed as either relaxed cluster members or part of an infalling population. A significant fraction are low mass; the median stellar mass of the sample is $10^{9.6} M_{odot} $, and 25% have stellar masses below $10^9 M_{odot}$ (i.e. 133 dwarf galaxies). We also identify seven active galactic nuclei (AGN), four of which reside in dwarf host galaxies. We probe the evolution of star formation rates, based on H$alpha$ emission and continuum modeling, as a function of both mass and environment. We find that more star forming galaxies are observed at larger clustercentric distances, while infalling galaxies show evidence for recently enhanced star forming activity. Main sequence galaxies, defined by their continuum star formation rates, show different evolutionary behavior based on their mass. At the low mass end, the galaxies have had their star formation recently quenched, while more massive galaxies show no significant change. The timescales probed here favor fast quenching mechanisms, such as ram-pressure stripping. Galaxies within the green valley, defined similarly, do not show evidence of quenching. Instead, the low mass galaxies maintain their levels of star forming activity, while the more massive galaxies have experienced a recent burst.
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