The Hydra I cluster core

Abstract

Context. The history of the mass assembly of brightest cluster galaxies may be studied by mapping the stellar populations at large radial distances from the galaxy centre, where the dynamical times are long and preserve the chemodynamical signatures of the accretion events. Aims. We provide extended and robust measurements of the stellar population parameters in NGC 3311, the cD galaxy at the centre of the Hydra I cluster, and out to three effective radii. We wish to characterize the processes that drove the build-up of the stellar light at all these radii. Methods. We obtained the spectra from several regions in NGC 3311 covering an area of ∼3 arcmin 2 in the wavelength range 4800 λ(Å) 5800, using the FORS2 spectrograph at the Very Large Telescope in the Mask exchange unit (MXU) mode. We measured the equivalent widths of seven absorption-features defined in the Lick/IDS system, which were modelled by single stellar populations, to provide luminosity-weighted ages, metallicities, and alpha element abundances. Results. The trends in the Lick indices and the distribution of the stellar population parameters indicate that the stars of NGC 3311 may be divided in two radial regimes, one within and the another beyond one effective radius, R e = 8.4 kpc, similar to the distinction between the inner galaxy and the external halo derived from the NGC 3311 velocity dispersion profile. The inner galaxy (R ≤ R e ) is old (age ∼14 Gyr), has negative metallicity gradients and positive alpha element gradients. The external halo is also very old, but has a negative age gradient. The metal and element abundances of the external halo both have a large scatter, indicating that stars from a variety of satellites with different masses have been accreted. The region in the extended halo associated with the off-centred envelope at 0• < PA < 90• has higher metallicity with respect to the symmetric external halo. Conclusions. The different stellar populations in the inner galaxy and extended halo reflect the dominance of in situ stars in the former and the accreted origin for the large majority of the stars in the latter. The low value of the velocity dispersion in the inner galaxy indicates that its stars are bound to the galaxy's gravitational potential, and the abundances and gradients suggest that the inner galaxy is formed in an outside-in scenario of merging gas-rich lumps, reminiscent of the first phase of galaxy formation. The external halo has a higher velocity dispersion, it is dynamically hotter than the galaxy and its stars are gravitationally driven by the cluster's gravitational potential. The stars in the external halo were removed from their parent galaxies, either disks with truncated star formation, or the outer regions of early-type galaxies. Late mass accretion at large radii is now coming from the tidal stripping of stars from dwarfs and S0 galaxies. These results provide supporting evidence for the recent theoretical models of formation of massive ellipticals as a two-phase process.

Detection of the Stellar Intracluster Medium in Perseus (Abell 426)

“…Consistent with its probable origin by stripped stars from all types of cluster galaxies, population synthesis shows that the ICL contains large numbers of old, relatively metal-rich stars such as are in giant ellipticals, but also a significant presence of younger, more metal-poor stars from smaller galaxies (Coccato et al 2011;Melnick et al 2012;DeMaio et al 2015;Edwards et al 2016;Barbosa et al 2016). Assessing the true fraction of the ICL component relative to the individual galaxies is a continuing and difficult issue since it requires careful separation of the ICL profile from the giant galaxies in the cluster, but is almost certainly in the range 10-50% for rich clusters and even groups (Gonzalez et al 2005; Da Rocha & Mendes de Oliveira 2005; Seigar et al 2007;Rudick et al 2011;Giallongo et al 2014;Jiménez-Teja & Dupke 2016).…”

Massive galaxies on the road to quenching: ALMA observations of powerful high redshift radio galaxies

“…Beyond just the necessary association of low mass galaxies, there are a number of lines of evidence that support the notion that massive early-type galaxies grew substantially through minor mergers. Some of these are: (1) the size evolution of massive galaxies in the early Universe to the present may be driven principally through minor mergers (e.g., Daddi et al 2005;van Dokkum et al 2008;Delaye et al 2014;Vulcani et al 2016;Hill et al 2017); (2) the change in the mass and luminosity function of galaxies with redshift and as a function of environment (e.g., Ilbert et al 2013;Sarron et al 2017); (3) the elemental abundance ratios, abundance gradients, and age gradients in the outer regions of local massive spheroids are consistent with accreting galaxies with a range of masses, perhaps predominately low mass, which had their star formation truncated early in their growth (Huang et al 2013;Greene et al 2013;Barbosa et al 2016); and (4) the mass of massive early-type galaxies grew by about a factor of four over the last ∼10 Gyr (e.g., van Dokkum et al 2010;Ilbert et al 2013).…”

The Burrell Schmidt Deep Virgo Survey: Tidal Debris, Galaxy Halos, and Diffuse Intracluster Light in the Virgo Cluster

“…Furthermore, connecting these measurements of the intracluster light distribution back to simulations of the stripped stellar mass in clusters requires additional knowledge of the stellar populations involved. Much recent progress has been made mapping out the kinematics of intracluster PNe to trace the ICL separately from the galaxy light, although mapping this back to a stellar ICL fraction requires knowledge of the underlying stellar populations in the ICL (Ventimiglia et al 2011, Longobardi et al 2015b, Barbosa et al 2016). All these complications make a unique determination of the ICL fraction extraordinarily difficult (see, for example, the extensive discussions in Puchwein et al 2010, Rudick et al 2011.…”