Article

A large population of recently-quenched red-sequence dwarf galaxies in the outskirts of the Coma Cluster

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 03/2008; 386(1). DOI: 10.1111/j.1745-3933.2008.00469.x
Source: arXiv

ABSTRACT

We analyse the stellar populations of 75 red-sequence dwarf galaxies in the Coma cluster, based on high signal-to-noise ratio
spectroscopy from the 6.5-m MMT. The sample covers a luminosity range 3–4 mag below M★, in the cluster core and in a field centred 1° to the south-west. We find a strong dependence of the absorption-line strengths
with location in the cluster. Galaxies further from the cluster centre have stronger Balmer lines than inner-field galaxies
of the same luminosity. The magnesium lines are weaker at large radius, while the iron lines are not correlated with radius.
Converting the line strengths into estimates of stellar age, metallicity and abundance ratios, we find the gradients are driven
by variations in age (>6σ significance) and in the iron abundance Fe/H (∼2.7σ significance). The light element (Mg, C, N,
Ca) abundances are almost independent of radius. At radius of times the virial radius), dwarf galaxies have ages ∼3.8 Gyr on average, compared to ∼6 Gyr near the cluster centre. The outer
dwarfs are also ∼50 per cent more iron-enriched, at given luminosity. Our results confirm earlier indications that the ages
of red-sequence galaxies depend on location within clusters, and in Coma in particular. The very strong trends found here
suggest that dwarf galaxies are especially susceptible to environmental ‘quenching’, and/or that the south-west part of Coma
is particularly a clear example of recent quenching in an infalling subcluster.

Download full-text

Full-text

Available from: Terry Bridges, Dec 10, 2012
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Aims: The present work is aimed at studying the distribution of galaxies of different types and luminosities along different structural scaling relations to see whether massive and dwarf ellipticals have been shaped by the same formation process. Methods: This exercise is done by comparing the distribution of Virgo cluster massive and dwarf ellipticals and star forming galaxies along the B band effective surface brightness and effective radius vs. absolute magnitude relations and the Kormendy relation to the predictions of models tracing the effects of ram-pressure stripping on disc galaxies entering the cluster environment and galaxy harassment. Results: Dwarf ellipticals might have been formed from low luminosity, late-type spirals that recently entered into the cluster and lost their gas because of a ram-pressure stripping event, stopping their star formation. The perturbations induced by the abrupt decrease of the star formation activity are sufficient to modify the structural properties of disc galaxies into those of dwarf ellipticals. Galaxy harassment induces a truncation of the disc and generally an increase of the effective surface brightness of the perturbed galaxies. The lack of dynamical simulations of perturbed galaxies spanning a wide range in luminosity prevents us from deriving any firm conclusion on a possible harassment-induced origin of the low surface brightness dwarf elliptical galaxy population inhabiting the Virgo cluster. Conclusions: Although the observed scaling relations are consistent with the idea that the distribution of elliptical galaxies along the mentioned scaling relation is due to a gradual variation with luminosity of the Sersic index n, the comparison with models indicates that dwarf ellipticals might have been formed by a totally different process to giant ellipticals.
    Preview · Article · Oct 2008 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present line-strength measurements for 74 early-type galaxies in the core of the Coma cluster reaching down to velocity dispersions, sigma, of 30 km/s. The index-sigma relations for our sample, including galaxies with sigma<100 km/s (low-sigma), differ in shape depending on which index is used. We notice two types of relations for the metallic indices: one showing a break in the slope around ~100 km/s, and another group with strong linear relations between an index and log sigma. We find no connection between the behavior of index-sigma relations with either alpha- or Fe-peak elements. However, we find indications that the relations are tighter for indices which do not depend on the micro-turbulent velocities of stellar atmospheres. We confirm previous results that low-sigma galaxies including dE/dS0s are on average younger, less metal rich, and have lower [alpha/Fe] in comparison to E/S0s. Our data show that these trends derived for high-sigma galaxies extend down to dE/dS0s. This is a factor of ~2 lower in sigma than previously published work. We confirm that the observed anti-correlation between age and metallicity for high-sigma galaxies is consistent with the effects of correlated errors. Low-sigma galaxies also show a similar relation between age and metallicity as a result of correlated errors. However, they are offset from this relationship so that, on average, they are less metal rich and younger than their high-sigma counterparts.
    Preview · Article · Oct 2008 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Early - type galaxies (ETGs) define a narrow strip in the size - mass plane because of the observed correlation between the effective radius $R_{eff}$ and the total stellar mass $M_{\star}$. When expressed in logarithmic units, a linear relation, $\log{R_{eff}} \propto \gamma \log{M_{\star}}$, is indeed observationally found, but the slope $\gamma$ deviates from the canonical $\gamma = 1/2$ value which can be naively predicted for a spherically symmetric isotropic system. We propose here that a transfer of angular momentum to the stellar component induces an anisotropy in the velocity space thus leading to a modified distribution function (DF). Assuming an Osipkov - Merritt like anisotropic DF, we derive an analytic relation between the slope $\gamma$ of the size - mass relation and the slope $\alpha$ of the angular momentum term in the DF. With this simple model, we are then able to recover the observed $\gamma$ value provided $\alpha$ is suitably set. It turns out that an anisotropy profile which is tangential inside $\sim 0.6 r_a$ and radial outside, with $r_a$ the anisotropy radius, is able to reproduce the observed size - mass relation observed for massive ($M_{\star} \ge 2 \times 10^{10} \ h^{-1} {\rm M}_{\odot}$) elliptical galaxies. Comment: 9 pages, 2 figures, submitted to MNRAS
    Full-text · Article · Jun 2009 · Monthly Notices of the Royal Astronomical Society
Show more