143 research outputs found
Caught in the act: cluster 'k+a' galaxies as a link between spirals and S0s
We use integral field spectroscopy of 13 disc galaxies in the cluster AC114 at z ∼ 0.31 in an attempt to disentangle the physical processes responsible for the transformation of spiral galaxies in clusters. Our sample is selected to display a dominant young stellar population, as indicated by strong Hδ absorption lines in their integrated spectra. Most of our galaxies lack the [O ii]λ3727 emission line, and hence ongoing star formation. They therefore possess ‘k+a’ spectra, indicative of a recent truncation of star formation, possibly preceded by a starburst. Discy ‘k+a’ galaxies are a promising candidate for the intermediate stage of the transformation from star-forming spiral galaxies to passive S0s. Our observations allow us to study the spatial distributions and the kinematics of the different stellar populations within the galaxies. We used three different indicators to evaluate the presence of a young population: the equivalent width of Hδ, the luminosity-weighted fraction of A stars, and the fraction of the galaxy light attributable to simple stellar populations with ages between 0.5 and 1.5 Gyr. We find a mixture of behaviours, but are able to show that in most of the galaxies the last episode of star formation occurred in an extended disc, similar to preceding generations of stars, though somewhat more centrally concentrated. We thus exclude nuclear starbursts and violent gravitational interactions as causes of the star formation truncation. Gentler mechanisms, such as ram-pressure stripping or weak galaxy–galaxy interactions, appear to be responsible for ending star formation in these intermediate-redshift cluster disc galaxies
Black hole growth and host galaxy morphology
We use data from large surveys of the local Universe (SDSS+Galaxy Zoo) to
show that the galaxy-black hole connection is linked to host morphology at a
fundamental level. The fraction of early-type galaxies with actively growing
black holes, and therefore the AGN duty cycle, declines significantly with
increasing black hole mass. Late-type galaxies exhibit the opposite trend: the
fraction of actively growing black holes increases with black hole mass.Comment: 4 pages, 2 figures. Proceedings of the IAU Symposium no. 267,
"Co-Evolution of Central Black Holes and Galaxies: Feeding and Feedback",
eds. B.M. Peterson, R.S. Somerville and T. Storchi-Bergman
Galaxy Zoo: Disentangling the Environmental Dependence of Morphology and Colour
We analyze the environmental dependence of galaxy morphology and colour with
two-point clustering statistics, using data from the Galaxy Zoo, the largest
sample of visually classified morphologies yet compiled, extracted from the
Sloan Digital Sky Survey. We present two-point correlation functions of spiral
and early-type galaxies, and we quantify the correlation between morphology and
environment with marked correlation functions. These yield clear and precise
environmental trends across a wide range of scales, analogous to similar
measurements with galaxy colours, indicating that the Galaxy Zoo
classifications themselves are very precise. We measure morphology marked
correlation functions at fixed colour and find that they are relatively weak,
with the only residual correlation being that of red galaxies at small scales,
indicating a morphology gradient within haloes for red galaxies. At fixed
morphology, we find that the environmental dependence of colour remains strong,
and these correlations remain for fixed morphology \textit{and} luminosity. An
implication of this is that much of the morphology--density relation is due to
the relation between colour and density. Our results also have implications for
galaxy evolution: the morphological transformation of galaxies is usually
accompanied by a colour transformation, but not necessarily vice versa. A
spiral galaxy may move onto the red sequence of the colour-magnitude diagram
without quickly becoming an early-type. We analyze the significant population
of red spiral galaxies, and present evidence that they tend to be located in
moderately dense environments and are often satellite galaxies in the outskirts
of haloes. Finally, we combine our results to argue that central and satellite
galaxies tend to follow different evolutionary paths.Comment: 19 pages, 18 figures. Accepted for publication in MNRA
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