Friday, 30 January 2009
Fig 10. Ruhland et al. http://arxiv.org/abs/0901.4340
They present first measurements of the evolution of the scatter of the cosmic average early-type galaxy color-magnitude relation (CMR) from z=1 to the present day, finding that it is consistent with models in which galaxies are constantly being added to the red sequence through truncation of star formation in blue cloud galaxies.
This fig. shows comparison between observations and a model in which new red sequence galaxies are being constantly added at the rate required to match the observed number density evolution The model predicts the correct CMR scatter and its evolution.
From Busha et al. (http://arxiv.org/abs/0901.3553)
The authors use the Via Lactea + a cosmological N-Body simulation to model the effects of reionization on the dwarf galaxy population. Previous studies have claimed that reionization did not reduce the number of dwarf galaxies enough. They find they can account for a number as well as the distribution of satellites. One particular way they differ from other studies is that other studies allowed pre-reionization collapsed objects to form stars with a much higher effeciency, while they claim that these objects would have their cold gas photo-evaporated on a short timescale.
The above figure shows the sub-halo population from the simulations (solid black line) and their inferred dwarf distribution (linestyles). The observations are given by the points and the cyan region.
On the formation of massive galaxies: A simultaneous study of number density, size and intrinsic colour evolution in GOODS
Ignacio Ferreras, Thorsten Lisker, Anna Pasquali, Sadegh Khochfar, Sugata Kaviraj
The evolution of number density, size and intrinsic colour is determined for a volume-limited sample of visually classified early-type galaxies selected from the HST/ACS images of the GOODS North and South fields (version 2). The sample comprises 457 galaxies over 320 arcmin2 with stellar masses above 3E10 Msun in the redshift range 0.4
Friday, 23 January 2009
Previous observations have shown that galaxies in clusters tend to exhibit AGN activity more frequently than field galaxies, and so the cluster environment may play a role in the triggering of AGN. However, the radial distribution of AGN in clusters has been a matter of debate: specifically, there have been claims that AGN activity is more prominent in the centers of clusters, while others do not see this.
The above figure shows the excess number of X-ray point sources (relative to blank fields) as a function of cluster-centric radius in 148 X-ray detected clusters. When the central cluster galaxy is disregarded, no excess of sources is seen in the center. Most of the clusters studied by other groups were included in this sample; the authors confirm the previous results (excess of central AGN) with those specific samples, but conclude that those results were due to sample bias and/or cosmic variance.
Bottom: Fraction of "Red and Dead" galaxies (defined as SFR/stellar mass <10^-11 yr-1) as a function of redshift. Sample is selected to include only galaxies with stellar masses greater than 7x10^10 solar masses. Filled points are data, lines and shaded region refer to theoretical predictions. Upper panel: Fraction of "Quiescent galaxies" (defined as SFR/stellar mass < age of Universe at redshift of galaxy) for the same mass selected sample.
Friday, 16 January 2009
The authors perform a comprehensive analysis of the "missing satellite problem", taking into account realistic selection effects from the SDSS. As the figure above shows, with relatively simple prescriptions for star formation in these halos (e.g. suppression of gas accretion after reionization in halos with v<35 km/s), the data (black bars) actually agree with the fiducial model (green band).
This paper investigates the evolution of the color-magnitude relation
slope in massive galaxy clusters using two samples: LARCS at z~0.1
and MACS at z~0.5. The observed slope evolution is compared to
predictions from the Bower et al. (2006) semianalytic model (taking
into account AGN feedback and "strangulation). The above figure
shows the rest-frame slope evolution, including the data themselves
(points), a fit to the data (solid line), and the Bower et al.
prediction (dashed line). The discrepancy between the models and
observations is attributed to either the shutdown of star formation
in clusters being stronger than the models predict, or possibly
differential chemical evolution (e.g. faint galaxies have
preferentially higher chemical enrichment rates). Oddly, the
observed-frame red sequence slope matches the models very well;
apparently this is because the observed evolution is dominated by the