The Ages of Elliptical Galaxies in a Merger Model

The tightness of the observed colour-magnitude and Mg$_{2}$- velocity dispersion relations for elliptical galaxies has often been cited as an argument against a picture in which ellipticals form by the merging of spiral disks. A common view is that merging would mix together stars of disparate ages and produce a large scatter in these relations. Here I use semi-analytic models of galaxy formation to derive the distribution of the mean ages, colours and metallicities of the stars in elliptical galaxies formed by mergers in a flat CDM universe. It is seen that most of the stars in ellipticals form at relatively high redshift (z > 1.9) and that the predicted scatter in the colour-magnitude and Mg_2 - sigma relations falls within observational bounds. I conclude that the apparent homogeneity in the properties of the stellar populations of ellipticals is not inconsistent with a merger scenario for the origin of these systems.Comment: latex file, figures available upon reques

The Evolving Faint-End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, $\alpha$, using semi-analytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by $\alpha (z) =a+b z$, with a=-1.13 and b=-0.1. The main driver for the evolution in $\alpha$ is the evolution in the underlying dark matter mass function. Sub-L_* galaxies reside in dark matter halos that occupy a different part of the mass function. At high redshifts, this part of the mass function is steeper than at low redshifts and hence $\alpha$ is steeper. Supernova feedback in general causes the same relative flattening with respect to the dark matter mass function. The faint-end slope at low redshifts is dominated by field galaxies and at high redshifts by cluster galaxies. The evolution of $\alpha(z)$ in each of these environments is different, with field galaxies having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from cluster-dominated to field-dominated faint-end slope occurs roughly at a redshift $z_* \sim 2$, and suggests that a single linear fit to the overall evolution of $\alpha(z)$ might not be appropriate. Furthermore, this result indicates that tidal disruption of dwarf galaxies in clusters cannot play a significant role in explaining the evolution of $\alpha(z)$ at z< z_*. In addition we find that different star formation efficiencies a_* in the Schmidt-Kennicutt-law and supernovae-feedback efficiencies $\epsilon$ generally do not strongly influence the evolution of $\alpha(z)$.Comment: 4 pages, replaced with version accepted to ApJL, minor changes to figure

Evolution since z = 0.5 of the Morphology-Density relation for Clusters of Galaxies

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the `morphology-density' relationship, M-D, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the M-D relation is strong in centrally concentrated ``regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low redshift clusters where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters, and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.Comment: 35 pages, 19 figures, uses psfig. Accepted for publication in Ap

Decarboxylation of Carbon Compounds as a Potential Source for CO2 and CO Observed by SAM at Yellowknife Bay, Gale Crater, Mars

Martian carbon was detected in the Sheepbed mudtsone at Yellowknife Bay, Gale Crater, Mars by the Sample Analysis at Mars (SAM) instrument onboard Curiosity, the rover of the Mars Science Laboratory missio]. The carbon was detected as CO2 thermally evolved from drilled and sieved rock powder that was delivered to SAM as a <150-micron-particle- size fraction. Most of the CO2 observed in the Cumberland (CB) drill hole evolved between 150deg and 350deg C. In the John Klein (JK) drill hole, the CO2 evolved up to 500deg C. Hypotheses for the source of the the CO2 include the breakdown of carbonate minerals reacting with HCl released from oxychlorine compounds, combustion of organic matter by O2 thermally evolved from the same oxychlorine minerals, and the decarboxylation of organic molecules indigenous to the martian rock sample. Here we explore the potential for the decarboxylation hypothesis

Example Based Learning for View-Based Human Face Detection

We present an example-based learning approach for locating vertical frontal views of human faces in complex scenes. The technique models the distribution of human face patterns by means of a few view-based "face'' and "non-face'' prototype clusters. At each image location, the local pattern is matched against the distribution-based model, and a trained classifier determines, based on the local difference measurements, whether or not a human face exists at the current image location. We provide an analysis that helps identify the critical components of our system

Formation of Galaxy Clusters

In this review, we describe our current understanding of cluster formation: from the general picture of collapse from initial density fluctuations in an expanding Universe to detailed simulations of cluster formation including the effects of galaxy formation. We outline both the areas in which highly accurate predictions of theoretical models can be obtained and areas where predictions are uncertain due to uncertain physics of galaxy formation and feedback. The former includes the description of the structural properties of the dark matter halos hosting cluster, their mass function and clustering properties. Their study provides a foundation for cosmological applications of clusters and for testing the fundamental assumptions of the standard model of structure formation. The latter includes the description of the total gas and stellar fractions, the thermodynamical and non-thermal processes in the intracluster plasma. Their study serves as a testing ground for galaxy formation models and plasma physics. In this context, we identify a suitable radial range where the observed thermal properties of the intra-cluster plasma exhibit the most regular behavior and thus can be used to define robust observational proxies for the total cluster mass. We put particular emphasis on examining assumptions and limitations of the widely used self-similar model of clusters. Finally, we discuss the formation of clusters in non-standard cosmological models, such as non-Gaussian models for the initial density field and models with modified gravity, along with prospects for testing these alternative scenarios with large cluster surveys in the near future.Comment: 66 pages, 17 figures, review to be published in 2012 Annual Reviews of Astronomy & Astrophysic

Simulation of networks of spiking neurons: A review of tools and strategies

We review different aspects of the simulation of spiking neural networks. We start by reviewing the different types of simulation strategies and algorithms that are currently implemented. We next review the precision of those simulation strategies, in particular in cases where plasticity depends on the exact timing of the spikes. We overview different simulators and simulation environments presently available (restricted to those freely available, open source and documented). For each simulation tool, its advantages and pitfalls are reviewed, with an aim to allow the reader to identify which simulator is appropriate for a given task. Finally, we provide a series of benchmark simulations of different types of networks of spiking neurons, including Hodgkin-Huxley type, integrate-and-fire models, interacting with current-based or conductance-based synapses, using clock-driven or event-driven integration strategies. The same set of models are implemented on the different simulators, and the codes are made available. The ultimate goal of this review is to provide a resource to facilitate identifying the appropriate integration strategy and simulation tool to use for a given modeling problem related to spiking neural networks.Comment: 49 pages, 24 figures, 1 table; review article, Journal of Computational Neuroscience, in press (2007

Evolution in the Disks and Bulges of Group Galaxies since z=0.4

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness--fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Redshift survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disk--dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z=0.4 groups have ~ 5.5 +/- 2 % fewer disk--dominated galaxies than the field, while by z=0.1 this difference has increased to ~ 19 +/- 6 %. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disk population. At both redshifts, the disks of group galaxies have similar scaling relations and show similar median asymmetries as the disks of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge--dominated galaxies is 6 +/- 3 % higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z=0.4 and z=0 using the semi-analytic galaxy catalogues of Bower et al (2006). This model accurately reproduces the B/T distributions of the group and field at z=0.1. However, the model does not reproduce our finding that the deficit of disks in groups has increased significantly since z=0.4.Comment: Accepted for publication in MNRAS. 20 pages, 17 figure

The Evolution of Early-Type Galaxies in Distant Clusters III.: M/L_V Ratios in the z=0.33 Cluster CL1358+62

Keck spectroscopy and Hubble Space Telescope WFPC2 imaging over a 1.5x1.5 Mpc field of CL1358+62 at z=0.33 are used to study the Fundamental Plane of galaxies based on a new, large sample of 53 galaxies. First, we have constructed the Fundamental Plane for the 30 E and S0 galaxies and find that it has the following shape: r_e = sigma**(1.31+-0.13) * _e**(-0.86+-0.10), similar to that found locally. The 1-sigma intrinsic scatter about this plane is 14% in M/L(V), comparable to that observed in Coma. We conclude that these E and S0 galaxies are structurally mature and homogeneous, like those observed in nearby clusters. The M/L(V) ratios of these early-type galaxies are offset from the Coma Fundamental Plane by delta log M/L(V) = -0.13+- 0.03 (q0=0.1), indicative of mild luminosity evolution. This evolution suggests a formation epoch for the stars of z > 1. We have also analyzed the M/L(V) ratios of galaxies of type S0/a and later. These early-type spirals follow a different plane from the E and S0 galaxies, with a scatter that is twice as large as the scatter for the E/S0s. The difference in the tilt between the plane of the spirals and the plane of the E/S0s is shown to be due to a systematic correlation of velocity dispersion with residual from the plane of the early-type galaxies. These residuals also correlate with the residuals from the Color-Magnitude relation. Thus for spirals in clusters, we see a systematic variation in the luminosity-weighted mean properties of the stellar populations with central velocity dispersion. If this is a relative age trend, then luminosity-weighted age is positively correlated with dispersion. [abridged version]Comment: 18 pages, 8 figures; revised version, accepted by ApJ on 13 August 199