Neil Trentham

The evolution of stellar mass and the implied star formation history

We present a compilation of measurements of the stellar mass density as a function of redshift. Using this stellar mass history we obtain a star formation history and compare it to the instantaneous star formation history. For z < 0.7 there is good agreement between the two star formation histories. At higher redshifts the instantaneous indicators suggest star formation rates larger than that implied by the evolution of the stellar mass density. This discrepancy peaks at z= 3 where instantaneous indicators suggest a star formation rate around 0.6 dex higher than those of the best fit to the stellar mass history. We discuss a variety of explanations for this inconsistency, such as inaccurate dust extinction corrections, incorrect measurements of stellar masses and a possible evolution of the stellar initial mass function.

Squelched Galaxies and Dark Halos

There is accumulating evidence that the faint end of the galaxy luminosity function might be very different in different locations. The luminosity function might be rising in rich clusters and flat in regions of low density. If galaxies form according to the model of hierarchical clustering, then there should be many small halos compared to the number of big halos. If this theory is valid, then there must be a mechanism that eliminates at least the visible component of galaxies in low-density regions. A plausible mechanism is photoionization of the intergalactic medium at a time before the epoch that most dwarf galaxies form in low-density regions but after the epoch of formation for similar systems that ultimately end up in rich clusters. The dynamical timescales are found to accommodate this hypothesis in a flat universe with Ωm 0.4. If small halos exist but simply cannot be located because they have never become the sites of significant star formation, they still might have dynamical manifestations. These manifestations are hard to identify in normal groups of galaxies because small halos do not make a significant contribution to the global mass budget. It could be entertained, however, that there are clusters of halos where there are only small systems, clusters that are at the low-mass end of the hierarchical tree. There may be places where only a few small galaxies managed to form, enough for us to identify and use as test probes of the potential. It turns out that such environments might be common. Four probable groups of dwarfs are identified within 5 Mpc, and the assumption that they are gravitationally bound suggests M/LB ~ 300-1200 M☉/L☉, a factor of 6 ± 2 times higher than typical values for groups with luminous galaxies.

The faint end of the galaxy luminosity function

We present and discuss optical measurements of the faint end of the galaxy luminosity function down to M R = -10 in five different local environments of varying galaxy density and morphological content. The environments we studied, in order of decreasing galaxy density, are the Virgo Cluster, the NGC 1407 Group, the Coma I Group, the Leo Group and the NGC 1023 Group. Our results come from a deep wide-angle survey with the National Astronomical Observatories of Japan Subaru 8-m Telescope on Mauna Kea and are sensitive down to very faint surface-brightness levels. Galaxies were identified as group or cluster members on the basis of their surface brightness and morphology. The faintest galaxies in our sample have R ∼ 22.5. There were thousands of fainter galaxies but we cannot distinguish cluster members from background galaxies at these faint limits so do not attempt to determine a luminosity function fainter than M R = -10. In all cases, there are far fewer dwarfs than the numbers of low-mass haloes anticipated by cold dark matter theory. The mean logarithmic slope of the luminosity function between M R = -18 and M R = -10 is α ≃ -1.2, far shallower than the cold dark matter mass function slope of α ≃ -1.8. We would therefore need to be missing about 90 per cent of the dwarfs at the faint end of our sample in all the environments we study to achieve consistency with CDM theory. It is unlikely that such large numbers of dwarfs are missed because (i) the data are deep enough that we are sensitive to very low surface brightness galaxies, and (ii) the seeing is good enough that we can have some confidence in our ability to distinguish high surface brightness dwarfs from background galaxies brighter than R = 22.5. One caveat is that we miss compact members taken to be background galaxies, but such objects (like M32) are thought to be rare.

The luminosity function of the Virgo Cluster from MB=‐22 to −11

We measure the galaxy luminosity function (LF) for the Virgo Cluster between blue magnitudes M B = -22 and -11 from wide-field charge-coupled device (CCD) imaging data. The LF is only gradually rising for -22 < M B < -16.Between M B = -16 and - 14 it rises steeply, with a logarithmic slope of α∼ - 1.6. Fainter than M B = -14, the LF flattens again. This LF is shallower (although turning up at brighter absolute magnitudes) than the R-band LF measured recently by Phillipps et al., who found α ∼ -2.2 to be fainter than M R = -13. It is similar, however, to the LF determined from the Virgo Cluster Catalog by Sandage et al. A few faint galaxies are found that Sandage et al. missed because their surface brightness threshold for detection was too high, but these do not dominate the luminosity function at any magnitude. Most of the faint galaxies we find are dwarf elliptical, alternatively called dwarf spheroidal, galaxies. The most important potential source of systematic error is that we may have rejected some high surface brightness galaxies from the cluster sample because we think that they are background galaxies. This is quite different from what has conventionally been regarded as the most serious source of systematic error in this kind of study: that we are missing many low surface brightness galaxies because they are never visible above the sky. There are ∼ 2.5 times more dwarfs per giant galaxy in Virgo than in the Ursa Major Cluster, a diffuse group of ∼ 80 spiral galaxies at the same distance as Virgo, or the Local Group. The Virgo and Ursa Major Cluster LFs are inconsistent with each other at a high level of significance. These results add weight to the hypothesis that is developing that dwarf galaxies are more common relative to giant galaxies in dense environments than diffuse ones. Both LFs are highly inconsistent with cold dark matter theory, which has been so successful at reproducing observations on large scales. Possible theoretical explanations for this discrepancy, and for the detailed shape of the Virgo Cluster LF, are investigated.

Fe K emission in the ultraluminous infrared galaxy Arp 220

Prominent Fe Kα line emission is detected in the XMM‐Newton spectrum of the ultraluminous infrared galaxy Arp 220. The centroid of the line is found at an energy of 6.7 keV and the equivalent width of the line is EW ∼ 1.9 keV (at 3.5σ significance). A few other spectral features are found at various degrees of significance in the lower energy range on a hard 2.5‐ 10 keV continuum (� ∼ 1). The large EW of the Fe K line poses a problem with interpreting the hard X-ray emission as integrated X-ray binary emission. A thermal emission spectrum with a temperature of kT ∼ 7k eV modified by absorption of N H � 3 × 10 22 cm −2 , can describe the 2.5‐10 keV continuum shape and the Fe K emission. A hot bubble that is shocked internally in a starburst region would have a similar temperature and gives a good explanation for the observed X-ray properties with a high star formation rate. An ensemble of radio supernovae in a dense environment, as suggested from VLBI imaging, could be another possibility, if such powerful supernovae are produced continuously at a high rate. However, the apparent lack of emission from X-ray binaries is incompatible with the high supernova rate (∼2 SNe yr −1 ) required by both interpretations. Highly photoionized, low-density gas illuminated by a hidden Compton-thick active galactic nucleus is a possible alternative for the hard X-ray emission, which can be tested by examining whether radiative recombination continua from highly ionized Ca and Fe are present in better quality data from a forthcoming observation. Ke yw ords: galaxies: individual: Arp 220 ‐ X-rays: galaxies.

THE HST/ACS COMA CLUSTER SURVEY. II. DATA DESCRIPTION AND SOURCE CATALOGS ∗

The Coma cluster, Abell 1656, was the target of an HST-ACS Treasury program designed for deep imaging in the F475W and F814W passbands. Although our survey was interrupted by the ACS instrument failure in early 2007, the partially completed survey still covers ~50% of the core high-density region in Coma. Observations were performed for 25 fields that extend over a wide range of cluster-centric radii (~1.75 Mpc or 1°) with a total coverage area of 274 arcmin2. The majority of the fields are located near the core region of Coma (19/25 pointings) with six additional fields in the southwest region of the cluster. In this paper, we present reprocessed images and SEXTRACTOR source catalogs for our survey fields, including a detailed description of the methodology used for object detection and photometry, the subtraction of bright galaxies to measure faint underlying objects, and the use of simulations to assess the photometric accuracy and completeness of our catalogs. We also use simulations to perform aperture corrections for the SEXTRACTOR Kron magnitudes based only on the measured source flux and its half-light radius. We have performed photometry for ~73,000 unique objects; approximately one-half of our detections are brighter than the 10σ point-source detection limit at F814W = 25.8 mag (AB). The slight majority of objects (60%) are unresolved or only marginally resolved by ACS. We estimate that Coma members are 5%-10% of all source detections, which consist of a large population of unresolved compact sources (primarily globular clusters but also ultra-compact dwarf galaxies) and a wide variety of extended galaxies from a cD galaxy to dwarf low surface brightness galaxies. The red sequence of Coma member galaxies has a color-magnitude relation with a constant slope and dispersion over 9 mag (–21 < M F814W < –13). The initial data release for the HST-ACS Coma Treasury program was made available to the public in 2008 August. The images and catalogs described in this study relate to our second data release.

Extragalactic constraints on the initial mass function

The local stellar mass density is observed to be significantly lower than the value obtained from integrating the cosmic star formation history (SFH), assuming that all the stars formed with a Salpeter initial mass function (IMF). Even other favoured IMFs, more successful in reconciling the observed z= 0 stellar mass density with that inferred from the SFH, have difficulties in reproducing the stellar mass density observed at higher redshift. In this study, we investigate to what extent this discrepancy can be alleviated for any universal power-law IMF. We find that an IMF with a high-mass slope shallower (2.15) than the Salpeter slope (2.35) reconciles the observed stellar mass density with the cosmic SFH, but only at low redshifts. At higher redshifts z > 0.5, we find that the observed stellar mass densities are systematically lower than predicted from the cosmic SFH, for any universal power-law IMF.

The HST/ACS coma cluster survey – V. Compact stellar systems in the coma cluster

The Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) Coma Cluster Treasury Survey is a deep two-passband imaging survey of the nearest very rich cluster of galaxies, covering a range of galaxy density environments. The imaging is complemented by a recent wide field redshift survey of the cluster conducted with Hectospec on the 6.5-m Monolithic Mirror Telescope (MMT). Among the many scientific applications for these data is the search for compact galaxies. In this paper, we present the discovery of seven compact (but quite luminous) stellar systems, ranging from M32-like galaxies down to ultra-compact dwarfs (UCDs)/dwarf to globular transition objects (DGTOs). We find that all seven compact galaxies require a two-component fit to their light profile and have measured velocity dispersions that exceed those expected for typical early-type galaxies at their luminosity. From our structural parameter analysis, we conclude that three of the samples should be classified as compact ellipticals or M32-like galaxies, and the remaining four being less extreme systems. The three compact ellipticals are all found to have old luminosity weighted ages (≳12 Gyr), intermediate metallicities (−0.6 < [Fe/H] < −0.1) and high [Mg/Fe] (≳0.25). Our findings support a tidal stripping scenario as the formation mode of compact galaxies covering the luminosity range studied here. We speculate that at least two early-type morphologies may serve as the progenitor of compact galaxies in clusters.

On the enrichment of the intergalactic medium by galactic winds

Observations of metal lines in

THE DWARF GALAXY POPULATION OF THE COMA CLUSTER TO MR = -11 : A DETAILED DESCRIPTION

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