X-ray Observations of Distant Optically Selected Cluster

We have measured fluxes or flux limits for 31 of the 79 cluster candidates in the Palomar Distant Cluster Survey (PDCS) using archival ROSAT/PSPC pointed observations. Our X-ray survey reaches a flux limit of $\simeq 3 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ (0.4 - 2.0 keV), which corresponds to luminosities of $L_x\simeq 5 \times 10^{43}$ erg s$^{-1}$ ($H_o$ = 50 km s$^{-1}$ Mpc$^{-1}$, $q_o$ = ${1/2}$), if we assume the PDCS estimated redshifts. Of the 31 cluster candidates, we detect six at a signal-to-noise greater than three. We estimate that $2.9^{+3.3}_{-1.4}$ (90% confidence limits) of these six detections are a result of X-ray emission from objects unrelated to the PDCS cluster candidates. The net surface density of X-ray emitting cluster candidates in our survey, $1.71^{+0.91}_{-2.19}$ clusters deg$^{-2}$, agrees with that of other, X-ray selected, surveys. It is possible, given the large error on our contamination rate, that we have not detected X-ray emission from any of our observed PDCS cluster candidates. We find no statistically significant difference between the X-ray luminosities of PDCS cluster candidates and those of Abell clusters of similar optical richness. This suggests that the PDCS contains objects at high redshift similar to the low redshift clusters in the Abell catalogs. We show that the PDCS cluster candidates are not bright X-ray sources, the average luminosity of the six detected candidates is only $\bar{L_x}=0.9\times10^{44}$ erg s$^{-1}$ (0.4-2.0 keV). This finding is in agreement with previous X-ray studies of high redshift, optically selected, rich clusters of galaxies.Comment: 19 pages, LaTeX with AAS Preprint Macros (v. 4), 3 embedded postscript figures, 3 Seperate Tables using aj_pt4.sty, Accepted by the Astronomical Journal for November 199

A Turn-over in the Galaxy Luminosity Function of the Coma Cluster Core?

Our previous study of the faint end (R$\leq$21.5) of the galaxy luminosity function (GLF) was based on spectroscopic data in a small region near the Coma cluster center. In this previous study Adami et al. (1998) suggested, with moderate statistical significance, that the number of galaxies actually belonging to the cluster was much smaller than expected. This led us to increase our spectroscopic sample. Here, we have improved the statistical significance of the results of the Coma GLF faint end study (R$\leq$22.5) by using a sample of 85 redshifts. This includes both new spectroscopic data and a literature compilation. The relatively small number of faint galaxies belonging to Coma that was suggested by Adami et al. (1998) and Secker et al. (1998) has been confirmed with these new observations. We also confirm that the color-magnitude relation is not well suited for finding the galaxies inside the Coma cluster core, close to the center at magnitudes fainter than R$\sim$19. We show that there is an enhancement in the Coma line of sight of field galaxies compared to classical field counts. This can be explained by the contribution of groups and of a distant $z\sim 0.5$ cluster along the line of sight. The result is that the Coma GLF appears to turn-over or at least to become flat for the faint galaxies. We suggest that this is due to environmental effects.Comment: 8 pages, 6 postscript figures, accepted in A&A, new table 1, updated figure

The Evolution of the Field and Cluster Morphology-Density Relation for Mass-Selected Samples of Galaxies

The Sloan Digital Sky Survey (SDSS) and photometric/spectroscopic surveys in the GOODS-South field (the Chandra Deep Field-South, CDFS) are used to construct volume-limited, stellar mass-selected samples of galaxies at redshifts 0<z<1. The CDFS sample at 0.6<z<1.0 contains 207 galaxies complete down to M=4x10^10 Msol (for a ``diet'' Salpeter IMF), corresponding to a luminosity limit for red galaxies of M_B=-20.1. The SDSS sample at 0.020<z<0.045 contains 2003 galaxies down to the same mass limit, which corresponds to M_B=-19.3 for red galaxies. Morphologies are determined with an automated method, using the Sersic parameter n and a measure of the residual from the model fits, called ``bumpiness'', to distinguish different morphologies. These classifications are verified with visual classifications. In agreement with previous studies, 65-70% of the galaxies are located on the red sequence, both at z~0.03 and at z~0.8. Similarly, 65-70% of the galaxies have n>2.5. The fraction of E+S0 galaxies is 43+/-3%$ at z~0.03 and 48+/-7% at z~0.8, i.e., it has not changed significantly since z~0.8. When combined with recent results for cluster galaxies in the same redshift range, we find that the morphology-density relation for galaxies more massive than 0.5M* has remained constant since at least z~0.8. This implies that galaxies evolve in mass, morphology and density such that the morphology-density relation does not change. In particular, the decline of star formation activity and the accompanying increase in the stellar mass density of red galaxies since z~1 must happen without large changes in the early-type galaxy fraction in a given environment.Comment: 16 pages, 13 figures, 2 tables. Updated to match journal version. Will appear in ApJ (vol. 670, p. 206

Controls on the spatial distribution of oceanic <i>δ</i>¹³C_DIC

We describe the design and evaluation of a large ensemble of coupled climate–carbon cycle simulations with the Earth system model of intermediate complexity GENIE. This ensemble has been designed for application to a range of carbon cycle questions, including the causes of late- Quaternary fluctuations in atmospheric CO2. Here we evaluate the ensemble by applying it to a transient experiment over the recent industrial era (1858 to 2008 AD). We employ singular vector decomposition and principal component emulation to investigate the spatial modes of ensemble variability of oceanic dissolved inorganic carbon (DIC) δ13C, considering both the spun-up pre-industrial state and the transient change. These analyses allow us to separate the natural (preindustrial) and anthropogenic controls on the δ13CDIC distribution. We apply the same dimensionally reduced emulation techniques to consider the drivers of the spatial uncertainty in anthropogenic DIC. We show that the sources of uncertainty related to the uptake of anthropogenic δ13CDIC and DIC are quite distinct. Uncertainty in anthropogenic δ13C uptake is controlled by air–sea gas exchange, which explains 63% of modelled variance. This mode of variability is largely absent from the ensemble variability in CO2 uptake, which is rather driven by uncertainties in thermocline ventilation rates. Although the need to account for air–sea gas exchange is well known, these results suggest that, to leading order, uncertainties in the ocean uptake of anthropogenic 13C and CO2 are governed by very different processes. This illustrates the difficulties in reconstructing one from the other, and furthermore highlights the need for careful targeting of both δ13CDIC and DIC observations to better constrain the ocean sink of anthropogenic CO2

Downregulation of genes with a function in axon outgrowth and synapse formation in motor neurones of the VEGF(delta/delta) mouse model of amyotrophic lateral sclerosis

Background: Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen that stimulates vasculogenesis. It has also been shown to act as a neurotrophic factor in vitro and in vivo. Deletion of the hypoxia response element of the promoter region of the gene encoding VEGF in mice causes a reduction in neural VEGF expression, and results in adult-onset motor neurone degeneration that resembles amyotrophic lateral sclerosis (ALS). Investigating the molecular pathways to neurodegeneration in the VEGF(delta/delta) mouse model of ALS may improve understanding of the mechanisms of motor neurone death in the human disease. Results: Microarray analysis was used to determine the transcriptional profile of laser captured spinal motor neurones of transgenic and wild-type littermates at 3 time points of disease. 324 genes were significantly differentially expressed in motor neurones of presymptomatic VEGF(delta/delta) mice, 382 at disease onset, and 689 at late stage disease. Massive transcriptional downregulation occurred with disease progression, associated with downregulation of genes involved in RNA processing at late stage disease. VEGF(delta/delta) mice showed reduction in expression, from symptom onset, of the cholesterol synthesis pathway, and genes involved in nervous system development, including axonogenesis, synapse formation, growth factor signalling pathways, cell adhesion and microtubule-based processes. These changes may reflect a reduced capacity of VEGF(delta/delta) mice for maintenance and remodelling of neuronal processes in the face of demands of neural plasticity. The findings are supported by the demonstration that in primary motor neurone cultures from VEGF(delta/delta) mice, axon outgrowth is significantly reduced compared to wild-type littermates. Conclusions: Downregulation of these genes involved in axon outgrowth and synapse formation in adult mice suggests a hitherto unrecognized role of VEGF in the maintenance of neuronal circuitry. Dysregulation of VEGF may lead to neurodegeneration through synaptic regression and dying-back axonopathy