Probing the gravitational potential of a nearby lensing cluster Abell 2104

The cluster Abell 2104 is one of the lowest redshift clusters (z=0.153) known to have a gravitational lensing arc. We present detailed analysis of the cluster properties such as the gravitational potential using the X-ray data from ROSAT (HRI) and ASCA, as well as optical imaging and spectroscopic data from the CFHT. The cluster is highly luminous in the X-ray with a bolometric luminosity of Lx ~3x10^{45} ergs/s and a high gas temperature of ~10.4 keV. The X-ray emission extending out to at least a radius of 1.46 Mpc, displays significant substructure. The total mass deduced from the X-ray data under the assumption of hydrostatic equilibrium and isothermal gas, is found to be M_tot(r<1.46Mpc) ~(8.0 +/- 0.8)x10^{14} solar mass. The gas fraction within a radius of 1.46 Mpc is ~5-10%. The cluster galaxy velocity distribution has a dispersion of 1200 +/- 200 km/s with no obvious evidence for substructure. The total mass within 1.46 Mpc, deduced from Jean's equation using the observed galaxy number density distribution and velocity dispersion, is found to be \~6.8x10^{14} solar mass to ~2.6x10^{15} solar mass marginally consistent with the X-ray deduced total mass.Comment: 13 pages, 10 postscript figures, use aa.sty, to appear in Astronomy and Astrophysic

Group-cluster merging and the formation of starburst galaxies

A significant fraction of clusters of galaxies are observed to have substructure, which implies that merging between clusters and subclusters is a rather common physical process of cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We report the results of numerical simulations, which show the dynamical evolution of a gas-rich late-type spiral in a merger between a small group of galaxies and a cluster. The simulations demonstrate that time-dependent tidal gravitational field of the merging excites non-axisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the central region, and finally triggers a secondary starburst. This result provides not only a new mechanism of starbursts but also a close physical relationship between the emergence of starburst galaxies and the formation of substructure in clusters. We accordingly interpret post-starburst galaxies located near substructure of the Coma cluster as one observational example indicating the global tidal effects of group-cluster merging. Our numerical results furthermore suggest a causal link between the observed excess of blue galaxies in distant clusters and cluster virialization process through hierarchical merging of subclusters.Comment: 5 pages 3 color figures, ApJL in pres

Magellan Spectroscopy of the Galaxy Cluster RX J1347.5-1145: Redshift Estimates for the Gravitationally Lensed Arcs

We present imaging and spectroscopic observations of the gravitationally lensed arcs in the field of RX J1347.5-1145, the most X-ray luminous galaxy cluster known. Based on the detection of the [OII] 3727 emission line, we confirm that the redshift of one of the arcs is z = 0.806. Its color and [OII] line strength are consistent with those of distant, actively star forming galaxies. In a second arc, we tentatively identify a pair of absorption lines superposed on a red continuum; the lines are consistent with Ca II H & K at z = 0.785. We detected a faint blue continuum in two additional arcs, but no spectral line features could be measured. We establish lower limits to their redshifts based on the absence of [OII] emission, which we argue should be present and detectable in these objects. Redshifts are also given for a number of galaxies in the field of the cluster.Comment: To appear in The Astrophysical Journal (September 2002). 6 page

Detecting high redshift evolved galaxies as the hosts of optically faint hard X-ray sources

We combine deep Subaru near-infrared images of the massive lensing clusters A2390 and A370 with Keck optical data to map the spectral energy distributions (SEDs) of Chandra X-ray sources lying behind the clusters. The three sources behind A2390 are found to have extremely red colors with SEDs consistent with evolved galaxies at redshifts z>1.4. One source has extremely anomalous colors, which we interpret as evidence for a type Sa SED at a redshift around 2.5. The photometric redshift of another source has been confirmed at z=1.467 from near-infrared spectroscopy using the CISCO spectrograph on Subaru. Mapping of optically faint hard X-ray sources may prove to be an extremely efficient way to locate luminous evolved galaxies at high redshifts.Comment: 5 pages, ApJ Letters, in pres

A study of diffuse radio sources and X-ray emission in six massive clusters

The goal of this study is to extend our current knowledge of the diffuse radio source (halo and relic) populations to z > 0.3. Here, we report GMRT and EVLA radio observations of six galaxy clusters taken from the MAssive Cluster Survey (MACS) catalogue to detect diffuse radio emission. We used archival GMRT (150, 235, and 610 MHz) and EVLA (L band) data and made images at multiple radio frequencies of the following six clusters – MACSJ0417.5−1154, MACSJ1131.8−1955, MACSJ0308.9+2645, MACSJ2243.3−0935, MACSJ2228.5+2036, and MACSJ0358.8−2955. We detect diffuse radio emission (halo or relic, or both) in the first four clusters. In the last two clusters, we do not detect any diffuse radio emission but we put stringent upper limits on their radio powers. We also use archival Chandra X-ray data to carry out morphology and substructure analysis of these clusters. We find that based on X-ray data, these MACS clusters are non-relaxed and show substructures in their temperature distribution. The radio powers of the first four MACS clusters are consistent with their expected values in the LX–P1.4 GHz plot. However, we found ultrasteep spectrum radio halo in the MACSJ0417.5−1154 cluster whose rest-frame cut-off frequency is at ∼900 MHz. The remaining two clusters whose radio powers are ∼11 times below the expected values are most likely to be in the ‘off-state’ as has been postulated in some of the models of radio halo formation

Resolving the Stellar Populations in a z=4 Lensed Galaxy

We present deep near-infrared Keck/NIRC imaging of a recently-discovered z=4.04 galaxy (Frye & Broadhurst 1998). This is lensed by the rich foreground cluster Abell~2390 (z~0.23) into highly-magnified arcs 3-5arcsec in length. Our H- and K'-band NIRC imaging allows us to map the Balmer+4000Ang break amplitude. In combination with high-quality archival HST/WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which correspond to sites of active star formation. However, there are considerable portions of the arcs are significantly redder, consistent with being observed >100Myr after star formation has ceased. Keck/LRIS long-slit spectroscopy along the arcs reveals that the Ly-alpha emission is spatially offset by ~1arcsec from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring HII regions, and that the z=4 system is unlikely to be an AGN.Comment: Accepted for publication in the Astrophysical Journal. Uses emulateapj.sty and graphics.sty (included). 34 pages - has 5 tables and 21 encapsulated postscript figures, 4 in colour mail (B&W versions also provided

The local star formation rate and radio luminosity density

We present a new determination of the local volume-averaged star formation rate from the 1.4 GHz luminosity function of star forming galaxies. Our sample, taken from the B<=12 Revised Shapley-Ames catalogue (231 normal spiral galaxies over effective area 7.1 sr) has ~100% complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fit Schechter function has a faint-end slope of -1.27+/-0.07 in agreement with the local Halpha luminosity function, characteristic luminosity L*=(2.6+/-0.7)*10^{22} W/Hz and density phi* = (4.8 +/-1.1)*10^{-4} / Mpc^3. The inferred local radio luminosity density of (1.73+/-0.37+/-0.03)*10^{19} W/Hz/Mpc^3 (Poisson noise, large scale structure fluctuations) implies a volume averaged star formation rate ~2 x larger than the Gallego et al. Halpha estimate, i.e. rho(1.4 GHz} = (2.10+/-0.45+/-0.04) *10^{-2}$ Msun/yr/Mpc^3 for a Salpeter initial mass function from 0.1-125 Msun and Hubble constant of 50 km/s/Mpc. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical-UV SFRs are easily underestimated, particularly at high redshift.Comment: MNRAS in press. 1 figure. Uses BoxedEPS and mn2e (included). Finally got round to the correction