54 research outputs found
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
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