757 research outputs found
ESO 3060170 -- a massive fossil galaxy group with a heated gas core?
We present a detailed study of the ESO 3060170 galaxy group combining
Chandra, XMM and optical observations. The system is found to be a fossil
galaxy group. The group X-ray emission is composed of a central dense cool core
(10 kpc in radius) and an isothermal medium beyond the central 10 kpc. The
region between 10 and 50 kpc (the cooling radius) has the same temperature as
the gas from 50 kpc to 400 kpc although the gas cooling time between 10 and 50
kpc (2 - 6 Gyr) is shorter than the Hubble time. Thus, the ESO 3060170 group
does not have a group-sized cooling core. We suggest that the group cooling
core may have been heated by a central AGN outburst in the past and the small
dense cool core is the truncated relic of a previous cooling core. The Chandra
observations also reveal a variety of X-ray features in the central region,
including a ``finger'', an edge-like feature and a small ``tail'', all aligned
along a north-south axis, as are the galaxy light and group galaxy
distribution. The proposed AGN outburst may cause gas ``sloshing'' around the
center and produce these asymmetric features. The observed flat temperature
profile to 1/3 R_vir is not consistent with the predicted temperature profile
in recent numerical simulations. We compare the entropy profile of the ESO
3060170 group with those of three other groups and find a flatter relation than
that predicted by simulations involving only shock heating, S r. This is direct evidence for the importance of non-gravitational
processes in group centers. We derive the mass profiles within 1/3 R_vir and
find the ESO 3060170 group is the most massive fossil group known (1 - 2 X
10 M). The M/L ratio of the system, ~ 150 at 0.3 R_vir, is
normal.Comment: 17 pages, 12 figures, to appear in ApJ. A high-resolution version can
be downloaded from http://cxc.harvard.edu/~msun/esoa.p
CHANDRA observations of the NGC 1550 galaxy group -- implication for the temperature and entropy profiles of 1 keV galaxy groups
We present a detailed \chandra study of the galaxy group NGC 1550. For its
temperature (1.370.01 keV) and velocity dispersion ( 300 km
s), the NGC 1550 group is one of the most luminous known galaxy groups
(L = 1.65 erg s within 200 kpc, or 0.2 \rv).
We find that within kpc, where the gas cooling time is less than a
Hubble time, the gas temperature decreases continuously toward the center,
implying the existence of a cooling core. The temperature also declines beyond
100 kpc (or 0.1 \rv). There is a remarkable similarity of the
temperature profile of NGC 1550 with those of two other 1 keV groups with
accurate temperature determination. The temperature begins to decline at 0.07 -
0.1 \rv, while in hot clusters the decline begins at or beyond 0.2 \rv. Thus,
there are at least some 1 keV groups that have significantly different
temperature profiles from those of hot clusters, which may reflect the role of
non-gravitational processes in ICM/IGM evolution. NGC 1550 has no isentropic
core in its entropy profile, in contrast to the predictions of `entropy-floor'
simulations. We compare the scaled entropy profiles of three 1 keV groups
(including NGC 1550) and three 2 - 3 keV groups. The scaled entropy profiles of
1 keV groups show much larger scatter than those of hotter systems, which
implies varied pre-heating levels. We also discuss the mass content of the NGC
1550 group and the abundance profile of heavy elements.Comment: emulateapj5.sty, 18 pages, 11 figures (including 4 color), to appear
in ApJ, v598, n1, 20 Nov 200
X-ray Over-Luminous Elliptical Galaxies: A New Class of Mass Concentrations in the Universe?
We detect four isolated, X-ray over-luminous (Lx>2e43 [h/0.5]**-2 erg/s)
elliptical galaxies (OLEGs) in our 160 square degree ROSAT PSPC survey. The
extent of their X-ray emission, total X-ray luminosity, total mass, and mass of
the hot gas in these systems correspond to poor clusters, and the optical
luminosity of the central galaxies (M_R<-22.5 + 5 lg h) is comparable to that
of cluster cDs. However, there are no detectable fainter galaxy concentrations
around the central elliptical. The mass-to-light ratio within the radius of
detectable X-ray emission is in the range 250-450 Msun/Lsun, which is 2-3 times
higher than typically found in clusters or groups. These objects can be the
result of galaxy merging within a group. However, their high M/L values are
difficult to explain in this scenario. OLEGs must have been undisturbed for a
very long time, which makes them the ultimate examples of systmes in
hydrostatic equilibrium. The number density of OLEGs is n=2.4(+3.1-1.2}x10**-7
(h/0.5)**-3 Mpc**-3 at the 90% confidence. They comprise 20% of all clusters
and groups of comparable X-ray luminosity, and nearly all galaxies brighter
than M_R=-22.5. The estimated contirubution of OLEGs to the total mass density
in the Universe is close to that of T>7 keV clusters.Comment: 4 pages, 2 figures, uses emulateapj.sty, submitted to ApJ Letter
Evolution of the Cluster X-ray Luminosity Function
We report measurements of the cluster X-ray luminosity function out to z=0.8
based on the final sample of 201 galaxy systems from the 160 Square Degree
ROSAT Cluster Survey. There is little evidence for any measurable change in
cluster abundance out to z~0.6 at luminosities less than a few times 10^44
ergs/s (0.5-2.0 keV). However, between 0.6 < z < 0.8 and at luminosities above
10^44 ergs/s, the observed volume densities are significantly lower than those
of the present-day population. We quantify this cluster deficit using
integrated number counts and a maximum-likelihood analysis of the observed
luminosity-redshift distribution fit with a model luminosity function. The
negative evolution signal is >3 sigma regardless of the adopted local
luminosity function or cosmological framework. Our results and those from
several other surveys independently confirm the presence of evolution. Whereas
the bulk of the cluster population does not evolve, the most luminous and
presumably most massive structures evolve appreciably between z=0.8 and the
present. Interpreted in the context of hierarchical structure formation, we are
probing sufficiently large mass aggregations at sufficiently early times in
cosmological history where the Universe has yet to assemble these clusters to
present-day volume densities.Comment: 15 pages, 10 figures, accepted for publication in Ap
Dark matter line emission constraints from NuSTAR observations of the Bullet Cluster
Line emission from dark matter is well motivated for some candidates e.g.
sterile neutrinos. We present the first search for dark matter line emission in
the 3-80keV range in a pointed observation of the Bullet Cluster with NuSTAR.
We do not detect any significant line emission and instead we derive upper
limits (95% CL) on the flux, and interpret these constraints in the context of
sterile neutrinos and more generic dark matter candidates. NuSTAR does not have
the sensitivity to constrain the recently claimed line detection at 3.5keV, but
improves on the constraints for energies of 10-25keV.Comment: 7 pages, 5 figures, submitted to Ap
The 160 Square Degree ROSAT Survey: the Revised Catalog of 201 Clusters with Spectroscopic Redshifts
We present the revised catalog of galaxy clusters detected as extended X-ray
sources in the 160 Square Degree ROSAT Survey, including spectroscopic
redshifts and X-ray luminosities for 200 of the 201 members. The median
redshift is z~0.25 and the median X-ray luminosity is 4.2e+43 erg/s/h50^2
(0.5-2.0 keV). This is the largest high-redshift sample of X-ray selected
clusters published to date. There are 73 objects at z>0.3 and 22 objects at
z>0.5 drawn from a statistically complete flux-limited survey with a median
object flux of 1.4d-13 erg/cm^2/s. We describe the optical follow-up of these
clusters with an emphasis on our spectroscopy which has yielded 155 cluster
redshifts, 110 of which are presented here for the first time. These
measurements combined with 45 from the literature and other sources provide
near-complete spectroscopic coverage for our survey. We discuss the final
optical identifications for the extended X-ray sources in the survey region and
compare our results to similar X-ray cluster searches.Comment: 17 pages, 7 figs, accepted for publication in ApJ, a version with
full resolution images is available at
http://www.eso.org/~cmullis/papers/160sd-catalog.ps.gz, machine-readable
versions of the catalog are available at
http://www.eso.org/~cmullis/research/160sd-catalog.htm
A Richness Study of 14 Distant X-ray Clusters From the 160 Square Degree Survey
We have measured the surface density of galaxies toward 14 X-ray-selected
cluster candidates at redshifts greater than z=0.46, and we show that they are
associated with rich galaxy concentrations. We find that the clusters range
between Abell richness classes 0-2, and have a most probable richness class of
one. We compare the richness distribution of our distant clusters to those for
three samples of nearby clusters with similar X-ray luminosities. We find that
the nearby and distant samples have similar richness distributions, which shows
that clusters have apparently not evolved substantially in richness since
redshift z =0.5. We compare the distribution of distant X-ray clusters in the
L_x--richness plane to the distribution of optically-selected clusters from the
Palomar Distant Cluster Survey. The optically-selected clusters appear overly
rich for their X-ray luminosities when compared to X-ray-selected clusters.
Apparently, X-ray and optical surveys do not necessarily sample identical mass
concentrations at large redshifts. This may indicate the existence of a
population of optically rich clusters with anomalously low X-ray emission. More
likely, however, it reflects the tendency for optical surveys to select
unvirialized mass concentrations, as might be expected when peering along
large-scale filaments.Comment: The abstract has been abridged. Accepted for publication in the
Astrophysical Journa
INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane
The center of our Galaxy is a known strong source of electron-positron
511-keV annihilation radiation. Thus far, however, there have been no reliable
detections of annihilation radiation outside of the central radian of our
Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy
Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study
of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution
coded-aperture gamma-ray telescope with an unprecedented combination of
sensitivity, angular resolution and energy resolution. We report results from
the first 10 months of observation. During this period a significant fraction
of the observing time was spent in or near the Galactic Plane. No positive
annihilation flux was detected outside of the central region (|l| > 40 deg) of
our Galaxy. In this paper we describe the observations and data analysis
methods and give limits on the 511-keV flux.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 3
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