1,210 research outputs found
The Fundamental Plane of Galaxy Clusters
Velocity dispersion , radius and luminosity of elliptical
galaxies are known to be related, leaving only two degrees of freedom and
defining the so-called ``fundamental plane". In this {\em Letter} we present
observational evidence that rich galaxy clusters exhibit a similar behaviour.
Assuming a relation , the best-fit values
of and are very close to those defined by galaxies. The
dispersion of this relation is lower than 10 percent, i.e. significantly
smaller than the dispersion observed in the and relations. We
briefly suggest some possible implications on the spread of formation times of
objects and on peculiar velocities of galaxy clusters.Comment: 11pp., 4 figures (available on request), LaTeX, BAP-04-1993-015-OA
Multiple merging events in Abell 521
We present a detailed spatial and dynamical analysis of the central
~2.2~\h~Mpc region of the galaxy cluster Abell~521 (z=0.247), based on
238 spectra obtained at the 3.6~m Telescope of ESO and at the CFHT. From the
analysis of the 125 galaxies confirmed members of the cluster, we derive a mean
velocity of km/s and detect a complex velocity
distribution with high velocity dispersion, km/s), but
clear departure from a single gaussian component. The general structure of the
cluster follows a NW/SE direction, crossed by a perpendicular high density
``ridge'' of galaxies in the core region. The northern region of the cluster is
characterized by a lower velocity dispersion as compared to the whole cluster
value; it hosts the BCG and a dynamically bound complex of galaxies, and it is
associated to a group detected in X-ray (Arnaud et al 2000). This region could
be in pre-merger stage onto the main cluster nearly in the plane of the sky.
These results, taken together with the fact that most of the clumps detected on
the isodensity maps, as well as the early type galaxies and the brightest ones
are aligned, suggest that this NW/SE direction is the preferred one for the
formation of this cluster. The central high dense region shows a lower velocity
location ( km/s) and significantly higher scale ( km/s) as compared to the whole cluster values. This is due to
the presence of a low-velocity group of galaxies with a high fraction of
emission line objects. This can be explained in a scenario in which a merging
of subclusters has recently occurred along the direction of the ``ridge'' with
a significant component along the line of sight.Comment: 21 pages, 32 figures, uses aa.cls style, Latex. Accepted for
publication in A&
High-Order Correlations of Rich Galaxy Clusters
We analyse the two--dimensional all--sky distribution of rich Abell and ACO
galaxy clusters by using counts in cells and measuring the high--order
area--averaged angular correlation functions. Confirming previous results, we
find a well defined hierarchical relation between the two and three--point
correlation functions, remarkably constant with scale. In the angular range
, the southern sample, limited at and including both Abell and ACO clusters, shows a remarkable
hierarchical behavior up to the 6th order, while northern Abell clusters give
positive correlations in the same range only up to the 4th order. The inferred
deprojected values of the 3--D coefficients , where , are similar to those measured for the galaxy
distribution, and consistent with theoretical predictions. These results are
confirmed to the 4th order by our analysis of a 3--D sample of Abell and ACO
clusters. Assuming that selection effects and / or the absence of a cluster
fair sample are the reason of the difference between the two galactic
hemispheres, and between Abell and ACO clusters, our results indicate that the
statistical properties of the cluster distribution originate from the
underlying galaxy distribution and show that the biasing between clusters and
galaxies is non--linear.Comment: 29 pp., (ApJ, accepted for publication). This is a uuencoded
compressed postscript file including figure
Properties of Very Luminous Galaxies
Recent analysis of the SSRS2 data based on cell-counts and two-point
correlation function has shown that very luminous galaxies are much more
strongly clustered than fainter galaxies. In fact, the amplitude of the
correlation function of very luminous galaxies () asymptotically
approaches that of clusters. In this paper we investigate the
properties of the most luminous galaxies, with blue absolute magnitude . We find that: 1) the population mix is comparable to that in other ranges
of absolute magnitudes; 2) only a small fraction are located in bona fide
clusters; 3) the bright galaxy-cluster cross-correlation function is
significantly higher on large scales than that measured for fainter galaxies;
4) the correlation length of galaxies brighter than \MB ,
expressed as a function of the mean interparticle distance, appears to follow
the universal dimensionless correlation function found for clusters and radio
galaxies; 5) a large fraction of the bright galaxies are in interacting pairs,
others show evidence for tidal distortions, while some appear to be surrounded
by faint satellite galaxies. We conclude that very luminous optical galaxies
differ from the normal population of galaxies both in the clustering and other
respects. We speculate that this population is highly biased tracers of mass,
being associated to dark halos with masses more comparable to clusters than
typical loose groups.Comment: 29 pages (6 figures) + 2 tables; paper with all figures and images
available at http://boas5.bo.astro.it/~cappi/papers.html; The Astronomical
Journal, in pres
ASCA Observations of the Seyfert 2 Galaxy NGC 7582: An Obscured and Scattered View of the Hidden Nucleus
ASCA observations of the Seyfert 2 galaxy NGC 7582 revealed it was highly
variable on the timescale of s in the hard X-ray (2-10 keV)
band, while the soft X-ray (0.5-2 keV) flux remained constant during the
observations.
The spectral analysis suggests that this object is seen through an obscuring
torus with the thickness of N. The
hard X-ray is an absorbed direct continuum from a hidden Seyfert 1 nucleus; the
soft X-ray is dominated by the scattered central continuum from an extended
spatial region. Thus we have an obscured/absorbed and a scattered view of this
source as expected from the unification model for Seyfert galaxies.
More interestingly, the inferred X-ray column was observed to increase by
from 1994 to 1996, suggesting a ``patchy''
torus structure, namely the torus might be composed of many individual clouds.
The observed iron line feature near 6.4 keV with the equivalent width of 170 eV
is also consistent with the picture of the transmission of nuclear X-ray
continuum through a non-uniform torus.Comment: 10 pages, 6 figures. To be appear in PASJ 50 No.5 (1998 Oct.25 issue
The Wide-Angle Outflow of the Lensed z = 1.51 AGN HS 0810+2554
We present results from X-ray observations of the gravitationally lensed z =
1.51 AGN HS 0810+2554 performed with the Chandra X-ray Observatory and
XMM-Newton. Blueshifted absorption lines are detected in both observations at
rest-frame energies ranging between ~1-12 keV at > 99% confidence. The inferred
velocities of the outflowing components range between ~0.1c and ~0.4c. A strong
emission line at ~6.8 keV accompanied by a significant absorption line at ~7.8
keV is also detected in the Chandra observation. The presence of these lines is
a characteristic feature of a P-Cygni profile supporting the presence of an
expanding outflowing highly ionized iron absorber in this quasar. Modeling of
the P-Cygni profile constrains the covering factor of the wind to be > 0.6,
assuming disk shielding. A disk-reflection component is detected in the
XMM-Newton observation accompanied by blueshifted absorption lines. The
XMM-Newton observation constrains the inclination angle to be < 45 degrees at
90% confidence, assuming the hard excess is due to blurred reflection from the
accretion disk. The detection of an ultrafast and wide-angle wind in an AGN
with intrinsic narrow absorption lines (NALs) would suggest that quasar winds
may couple efficiently with the intergalactic medium and provide significant
feedback if ubiquitous in all NAL and BAL quasars. We estimate the mass-outflow
rate of the absorbers to lie in the range of 1.5 and 3.4 Msolar/yr for the two
observations. We find the fraction of kinetic to electromagnetic luminosity
released by HS 0810+2554 is large (epsilon = 9 (-6,+8)) suggesting that
magnetic driving is likely a significant contributor to the acceleration of
this outflow.Comment: 27 pages, 13 figures, Accepted for publication in Ap
On the nature of the X-ray absorption in the Seyfert 2 galaxy NGC 4507
We present results of the ASCA observation of the Seyfert 2 galaxy NGC 4507.
The 0.5-10 keV spectrum is rather complex and consists of several components:
(1) a hard X-ray power law heavily absorbed by a column density of about 3
10^23 cm^-2, (2) a narrow Fe Kalpha line at 6.4 keV, (3) soft continuum
emission well above the extrapolation of the absorbed hard power law, (4) a
narrow emission line at about 0.9 keV. The line energy, consistent with highly
ionized Neon (NeIX), may indicate that the soft X-ray emission derives from a
combination of resonant scattering and fluorescence in a photoionized gas. Some
contribution to the soft X-ray spectrum from thermal emission, as a blend of Fe
L lines, by a starburst component in the host galaxy cannot be ruled out with
the present data.Comment: 8 pages, LateX, 5 figures (included). Uses mn.sty and epsfig.sty. To
appear in MNRA
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