327 research outputs found
Reconstructing Positions \& Peculiar Velocities of Galaxy Clusters within 25000 km/sec: The Bulk Velocity
Using a dynamical 3-D reconstruction procedure we estimate the peculiar
velocities of Abell/ACO galaxy clusters from their measured redshift
within 25000 km/sec. The reconstruction algorithm relies on the linear
gravitational instability hypothesis, assumes linear biasing and requires an
input value of the cluster -parameter (), which we estimated in Branchini \& Plionis (1995)
to be . The resulting cluster velocity field is dominated
by a large scale streaming motion along the Perseus Pisces--Great Attractor
base-line directed towards the Shapley concentration, in qualitative agreement
with the galaxy velocity field on smaller scales. Fitting the predicted cluster
peculiar velocities to a dipole term, in the local group frame and within a
distance of km/sec, we recover extremely well both the local group
velocity and direction, in disagreement with the Lauer \& Postman (1994)
observation. However, we find a probability that their observed
velocity field could be a realization of our corresponding one, if the latter
is convolved with their large distance dependent errors. Our predicted cluster
bulk velocity amplitude agrees well with that deduced by the POTENT and the da
Costa et al. (1995) analyses of observed galaxy motions at
km/sec; it decreases thereafter while at the Lauer \& Postman limiting depth
( km/sec) its amplitude is km/sec, in comfortable
agreement with most cosmological models.Comment: 8 pages, uuencoded compressed tarred postscript file uncluding text
and 3 figures. Accepted in ApJ Letter
The angular correlation function of the ROSAT All Sky Survey Bright Source Catalogue
We have derived the angular correlation function of a sample of 2096 sources
detected in the ROSAT All Sky Survey Bright Source Catalogue, in order to
investigate the clustering properties of AGN in the local Universe. Our sample
is constructed by rejecting all known stars, as well as extended X-ray sources.
Areas with |b|<30 deg. and declination <-30 deg. are also rejected due to the
high or uncertain neutral hydrogen absorption. Cross-correlation of our sample
with the Hamburg/RASS optical identification catalogue, suggests that the vast
majority of our sources are indeed AGN. A 4.1 sigma correlation signal between
0 and 8 degrees was detected with w(theta<8 deg.)=0.025 +- 0.006. Assuming the
usual power-law form of the 2-point correlation function we find an angular
correlationlength of 0.062 degrees. Deprojection on 3 dimensions, using the
Limber's equation, yields a spatial correlation length of 6.0+- 1.6 h^-1 Mpc.
This is consistent with the AGN clustering results derived at higher redshifts
in optical surveys and suggests a comoving model for the clustering evolution.Comment: 5 pages, revised version accepted in MNRA
The X-ray luminosity function of local galaxies
We present an estimate of the local X-ray luminosity function and emissivity
for different subsamples of galaxies namely Seyferts, LINERS, star-forming and
passive (no-emission-line) galaxies. This is performed by convolving their
optical luminosity function, as derived from the Ho et al. spectroscopic sample
of nearby galaxies with the corresponding L_x/L_B relation. The local galaxy
emissivity is about 1.6 X 10^{39} h erg/sec Mpc^3 in agreement with the results
of Lahav et al. derived from cross-correlation techniques of the X-ray
background with optical and infrared galaxy catalogues. From our analysis, it
becomes evident that the largest fraction of the galaxy emissivity comes from
galaxies associated with AGN (Seyferts but also LINERS) while the contribution
of star-forming and passive galaxies is small. This independently supports the
view that most of the yet unidentified X-ray sources in deep \rosat fields
which are associated with faint optical galaxies, do harbour an AGN.Comment: 4 pages, 2 figures, MNRAS Pink pages (in press
The Clustering of XMM-Newton Hard X-ray Sources
We present the clustering properties of hard (2-8 keV) X-ray selected sources
detected in a wide field (~2 deg^{2}) shallow [f(2-8 keV)~ 10^{-14} erg cm^{-2}
s^{-1}] and contiguous XMM-Newton survey. We perform an angular correlation
function analysis using a total of 171 sources to the above flux limit. We
detect a ~ 4\sigma correlation signal out to 300 arcsec with w(theta <
300^{''}) ~ 0.13 +- 0.03. Modeling the two point correlation function as a
power law of the usual form we find: theta_o=48.9^{+15.8}_{-24.5} arcsec and
gamma=2.2 +- 0.30. Fixing the correlation function slope to gamma=1.8 we obtain
theta_o=22.2^{+9.4}_{-8.6} arcsec. Using Limber's integral equation and a
variety of possible luminosity functions of the hard X-ray population, we find
a relatively large correlation length, ranging from r_o ~ 9 to 19 h^{-1} Mpc
(for gamma=1.8 and the concordance cosmological model), with this range
reflecting also different evolutionary models for the source luminosities and
clustering characteristics.Comment: In "Multiwavelength AGN Surveys" (Cozumel, December 8-12 2003), ed.
R. Maiolino and R. Mujica, Singapore: World Scientific, 200
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