609 research outputs found
Coma cluster object populations down to M_R~-9.5
This study follows a recent analysis of the galaxy luminosity functions and
colour-magnitude red sequences in the Coma cluster (Adami et al. 2007). We
analyze here the distribution of very faint galaxies and globular clusters in
an east-west strip of arcmin crossing the Coma cluster
center (hereafter the CS strip) down to the unprecedented faint absolute
magnitude of M. This work is based on deep images obtained at the
CFHT with the CFH12K camera in the B, R, and I bands. The analysis shows that
the observed properties strongly depend on the environment, and thus on the
cluster history. When the CS is divided into four regions, the westernmost
region appears poorly populated, while the regions around the brightest
galaxies NGC 4874 and NGC 4889 (NGC 4874 and NGC 4889 being masked) are
dominated by faint blue galaxies. They show a faint luminosity function slope
of -2, very significantly different from the field estimates. Results are
discussed in the framework of galaxy destruction (which can explain part of the
very faint galaxy population) and of structures infalling on to Coma.Comment: To be published in A&
The CFH Optical PDCS survey (COP) I: The Data
This paper presents and gives the COP (COP: CFHT Optical PDCS; CFHT:
Canada-France-Hawaii Telescope; PDCS: Palomar Distant Cluster Survey) survey
data. We describe our photometric and spectroscopic observations with the MOS
multi-slit spectrograph at the CFH telescope. A comparison of the photometry
from the PDCS (Postman et al. 1996) catalogs and from the new images we have
obtained at the CFH telescope shows that the different magnitude systems can be
cross-calibrated. After identification between the PDCS catalogues and our new
images, we built catalogues with redshift, coordinates and V, I and
Rmagnitudes. We have classified the galaxies along the lines of sight into
field and structure galaxies using a gap technique (Katgert et al. 1996). In
total we have observed 18 significant structures along the 10 lines of sight.Comment: 40 pages, 13 figures, accepted in A
A Turn-over in the Galaxy Luminosity Function of the Coma Cluster Core?
Our previous study of the faint end (R21.5) of the galaxy luminosity
function (GLF) was based on spectroscopic data in a small region near the Coma
cluster center. In this previous study Adami et al. (1998) suggested, with
moderate statistical significance, that the number of galaxies actually
belonging to the cluster was much smaller than expected. This led us to
increase our spectroscopic sample. Here, we have improved the statistical
significance of the results of the Coma GLF faint end study (R22.5) by
using a sample of 85 redshifts. This includes both new spectroscopic data and a
literature compilation. The relatively small number of faint galaxies belonging
to Coma that was suggested by Adami et al. (1998) and Secker et al. (1998) has
been confirmed with these new observations. We also confirm that the
color-magnitude relation is not well suited for finding the galaxies inside the
Coma cluster core, close to the center at magnitudes fainter than R19. We
show that there is an enhancement in the Coma line of sight of field galaxies
compared to classical field counts. This can be explained by the contribution
of groups and of a distant cluster along the line of sight. The
result is that the Coma GLF appears to turn-over or at least to become flat for
the faint galaxies. We suggest that this is due to environmental effects.Comment: 8 pages, 6 postscript figures, accepted in A&A, new table 1, updated
figure
Searching for rotating galaxy clusters in SDSS and 2dFGRS
We present a result of searching for galaxy clusters that show an indication
of global rotation using a spectroscopic sample of galaxies in SDSS and 2dFGRS.
We have determined the member galaxies of 899 Abell clusters covered in SDSS
and 2dFGRS using the redshift and the positional data of galaxies, and have
estimated the ratio of the cluster rotation amplitude to the cluster velocity
dispersion and the velocity gradient across the cluster. We have found 12
tentative rotating clusters that have large ratios of rotation amplitude to
dispersion and large velocity gradients. We have determined the morphological
parameters for 12 tentative rotating clusters using the positional information
of the member galaxies: the ellipticity of the dispersion ellipse is in the
range of 0.080.57, and the position angle of major or minor axis does not
appear to be related to the position angle of rotation axis. We have
investigated the substructures in the sample of tentative rotating clusters,
finding from the Dressler-Shectman plots that the majority (9 out of 12) of
clusters show an evidence of substructure due to the spatially correlated
velocities of galaxies. We have selected six probable rotating clusters (A0954,
A1139, A1399, A2162, A2169, and A2366) that show a single number density peak
around the cluster center with a spatial segregation of the high and low
velocity galaxies. We have found no strong evidences of a recent merging for
the probable rotating clusters: the probable rotating clusters do not deviate
significantly from the relation of the X-ray luminosity and the velocity
dispersion or the virial mass of the clusters, and two probable rotating
clusters (A0954 and A1399) have small values of the peculiar velocities and the
clustercentric distances of the brightest cluster galaxies.Comment: 33 pages, 9 figures. To appear in Ap
Constraining Omega with Cluster Evolution
We show that the evolution of the number density of rich clusters of galaxies
breaks the degeneracy between Omega (the mass density ratio of the universe)
and sigma_{8} (the normalization of the power spectrum), sigma_{8}Omega^{0.5}
\simeq 0.5, that follows from the observed present-day abundance of rich
clusters. The evolution of high-mass (Coma-like) clusters is strong in Omega=1,
low-sigma_{8} models (such as the standard biased CDM model with sigma_{8}
\simeq 0.5), where the number density of clusters decreases by a factor of \sim
10^{3} from z = 0 to z \simeq 0.5; the same clusters show only mild evolution
in low-Omega, high-sigma_{8} models, where the decrease is a factor of \sim 10.
This diagnostic provides a most powerful constraint on Omega. Using
observations of clusters to z \simeq 0.5-1, we find only mild evolution in the
observed cluster abundance. We find Omega = 0.3 \pm 0.1 and sigma_{8} = 0.85
\pm 0.15 (for Lambda = 0 models; for Omega + Lambda = 1 models, Omega = 0.34
\pm 0.13). These results imply, if confirmed by future surveys, that we live in
a low-den sity, low-bias universe.Comment: 14 pages, 3 Postscript figures, ApJ Letters, accepte
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