89 research outputs found
Galaxy groups and the modified dynamics
I estimate Modified-Dynamics (MOND), median M/L values for recently published
catalogues of galaxy groups. While the median, Newtonian M/L values quoted for
these catalogues are 110-200 solar units, the corresponding values for MOND are
less than 10 solar units.Comment: 5 pages, Latex, to appear in Astrophys. J. Let
The Dependence of the Galaxy Luminosity Function on Environment
We present luminosity functions for galaxies in loose groups in the Las
Campanas Redshift Survey, differentiated by their environment (defined by the
line-of-sight velocity dispersion sigma of the host groups) and also by their
spectral type (emission or non-emission, defined by the equivalent width of the
3727-Angstrom [OII] line).
We find systematic variations in the Schechter parameters alpha and M* for
non-emission line galaxies over a range of 0 < sigma < 800 km/s. Alpha varies
from 0.20 to -0.91, indicating an increase in the steepness of the faint end
slope with increasing sigma. The accompanying variation in M* appears to be
accounted for by the intrinsic correlation with alpha and does not indicate a
significant physical variation in the bright end of the luminosity function.
For emission line galaxies, we find no significant systematic variation of the
luminosity function with the environment. Our results show that emission and
non-emission galaxies generally occupy two distinct regions in the alpha-M*
parameter space. From our luminosity functions, we derive the number ratios of
emission to non-emission galaxies as a function of environment and absolute
magnitude, showing that the relative abundance of non-emission line galaxies
generally increases for all magnitudes -23 < M_R < -17.5 towards high-sigma
environments, from ~80% to >90% at M_R = -22 and from ~10% to >50% at M_R = -18
(H_0 = 100 km s^{-1} Mpc^{-1} and q_0 = 0.5).Comment: 34 pages, 10 figures; accepted for publication in the Ap
Galaxy Groups in Cold + Hot and CDM Universes: Comparison with CfA
This letter presents results of new high resolution Cold + Hot
Dark Matter (CHDM) and Cold Dark Matter (CDM) simulations. Properties of groups
in these simulations reflect the lower small-scale velocities and the greater
tendency to form distinct filaments on both small and large scales in CHDM as
compared to CDM. The fraction of galaxies in groups and the median group rms
velocity are found to be powerful discriminators between models. We combine
these two features into a very robust statistic, median group rms velocity
as a function of the fraction of galaxies
in groups. Using this statistic, we compare ``observed'' simulations to CfA
data in redshift space in a careful and consistent way. We find that CHDM
remains a promising model, with for example v_{\rmgr}(0.45) \approx 125 \pm 25
\kms in agreement with the CfA data, while CDM with bias b=1.0
(COBE-compatible) or b=1.5, both giving v_{\rm gr}(0.45) \approx 400 \pm 25
\kms, can be virtually ruled out. Using median , the observed value of
is (CHDM) to (CDM).Comment: to appear in Ap J Letters, 12 pages including 3 figures, uuencoded
compressed postscript, preprint SCIPP 93/4
Losing Weight: A KECK Spectroscopic Survey of the Massive Cluster of Galaxies RX J1347-1145
We present a sample of 47 spectroscopically confirmed members of RX
J1347-1145, the most luminous X-ray cluster of galaxies discovered to date.
With two exceptions, all the galaxies in this sample have red B-R colors and
red spectral indices, with spectra similar to old local ellipticals. Using all
47 cluster members, we derive a mean redshift of 0.4509\pm 0.003, and a
velocity dispersion of 910\pm130 km/sec, which corresponds to a virial mass of
4.4 x 10^{14} h^{-1} Solar masses with an harmonic radius of 380 h^{-1} kpc.
The derived total dynamical mass is marginally consistent with that deduced
from the cluster's X-ray emission based on the analysis of ROSAT/ASCA images
(Schindler et al. 1997), but not consistent with the more recent X-ray analyses
of Allen (2000), Ettori, Allen & Fabian (2001) and Allen, Schmidt & Fabian
(2002). Furthermore, the dynamical mass is significantly smaller than that
derived from weak lensing (Fischer & Tyson 1997) and from strong lensing (Sahu
et al. 1998). We propose that these various discrepant mass estimates may be
understood if RX J1347-1145 is the product of two clusters caught in the act of
merging in a direction perpendicular to the line of sight, although there is no
evidence from the galaxy redshift distribution supporting this hypothesis. Even
with this hypothesis, a significant part of the extremely high X-ray luminosity
must still arise from non-virialized, presumably shocked, gas. Finally, we
report the serendipitous discovery of a lensed background galaxy at z=4.083
which will put strong constraints on the lensing mass determination once its
counter-image is securely identified.Comment: Minor changes to conform to version accepted by Ap
Galaxy Clustering and Large-Scale Structure from z = 0.2 to z = 0.5 in Two Norris Redshift Surveys
(abridged) We present a study of the nature and evolution of large-scale
structure based on two independent redshift surveys of faint field galaxies
conducted with the 176-fiber Norris Spectrograph on the Palomar 200-inch
telescope. The two surveys together sparsely cover ~20 sq. degrees and contain
835 r < 21 mag galaxies with redshifts 0.2 < z < 0.5. Both surveys have a
median redshift of z = 0.30. In order to obtain a rough estimate of the cosmic
variance, we analyze the two surveys independently. We measure the comoving
correlation length to be 3.70 +/- 0.13 h^-1 Mpc at z = 0.30 with a power-law
slope gamma = 1.77 +/- 0.05. Dividing the sample into low (0.2 < z < 0.3) and
high (0.32 < z < 0.5) redshift intervals, we see no evidence for a change in
the comoving correlation length over the redshift range 0.2 < z < 0.5. Similar
to the well-established results in the local universe, we find that
intrinsically bright galaxies are more strongly clustered than intrinsically
faint galaxies and that galaxies with little ongoing star formation, as judged
from the rest-frame equivalent width of the [OII]3727, are more strongly
clustered than galaxies with significant ongoing star formation. The rest-frame
pairwise velocity dispersion of the sample is 326^+67_-52 km s^-1, ~25% lower
than typical values measured locally. The appearance of the galaxy
distribution, particularly in the more densely sampled Abell 104 field, is
quite striking. The pattern of sheets and voids which has been observed locally
continues at least to z ~ 0.5. A friends-of-friends analysis of the galaxy
distribution supports the visual impression that > 90% of all galaxies at z <
0.5 are part of larger structures with overdensities of > 5.Comment: 40 pages including 26 Postscript figures; revised version to match
version accepted by Ap
Correlation Function of Galaxy Groups
We use the Updated Zwicky Catalog of galaxies (Falco et al. 1999) to generate
a catalog of groups, by means of a friend-of-friend algorithm. The correlation
length of the total sample is well fitted with a power law with parameters and for values of . Three subsamples defined by
the range of group virial masses were used to have their clustering
properties examined throughout the autocorrelation function. We find an
increase of the amplitude of the correlation function according to the group
masses which extends the results of the relation for galaxy systems
at small . For completeness we have also analyzed a sample of groups
obtained from the Southern Sky Redshift Survey (da Costa et al.1998) in the
range of virial masses to compare the results with those obtained from
GUZC.Comment: 9 figures, accepted for publication in Ap
Groups of Galaxies in the Two Micron All-Sky Redshift Survey
We present the results of applying a percolation algorithm to the initial
release of the Two Micron All-Sky Survey Extended Source Catalog, using
subsequently measured redshifts for almost all of the galaxies with K < 11.25
mag. This group catalog is based on the first near-IR all-sky flux-limited
survey that is complete to |b| = 5 deg. We explore the dependence of the
clustering on the length and velocity scales involved. The paper describes a
group catalog, complete to a limiting redshift of 10,000 km/s, created by
maximizing the number of groups containing 3 or more members. A second catalog
is also presented, created by requiring a minimum density contrast of 80 to
identify groups. We identify known nearby clusters in the catalogs and contrast
the groups identified in the two catalogs. We examine and compare the
properties of the determined groups and verify that the results are consistent
with the UZC-SSRS2 and northern CfA redshift survey group catalogs. The all-sky
nature of the catalog will allow the development of a flow-field model based on
the density field inferred from the estimated cluster masses.Comment: Accepted for publication in ApJ (29 pages including 13 figures). A
version with high-resolution figures is available at
http://www.cfa.harvard.edu/~acrook/preprints
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