240 research outputs found
A revised catalog of CfA galaxy groups in the Virgo/Great Attractor flow field
A new identification of groups and clusters in the CfAl Catalog of Huchra, et al. (1983) is presented, using a percolation algorithm to identify density enhancements. The procedure differs from that of the original Geller and Huchra (1983; GH) catalog in several important respects; galaxy distances are calculated from the Virgo-Great Attractor flow model of Faber and Burnstein (1988), the adopted distance linkage criteria is only approx. 1/4 as large as in the Geller and Huchra catalog, the sky link relation is taken from Nolthenius and White (1987), correction for interstellar extinction is included, and 'by-hand' adjustments to group memberships are made in the complex regions of Virgo/Coma I/Ursa Major and Coma/A1367 (to allow for varying group velocity dispersions and to trim unphysical 'spider arms'). Since flow model distances are poorly determined in these same regions, available distances from the IR Tully-Fisher planetary nebula luminosity function and surface brightness resolution methods are adopted if possible
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
Shock waves in tidally compressed stars by massive black holes
We interest in the case of a main-sequence star deeply penetrating within the
tidal radius of a massive black hole. We focus on the compression phase leading
to a so-called pancake configuration of the star at the instant of maximal
compression. The aim is to study the tidal compression process paying
particular attention to the development of shock waves;to deduce reliable
estimates of the thermodynamical quantities involved in the pancake star; and
to solve a controversy about whether or not thermonuclear reactions can be
triggered in the core of a tidally compressed star. We have set up a
one-dimensional hydrodynamical model well-adapted to the geometry of the
problem. Based on the high-resolution shock-capturing Godunov-type approach, it
allows to study the compression phase undergone by the star in the direction
orthogonal to its orbital plane. We show the existence of two regimes depending
on whether shock waves develop before or after the instant of maximal core
compression. In both cases we confirm high compression and heating factors in
the stellar core able to trigger a thermonuclear explosion. Moreover, we show
that the shock waves carry outwards a brief but very high peak of temperature
from the centre to the surface of the star. We tentatively conclude that the
phenomenon could give rise to hard electromagnetic radiation, to be compared to
some X-ray flares already observed in some galactic nuclei harbouring massive
black holes. Finally, we estimate that the rate of pancake stars should be
about per galaxy per year. If generated in hard X- or -ray
band, several events of this kind per year should be detectable within the full
observable universe.Comment: 19 pages, 38 figures, 7 tables; v2 : corrected to match version
accepted in Astron. Astrophys. Tables and references added, new simulations
also performed for adiabatic index 4/
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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