812 research outputs found
Far-infrared properties of cluster galaxies
Far-infrared properties are derived for a sample of over 200 galaxies in seven clusters: A262, Cancer, A1367, A1656 (Coma), A2147, A2151 (Hercules), and Pegasus. The IR-selected sample consists almost entirely of IR normal galaxies, with Log of L(FIR) = 9.79 solar luminosities, Log of L(FIR)/L(B) = 0,79, and Log of S(100 microns)/S(60 microns) = 0.42. None of the sample galaxies has Log of L(FIR) greater than 11.0 solar luminosities, and only one has a FIR-to-blue luminosity ratio greater than 10. No significant differences are found in the FIR properties of HI-deficient and HI-normal cluster galaxies
Galaxy Peculiar Velocities and Infall onto Groups
We perform statistical analyses to study the infall of galaxies onto groups
and clusters in the nearby Universe. The study is based on the UZC and SSRS2
group catalogs and peculiar velocity samples. We find a clear signature of
infall of galaxies onto groups over a wide range of scales 5 h^{-1} Mpc<r<30
h^{-1} Mpc, with an infall amplitude on the order of a few hundred kilometers
per second. We obtain a significant increase in the infall amplitude with group
virial mass (M_{V}) and luminosity of group member galaxies (L_{g}). Groups
with M_{V}<10^{13} M_{\odot} show infall velocities V_{infall} \simeq 150 km
s^{-1} whereas for M_{V}>10^{13} M_{\odot} a larger infall is observed,
V_{infall} \simeq 200 km s^{-1}. Similarly, we find that galaxies surrounding
groups with L_{g}<10^{15} L_{\odot} have V_{infall} \simeq 100 km s^{-1},
whereas for L_{g}>10^{15} L_{\odot} groups, the amplitude of the galaxy infall
can be as large as V_{infall} \simeq 250 km s^{-1}. The observational results
are compared with the results obtained from mock group and galaxy samples
constructed from numerical simulations, which include galaxy formation through
semianalytical models. We obtain a general agreement between the results from
the mock catalogs and the observations. The infall of galaxies onto groups is
suitably reproduced in the simulations and, as in the observations, larger
virial mass and luminosity groups exhibit the largest galaxy infall amplitudes.
We derive estimates of the integrated mass overdensities associated with groups
by applying linear theory to the infall velocities after correcting for the
effects of distance uncertainties obtained using the mock catalogs. The
resulting overdensities are consistent with a power law with \delta \sim 1 at r
\sim 10 h^{-1}Mpc.Comment: 25 pages, 10 figure
NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey
We present an HI map constructed from the Arecibo Legacy Fast ALFA (ALFALFA)
survey of the surroundings of the strongly asymmetric Virgo cluster Sc galaxy
NGC 4254. Noted previously for its lopsided appearance, rich interstellar
medium, and extradisk HI emission, NGC 4254 is believed to be entering the
Virgo environment for the first time and at high speed. The ALFALFA map clearly
shows a long HI tail extending ~250 kpc northward from the galaxy. Embedded as
one condensation within this HI structure is the object previously identified
as a "dark galaxy": Virgo HI21 (Davies et al. 2004). A body of evidence
including its location within and velocity with respect to the cluster and the
appearance and kinematics of its strong spiral pattern, extra-disk HI and
lengthy HI tail is consistent with a picture of "galaxy harassment" as proposed
by Moore et al. (1996a,b; 1998). The smoothly varying radial velocity field
along the tail as it emerges from NGC 4254 can be used as a timing tool, if
interpreted as resulting from the coupling of the rotation of the disk and the
collective gravitational forces associated with the harassment mechanism.Comment: accepted for publication in Ap.J.(Lett.). higher resolution figure
available at http://egg.astro.cornell.edu/alfalfa/pubs/figs/n4254_f1.ep
The WSRT wide-field HI survey: I. The background galaxy sample
We have used the Westerbork array to carry out an unbiased wide-field survey
for HI emission features, achieving an RMS sensitivity of about 18 mJy/Beam at
a velocity resolution of 17 km/s over 1800 deg^2 and between -1000 <
V_Hel<+6500 km/s. The primary data consists of auto-correlation spectra with an
effective angular resolution of 49' FWHM. We detect 155 external galaxies in
excess of 8 sigma in integrated HI flux density. Plausible optical associations
are found within a 30' search radius for all but one of our HI detections in
DSS images, although several are not previously cataloged or do not have
published red-shift determinations. Twenty-three of our objects are detected in
HI for the first time. We classify almost half of our detections as
``confused'', since one or more companions is cataloged within a radius of 30'
and a velocity interval of 400 km/s. We identify a handful of instances of
significant positional offsets exceeding 10 kpc of unconfused optical galaxies
with the associated HI centroid, possibly indicative of severe tidal
distortions or uncataloged gas-rich companions. A possible trend is found for
an excess of detected HI flux in unconfused galaxies within our large survey
beam relative to that detected previously in smaller telescope beams, both as
function of increasing distance and increasing gas mass. This may be an
indication for a diffuse gaseous component on 100 kpc scales in the environment
of massive galaxies or a population of uncataloged low mass companions. We use
our galaxy sample to estimate the HI mass function from our survey volume. Good
agreement is found with the HIPASS BGC results, but only after explicit
correction for galaxy density variations with distance.Comment: 27 pages, 14 figures, accepted for publication in A&
Distances from the Correlation between Galaxy Luminosities and Rotation Rates
A large luminosity--linewidth template sample is now available, improved
absorption corrections have been derived, and there are a statistically
significant number of galaxies with well determined distances to supply the
zero point. A revised estimate of the Hubble Constant is H_0=77 +-4 km/s/Mpc
where the error is the 95% probability statistical error. Systematic
uncertainties are potentially twice as large.Comment: 21 pages, 9 figures. Invited chapter for the book `Post-Hipparcos
Cosmic Candles', Eds. F. Caputo and A. Heck (Kluwer Academic Publishers,
Dordrecht
Slow shocks and conduction fronts from Petschek reconnection of skewed magnetic fields: two-fluid effects
In models of fast magnetic reconnection, flux transfer occurs within a small
portion of a current sheet triggering stored magnetic energy to be thermalized
by shocks. When the initial current sheet separates magnetic fields which are
not perfectly anti-parallel, i.e. they are skewed, magnetic energy is first
converted to bulk kinetic energy and then thermalized in slow magnetosonic
shocks. We show that the latter resemble parallel shocks or hydrodynamic shocks
for all skew angles except those very near the anti-parallel limit. As for
parallel shocks, the structures of reconnection-driven slow shocks are best
studied using two-fluid equations in which ions and electrons have independent
temperature. Time-dependent solutions of these equations can be used to predict
and understand the shocks from reconnection of skewed magnetic fields. The
results differ from those found using a single-fluid model such as
magnetohydrodynamics. In the two-fluid model electrons are heated indirectly
and thus carry a heat flux always well below the free-streaming limit. The
viscous stress of the ions is, however, typically near the fluid-treatable
limit. We find that for a wide range of skew angles and small plasma beta an
electron conduction front extends ahead of the slow shock but remains within
the outflow jet. In such cases conduction will play a more limited role in
driving chromospheric evaporation than has been predicted based on
single-fluid, anti-parallel models
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