404 research outputs found
The Baryonic Fraction in Groups of Galaxies from X-Ray Measurements
The recent {\sl ROSAT \/} X-ray detections of hot intergalactic gas in three
groups of galaxies are reviewed and the resulting baryonic fraction in these
groups is reevaluated. We show that the baryonic fraction obtained, assuming
hydrostatic equilibrium, should depend, perhaps sensitively, on the radius out
to which the X-rays are detected, and the temperature profile of the gas. We
find that the NGC 2300 group has a baryonic fraction out to of at least
20\%, thus over five times higher than in the original analysis of Mulchaey
\etal\ (1993), and also much higher than one would obtain from big-bang
nucleosynthesis, but similar to the other two groups as well as rich clusters.
With this baryonic fraction, groups would be fair tracers of the distribution
of baryons in the Universe if . A baryonic fraction that
increases with radius is consistent with the X-ray data from all three groups.
However, a detailed analysis of the NGC 2300 group shows that the dependence of
baryonic fraction on radius is not well constrained by the data, in part
because of uncertainties in the estimated background.Comment: ApJ Lett 421 (Feb 1 1994), in press. 4 pages of uuencoded compressed
Postscript (extract on UNIX with 'csh' after removing header: 1st line should
be '#/bin/csh -f') with 3 figures. POP-DAEC-9306
Lost and found dark matter in elliptical galaxies
The kinematical properties of elliptical galaxies formed during the mergers
of equal mass, stars+gas+dark matter spiral galaxies are compared to the
observed low velocity dispersions found for planetary nebulae on the outskirts
of ellipticals, which have been interpreted as pointing to a lack of dark
matter in ellipticals (which poses a problem for the standard model of galaxy
formation). We find that the velocity dispersion profiles of the stars in the
simulated ellipticals match well the observed ones. The low outer stellar
velocity dispersions are mainly caused by the radial orbits of the outermost
stars, which, for a given binding energy must have low angular momentum to
reach their large radial distances, usually driven out along tidal tails.Comment: Talk presented at 21st IAP meeting, Mass Profiles andShapes of
Cosmological Structures. Ed. G. A. Mamon, F. Combes, C. Deffayet & B. Fort
(Paris: EDP), 4 pages, 3 figures (4 plots
The evolution of galaxy groups and of galaxies therein
Properties of groups of galaxies depend sensitively on the algorithm for
group selection, and even the most recent catalogs of groups built from
redshift-space selection should suffer from projections and infalling galaxies.
The cosmo-dynamical evolution of groups from initial Hubble expansion to
collapse and virialization leads to a fundamental track (FT) in
virial-theorem-M/L vs crossing time. The increased rates of mergers, both
direct and after dynamical friction, in groups relative to clusters, explain
the higher fraction of elliptical galaxies at given local number density in
X-ray selected groups, relative to clusters, even when the hierarchical
evolution of groups is considered. Galaxies falling into groups and clusters
should later travel outwards to typically 2 virial radii, which is somewhat
less than the outermost radius where observed galaxy star formation
efficiencies are enhanced relative to field galaxies of same morphological
type. An ongoing analysis of the internal kinematics of X-ray selected groups
suggests that the radial profiles of line of sight velocity dispersion are
consistent with isotropic NFW distributions for the total mass density, with
higher (lower) concentrations than LambdaCDM predictions in groups of high
(low) mass. The critical mass, at M200 ~ 10^13 M_sun is consistent with
possible breaks in the X-ray luminosity-temperature and Fundamental Plane
relations. The internal kinematics of groups indicate that the M-T relation of
groups should agree with that extrapolated from clusters with no break at the
group scale. The analyses of observed velocity dispersion profiles and of the
FT both suggest that low velocity dispersion groups (compact and loose, X-ray
emitting or undetected) are quite contaminated by chance projections.Comment: Invited review, ESO workshop "Groups of Galaxies in the Nearby
Universe", held in Santiago, Chile, 5-9 December 2005, ed. I. Saviane, V.
Ivanov & J. Borissova, 16 page
Deep Chandra Observations of HCG 16 - II. The Development of the Intra-group Medium in a Spiral-Rich Group
We use a combination of deep Chandra X-ray observations and radio continuum
imaging to investigate the origin and current state of the intra-group medium
in the spiral-rich compact group HCG 16. We confirm the presence of a faint
(=1.8710 erg/s), low
temperature (0.30 keV) intra-group medium (IGM) extending
throughout the ACIS-S3 field of view, with a ridge linking the four original
group members and extending to the southeast, as suggested by previous Rosat
and XMM-Newton observations. This ridge contains
6.610 solar masses of hot gas and is at least
partly coincident with a large-scale HI tidal filament, indicating that the IGM
in the inner part of the group is highly multi-phase. We present evidence that
the group is not yet virialised, and show that gas has probably been
transported from the starburst winds of NGC 838 and NGC 839 into the
surrounding IGM. Considering the possible origin of the IGM, we argue that
material ejected by galactic winds may have played a significant role,
contributing 20-40% of the observed hot gas in the system.Comment: 11 pages, 6 figures, 1 table, accepted for publication in ApJ;
updated references and fixed typos identified at proof stag
Deep Chandra Observations of HCG 16 - I. Active Nuclei, Star formation and Galactic Winds
We present new, deep Chandra X-ray and Giant Metrewave Radio Telescope
610~MHz observations of the spiral-galaxy-rich compact group HCG 16, which we
use to examine nuclear activity, star formation and the high luminosity X-ray
binary populations in the major galaxies. We confirm the presence of obscured
active nuclei in NGC 833 and NGC 835, and identify a previously unrecognized
nuclear source in NGC 838. All three nuclei are variable on timescales of
months to years, and for NGC 833 and NGC 835 this is most likely caused by
changes in accretion rate. The deep Chandra observations allow us to detect for
the first time an Fe-K emission line in the spectrum of the Seyfert 2
nucleus of NGC 835. We find that NGC 838 and NGC 839 are both
starburst-dominated systems, with only weak nuclear activity, in agreement with
previous optical studies. We estimate the star formation rates in the two
galaxies from their X-ray and radio emission, and compare these results with
estimates from the infra-red and ultra-violet bands to confirm that star
formation in both galaxies is probably declining after galaxy-wide starbursts
were triggered ~400-500 Myr ago. We examine the physical properties of their
galactic superwinds, and find that both have temperatures of ~0.8 keV. We also
examine the X-ray and radio properties of NGC 848, the fifth largest galaxy in
the group, and show that it is dominated by emission from its starburst.Comment: 18 pages, 11 figures, 11 tables, accepted for publication in ApJ;
updated references and fixed typos identified at proof stag
The origin of HI-deficiency in galaxies on the outskirts of the Virgo cluster. I. How far can galaxies bounce out of clusters?
Spiral galaxies that are deficient in neutral Hydrogen are observed on the
outskirts of the Virgo cluster. If their orbits have crossed the inner parts of
the cluster, their interstellar gas may have been lost through ram pressure
stripping by the hot X-ray emitting gas of the cluster. We estimate the maximum
radius out to which galaxies can bounce out of a virialized system using
analytical arguments and cosmological N-body simulations. In particular, we
derive an expression for the turnaround radius in a flat cosmology with a
cosmological constant that is simpler than previously derived expressions. We
find that the maximum radius reached by infalling galaxies as they bounce out
of their cluster is roughly between 1 and 2.5 virial radii. Comparing to the
virial radius of the Virgo cluster, which we estimate from X-ray observations,
these HI-deficient galaxies appear to lie significantly further away from the
cluster center. Therefore, if their distances to the cluster core are correct,
the HI-deficient spiral galaxies found outside of the Virgo cluster cannot have
lost their gas by ram pressure from the hot intracluster gas.Comment: A&A, in press. 8 pages LaTeX2e with 1 figur
SDSSJ124155.33+114003.7 -- a Missing Link Between Compact Elliptical and Ultracompact Dwarf Galaxies
We report the discovery of a compact object (R_e = 32 pc, M_B = -12.34 mag)
at a projected distance of 9 kpc from Messier 59, a giant elliptical in the
Virgo cluster. Using HST imaging and SDSS spectroscopy, both available in the
Virtual Observatory, we find that this object has a blue core containing
one-quarter of the light, and a redder n=1 Sersic envelope, as well as
luminosity-weighted age of 9.3 +/- 1.4 Gyr, a metallicity of -0.03 +/- 0.04 dex
and a velocity dispersion of 48 +/- 5 km/s. While ultra-compact dwarfs (UCDs)
in the face-on view of the Fundamental Plane are found to form a sequence
connecting the highest-luminosity globular clusters with the lowest-luminosity
dwarf ellipticals, the compact object near M59 lies in between this UCD
sequence and the positions of compact ellipticals. Its stellar age,
metallicity, and effective surface brightness are similar to low-luminosity
ellipticals and lenticulars, suggesting that SDSSJ124155.33+114003.7 is a
result of the tidal stripping of such an object.Comment: 5 pages, 4 figures, accepted to MNRAS Letter
Suppressed radio emission in supercluster galaxies: enhanced ram pressure in merging clusters?
The environmental influence on the 1.4 GHz continuum radio emission of
galaxies is analyzed in a 600 deg2 region of the local Universe containing the
Shapley Supercluster (SSC). Galaxies in the FLASH and 6dFGS redshift surveys
are cross-identified with NVSS radio sources, selected in a subsample doubly
complete in volume and luminosity. Environmental effects are studied through a
smoothed density field (normalized with random catalogs with the same survey
edges and redshift selection function) and the distance to the nearest cluster
(R/r200, where r200 is the virial radius, whose relation to the aperture
velocity dispersion is quantified). The fraction of high radio loudness
(R_K=L_radio/L_K) galaxies in the 10 Mpc Abell 3558 cluster complex at the core
of the SSC (SSC-CR) is half as large than elsewhere. In the SSC-CR, R_K is
anti-correlated with the density of the large-scale environment and correlated
with R/r200: central brightest cluster galaxies (BCGs) in the SSC-CR are 10x
less radio-loud than BCGs elsewhere, with signs of suppressed radio loudness in
the SSC-CR also present beyond the BCGs, out to at least 0.3 r200. This
correlation is nearly as strong as the tight correlation of L_K with R/r200
(K-luminosity segregation), inside the SSC-CR. The suppression of radio
loudness in SSC-CR BCGs can be attributed to cluster-cluster mergers that
destroy the cool core and thus the supply of gas to the central AGN. We
analytically demonstrate that the low radio loudness of non-BCG galaxies within
SSC-CR clusters cannot be explained by direct major galaxy mergers or rapid
galaxy flyby collisions, but by the loss of gas supply through the enhanced ram
pressure felt when these galaxies cross the shock front between the 2 merging
clusters and are later subjected to the stronger wind from the 2nd cluster.Comment: Version consolidated with Erratum A&A 499, 4
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