201 research outputs found
The Selection of Tenured Astronomers in France
The organization of the recruitment of tenured astronomers and
astrophysicists in France is presented and compared with the setup in other
countries. The ages for getting tenure have increased from 27-28 in 1980 to 31
today. Foreign scientists constitute at least 11% of the recruits and the delay
in their hiring is quantified. The large reliance on national tenure committees
is justified, while the increased targeting of positions is questioned and a
compromise proposed.Comment: Final refereed version. One sentence added plus several other
modifications. 20 page
Dynamical theory of dense groups of galaxies
It is well known that galaxies associate in groups and clusters. Perhaps 40% of all galaxies are found in groups of 4 to 20 galaxies (e.g., Tully 1987). Although most groups appear to be so loose that the galaxy interactions within them ought to be insignificant, the apparently densest groups, known as compact groups appear so dense when seen in projection onto the plane of the sky that their members often overlap. These groups thus appear as dense as the cores of rich clusters. The most popular catalog of compact groups, compiled by Hickson (1982), includes isolation among its selection critera. Therefore, in comparison with the cores of rich clusters, Hickson's compact groups (HCGs) appear to be the densest isolated regions in the Universe (in galaxies per unit volume), and thus provide in principle a clean laboratory for studying the competition of very strong gravitational interactions. The $64,000 question here is then: Are compact groups really bound systems as dense as they appear? If dense groups indeed exist, then one expects that each of the dynamical processes leading to the interaction of their member galaxies should be greatly enhanced. This leads us to the questions: How stable are dense groups? How do they form? And the related question, fascinating to any theorist: What dynamical processes predominate in dense groups of galaxies? If HCGs are not bound dense systems, but instead 1D change alignments (Mamon 1986, 1987; Walke & Mamon 1989) or 3D transient cores (Rose 1979) within larger looser systems of galaxies, then the relevant question is: How frequent are chance configurations within loose groups? Here, the author answers these last four questions after comparing in some detail the methods used and the results obtained in the different studies of dense groups
How well does the Friends-of-Friends algorithm recover group properties from galaxy catalogs limited in both distance and luminosity?
We test the Friends-of-Friends (FoF) grouping algorithm, which depends on two
linking lengths (LLs), plane-of-sky and line-of-sight (LOS), normalized to the
mean nearest neighbor separation of field galaxies. We argue, on theoretical
grounds, that LLs should be , and
to recover 95% of all galaxies with projected radii within the virial radius
and 95% of the galaxies along the LOS. We then predict that 80 to 90%
of the galaxies in FoF groups should lie within their parent real-space groups
(RSGs), defined within their virial spheres. We test the FoF extraction for
16x16 pairs of LLs, using subsamples of galaxies, doubly complete in distance
and luminosity, of a flux-limited mock SDSS galaxy catalog. We find that
massive RSGs are more prone to fragmentation, while the fragments typically
have low estimated mass, with typically 30% of groups of low and intermediate
estimated mass being fragments. Group merging rises drastically with estimated
mass. For groups of 3 or more galaxies, galaxy completeness and reliability are
both typically better than 80% (after discarding the fragments). Estimated
masses of extracted groups are biased low, by up to a factor 4 at low richness,
while the inefficiency of mass estimation improves from 0.85 dex to 0.2 dex
when moving from low to high multiplicity groups. The optimal LLs depend on the
scientific goal for the group catalog. We propose , with
for studies of environmental effects, for cosmographic studies and for followups
of individual groups.Comment: Revised version resubmitted to MNRAS. 16 pages. Smaller optimal
linking lengths. Two extra figures. Comments welcome
A finer view of the conditional galaxy luminosity function and magnitude-gap statistics
The gap between first and second ranked galaxy magnitudes in groups is often
considered a tracer of their merger histories, which in turn may affect galaxy
properties, and also serves to test galaxy luminosity functions (LFs). We
remeasure the conditional luminosity function (CLF) of the Main Galaxy Sample
of the SDSS in an appropriately cleaned subsample of groups from the Yang
catalog. We find that, at low group masses, our best-fit CLF have steeper
satellite high ends, yet higher ratios of characteristic satellite to central
luminosities in comparison with the CLF of Yang et al. (2008). The observed
fractions of groups with large and small magnitude gaps as well as the Tremaine
& Richstone (1977) statistics, are not compatible with either a single
Schechter LF or with a Schechter-like satellite plus lognormal central LF.
These gap statistics, which naturally depend on the size of the subsamples, and
also on the maximum projected radius, , for defining the 2nd
brightest galaxy, can only be reproduced with two-component CLFs if we allow
small gap groups to preferentially have two central galaxies, as expected when
groups merge. Finally, we find that the trend of higher gap for higher group
velocity dispersion, , at given richness, discovered by Hearin
et al. (2013), is strongly reduced when we consider in bins of
richness, and virtually disappears when we use group mass instead of
. This limits the applicability of gaps in refining
cosmographic studies based on cluster counts.Comment: 18 pages, 13 figures, 5 tables, submitted to MNRAS, comments are
welcom
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
Mass, velocity anisotropy, and pseudo phase-space density profiles of Abell 2142
Aim: We aim to compute the mass and velocity anisotropy profiles of Abell
2142 and, from there, the pseudo phase--space density profile and the
density slope - velocity anisotropy relation, and then to
compare them with theoretical expectations. Methods: The mass profiles were
obtained by using three techniques based on member galaxy kinematics, namely
the caustic method, the method of Dispersion - Kurtosis, and MAMPOSSt. Through
the inversion of the Jeans equation, it was possible to compute the velocity
anisotropy profiles. Results: The mass profiles, as well as the virial values
of mass and radius, computed with the different techniques agree with one
another and with the estimates coming from X-ray and weak lensing studies. A
concordance mass profile is obtained by averaging the lensing, X-ray, and
kinematics determinations. The cluster mass profile is well fitted by an NFW
profile with . The population of red and blue galaxies appear to
have a different velocity anisotropy configuration, since red galaxies are
almost isotropic, while blue galaxies are radially anisotropic, with a weak
dependence on radius. The profile for the red galaxy population agrees
with the theoretical results found in cosmological simulations, suggesting that
any bias, relative to the dark matter particles, in velocity dispersion of the
red component is independent of radius. The relation for red
galaxies matches the theoretical relation only in the inner region. The
deviations might be due to the use of galaxies as tracers of the gravitational
potential, unlike the non--collisional tracer used in the theoretical relation.Comment: 14 pages, 14 figures. Consolidated version including the Corrigendum
published on A&
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