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

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, $R_{\rm max}$, 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, $\sigma_{\rm v}$, at given richness, discovered by Hearin et al. (2013), is strongly reduced when we consider $\sigma_{\rm v}$ in bins of richness, and virtually disappears when we use group mass instead of $\sigma_{\rm v}$. 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 $25'$ 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 $\Omega h_{50}^2 = 0.3$. 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

Properties of spherical galaxies and clusters with an NFW density profile

Using the standard dynamical theory of spherical systems, we calculate the properties of spherical galaxies and clusters whose density profiles obey the universal form first obtained in high resolution cosmological N-body simulations by Navarro, Frenk & White. We adopt three models for the internal kinematics: isotropic velocities, constant anisotropy and increasingly radial Osipkov-Merritt anisotropy. Analytical solutions are found for the radial dependence of the mass, gravitational potential, velocity dispersion, energy and virial ratio and we test their variability with the concentration parameter describing the density profile and amount of velocity anisotropy. We also compute structural parameters, such as half-mass radius, effective radius and various measures of concentration. Finally, we derive projected quantities, the surface mass density and line-of-sight as well as aperture velocity dispersion, all of which can be directly applied in observational tests of current scenarios of structure formation. On the mass scales of galaxies, if constant mass-to-light is assumed, the NFW surface density profile is found to fit well Hubble-Reynolds laws. It is also well fitted by Sersic R^(1/m) laws, for m ~ 3, but in a much narrower range of m and with much larger effective radii than are observed. Assuming in turn reasonable values of the effective radius, the mass density profiles imply a mass-to-light ratio that increases outwards at all radii.Comment: 13 pages, 13 figures, uses mn.sty and epsf.tex, revised version accepted by MNRAS (results for anisotropic models added, comparison with observations extended

Tangential Large Scale Structure as a Standard Ruler: Curvature Parameters from Quasars

Several observational analyses suggest that matter is spatially structured at $\approx 130h^{-1}Mpc$ at low redshifts. This peak in the power spectrum provides a standard ruler in comoving space which can be used to compare the local geometry at high and low redshifts, thereby constraining the curvature parameters. It is shown here that this power spectrum peak is present in the observed quasar distribution at $z\sim 2$: qualitatively, via wedge diagrams which clearly show a void-like structure, and quantitatively, via one-dimensional Fourier analysis of the quasars' tangential distribution. The sample studied here contains 812 quasars. The method produces strong constraints (68% confidence limits) on the density parameter $\Omega_0$ and weaker constraints on the cosmological constant $\lambda_0$, which can be expressed by the relation $\Omega_0 = (0.24 \pm0.15) + (0.10\pm0.08) \lambda_0$. Independently of $\lambda_0$ (in the range $\lambda_0 \in [0,1]$), the constraint is $0.1 < \Omega_0 < 0.45$. Combination of the present results with SN Type Ia results yields $\Omega_0 = (0.30\pm0.11) + (0.57\pm0.11) (\lambda_0-0.7),$ $0.55 < \lambda_0 < 0.95,$ (68% confidence limits). This strongly supports the possibility that the observable universe satisfies a nearly flat, perturbed Friedmann-Lema\^{\i}tre-Robertson-Walker model, independently of any cosmic microwave background observations.Comment: 15 pages, 15 figures; v2 has several minor modifications but conclusions unchanged; accepted by Astronomy & Astrophysic

The nature of the nearest compact group of galaxies from precise distance measurements

Compact groups (CGs) of galaxies, similar to those catalogued by Hickson, appear to be the densest galaxy structures in the Universe. Redshift information is insufficient to determine whether a CG is roughly as dense in 3D as it appears in projection, or whether it is caused by a chance alignment (CA) along the line of sight within a larger galaxy system. Recent precise distance measurements help probe the nature of the nearest CG, situated in the Virgo cluster, whose dominant member is M60. The isolated status of the CG is reassessed with recent photometry and a statistical analysis is performed on the surface brightness fluctuation (SBF) distances measured by Mei et al. in Virgo, for 4 of the 5 CG members. The neighboring galaxy NGC 4606 appears (with 80-90% confidence) to be too faint to affect the isolated status of the CG. Taken at face value, the SBF distances suggest that M59 and NGC 4660 lie roughly 2 Mpc to the foreground of M60, NGC 4638, and the bulk of the Virgo cluster. The statistical analysis confirms that the CG is, indeed, the result of a CA of its galaxies, with NGC 4638 lying at least 800 kpc further away (with 99% confidence) than either M59 or NGC 4660. The first two galaxy distances are consistent with M59 and NGC 4660 constituting a tight pair. The dominant galaxy, M60, is at least 440 kpc more distant (95% confidence) than the M59+NGC 4660 pair, and over 1 Mpc (99% confidence) more distant if one uses the broken linear calibration of the SBF distances. This work constitutes the first direct analysis of the nature of a compact group of galaxies. CAs of galaxies represent a realistic alternative to truly dense groups to explain the nature of CGs. With current SBF distance accuracies, one could determine the nature of HCG 68 in the same way.Comment: 5 pages, 4 figures, accepted as Research Note in A&A, final version with corrected Englis