Polar-ring galaxies: the SDSS view on the symbiotic galaxies

Polar-ring galaxies are multi-spin systems, showing star formation in a blue late-type component, perpendicular to a red early-type one, revealing how galaxy formation can sometimes occur in successive steps. We perform two-dimensional decomposition in the $g$, $r$, $i$ bandpasses of 50 polar-ring galaxies (PRGs) from the Sloan Digital Sky Survey. Each object was fit with a S\'ersic host galaxy and a S\'ersic ring. Our general results are: (i) The central (host) galaxies of the PRGs are non-dwarf sub-$L^{\ast}$ galaxies with colors typical for early-type galaxies. (ii) Polar structures in our sample are, on average, fainter and bluer than their host galaxies. (iii) In most galaxies, the stellar mass M$_*$ of the polar component is not negligible in comparison with that of the host. (iv) The distributions of the host galaxies on the size -- luminosity and Kormendy diagrams are shifted by $\sim 1^m$ to fainter magnitudes in comparison with E/S0 galaxies. It means that the PRGs hosts are more similar to quenched disks than to ordinary early-type galaxies. (v) All the PRGs in our sample are detected in mid-infrared by WISE, and we derive from the 22$\mu$m luminosity their star formation rate (SFR). Their SFR/M$_*$ ratio is larger than for the early-type galaxy sample of Atlas$^{\rm 3D}$, showing that the star forming disk brings a significant contribution to the new stars. Globally, PRGs appear frequently on the green valley in the mass-color diagram, revealing the symbiotic character between a red-sequence host and a blue cloud ring.Comment: 8 pages, 11 figures, accepted for publication in MNRA

On the global structure of distant galactic disks

Radial and vertical profiles are determined for a sample of 34 edge-on disk galaxies in the HDFs, selected for their apparent diameter larger than 1.3 arcsec and their unperturbed morphology. The thickness and flatness of their galactic disks are determined and discussed with regard to evolution with redshift. We find that sub-L* spiral galaxies with z \sim 1 have a relative thickness or flatness (characterized by h_z/h the scaleheight to scalelength ratio) globally similar to those in the local Universe. A slight trend is however apparent, with the h_z/h flatness ratio larger by a factor of \sim 1.5 in distant galaxies if compared to local samples. In absolute value, the disks are smaller than in present-day galaxies. About half of the z \sim 1 spiral disks show a non-exponential surface brightness distribution.Comment: 13 pages, 8 figures, accepted in A and

Molecular gas and star formation in M81

We present IRAM 30m observations of the central 1.6 kpc of the spiral M81 galaxy. The molecular gas appears weak and with an unusual excitation physics. We discuss a possible link between low CO emission and weak FUV surface brightness.Comment: 2 pages, 2 figures, to appear in "Pathways through an eclectic Universe", J. H. Knapen, T. J. Mahoney, and A. Vazdekis (Eds.), ASP Conf. Ser., 200

First Detection of Molecular Gas in the Shells of CenA

Shells are faint arc-like stellar structures, which have been observed around early type galaxies and are thought to be the result of an interaction. HI gas has recently been detected in shells, a surprising result in view of the theoretical predictions that most of the gas should decouple from stars and fall into the nucleus in such interactions. Here we report the first detection of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128 (CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The ratio between CO and HI emission in the shells is the same as that found in the central regions, which is unexpected given the metallicity gradient usually observed in galaxies. We propose that the dynamics of the gas can be understood within the standard picture of shell formation if one takes into account that the interstellar medium is clumpy and hence not highly dissipative. The observed metal enrichment could be due to star formation induced by the AGN jet in the shells. Furthermore our observations provide evidence that molecular gas in mergers may be spread out far from the nuclear regions.Comment: Accepted for publication in Astronomy & Astrophysics Letters, (Vol. 356), 4 pages + 1 color figur

Gravitational Lensing & Stellar Dynamics

Strong gravitational lensing and stellar dynamics provide two complementary and orthogonal constraints on the density profiles of galaxies. Based on spherically symmetric, scale-free, mass models, it is shown that the combination of both techniques is powerful in breaking the mass-sheet and mass-anisotropy degeneracies. Second, observational results are presented from the Lenses Structure & Dynamics (LSD) Survey and the Sloan Lens ACS (SLACS) Survey collaborations to illustrate this new methodology in constraining the dark and stellar density profiles, and mass structure, of early-type galaxies to redshifts of unity.Comment: 6 pages, 2 figures; Invited contribution in the Proceedings of XXIst IAP Colloquium, "Mass Profiles & Shapes of Cosmological Structures" (Paris, 4-9 July 2005), eds G. A. Mamon, F. Combes, C. Deffayet, B. Fort (Paris: EDP Sciences