Molecular content of a type-Ia SN host galaxy at z=0.6

We study the properties and the molecular content of the host of a type-Ia supernova (SN1997ey). This z=0.575 host is the brightest submillimetre source of the sample of type-Ia supernova hosts observed at 450um and 850um by Farrah et al.. Observations were performed at IRAM-30m to search for CO(2-1) and CO(3-2) lines in good weather conditions but no signal was detected. The star formation rate cannot exceed 50 M_sol/yr. These negative results are confronted with an optical analysis of a Keck spectrum and other data archives. We reach the conclusion that this galaxy is a late-type system (0.7 L^B_*), with a small residual star-formation activity (0.2 M_sol/yr) detected in the optical. No source of heating (AGN or starburst) is found to explain the submillimetre-continuum flux and the non-CO detection excludes the presence of a large amount of cold gas. We thus suggest that either the star formation activity is hidden in the nucleus (with A_V ~ 4) or this galaxy is passive or anemic and this flux might be associated with a background galaxy.Comment: 8 pages, 7 figures, accepted for publication in MNRA

CO investigation of z=0.4-1.5 galaxies

We report on the results of an IRAM-30m search for CO emission lines in three galaxies at intermediate redshifts. The idea was to investigate the molecular content of galaxies bright in the infrared at z=0.4-1.5, a redshift desert for molecular line studies, poorly investigated as of yet. We integrated 8-10h per source and did not succeed in detecting any of the sources. From our upper limits, we are able to constrain the molecular gas content in these systems to less than 4 to 8 x 10^9 Mo, assuming a CO-to-H_2 conversion factor (\alpha=0.8 Mo/(K km s^-1 pc^2)). We stress the current difficulty of selecting sources with a detectable molecular content, a problem that will be faced by the ALMA First Science projects.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy and Astrophysic

Molecular Gas and Star Formation in the SAURON Early-type Galaxies

We present the results of a survey of CO emission in 43 of the 48 representative E/S0 galaxies observed in the optical with the SAURON integral-field spectrograph. The CO detection rate is 12/43 or 28%. This is lower than previous studies of early-types but can probably be attributed to different sample selection criteria. As expected, earlier type, more luminous and massive galaxies have a relatively lower molecular gas content. We find that CO-rich galaxies tend to have higher H\beta but lower Fe5015 and Mgb absorption indices than CO-poor galaxies. Those trends appear primarily driven by the age of the stars, an hypothesis supported by the fact that the galaxies with the strongest evidence of star formation are also the most CO-rich. In fact, the early-type galaxies from the current sample appear to extend the well-known correlations between FIR luminosity, dust mass and molecular mass of other galaxy types. The star formation interpretation is also consistent with the SAURON galaxies' radio continuum and FIR flux ratios, and their inferred star formation efficiencies are similar to those in spiral galaxies. It thus appears that we have identified the material fueling (residual) star formation in early-type galaxies, and have demonstrated that it is actively being transformed. Nevertheless, the lack of strong correlations between the CO content and most stellar parameters is compatible with the idea that, in a significant number of sample galaxies, the molecular gas has been accreted from the outside and has properties rather independent from the old, pre-existing stellar component.Comment: 14 pages, 9 figures, accepted in MNRA

Conductivity and the current-current correlation measure

We review various formulations of conductivity for one-particle Hamiltonians and relate them to the current-current correlation measure. We prove that the current-current correlation measure for random Schr\"odinger operators has a density at coincident energies provided the energy lies in a localization regime. The density vanishes at such energies and an upper bound on the rate of vanishing is computed. We also relate the current-current correlation measure to the localization length

Star formation efficiency in galaxy interactions and mergers: a statistical study

We investigate the enhancement of star formation efficiency in galaxy interactions and mergers, by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are: (1) retrograde encounters have a larger star formation efficiency (SFE) than direct encounters; (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase; (3) there is an anticorrelation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions; (4) globally, the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies, interacting pairs and mergers; (5) the enhanced star formation is essentially occurring in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these statistical results, we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic

Gravitational torques in spiral galaxies: gas accretion as a driving mechanism of galactic evolution

The distribution of gravitational torques and bar strengths in the local Universe is derived from a detailed study of 163 galaxies observed in the near-infrared. The results are compared with numerical models for spiral galaxy evolution. It is found that the observed distribution of torques can be accounted for only with external accretion of gas onto spiral disks. Accretion is responsible for bar renewal - after the dissolution of primordial bars - as well as the maintenance of spiral structures. Models of isolated, non-accreting galaxies are ruled out. Moderate accretion rates do not explain the observational results: it is shown that galactic disks should double their mass in less than the Hubble time. The best fit is obtained if spiral galaxies are open systems, still forming today by continuous gas accretion, doubling their mass every 10 billion years.Comment: 4 pages, 2 figures, Astronomy and Astrophysics Letters (accepted