Molecular lines studies at redshift greater than 1

Observations of CO molecules in the millimetrer domain at high redshift (larger than 1), have provided interesting informations about star formation efficiency, and its evolution with redshift. Due to the difficulty of the detections, selection effects are important. The detection if often due to gravitational amplification. Objects selected by their (far)infrared flux, are in general associated to ULIRGS, mergers with starburst in the nuclear regions. Quasars have been selected as powerful optical sources, and have been found to be associated to starbursts, rich in gas. The gas fraction appears to be much higher at redshift greater than 1. Quasars allow to probe the end of the reionisation period, and the relation between bulge and black hole mass. However these selection bias could have led us to miss some gaseous galaxies, with low-efficiency of star formation, such as the more quiescent objects selected by their BzK colors at z=1.5 or 2.Comment: 3 pages, to appear in IAU highlights, JD14 "FIR2009, The ISM of galaxies in the Far-infrared and Sub-millimeter

Deep CO Observations and the CO-to-H_2 Conversion Factor in DDO 154, a Low Metallicity Dwarf Irregular Galaxy

We present a deep spectroscopic search for CO emission in the dwarf irregular galaxy DDO154, which has an Oxygen abundance of only 1/20 the solar value. The observations were conducted in order to constrain the CO-to-$\mathrm{H_2}$ conversion factor at low metallicity. No CO was detected, however, despite being one of the sensitive observations done towards galaxies of this type. We succeed in putting a strong lower limit on the conversion factor, at least 10 times the Galactic value. Our result supports previous studies which argue for a high conversion factor at low metallicity.Comment: 11 pages, 4 figures. Accepted for publication in PAS

Physical Properties, Star Formation, and Active Galactic Nucleus Activity in Balmer Break Galaxies at 0 < z < 1

We present a spectroscopic study with the derivation of the physical properties of 37 Balmer break galaxies, which have the necessary lines to locate them in star-forming-AGN diagnostic diagrams. These galaxies span a redshift range from 0.045 to 0.93 and are somewhat less massive than similar samples of previous works. The studied sample has multiwavelength photometric data coverage from the ultraviolet to MIR Spitzer bands. We investigate the connection between star formation and AGN activity via optical, mass-excitation (MEx) and MIR diagnostic diagrams. Through optical diagrams, 31 (84%) star-forming galaxies, 2 (5%) composite galaxies and 3 (8%) AGNs were classified, whereas from the MEx diagram only one galaxy was classified as AGN. A total of 19 galaxies have photometry available in all the IRAC/Spitzer bands. Of these, 3 AGN candidates were not classified as AGN in the optical diagrams, suggesting they are dusty/obscured AGNs, or that nuclear star formation has diluted their contributions. Furthermore, the relationship between SFR surface density (\Sigma_{SFR}) and stellar mass surface density per time unit (\Sigma_{M_{\ast}/\tau}) as a function of redshift was investigated using the [OII] \lambda3727, 3729, H\alpha \lambda6563 luminosities, which revealed that both quantities are larger for higher redshift galaxies. We also studied the SFR and SSFR versus stellar mass and color relations, with the more massive galaxies having higher SFR values but lower SSFR values than less massive galaxies. These results are consistent with previous ones showing that, at a given mass, high-redshift galaxies have on average larger SFR and SSFR values than low-redshift galaxies. Finally, bluer galaxies have larger SSFR values than redder galaxies and for a given color the SSFR is larger for higher redshift galaxies.Comment: preprint version, 36 pages, 17 figures, 3 tables, accepted for publication in the Astrophysical Journa

Constraint on the inflow/outflow rates in star-forming galaxies at z~1.4 from molecular gas observations

We constrain the rate of gas inflow into and outflow from a main-sequence star-forming galaxy at z~1.4 by fitting a simple analytic model for the chemical evolution in a galaxy to the observational data of the stellar mass, metallicity, and molecular gas mass fraction. The molecular gas mass is derived from CO observations with a metallicity-dependent CO-to-H2 conversion factor, and the gas metallicity is derived from the H{\alpha} and [NII]{\lambda} 6584 emission line ratio. Using a stacking analysis of CO integrated intensity maps and the emission lines of H{\alpha} and [NII], the relation between stellar mass, metallicity, and gas mass fraction is derived. We constrain the inflow and outflow rates with least-chi-square fitting of a simple analytic chemical evolution model to the observational data. The best-fit inflow and outflow rates are ~1.7 and ~0.4 in units of star-formation rate, respectively. The inflow rate is roughly comparable to the sum of the star-formation rate and outflow rate, which supports the equilibrium model for galaxy evolution; i.e., all inflow gas is consumed by star formation and outflow.Comment: 5 pages, 2 figures, Accepted for publication in the Ap