Monday 09/10/2012

Spectral Energy Distributions of Local Luminous And Ultraluminous Infrared Galaxies

Vivian U, David B. Sanders, Joseph M. Mazzarella, Aaron S. Evans, Justin H. Howell, Jason A. Surace, Lee Armus, Kazushi Iwasawa, Dong-Chan Kim, Caitlin M. Casey, Tatjana Vavilkin, Michele Dufault, Kirsten Larson, Joshua E. Barnes, Ben H. P. Chan, David T. Frayer, Sebastian Haan, Hanae Inami, Cathy M. Ishida,Jeyhan S. Kartaltepe, Jason L. Melbourne, Andreea Petric
(Submitted on 7 Sep 2012)

Luminous and ultraluminous infrared galaxies ((U)LIRGs) are the most extreme star forming galaxies in the universe. The local (U)LIRGs provide a unique opportunity to study their multi-wavelength properties in detail for comparison to their more numerous counterparts at high redshifts. We present common large aperture photometry at radio through X-ray wavelengths, and spectral energy distributions (SEDs) for a sample of 53 nearby LIRGs and 11 ULIRGs spanning log (LIR/Lsun) = 11.14-12.57 from the flux-limited Great Observatories All-sky LIRG Survey (GOALS). The SEDs for all objects are similar in that they show a broad, thermal stellar peak and a dominant FIR thermal dust peak, where nuLnu(60um) / nuLnu(V) increases from ~2-30 with increasing LIR. When normalized at IRAS-60um, the largest range in the luminosity ratio, R(lambda)=log[nuLnu(lambda)/nuLnu(60um)] observed over the full sample is seen in the Hard X-rays (HX=2-10 keV). A small range is found in the Radio (1.4GHz), where the mean ratio is largest. Total infrared luminosities, LIR(8-1000um), dust temperatures, and dust masses were computed from fitting thermal dust emission modified blackbodies to the mid-infrared (MIR) through submillimeter SEDs. The new results reflect an overall ~0.02 dex lower luminosity than the original IRAS values. Total stellar masses were computed by fitting stellar population synthesis models to the observed near-infrared (NIR) through ultraviolet (UV) SEDs. Mean stellar masses are found to be log(M/Msun) = 10.79+/-0.40. Star formation rates have been determined from the infrared (SFR_IR~45Msun/yr) and from the monochromatic UV luminosities (SFR_UV~1.3Msun/yr), respectively. Multiwavelength AGN indicators have be used to select putative AGN: about 60% of the ULIRGs would have been classified as an AGN by at least one of the selection criteria.

Chemical abundances in M31 from HII regions

Almudena Zurita, Fabio Bresolin
(Submitted on 7 Sep 2012)

We have obtained multi-slit spectroscopic observations from 3700A to 9200A with LRIS at the Keck I telescope for 31 HII regions in the disk of the Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9 kpc to 16.1 kpc. In 9 HII regions we measure one or several auroral lines ([OIII]4363, [NII]5755, [SIII]6312, [OII]7325), from which we determine the electron temperature (Te) of the gas and derive chemical abundances using the ‘direct Te-based method’. We analyze, for the first time in M31, abundance trends with galactocentric radius from the ‘direct’ method, and find that the Ne/O, Ar/O, N/O and S/O abundance ratios are consistent with a constant value across the M31 disc, while the O/H abundance ratio shows a weak gradient. We have combined our data with all spectroscopic observations of HII regions in M31 available in the literature, yielding a sample of 85 HII regions spanning distances from 3.9 kpc to 24.7 kpc (0.19 – 1.2 R25) from the galaxy center. We have tested a number of empirical calibrations of strong emission line ratios. We find that the slope of the oxygen abundance gradient in M31 is -0.023+/-0.002 dex/kpc, and that the central oxygen abundance is in the range 12+log(O/H) = 8.71 – 8.91 dex (i.e. between 1.05 and 1.66 times the solar value, for 12+log(O/H)_solar=8.69), depending on the calibration adopted. The HII region oxygen abundances are compared with the results from other metallicity indicators (supergiant stars and planetary nebulae). The comparison shows that HII region O/H abundances are systematically ~0.3 dex below the stellar ones. This discrepancy is discussed in terms of oxygen depletion onto dust grains and possible biases affecting Te-based oxygen abundances at high metallicity.

The Molecular Gas Content of z = 3 Lyman Break Galaxies; Evidence of a non Evolving Gas Fraction in Main Sequence Galaxies at z > 2

G. E. Magdis (Oxford), E. Daddi (CEA), M. Sargent (CEA), D. Elbaz (CEA), R. Gobat (CEA), H. Dannerbauer(Universitat Wien), Q. Tan (CEA), C. Feruglio (IRAM), D. Rigopoulou (Oxford, RAL), V. Charmandaris (Univ. of Crete, IESL/FORTH), M. Dickinson (NOAO), N. Reddy (Univ. of California), H. Aussel (CNRS)
(Submitted on 7 Sep 2012 (v1), last revised 10 Sep 2012 (this version, v2))

We present observations of the CO[3-2] emission towards two massive and infrared luminous Lyman Break Galaxies at z = 3.21 and z = 2.92, using the IRAM Plateau de Bure Interferometer, placing first constraints on the molecular gas masses (Mgas) of non-lensed LBGs. Their overall properties are consistent with those of typical (Main-Sequence) galaxies at their redshifts, with specific star formation rates ~1.6 and ~2.2 Gyr^(-1), despite their large infrared luminosities L_IR ~2-3 x 10^12 Lsun derived from Herschel. With one plausible CO detection (spurious detection probability of 10^(-3)) and one upper limit, we investigate the evolution of the molecular gas-to-stellar mass ratio (Mgas/M*) with redshift. Our data suggest that the steep evolution of Mgas/M* of normal galaxies up to z~2 is followed by a flattening at higher redshifts, providing supporting evidence for the existence of a plateau in the evolution of the specific star formation rate at z > 2.5.

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