Tuesday, 04/02/13

For the top of today’s list: [CII] emission in M33, a study on gas and dust cooling (Kramer et al.) Enough said, have a look. Next is a paper studying the CO(1-0) and CO(3-2) emission in 31 intermed-redshift (z=0.05-0.5) galaxies (Bauermeister et al.) and one that is very related to the paper I posted last week on the mass-metallicity relationship, except this one is longer, by the guy that does the theoretical calibration (Pilyugin) and focuses more on the specific SFR than redshift evolution, it looks like… Enjoy! – Alison

Title: Gas and dust cooling along the major axis of M33 (HerM33es): ISO/LWS CII observations
Authors: Kramer, C.; Abreu-Vicente, J.; Garcia-Burillo, S.; Relano, M.; Aalto, S.; Boquien, M.; Braine, J.; Buchbender, C.; Gratier, P.; Israel, F. P.; Nikola, T.; Roellig, M.; Verley, S.; van der Werf, P.;Xilouris, E. M.
Comment: (16 pages, a version with higher resolution figures may be downloaded from http://www.iram.es/IRAMES/hermesWiki/HermesPublications)
Bibliographic Code: 2013arXiv1304.0199K


We aim to better understand the heating of the gas by observing the prominent gas cooling line [CII] at 158um in the low-metallicity environment of the Local Group spiral galaxy M33 at scales of 280pc. In particular, we aim at describing the variation of the photoelectric heating efficiency with galactic environment. In this unbiased study, we used ISO/LWS [CII] observations along the major axis of M33, in combination with Herschel PACS and SPIRE continuum maps, IRAM 30m CO 2-1 and VLA HI data to study the variation of velocity integrated intensities. The ratio of [CII] emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, FUV radiation fields, and average column densities of the molecular clouds. The heating efficiency stays constant at 0.8% in the inner 4.5kpc radius of the galaxy where it starts to increase to reach values of ~3% in the outskirts at about 6kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields, for optical extinctions of only a few magnitudes at constant metallicity. In view of the significant fraction of HI emission stemming from PDRs, and for typical pressures found in the Galactic cold neutral medium (CNM) traced by HI emission, the CNM contributes ~15% to the observed [CII] emission in the inner 2kpc radius of M33. The CNM contribution remains largely undetermined in the south, while positions between 2 and 7.3kpc radial distance in the north of M33 show a contribution of ~40%+-20%.

Title: The EGNoG Survey: Molecular Gas in Intermediate-Redshift Star-Forming Galaxies
Authors: Bauermeister, Amber; Blitz, Leo; Bolatto, Alberto D.; Bureau, Martin; Leroy, Adam; Ostriker, Eve; Teuben, Peter J.; Wong, Tony; Wright, Melvyn C. H.
Comment: Accepted for publication in the Astrophysical Journal; 29 pages, 20 figures, 6 tables
Bibliographic Code: 2013arXiv1304.0010B


We present the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey, an observational study of molecular gas in 31 star-forming galaxies from z=0.05 to z=0.5, with stellar masses of (4-30)x10^10 M_Sun and star formation rates of 4-100 M_Sun yr^-1. This survey probes a relatively un-observed redshift range in which the molecular gas content of galaxies is expected to have evolved significantly. To trace the molecular gas in the EGNoG galaxies, we observe the CO(1-0) and CO(3-2) rotational lines using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We detect 24 of 31 galaxies and present resolved maps of 10 galaxies in the lower redshift portion of the survey. We use a bimodal prescription for the CO to molecular gas conversion factor, based on specific star formation rate, and compare the EGNoG galaxies to a large sample of galaxies assembled from the literature. We find an average molecular gas depletion time of 0.76 \pm 0.54 Gyr for normal galaxies and 0.06 \pm 0.04 Gyr for starburst galaxies. We calculate an average molecular gas fraction of 7-20% at the intermediate redshifts probed by the EGNoG survey. By expressing the molecular gas fraction in terms of the specific star formation rate and molecular gas depletion time (using typical values), we also calculate the expected evolution of the molecular gas fraction with redshift. The predicted behavior agrees well with the significant evolution observed from z~2.5 to today.

Title: The metallicity – redshift relations for emission-line SDSS galaxies: examination of the dependence on the star formation rate
Authors: Pilyugin, L. S.; Lara-Lopez, M. A.; Grebel, E. K.; Kehrig, C.; Zinchenko, I. A.; Lopez-Sanchez, A. R.; Vilchez, J. M.; Mattsson, L.
Comment: 14 pages, 7 figures, accepted for publication in the MNRAS
Bibliographic Code: 2013arXiv1304.0191P


We analyse the oxygen abundance and specific star formation rates (sSFR) variations with redshift in star-forming SDSS galaxies of different masses. We find that the maximum value of the sSFR, sSFRmax, decreases when the stellar mass, Ms, of a galaxy increases, and decreases with decreasing of redshift. The sSFRmax can exceed the time-averaged sSFR by about an order of magnitude for massive galaxies. The metallicity – redshift relations for subsamples of galaxies with sSFR = sSFRmax and with sSFR = 0.1sSFRmax coincide for massive (log(Ms/Mo) > 10.5, with stellar mass Ms in solar units) galaxies and differ for low-mass galaxies. This suggests that there is no correlation between oxygen abundance and sSFR in massive galaxies and that the oxygen abundance correlates with the sSFR in low-mass galaxies. We find evidence in favour of that the irregular galaxies show, on average, higher sSFR and lower oxygen abundances than the spiral galaxies of similar masses and that the mass – metallicity relation for spiral galaxies differs slightly from that for irregular galaxies. The fact that our sample of low-mass galaxies is the mixture of spiral and irregular galaxies can be responsible for the dependence of the metallicity – redshift relation on the sSFR observed for the low-mass SDSS galaxies. The mass – metallicity and luminosity – metallicity relations obtained for irregular SDSS galaxies agree with corresponding relations for nearby irregular galaxies with direct abundance determinations. We find that the aperture effect does not make a significant contribution to the redshift variation of oxygen abundances in SDSS galaxies.

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