Thursday July 19th, 2012

Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: NGC 628 and NGC 6946

Authors: G. Aniano, B. T. Draine, D. Calzetti, D. A. Dale, C. W. Engelbracht, K. D. Gordon, L. K. Hunt, R. C. Kennicutt, O. Krause, A. K. Leroy, H-W. Rix, H. Roussel, K. Sandstrom, M. Sauvage, F. Walter, L. Armus, A. D. Bolatto, A. Crocker, J. Donovan Meyer, M. Galametz, G. Helou, J. Hinz, B. D. Johnson, J. Koda, E. Montiel, E. J. Murphy, R. Skibba, J.-D.T. Smith, M. G. Wolfire
(Submitted on 17 Jul 2012)

Abstract: We characterize the dust in NGC628 and NGC6946, two nearby spiral galaxies in the KINGFISH sample. With data from 3.6um to 500um, dust models are strongly constrained. Using the Draine & Li (2007) dust model, (amorphous silicate and carbonaceous grains), for each pixel in each galaxy we estimate (1) dust mass surface density, (2) dust mass fraction contributed by polycyclic aromatic hydrocarbons (PAH)s, (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in regions with high starlight intensity. We obtain maps for the dust properties, which trace the spiral structure of the galaxies. The dust models successfully reproduce the observed global and resolved spectral energy distributions (SEDs). The overall dust/H mass ratio is estimated to be 0.0082+/-0.0017 for NGC628, and 0.0063+/-0.0009 for NGC6946, consistent with what is expected for galaxies of near-solar metallicity. Our derived dust masses are larger (by up to a factor 3) than estimates based on single-temperature modified blackbody fits. We show that the SED fits are significantly improved if the starlight intensity distribution includes a (single intensity) "delta function" component. We find no evidence for significant masses of cold dust T<12K. Discrepancies between PACS and MIPS photometry in both low and high surface brightness areas result in large uncertainties when the modeling is done at PACS resolutions, in which case SPIRE, MIPS70 and MIPS160 data cannot be used. We recommend against attempting to model dust at the angular resolution of PACS.

Comments: To be published in Apj, September 2012. See the full version at this http URL
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)
Cite as: arXiv:1207.4186v1 [astro-ph.CO]

The role of cold gas and environment on the stellar mass – metallicity relation of nearby galaxies

Authors: T. M. Hughes, L. Cortese, A. Boselli, G. Gavazzi, J. I. Davies
(Submitted on 17 Jul 2012)

Abstract: We investigate the relationship between stellar mass, metallicity and gas content for a magnitude- and volume-limited sample of 260 nearby late-type galaxies in different environments, from isolated galaxies to Virgo cluster members. We derive oxygen abundance estimates using new integrated, drift-scan optical spectroscopy and the base metallicity calibrations of Kewley & Ellison (2008). Combining these measurements with ultraviolet to near-infrared photometry and HI 21 cm line observations, we examine the relations between stellar mass, metallicity, gas mass fraction and star formation rate. We find that, at fixed stellar mass, galaxies with lower gas fractions typically also possess higher oxygen abundances. We also observe a relationship between gas fraction and metal content, whereby gas-poor galaxies are typically more metal-rich, and demonstrate that the removal of gas from the outskirts of spirals increases the observed average metallicity by approximately 0.1 dex. Although some cluster galaxies are gas-deficient objects, statistically the stellar-mass metallicity relation is nearly invariant to the environment, in agreement with recent studies. These results indicate that internal evolutionary processes, rather than environmental effects, play a key role in shaping the stellar mass-metallicity relation. In addition, we present metallicity estimates based on observations of 478 nearby galaxies.

Comments: Accepted for publication in A&A. 28 pages, including 10 figures and 3 tables
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1207.4191v1 [astro-ph.CO]
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