Tuesday 02/26/13

Two dusty papers today. The first, Eric Murphy et al., compares the radio and MIR properties of LIRG and ULIRGs. Interestingly, the radio spectral index correlates with the compactness of the starburst region, as measured by size of the radio emission, increased dust obscuration by the silicate absorption or small 6.2 micron PAH equivalent widths (suggesting that EWs might be low due to lots of hot dust).

The second paper is by Simone Bianchi expounding on why single-temp dust fits (modified blackbodies) are indeed ok. At least partly in reply to Dale et al. 2012….

Title: Radio and Mid-Infrared Properties of Compact Starbursts: Distancing Themselves from the Main Sequence
Authors: Murphy, E. J.; Stierwalt, S.; Armus, L.; Condon, J. J.; Evans, A. S.
Bibliographic Code: 2013arXiv1302.5704M

Abstract

We investigate the relationship between 8.44\,GHz brightness temperatures and 1.4 to 8.44\,GHz radio spectral indices with 6.2\,$\mu$m polycyclic aromatic hydrocarbon (PAH) emission and 9.7\,$\mu$m silicate absorption features for a sample of 36 local luminous and ultra-luminous infrared galaxies. We find that galaxies having small 6.2\,$\mu$m PAH equivalent widths (EQWs), which signal the presence of weak PAH emission and/or an excess of very hot dust, also have flat spectral indices. The three active galactic nuclei (AGN) identified through their excessively large 8.44\,GHz brightness temperatures are also identified as AGN via their small 6.2\,$\mu$m PAH EQWs. We also find that the flattening of the radio spectrum increases with increasing silicate optical depth, 8.44\,GHz brightness temperature, and decreasing size of the radio source even after removing potential AGN, supporting the idea that compact starbursts show spectral flattening as the result of increased free-free absorption. These correlations additionally suggest that the dust obscuration in these galaxies must largely be coming from the vicinity of the compact starburst itself, and is not distributed throughout the (foreground) disk of the galaxy. Finally, we investigate the location of these infrared-bright systems relative to the main sequence (star formation rate vs. stellar mass) of star-forming galaxies in the local universe. We find that the radio spectral indices of galaxies flattens with increasing distance above the main sequence, or in other words, with increasing specific star formation rate. This indicates that galaxies located above the main sequence, having high specific star formation rates, are typically compact starbursts hosting deeply embedded star formation that becomes more optically thick in the radio and infrared with increased distance above the main sequence.

Title: Vindicating single-T modified blackbody fits to Herschel SEDs (Research Note)
Authors: Bianchi, Simone
Bibliographic Code: 2013arXiv1302.5699B

Abstract

I show here that the bulk of the dust mass in a galaxy can be equivalently estimated from: i) the full spectral energy distribution of dust emission, using the approach of Draine & Lee (2007) that includes a distribution of dust grains and a range of interstellar radiation field intensities; ii) the emission in the wavelength range 100um <= lambda <= 500um (covered by the Herschel Space Observatory), by fitting to the data a simpler single temperature modified blackbody. Recent claims on the contrary (Dale et al. 2012) should be interpreted as a caveat to use in the simpler fits an absorption cross section which is consistent both in the normalization and in the spectral index beta with that of the full dust model. I also show that the dust mass does not depend significantly on the choice of beta, if both the dust mass and the absorption cross section are derived with the same

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