Paradis, D. and Paladini, R. and Noriega-Crespo, A. and Mény, C. and Piacentini, F. and Thompson, M. A. and Marshall , D. J. and Veneziani, M. and Bernard, J. -P. and Molinari, S. (2012) Detection and characterization of a 500 μm dust emissivity excess in the Galactic plane using Herschel/Hi-GAL observations. Astronomy and Astrophysics, 537 . A113. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20120417-150941949
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Context. Past and recent observations have revealed unexpected variations in the far-infrared – millimeter (FIR-mm) dust emissivity in the interstellar medium. In the Herschel spectral range, those are often referred to as a 500 μm emission excess. Several dust emission models have been developed to interpret astrophysical data in the FIR-mm domain. However, these are commonly unable to fully reconcile theoretical predictions with observations. In contrast, the recently revised two level system (TLS) model, based on the disordered internal structure of amorphous dust grains, seems to provide a promising way of interpreting existing data. Aims. The newly available Herschel infrared GALactic (Hi-GAL) data, which covers most of the inner Milky Way, offers a unique opportunity to investigate possible variations in the dust emission properties both with wavelength and environment. The goal of our analysis is to constrain the internal structure of the largest dust grains on Galactic scales, in the framework of the TLS model. Methods. By combining the IRIS (Improved Reprocessing of the IRAS Survey) 100 μm with the Hi-GAL 160, 250, 350, and 500 μm data, we model the dust emission spectra in each pixel of the Hi-GAL maps, using both the TLS model and, for comparison, a single modified black-body fit. The effect of temperature mixing along the line of sight is investigated to test the robustness of our results. Results. We find a slight decrease in the dust temperature with distance from the Galactic center, confirming previous results. We also report the detection of a significant 500 μm emissivity excess in the peripheral regions of the plane (35° < |l| < 70°) of about 13–15% of the emissivity, which can reach up to 20% in some HII regions. We present the spatial distributions of the best-fit values for the two main parameters of the TLS model, i.e. the charge correlation length, lc, used to characterize the disordered charge distribution (DCD) part of the model, and the amplitude A of the TLS processes with respect to the DCD effect. These distributions illustrate the variations in the dust properties with environment, in particular the plausible existence of an overall gradient with distance to the Galactic center. A comparison with previous findings in the solar neighborhood shows that the local value of the excess is less than expected from the Galactic gradient observed here.
|Additional Information:||© 2012 ESO. Article published by EDP Sciences. Received 26 August 2011; Accepted 7 November 2011. Published online 17 January 2012. We would like to thank the anonymous referee for his invaluable comments. D.P. acknowledges grant support from the Centre National d’Études Spatiales (CNES). Herschel is an ESA space observatory with science instruments provided by European-led Principal investigator consortia and with important participation from NASA.|
|Subject Keywords:||infrared: ISM; dust, extinction; submillimeter: ISM|
|Official Citation:||Detection and characterization of a 500 μm dust emissivity excess in the Galactic plane using Herschel/Hi-GAL observations D. Paradis, R. Paladini, A. Noriega-Crespo, C. Mény, F. Piacentini, M. A. Thompson, D. J. Marshall, M. Veneziani, J.-P. Bernard and S. Molinari A&A 537 A113 (2012) DOI: http://dx.doi.org/10.1051/0004-6361/201117956|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||18 Apr 2012 20:52|
|Last Modified:||26 Dec 2012 15:04|
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