Letter to the Editor

Variations of the spectral index of dust emissivity from Hi-GAL observations of the Galactic plane^*

¹ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA e-mail: paradis@ipac.caltech.edu
² Dipartimento di Fisica, Universita di Roma 1 La Sapienza, 00185 Roma, Italy
³ Université de Toulouse, UPS, CESR, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
⁴ CNRS, UMR 5187, 31028 Toulouse, France
⁵ ASI Science Data Center, 00044 Frascati (Rome), Italy
⁶ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
⁷ Dipartimento di Fisica, Universita di Roma Tor Vergata, Rome, Italy
⁸ Centre for Astrophysics Research, Science & Technology Institute, University of Hertfordshire, Hatfield, AL10 9AB, UK
⁹ INAF - IFSI - via Fosso del Cavaliere 100, Rome, Italy

Received: 30 June 2010
Accepted: 13 September 2010

Abstract

Context. Variations in the dust emissivity are critical for gas mass determinations derived from far-infrared observations, but also for separating dust foreground emission from the Cosmic Microwave Background (CMB). Hi-GAL observations allow us for the first time to study the dust emissivity variations in the inner regions of the Galactic plane at resolution below 1°.

Aims. We present maps of the emissivity spectral index derived from the combined Herschel PACS 160 μm, SPIRE 250 μm, 350 μm, and 500 μm data, and the IRIS 100 μm data, and we analyze the spatial variations of the spectral index as a function of dust temperature and wavelength in the two science demonstration phase Hi-GAL fields, centered at l = 30° and l = 59°.

Methods. Applying two different methods, we determine both dust temperature and emissivity spectral index between 100 and 500 μm, at an angular resolution (θ) of 4'.

Results. Combining both fields, the results show variations of the emissivity spectral index in the range 1.8–2.6 for temperatures between 14 and 23 K. The median values of the spectral index are similar in both fields, i.e. 2.3 in the range 100–500 μm, while the median dust temperatures are equal to 19.1 K and 16.0 K in the l = 30° and l = 59° field, respectively. Statistically, we do not see any significant deviations in the spectra from a power law emissivity between 100 and 500 μm. We confirm the existence of an inverse correlation between the emissivity spectral index and dust temperature, found in previous analyses.