Letter to the Editor

Dust/gas correlations from Herschel observations ^*

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: duval@stsci.edu
² Sterrewacht Leiden, Leiden University, PO 9513, 2300 RA Leiden, The Netherlands e-mail: israel@strw.leidenuniv.nl
³ University of Maryland, Department of Astronomy, Lab for Millimeter Wave Astronomy, College Park, MD 20742, USA e-mail: bolatto@astro.umd.edu
⁴ Centre for Supercomputing and Astrophysics, Swinburne University of Technology, Hawthorn VIC 3122, Australia e-mail: ahughes@astro.swin.edu.au
⁵ CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
⁶ National Radio Astronomy Obsevatory, 20 Edgemont Road Charlottesville, VA 22903-2475, USA e-mail: aleroy@nrao.edu
⁷ CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France e-mail: [smadden;sacha.hony;marc.sauvage; Koryo.OKUMURA;pasquale.panuzzo]@cea.fr
⁸ Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125, USA e-mail: [paradis;reach]@ipac.caltech.edu
⁹ Department of Astrophysics, Nagoya University, Nagoya 464-8602, Japan e-mail: kawamura@a.phys.nagoya-u.ac.jp
¹⁰ University of Missouri, Department of Physics and Astronomy, 314 Physics Building, Columbia, MO 65211, USA e-mail: lia@missouri.edu
¹¹ University of Illinois, Dept. of Astronomy, MC 221, Urbana, IL 61801, USA e-mail: wongt@astro.illinois.edu
¹² Centre d'Étude Spatiale des Rayonnements, CNRS, 9 av. du Colonel Roche, BP 4346, 31028 Toulouse, France e-mail: Jean-Philippe.Bernard@cesr.fr
¹³ Steward Observatory, University of Arizona, 933 North Cherry Ave., Tucson, AZ 85721, USA e-mail: [cengelbracht;kmisselt]@as.arizona.edu
¹⁴ Sejong University, Astronomy & Space Science, 143-747, Seoul, South Korea e-mail: sek@sejong.ac.kr
¹⁵ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801-0387, USA e-mail: jott@nrao.edu
¹⁶ Jet Propulsion Laboratory M/S 169-507, 4800 Oak Grove Dr., Pasadena, CA 91109, USA e-mail: Jorge.Pineda@jpl.nasa.gov
¹⁷ Stratospheric Observatory for Infrared Astronomy, Universities Space Research Association, Mail Stop 211-3, Moffett Field, CA 94035, USA
¹⁸ Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile e-mail: rubio.monik@gmail.com

Received: 30 March 2010
Accepted: 16 April 2010

Abstract

Context. Previous Spitzer and IRAS observations of the LMC suggest an excess of FIR emission with respect to the gas surface density traced by ¹²CO rotational emission lines and H i 21 cm emission. This so-called “FIR excess” is especially noticeable near molecular clouds in the LMC, and has usually been interpreted as indicating the presence of a self-shielded H₂ component not traced by CO in the envelopes of molecular clouds.

Aims. Based on Herschel HERITAGE observations taken as part of the science demonstration phase, we examine the correlation between gas and dust surface densities at higher resolution than previously achieved. We consider three additional possible causes for the FIR excess: X factor, FIR dust emissivity, and gas-to-dust ratio variations between the diffuse and dense phases of the ISM.

Methods. We examine the structure of NT80 and NT71, two molecular clouds detected in the NANTEN ¹²CO survey of the LMC. Dust surface density maps were derived from the HERITAGE data. The gas phase is traced by MAGMA ¹²CO and ATCA+Parkes H i 21 cm observations of the LMC. These data provide unprecedented resolution (1’) to examine the structure of molecular clouds. The dust emissivity, gas-to-dust ratio, and X factor required to match the dust and gas surface densities are derived, and their correlations with the dust surface density are examined.

Results. We show that the dust surface density is spatially correlated with the atomic and molecular gas phases. The dust temperature is consistently lower in the dense phase of the ISM than in the diffuse phase. We confirm variations in the ratio of FIR emission to gas surface density derived from H i and CO observations. There is an excess of FIR emission, spatially correlated with regions of intermediate H i and dust surface densities (A_V = 1-2), and little or no CO. While there is no significant trend in the dust emissivity or gas-to-dust ratio with dust surface density, the X factor is enhanced at A_V = 1-2. We conclude that H₂ envelopes not traced by CO and X factor variations close to the CO boundary may be more likely to cause these deviations between FIR emission and gas surface density than gas-to-dust ratio or emissivity variations.