Letter to the Editor

Herschel-SPIRE observations of the Polaris flare: Structure of the diffuse interstellar medium at the sub-parsec scale ^*,**

¹ Institut d'Astrophysique Spatiale, CNRS/Université Paris-Sud 11, 91405 Orsay, France
² Canadian Institute for Theoretical Astrophysics, University of Toronto, ON M5S 3H8, Canada
³ Université de Toulouse; UPS; CESR; CNRS; UMR5187; 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
⁴ Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS & Université de Provence), 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
⁵ CEA, Saclay, France
⁶ CNRS/INSU, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, BP 89, 33271 Floirac Cedex, France
⁷ Radio Astronomy Laboratory, University of California, Berkeley, USA
⁸ IRAM, Grenoble, France
⁹ NOAC, PR China
¹⁰ Space Science Department, Rutherford Appleton Laboratory, Chilton, UK
¹¹ 
Blue Sky Spectrosocpy Inc, Lethbridge, Canada
¹² Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Canada
¹³ Department of Physics and Astronomy, Cardiff University, Cardiff, UK
¹⁴ CNR - Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
¹⁵ NASA - Goddard SFC, USA
¹⁶ Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK

Received: 31 March 2010
Accepted: 11 May 2010

Abstract

We present a power spectrum analysis of the Herschel-SPIRE observations of the Polaris flare, a high Galactic latitude cirrus cloud midway between the diffuse and molecular phases. The SPIRE images of the Polaris flare reveal for the first time the structure of the diffuse interstellar medium down to 0.01 parsec over a 10 square degrees region. These exceptional observations highlight the highly filamentary and clumpy structure of the interstellar medium even in diffuse regions of the map. The power spectrum analysis shows that the structure of the interstellar medium is well described by a single power law with an exponent of -2.7±0.1 at all scales from 30” to 8°. That the power spectrum slope of the dust emission is constant down to the SPIRE angular resolution is an indication that the inertial range of turbulence extends down to the 0.01 pc scale. The power spectrum analysis also allows the identification of a Poissonian component at sub-arcminute scales in agreement with predictions of the cosmic infrared background level at SPIRE wavelengths. Finally, the comparison of the SPIRE and IRAS 100 μm data of the Polaris flare clearly assesses the capability of SPIRE in maping diffuse emission over large areas.