Serendipity observations of far infrared cirrus emission in the Spitzer Infrared Nearby Galaxies Survey: Analysis of far-infrared correlations

📝 Abstract

We present an analysis of far-infrared dust emission from diffuse cirrus clouds. This study is based on serendipitous observations at 160 microns at high galactic latitude with the Multiband Imaging Photometer (MIPS) onboard the Spitzer Space Telescope by the Spitzer Infrared Nearby Galaxies Survey (SINGS). These observations are complemented with IRIS data at 100 and 60 microns and constitute one of the most sensitive and unbiased samples of far infrared observations at small scale of diffuse interstellar clouds. Outside regions dominated by the cosmic infrared background fluctuations, we observe a substantial scatter in the 160/100 colors from cirrus emission. We compared the 160/100 color variations to 60/100 colors in the same fields and find a trend of decreasing 60/100 with increasing 160/100. This trend can not be accounted for by current dust models by changing solely the interstellar radiation field. It requires a significant change of dust properties such as grain size distribution or emissivity or a mixing of clouds in different physical conditions along the line of sight. These variations are important as a potential confusing foreground for extragalactic studies.

💡 Analysis

We present an analysis of far-infrared dust emission from diffuse cirrus clouds. This study is based on serendipitous observations at 160 microns at high galactic latitude with the Multiband Imaging Photometer (MIPS) onboard the Spitzer Space Telescope by the Spitzer Infrared Nearby Galaxies Survey (SINGS). These observations are complemented with IRIS data at 100 and 60 microns and constitute one of the most sensitive and unbiased samples of far infrared observations at small scale of diffuse interstellar clouds. Outside regions dominated by the cosmic infrared background fluctuations, we observe a substantial scatter in the 160/100 colors from cirrus emission. We compared the 160/100 color variations to 60/100 colors in the same fields and find a trend of decreasing 60/100 with increasing 160/100. This trend can not be accounted for by current dust models by changing solely the interstellar radiation field. It requires a significant change of dust properties such as grain size distribution or emissivity or a mixing of clouds in different physical conditions along the line of sight. These variations are important as a potential confusing foreground for extragalactic studies.

📄 Content

arXiv:0901.0792v1 [astro-ph.GA] 7 Jan 2009 Serendipity observations of far infrared cirrus emission in the Spitzer Infrared Nearby Galaxies Survey: Analysis of far-infrared correlations∗ Caroline Bot1,5 and George Helou1 and Fran¸cois Boulanger2 and Guilaine Lagache2 and Marc-Antoine Miville-Deschenes2 and Bruce Draine3 and Peter Martin4 bot@astro.u-strasbg.fr ABSTRACT We present an analysis of far-infrared dust emission from diffuse cirrus clouds. This study is based on serendipitous observations at 160µm at high galactic lati- tude with the Multiband Imaging Photometer (MIPS) onboard the Spitzer Space Telescope by the Spitzer Infrared Nearby Galaxies Survey (SINGS). These obser- vations are complemented with IRIS data at 100 and 60µm and constitute one of the most sensitive and unbiased samples of far infrared observations at small scale of diffuse interstellar clouds. Outside regions dominated by the cosmic infrared background fluctuations, we observe a substantial scatter in the 160/100 colors from cirrus emission. We compared the 160/100 color variations to 60/100 colors in the same fields and find a trend of decreasing 60/100 with increasing 160/100. This trend can not be accounted for by current dust models by changing solely the interstellar radiation field. It requires a significant change of dust properties such as grain size distribution or emissivity or a mixing of clouds in different physical conditions along the line of sight. These variations are important as a potential confusing foreground for extragalactic studies. Subject headings: ISM:clouds — infrared:ISM *This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. 1California Institute of Technology, Pasadena CA 91125, USA 2Institut d’Astrophyisque Spatiale, 91405 Orsay, FRANCE 3Princeton University Observatory, Princeton, NJ08544, USA 4Canadian Institute for Theoretical Astrophysics, Toronto, Ontario, M5S 3H8, Canada 5Observatoire Astronomique de Strasbourg, 67000 Strasbourg, FRANCE – 2 – 1. Introduction The InfraRed Astronomical Satellite (IRAS) showed for the first time that extended in- frared emission was present at high galactic latitude, far from star forming regions (Low et al. 1984). In these diffuse regions, clouds are optically thin to stellar radiation and the radi- ation field is relatively uniform which results in very limited variations of dust equilibrium temperature (Boulanger et al. 1996; Arendt et al. 1998; Lagache et al. 1998; Schlegel et al. 1998). These high latitude cirrus also show a tight correlation between their infrared emission (100µm to 1mm observed by DIRBE and FIRAS) and the HI column density (Boulanger et al. 1996) and the dust emission is well characterized with a constant dust emissivity per hydrogen atom (τ/NH = 10−25(λ/250)−2cm2) close to the value expected from models of interstellar dust grains (Draine & Lee 1984). At shorter wavelength, the smaller dust grains emission is characterized by a ratio of I60µm/I100µm ∼0.2 (Laureijs et al. 1991; Abergel et al. 1996; Boulanger et al. 2000). All in all, dust emission from local cirrus is then seen as rather homogeneous and simply characterized on large scales. However, little is known about the dust properties (e.g. optical properties for absorption and emission, distribution,. . . ) in these high latitude clouds, at resolutions smaller than the DIRBE beam (0.7o). Smaller scale analysis of infrared colors have been done on individual regions and show clear variations of dust properties. Laureijs et al. (1996) and Abergel et al. (1994) observed a decrease of I60µm/I100µm toward dense clouds. Bernard et al. (1999) studied the far infrared emission at the arcminute scale in the Polaris flare with IRAS, ISOPHOT and PRONAOS (200 to 600µm) in a region where extended emission from cirrus is detected as well as a denser structure. The spectrum of the extended cirrus indicates a low dust temperature associated with a low 60/100 µm ratio. This was also observed in the Polaris flare toward moderately dense regions (AV ∼1) and in a denser filament in the Taurus complex (Cambr´esy et al. 2001; Stepnik et al. 2003). It might be explained by the formation of large dust aggregates through the adhesion of small dust particles onto the surface of larger grains, leading to a change of dust emissivity properties. In the dense regions the very small grains seem to have disappeared almost completely. However, all these observations were restricted to individual regions, most of which are much denser than the diffuse local interstellar medium seen at high galactic latitudes. By comparing near-infrared extinction and extinction deduced from far-infrared dust emission in the whole anticenter hemisphere, Cambr´esy et al. (2005) observed a discrepancy between the two quantities in regions above 1 mag. This effect is also interpreted by a change of dust emissivity due

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