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Astron. Astrophys. 315, L161-L164 (1996)

ISOCAM observations of NGC 6946: Mid-IR structure *

S. Malhotra1, G. Helou1, D. Van Buren1, M. Kong1, C.A. Beichman1, H. Dinerstein2, D.J. Hollenbach3, D.A. Hunter4, K.Y. Lo5, S.D. Lord1, N.Y. Lu1, R.H. Rubin3, G.J. Stacey6, H.A. Thronson Jr.7, and M.W. Werner8

1 IPAC, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
2 University of Texas, Astronomy Department, RLM 15.308, Texas, Austin, TX 78712, USA
3 NASA/Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA
4 Lowell Observatory, 1400 Mars Hill Rd., Flagstaff, AZ 86001, USA
5 University of Illinois, Astronomy Department, 1002 W. Green St., Urbana, IL 61801, USA
6 Cornell University, Astronomy Department, 220 Space Science Building, Ithaca, NY 14853, USA
7 University of Wyoming, Wyoming Infrared Observatory, Laramie, WY 82071, USA
8 Jet Propulsion Laboratory, MS 233-303, 4800 Oak Grove Rd., Pasadena, CA 91109, USA

Received 7 August 1996 / Accepted 20 August 1996

Abstract. The nearby spiral galaxy NGC 6946 was observed with ISO-CAM in the mid-infrared, achieving img1.gif resolution and sub- img2.gif sensitivity. Images taken with CAM filters LW2 ( img3.gifm) and LW3 ( img4.gifm) are analysed to determine the morphology of this galaxy and understand better the emission mechanisms. The mid-infrared emission follows an exponential disk with a scale length img5.gif . This is 60% of the scale length in the optical R-band and radio continuum. The nuclear starburst region is too bright for reliable measurement in these images. Its surface brightness exceeds the inner disk by at least a factor of 12. The arms and interarm regions are clearly detected, with each of these components contributing about equally to the disk emission. The arm-interarm contrast is 2-4 in the mid-IR, close to that measured in the visible R band light and lower than the contrast in img6.gif , suggesting that non-ionizing radiation contributes significantly to dust heating.

Key words: infrared: galaxies - infrared: ISM: continuum - galaxies: ISM - galaxies: spiral - galaxies: structure

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

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