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Astron. Astrophys. 315, L157-L160 (1996)

The mid-infrared color of NGC 6946*

G. Helou1, S. Malhotra,1, 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 31 July 1996 / Accepted 20 August 1996

Abstract. We analyze the new mid-infrared maps of NGC 6946 for variations in the color ratio of the 7-to-15 img1.gifm emission. Our preliminary findings are that this mid-infrared color is remarkably constant between arms and inter-arm regions, and as a function of radius in the disk, excluding the nuclear region. As surface brightness ranges by more than an order of magnitude and the radius runs from about 0.5 to img2.gif , the color ratio remains constant to about img3.gif . Our interpretation is that (1) hard UV radiation from OB stars does not dominate the heating of the grains radiating in the mid-infrared; and (2) that surface brightness variations are driven primarily by surface-filling fraction in the disk, and by radiation intensity increases in starburst environments, such as the nucleus of NGC 6946.

Key words: galaxies: individual: NGC 6946 - infrared: galaxies - galaxies: ISM

* 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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