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Astron. Astrophys. 315, L153-L156 (1996)

An ISOPHOT study of the disk of galaxy NGC6946: 60 img1.gifm infrared and radio continuum correlation*

N.Y. Lu1, G. Helou1, R. Tuffs2, C. Xu2, S. Malhotra1, M.W. Werner3, and H. Thronson4

1 Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125, USA
2 Max-Planck Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
3 Jet propulsion Laboratory, MS 233-303, California Institute of Technology, Pasadena, CA 91109, USA
4 Wyoming Infrared Obs., Univ. of Wyoming, Laramie, WY 82071, and Code SR, NASA HQs, Washington DC 20546, USA

Received 6 August 1996 / Accepted 28 August 1996

Abstract. We use the ISOPHOT 60 img1.gifm image of Tuffs et al.  (1996) to study the 60 img1.gifm light distribution and its correlation with radio continuum within the disk of spiral galaxy NGC6946, and to resolve the long-standing IRAS controversy as to whether the infrared-to-radio continuum ratio, Q, decreases radially out in galaxy disks. Our main results are: (1) The disk at 60 img1.gifm also appears to follow an exponential fall-off. Its e-folding scale-length lies between 1'.3 and an upper limit of 1'.9, obtained from the mean radial profile between 1'.5 and 4' in radius. This scale length is smaller than that derived from the radio continuum data at either 20cm or 6cm, with or without the thermal component removed, at a significance level of img3.gif . (2) From 1'.5 to 4' in radius, the 60 img1.gif m-to-radio continuum surface brightness ratio, Q, decreases by a factor of 1.5 to 1.7 on average. These quantitative results agree with that of Marsh & Helou (1995) using IRAS HiRes data, but not with that of Fitt et al.  (1992) based on IRAS CPC data. Our results also fit into the picture of the radio disk as a ``smeared'' version of the infrared disk, but are not consistent with the picture in which the variation of Q is a result of the thermal and non-thermal radio components having two distinct, constant values of the infrared-to-radio ratio.

Key words: galaxies: individual/NGC6946 - galaxies: ISM - infrared: galaxies - radio continuum: galaxies

* 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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