A submillimeter exponential disk in M 51: Evidence for an extended cold dust disk

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: meijerin@strw.leidenuniv.nl
² Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, Hawaii, 96720, USA
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 13 17, 85741 Garching, Germany

Received: 18 March 2004
Accepted: 21 September 2004

Abstract

A 850 μm map of the interacting spiral galaxy M 51 shows well-defined spiral arms, closely resembling the structures seen in CO and HI emission. However, most of the 850 μm emission originates in an underlying exponential disk, a component that has not been observed before in a face-on galaxy at these wavelengths. The scale-length of this disk is 5.45 kpc, which is somewhat larger than the scale-length of the stellar disk, but somewhat smaller than that of atomic hydrogen. Its profile cannot be explained solely by a radial disk temperature gradient but requires the underlying dust to have an exponential distribution as well. This reinforces the view that the submm emission from spiral galaxy disks traces total hydrogen column density, i.e. the sum of  H₂and H i. A canonical gas-to-dust ratio of is obtained for  g^-1 cm², where is the dust opacity at 850 μm.