The JCMT Nearby Galaxies Legacy Survey

VI. The distribution of gas and star formation in M 81

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: jrsg@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
³ Department of Physics, Laval University, Québec City, Québec, G1V 0A6, Canada
⁴ Department of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
⁵ International Centre for Radio Astronomy Research, M468, University of Western Australia, Crawley, WA, 6009, Australia
⁶ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁷ Physics and Astronomy Department, University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 3K7, Canada
⁸ Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
⁹ Korea Astronomy and Space Science Institute, 61-1 Hwaam-dong, Yuseong-gu, Daejeon 305-348, Republic of Korea
¹⁰ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Department of Physics & Astronomy, The Open University, Milton Keynes Mk7 6AA, UK
¹² Joint Astronomy Centre, 660 N. Aohoku Pl., University Park, Hilo, HI 96720, USA
¹³ Netherlands Organisation for Scientific Research, Laan van Nieuw Oost-Indie 300, 2509 AC The Hague, The Netherlands

Received: 4 May 2010
Accepted: 9 November 2010

Abstract

We present the first complete ¹²CO J = 3−2 map of M 81, observed as part of the Nearby Galaxies Legacy Survey being carried out at the James Clerk Maxwell Telescope. We detect nine regions of significant CO emission located at different positions within the spiral arms, and confirm that the global CO emission in the galaxy is low. We combine these data with a new Hα map obtained using the Isaac Newton Telescope and archival H i, 24μm, and FUV images to uncover a correlation between the molecular gas and star forming regions in M 81. For the nine regions detected in CO J = 3−2, we combine our CO J = 3−2 data with existing CO J = 1−0 data to calculate line ratios. We find that the ratio J = (3−2)/(1−0) is in agreement with the range of typical values found in the literature (0.2−0.8). Making reasonable assumptions, this allows us to constrain the hydrogen density to the range (10³−10⁴) cm^-3. We also estimated the amount of hydrogen produced in photo-dissociation regions near the locations where CO J = 3−2 was detected.