Structure and kinematics within the Starburst nucleus of M82: Searching for a bar

Abstract

The structure and kinematics of the ionized gas within the starburst nucleus of M82 are investigated through high spectral (30 km/s) and spatial (2 arcsec) resolution observations of the 12.8 micrometer line of (Ne II). The structure is dominated by a nuclear ring and two ionized 'lanes'. These components are found in other barred galaxies, supporting the suggestion that M82 may contain a nuclear bar. The ionized ring is approximately circular with a radius of 86 pc and an inclination of 73 deg. The center of the ring closely corresponds to M82's kinematic center and is located at 9h 51m 43.4sec +/- 0.5 secs, + 69 deg 55 min 0.1 sec +/- 0.5 arcsecs (1950). M82's rotation curve is searched for signs of the chi1 and chi2 orbital families which make up a nuclear bar. Unfortunately, the orientation of M82's proposed bar is such that the expected rotation curves from barred or circular orbits are very similar. Hints of chi1 and chi2 orbits are found in M82, but kinematic identification of a bar remains elusive. Observations of (Ar II) (8.99 micrometers), (S IV) (10.51 micrometers), and hydrogen Br-alpha measure a lower level of excitation than that found in most Galactic H II regions, and which remains fairly constant across the galaxy. The total neon abundance is 1-2 times solar. The temperature of the stars ionizing the starburst regions is approximately 33,000 K. The Br-alpha flux requires about 6 x 1053 Lyman continuum photons per second. Star formation has occurred either during a very brief burst (or bursts) or within an initial mass function with an upper mass cutoff approximately 25 solar mass.