M82 AS A GALAXY: MORPHOLOGY AND STELLAR CONTENT OF THE DISK AND HALO

For decades, the nuclear starburst has taken all the limelight in M82 with very little discussion on M82 as a galaxy. The situation is changing over the last decade, with the publication of some important results on the morphology and stellar content of its disk and halo. In this review, we discuss these recent findings in the framework of M82 as a galaxy. It is known for almost half a century that M82 as a galaxy doesn't follow the trends expected for normal galaxies that had prompted the morphologists to introduce a separate morphological type under the name Irr II or amorphous. It is now being understood that the main reasons behind its apparently distinct morphological appearance are its peculiar star formation history, radial distribution of gas density and the form of the rotation curve. The disk formed almost all of its stars through a burst mode around 500 Myr ago, with the disk star formation completely quenched around 100 Myr ago. The fossil record of the disk-wide burst lies in the form of hundreds of compact star clusters, similar in mass to that of the globular clusters in the Milky Way, but an order of magnitude younger. The present star formation is restricted entirely to the central 500 pc zone, that contains more than 200 young compact star clusters. The disk contains a non-star-forming spiral arm, hidden from the optical view by a combination of extinction and high inclination to the line of sight. The halo of M82 is also unusual in its stellar content, with evidence for star formation, albeit at low levels, occurring continuously for over a gigayear. We carefully examine each of the observed abnormality to investigate the overall effect of interaction on the evolution of M82.