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Register a new userM82 as a Galaxy: Morphology and Stellar Content of the Disk and Halo
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Y. Divakara Mayya
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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 doesnt 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.
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(Abridged) In this second paper of the series, we present the results from optical Gemini-North GMOS-IFU and WIYN DensePak IFU spectroscopic observations of the starburst and inner wind zones of M82, with a focus on the state of the T~10^4 K ionized interstellar medium. Our electron density maps show peaks of a few 1000 cm-3, local small spatial-scale variations, and a fall-off in the minor axis direction. We discuss the implications of these results with regards to the conditions/locations that may favour the escape of individual cluster winds. Our findings imply that the starburst environment is highly fragmented into a range of clouds from small/dense clumps with low filling factors (<1pc, n_e>10^4 cm-3) to larger filling factor, less dense gas. The near-constant state of the ionization state of the ~10^4 K gas throughout the starburst can be explained as a consequence of the small cloud sizes, which allow the gas conditions to respond quickly to any changes. We have examined in more detail both the broad (FWHM 150-350 km/s) line component found in Paper I that we associated with emission from turbulent mixing layers on the gas clouds, and the discrete outflow channel identified within the inner wind. The channel appears as a coherent, expanding cylindrical structure of length >120 pc and and width 35-50 pc and the walls maintain an approximately constant (but subsonic) expansion velocity of ~60 km/s. We use the channel to examine further the relationship between the narrow and broad component emitting gas within the inner wind. Within the starburst energy injection zone, we find that turbulent motions (as traced by the broad component) appear to play an increasing role with height.
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