The Star Formation History of the Disk of the Starburst Galaxy M82

Spectroscopic, photometric, and dynamical data of the inner 3 kpc of the starburst galaxy M82 are analyzed in order to investigate the star formation history of its disk. The long-slit spectra along the major axis are dominated by Balmer absorption lines in the region outside the nuclear starburst all the way up to ≈3.5 scale-lengths (μ_B = 22 mag arcsec^-2). Single stellar population (SSP) spectra of age 0.4-1.0 Gyr match the observed spectra in the 1-3 kpc zone well, with a marginally higher mean age of the stellar population in the outer parts. The mass in these populations, along with that in the gas component, makes up for the inferred dynamical mass in the same annular zone for a Kroupa initial mass function, with a low-mass cutoff of m_l = 0.4 M_☉. The observed ratio of the abundances of α-elements with respect to Fe is also consistent with the idea that almost all the stars in the M82 disk formed in a burst of short duration (0.3 Gyr) around 0.8 Gyr ago. We find that the optical/near-infrared colors and their gradients in the disk are determined by the reddening, with visual extinction exceeding 1 mag even in the outer parts of the disk, where there is apparently no current star formation. The disk-wide starburst activity was most likely triggered by the interaction of M82 with its massive neighbor M81, around 1 Gyr ago. The properties of the disk of M82 very much resemble the properties of the disks of luminous compact blue galaxies seen at z = 0.2-1.0.