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

Abstract

Spectroscopic, photometric, and dynamical data of the inner 3 kpc of thestarburst galaxy M82 are analyzed in order to investigate the starformation history of its disk. The long-slit spectra along the majoraxis are dominated by Balmer absorption lines in the region outside thenuclear 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 1-3kpc zone well, with a marginally higher mean age of the stellarpopulation in the outer parts. The mass in these populations, along withthat in the gas component, makes up for the inferred dynamical mass inthe same annular zone for a Kroupa initial mass function, with alow-mass cutoff of m_{l}=0.4 M_{solar}. The observedratio of the abundances of {α}-elements with respect to Fe is alsoconsistent with the idea that almost all the stars in the M82 diskformed in a burst of short duration (0.3 Gyr) around 0.8 Gyr ago. Wefind that the optical/near-infrared colors and their gradients in thedisk are determined by the reddening, with visual extinction exceeding 1mag even in the outer parts of the disk, where there is apparently nocurrent star formation. The disk-wide starburst activity was most likelytriggered by the interaction of M82 with its massive neighbor M81,around 1 Gyr ago. The properties of the disk of M82 very much resemblethe properties of the disks of luminous compact blue galaxies seen atz=0.2-1.0.