Spectroscopic , photometric and dynamical data of the inner 3Â kpc part of the starburst galaxy M 82 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 \approx 3.5 scalelengths ( \mu _ { B } = 22 Â mag arcsec ^ { -2 } ) .
Single Stellar Population ( SSP ) spectra of age 0.4–1.0Â Gyr match well the observed spectra in the 1–3Â kpc zone , with a mean age of the stellar population marginally higher in the outer parts .
The mass in these populations , along with that in the gas component , make up for the inferred dynamical mass in the same annular zone for a Kroupa initial mass function , with a low mass cut-off m _ { l } = 0.4 Â M _ { \sun } .
The observed ratio of the abundances of \alpha elements with respect to Fe , is also consistent with the idea that almost all the stars in M 82 disk formed in a burst of short duration ( 0.3Â Gyr ) around 0.8Â 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 1Â 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 M 82 with its massive neighbor M 81 around 1Â Gyr ago .
The properties of the disk of M 82 very much resemble the properties of the disks of luminous compact blue galaxies seen at 0.2–1.0 redshift .