NARROW-BAND IMAGING OF THE CIRCUMNUCLEAR EMISSION-LINE REGION OF M81

We present new flux-calibrated narrow-band images of the circumnuclear region in M81 = NGC 3031 in the light of [O III] lambda 5007, [OI] la mbda 6300, H alpha + [N II] lambda lambda 6548, 84 and [S II] lambda l ambda 6717 + 31 emission lines. The H alpha + [N II] and [O III] image s as well as [OI] and [S II] ones reveal the existence of an extended emission-line structure around the nucleus of M81. The H alpha + [N II ] lambda lambda 6548, 84 extends to similar to 8 arcsec west and simil ar to 13 arcsec east of the nucleus at the 5 sigma noise level of the image. The estimation of the filling factor, between 5 x 10(-4) and 3 x 10(-3), was made based on the emissivity in the [O III] and H alpha regions, respectively. An increase of the [O III]/H alpha ratio was de tected, growing more prominent farther from the nucleus. This is cause d by increasing [O III] lambda 5007 emission rather than decreasing H alpha + [N II] emission. The [O III]/H beta against [O I]/H alpha diag nostic diagram for the circumnuclear emission-line region (CELR) aroun d the nucleus implies that part of the ionization, particularly in the innermost nuclear regions, could be accounted for by an AGN-like powe r-law ionization spectrum. Another part of the ionization, particularl y in outer regions of the CELR, is caused by diffuse thermal X-ray con tinuum.