Astronomy and Astrophysics – Astronomy
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462..183h&link_type=abstract
Astrophysical Journal v.462, p.183
Astronomy and Astrophysics
Astronomy
102
Galaxies: Individual Messier Number: M81, Galaxies: Nuclei, Galaxies: Seyfert, Line: Profiles, Ultraviolet: Spectra, Spectrophotometry
Scientific paper
We discuss the physical conditions and excitation of the nucleus of M81 by combining ground-based optical spectra, archival Hubble Space Telescope (HST) Ultraviolet (UV) and optical spectral, and published X- ray measurements. This well known low-ionization nuclear emission-line region (LINER) is representative of a general class of objects thought to be the less luminous counterparts of active galactic nuclei (AGNs) present in Seyfert galaxies and quasars. A featureless UV continuum, most likely of non-stellar origin is detected unambiguously for the first time in M81. Compared with "classical", more luminous AGNs, the UV continuum is weak relative to the X-rays, the slope between 1200 and 3000 A is significantly steeper (fnu_ is proportional to ν^-2.0+/-0.3^ rather than ν^-0.5^ to ν^-1^), and the "big blue bump" is absent. The nonstellar continuum of the nucleus has an absolute blue magnitude of - 11.6, Only a factor of 5 brighter than that of the least luminous Seyfert 1 nucleus in NGC 4395. We suggest that these characteristics of the spectral energy distribution may be a generic property of galactic nuclei having low-level activity, perhaps being a manifestation of extremely low accretion rates. The UV-optical spectrum exhibits broad emission lines with FWHM ~ 2500 km s^-1^, further reinforcing the similarity to Seyfert 1 nuclei. Detailed properties have been derived for the regions emitting narrow and broad lines. The narrow-line region is characterized by velocities of 200-1000 km s^-1^, a large, radially decreasing gradient in density ranging from a few times 10^2^ to 10^7^ cm^-3^, and a spatial extent of less than 1 to several parsecs from the central ionizing source. We deduce that the gas has roughly solar metallicity, an ionization parameter of ~10^-3.3^, and electron temperatures typical of photoionized nebulae. Lacking information on the structure of the broad-line region, we can obtain only crude estimates of its density (10^9^-10^10^ cm^-3^), ionization parameter (<~ 10^-2.8^), and radius (0.001-0.004 pc). Combining the velocities and dimensions of the broad-line region, and assuming that gravity dominates the motions of the clouds, we derive a central mass of (0.7-3) x 10^6^ M_sun_ for the nucleus of M81. The low bolometric luminosity of the nucleus implies that it is radiating only at (2-10) x 10^-4^ of the Eddington limit. Our extensive ground-based monitoring reveals that the broad component of the Hα emission line has not varied significantly during the past 15 yr, although X-ray variability on timescales of years has been reported. This variability pattern is not yet understood. The broad Hα luminosity (1.8 x 10^39^ ergs s^-1^) is 15 times larger than in NGC 4395, assuming that the reddening intrinsic to the broad-line region is negligible in both objects.
Filippenko Alexei V.
Ho Luis C.
Sargent Wallace L. W.
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