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For refcode 1988ApJ...324..134F:
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1988ApJ...324..134F A DETAILED STUDY OF THE EMISSION LINES IN THE SEYFERT I NUCLEUS OF M81 ALEXEI V. FILIPPENKO Department of Astronomy, University of California, Berkeley AND WALLACE L. W. SARGENT Palomar Observatory, California Institute of Technology Received 1987 May 11; accepted 1987 June 22 ABSTRACT We present optical spectra of M81 having moderate resolution (1.6-4.5 A) and exceptionally high signal-to-noise ratios (S/N >~ 100/1). The broad component of H{alpha} emission first noticed by Peimbert and Torres- Peimbert is easily visible, confirming that M81 harbors an active galactic nucleus (AGN) of the Seyfert l type. Prominent forbidden lines are also present, but many features are severely contaminated by the underlying starlight. An absorption-line template galaxy, NGC 4339, is used to eliminate the stars, revealing the pure emission-line spectrum of M81. After carefully removing the narrow component of H{alpha} and the neighboring [N II] {lambda}{lambda}6548, 6583 lines, we measure full widths at half-maximum (FWHM) and near zero intensity (FWZI) of 2200 and 6900 km s^-1^, respectively, for the broad H{alpha} emission line. Its absolute luminosity, uncorrected for extinction, is 1.2 x 10^39^ ergs s^ -1^. This is ~0.05 of the broad H{alpha} in the faintest known classical Seyfert 1 nucleus, NGC 4051. We derive L_X_/L_H{alpha} ~ 23, well within the range observed in classical broad-line AGNs. Extrapolation of the observed X-ray spectrum to longer wavelengths fails to provide enough ionizing photons to account for the emission lines. In addition to the broad component of H{alpha} emission, the data convincingly show the corresponding component of H{beta} (FWHM ~ 1800 km s^-1^; FWZI ~ 5800 km s^-1^). The intensity ratio of broad H{alpha} to H{beta} is ~6.6, rather than the recombination ratio of ~ 3. If reddening is the sole reason for the discrepancy, an extinction A_V_ ~ 2 mag is calculated, but the observed lower limit to the broad H{gamma} line gives a much smaller value (A_V_ <~ 0.3 mag). Under the assumption that the width of H{beta} is produced by clouds in Keplerian orbits, the mass interior to the broad-line region (r = 0.0013-0.0036 pc) is calculated to be 3-8 x 10^5^ M_sun_. A single central object, presumably a black hole, probably accounts for most of this mass. Comparison of spectra obtained over several days, 1 month, 1 yr, and 3 yr reveals no variations in the strength or the shape of the broad H{alpha} emission line. Given its low luminosity, this is quite puzzling; the Balmer lines of other, brighter type 1 Seyferts are known to vary substantially. Moreover, at X-ray energies M81 has brightened by factors of 3-5 over the past 7 yr, and it has exhibited X-ray variability of a factor of 2 in 600 s. The rapid X-ray fluctuations may be produced by broad-line clouds moving across our line of sight to the continuum source. The forbidden lines in M81 exhibit a strong correlation between profile width and critical density. [O I] {lambda}6300, for example, is far broader than each of the [S II] {lambda}{lambda}6716, 673l lines. This implies that the narrow-line region (NLR) of M81 is composed of clouds having a wide range of electron densities (n_e_ ~10^2.5^-10^7.5^ cm^-3^). The densest clouds are optically thick, have the highest bulk motions, and live closest to the nucleus. Additional support for this interpretation is found from an analysis of the individual [S II] lines themselves: [S II] {lambda}6716 is noticeably narrower than [S II] {lambda}6731, whose critical density is a factor of 2.6 larger. The relative intensity of the two lines actually reaches the high-density limit (n_e_ >~ 10^5^ cm^-3^) in the extreme wings, which are produced by high-velocity gas. High densities and large optical depths in the NLR alleviate several previous problems with photoionization models of LINERs. The electron temperature is low, rather than high, in the O^++^ zone, and the great strength of [O I] {lambda}6300 is easily explained. Some of these results are inconsistent with the hypothesis that shock heating is the dominant excitation mechanism in the NLR of M8l. This supports the conclusion that the LINERs found in many galaxies may be genuine, albeit low-luminosity, AGNs in which gas is photoionized by a nonstellar continuum. Subject headings: galaxies: individual - galaxies: nuclei - galaxies: Seyfert - line profiles - spectrophotometry
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