A detailed study of the emission lines in the Seyfert 1 nucleus of M81

view Abstract Citations (127) References (106) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Detailed Study of the Emission Lines in the Seyfert 1 Nucleus of M81 Filippenko, Alexei V. ; Sargent, Wallace L. W. 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 Hα 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 Hα and the neighboring [N II] λλ6548, 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 Hα emission line. Its absolute luminosity, uncorrected for extinction, is 1.2 x 10^39^ ergs s^ -1^. This is ~0.05 of the broad Hα in the faintest known classical Seyfert 1 nucleus, NGC 4051. We derive L_X_/L_Hα ~ 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 Hα emission, the data convincingly show the corresponding component of Hβ (FWHM ~ 1800 km s^-1^; FWZI ~ 5800 km s^-1^). The intensity ratio of broad Hα to Hβ 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 Hγ line gives a much smaller value (A_V_ <~ 0.3 mag). Under the assumption that the width of Hβ 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 Hα 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] λ6300, for example, is far broader than each of the [S II] λλ6716, 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] λ6716 is noticeably narrower than [S II] λ6731, 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] λ6300 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. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165886 Bibcode: 1988ApJ...324..134F Keywords: Astronomical Spectroscopy; Emission Spectra; Galactic Nuclei; Line Spectra; Seyfert Galaxies; Visible Spectrum; Absorption Spectra; H Alpha Line; Signal To Noise Ratios; Spectral Resolution; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M81; GALAXIES: NUCLEI; GALAXIES: SEYFERT; LINE PROFILES; SPECTROPHOTOMETRY full text sources ADS | data products NED (8) SIMBAD (7)