AGN Emission Lines

10.3. Line Width vs. Continuum Luminosity: M/L for AGNs

10.3.1 Observed correlations. A superficial study of AGN spectra reveals the large diversity in line widths. The FWHM (Full Width Half Maximum) of a certain line, in objects of similar continuum luminosity, can differ by a factor of two or more. For example, the typical FWHM in radio-quiet Seyfert 1s is about 4000 km s^-1, while in radio-loud Seyfert 1s (the BLRGs), with similar continuum luminosity, it can exceed 10,000 km s^-1. Extremely broad emission lines seem to be typical of steep spectrum radio quasars. Many of these objects show also a bumpy, asymmetrical line profile.

The CIV lambda 1549 FWHM versus L₁₅₄₉ correlation is shown in Fig. 30. There is a weak tendency for brighter objects to have somewhat broader lines which, for the data in the diagram, is expressed by

(95)

The correlation is noisy and only marginally significant. More important, the above sample, which was collected from published line lists, does not include the BLRGs. Having a few such objects in the sample would increase the average FWHM at the low luminosity end, causing the above correlation to weaken, or disappear.

Figure 30. The FWHM of CIV lambda 1549 vs. continuum luminosity at 1549Å for 104 AGNs.

This and several published line width vs. L correlations, are all affected by the lack of well selected samples. It will take more observational effort, and better defined samples, to verify whether or not the line-widths are indeed correlated with the continuum luminosity. Since no firm conclusion can be drawn at this stage, we proceed by examining the consequences of two likely possibilities: FWHM independent of L and FWHM L^1/4.

10.3.2 M/L for AGNs. Assume that the emissivity weighted radius, r_av in (71), is the typical dimension of the BLR. Assume further that the Keplerian velocity at r_av, measured in units of 3000 km s^-1, is

(96)

For a bound Keplerian motion around a central mass M,

(97)

and from the definition of the ionization parameter (5)

(98)

where L₄₆(ion) is the ionizing luminosity in 10⁴⁶ erg s^-1, N₁₀ = N/10¹⁰ cm^-3 and nubar the mean energy of an ionizing photon, in Rydberg.

Assume now U propto L^a, v₃₀₀₀ propto L^b and (N₁₀ nubar ) which is independent of L(ion). Thus

(99)

The previous estimate of r_av (75) obtained from line reverberation studies, combined with typically observed FWHM, enable us to estimate that for Seyfert 1 galaxies M₁ ~ 10⁹ M.

The value of a is not well known, but it is likely to be in the range 0 to -1/2. For the case of a = 0 (U independent of L) there are two interesting possibilities: b = 0 (line width independent of luminosity), which results in M propto L^1/2, and b = 1/4, that gives M propto L.

The accretion rate dotm , expressed in units of L / L_Edd, is proportional to L / M ,

(100)

With the above estimate of M₁ we find L / L_Edd appeq 0.05, which is in the general accepted range for thin accretion disks. The estimates are still u ncertain and more measurements are needed to establish the values of a, b and M₁.

If radiation pressure is the major driving force of the clouds, the velocity field and L/M are somewhat changed. For example, in the optically thin outflow case, the acceleration is proportional to N_e and the velocity, at a distance r, is proportional to (Nr)^1/2. Assume that the measured FWHM is associated with a terminal velocity v₃₀₀₀ at a distance r_av, where the density is the "typical" BLR density. In this case v₃₀₀₀ propto r_av^1/2 propto L^1/4 and the M/L dependence is similar to the Keplerian case (99) with b = 1/4 and somewhat smaller M for a given L (the velocity at r_av is larger, for a given M, than the corresponding Keplerian velocity). The optically thick acceleration case is more complicated, but the v propto r^l/2 dependence is quite general.

10.3.3 Line width and radio properties. The radio emission of AGNs is observed to come from either a compact or an extended radio source. The compact source coincides with the optical continuum source. Its radiation is thought to be beamed, due to relativistic motion, and its apparent luminosity depends on the observer's viewing angle. The extended radio emission originates in a much larger volume, it is not beamed and the apparent luminosity is angle independent. Thus the ratio of the compact and extended radio fluxes is a measure of the orientation of the central radio source. This ratio (compact/extended) is found to be smaller in objects with broader emission lines. This may influence all line-width vs. continuum luminosity correlations in samples including radio-loud objects. It suggests that the emission lines in those radio-loud objects are emitted, preferentially, from material in a plane perpendicular to the direction of the radio beam, which may also be the plane of the central accretion disk.