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6. Statistical Properties of LINERs and Seyfert Nuclei

6.1. Emission-Line Luminosity

In lieu of direct measurement of the nonstellar featureless continuum at optical wavelengths, an almost impossible feat for the low-luminosity sources in question, one might use, as a substitute, an indirect measure such as the luminosity of a narrow emission line powered by the continuum. In luminous AGNs, whose nonstellar optical continuum generally overwhelms the stellar background, the Halpha luminosity scales linearly with the luminosity of the continuum (Searle & Sargent 1968; Yee 1980; Shuder 1981). However, using line luminosities derived from slit spectroscopy is not without complications, especially for the relatively narrow slit employed in the Palomar survey. For any given object, the amount of line emission sampled will depend on its distance as well as on the physical extent of the line-emitting region. Moreover, circumnuclear H II regions undoubtedly contaminate the line emission at some level (indeed, for transition objects, this is assumed a priori). These limitations notwithstanding, it can be argued that a statistical examination of such line luminosities might still be of value, as the individual ``fluctuations'' will tend to average out for sufficiently large samples. The Halpha luminosities for the various classes of emission-line nuclei are shown in Figure 5a. The incredible feebleness of the low-luminosity AGNs can be readily appreciated by realizing that a sizable fraction of H II nuclei and disk giant H II regions (e.g., Kennicutt 1988), not to mention starburst nuclei (e.g., Balzano 1983), in fact have much stronger Halpha luminosities than these sources. Remarkably, the distributions for LINERs and Seyferts appear very similar, both having a median L(Halpha) approx 6 x 1038 ergs s-1; transition objects tend to be somewhat less luminous, but the difference is insignificant according to the Kolmogorov-Smirnov (K-S) test. (The probability that the LINERs and transition objects are drawn from the same population, PKS, is 0.16.) The above comparison is not obviously affected by known systematic biases, since all three subclasses have virtually identical distance distributions, modest reddening corrections were consistently applied, and it was already shown (Section 5) that the host galaxies of LINERs and Seyferts are grossly similar.

Figure 5

Figure 5. (a) Distribution of dereddened luminosities for the narrow Halpha emission line. (b) Distribution of electron densities derived from [S II] lambdalambda 6716, 6731.

Heckman (1980b) claimed that the line luminosities of LINERs are smaller than those of Seyferts, contrary to what we and Stauffer (1982b) find. The principal reason for this difference appears to be that Heckman used for comparison a sample of Seyferts, taken from Adams & Weedman (1975), which was biased toward luminous sources.

One should be cautious in interpreting these results. Superficially, it appears that both LINERs and Seyferts have the same ``level'' of activity, if one assumes that the narrow-line luminosity is an appropriate yard stick for gauging the power output of the central source. It remains to be demonstrated, however, that the line-continuum relation of luminous AGNs continues to hold for low-luminosity sources. There is preliminary evidence, for instance, that the spectral energy distributions of LINERs and low-luminosity Seyferts may differ appreciably from those of high-luminosity sources (Ho, Filippenko, & Sargent 1996b). If so, the equivalent widths of emission lines, and hence the slope of the line-continuum relation, will vary systematically with luminosity.

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