Emission Line Regions and Jet Powers

Next, I want to investigate relations between emission line region luminosities and radio and X-ray jet powers for weak- and strong-lined radio-loud AGN.

Fig. 4.8 plots the narrow line region luminosity versus the extended radio lumi-nosity at 151 MHz (left panel) and versus the total X-ray lumilumi-nosity at 1 keV (right panel) for the sample of objects under study. For sources from the 2 Jy survey I have used the radio and X-ray information given in Morganti et al. (1993) and Siebert et al. (1996) respectively. I have calculated the extended radio luminosity at 151 MHz for the sample under study from the total radio luminosity at 5 GHz as follows. I have first used the relation between radio core dominance parameter and radio spectral index given in D’Elia et al. (2003) to compute the extended radio luminosity at 5 GHz from the total radio luminosity at this frequency and the radio spectral index. I have then transformed the extended radio luminosity at 5 GHz to 151 MHz assuming a radio spectral index α_r= 0.8 (where f_ν ∝ν^−α). I have used the extended radio luminosity at 151 MHz since from this an estimate of the jet power can be derived, asL_jet∝L^6/7₁₅₁ (Willott et al., 1999). For objects from the 2

86 A New Classification Scheme for Blazars Jy survey the ROSATX-ray band (0.1−2.4 keV) luminosity has been transformed to 1 keV assuming an X-ray spectral index α_x= 1.

For the strong-lined radio-loud AGN a significant (P > 99.9%) correlation is present between the narrow line region luminosity and the extended radio luminos-ity at 151 MHz (solid line in Fig. 4.8, left panel). I have verified also by means of a partial correlation analysis that the luminosity - luminosity correlation is not induced by a common redshift dependence. The large number of upper limits in the case of weak-lined radio-loud AGN do not allow for a similar investigation. There-fore, I want to quantify their relation to the strong-lined sources only qualitatively.

For this purpose Fig. 4.8, left panel, plots also the dispersion (2σ = 1.16) around the best-fit line for the observed correlation (dashed lines). This gives that 44%

(11/25) of the weak-lined AGN fall below the 2σ region of the correlation followed by the strong-lined sources. Moreover, the majority (70% or 10/14) of the remaining sources nominally within this region have only upper limits on their NLR luminosi-ties. Therefore, it appears that weak- and strong-lined radio-loud AGN form a parallel sequence in this luminosity - luminosity plane rather than weak-lined AGN being merely an extrapolation of the strong-lined sources down to lower emission line luminosities and radio jet powers. In fact, whereas weak- and strong-lined objects have mean NLR luminosities a factor of ∼ 300 apart, their mean extended radio luminosities differ only by a factor of∼10 (logL₁₅₁= 33.51±0.18 and 34.49±0.10 erg s⁻¹ Hz⁻¹ respectively). This means that weak-lined radio-loud AGN have un-derluminous narrow emission lines rather than weak radio jets.

Fig. 4.8, right panel, plots the NLR luminosity versus the total X-ray luminosity at 1 keV for the sample of objects under study. The X-ray emission of radio-loud AGN, contrary to their emission at radio frequencies, is believed to be made up of two components with different origins: an isotropic one, produced by the ambient hot gas (presumably from the group or cluster possibly associated with the source), and an anisotropic one, associated with the nucleus (e.g. Siebert et al., 1996). Then, which of these two components dominates the observed X-ray power will strongly depend on the object’s orientation with respect to our line of sight. At relatively large viewing angles, i.e. in the case of radio galaxies, we expect to observe mainly the isotropic component of the X-ray emission, while at relatively small viewing angles, i.e. for blazars, we expect the X-ray emission to be dominated by the (beamed) nuclear component (e.g. Baker et al., 1995). Therefore, I have split in this case the sample not only into weak- and strong-lined radio-loud AGN, but also into objects with Ca H&K break values C = 0 (circles) and C > 0 (squares), i.e. viewed at smaller and larger angles respectively. (Note that a similar split is shown for the

4.4 Weak- and Strong-Lined Radio-Loud AGN 87

Figure 4.8. The narrow line region luminosity versus the extended radio luminosity at 151 MHz (left panel) and the total X-ray luminosity at 1 keV (right panel) for objects from the DXRBS and 2 Jy surveys. Filled and open symbols indicate weak- and strong-lined radio-loud AGN respectively.

Circles and squares indicate objects with Ca H&K break valuesC = 0 and C >0 respectively. Arrows indicate upper limits. The solid line represents the observed correlation for the strong-lined radio-loud AGN (left panel) and the strong-lined radio-loud AGN withC= 0 (right panel). The dashed lines indicate the 2σdispersion of the correlation.

objects in the left panel of Fig. 4.8, although only for consistency reasons.)

For the strong-lined radio-loud AGN with C = 0 (40 objects) a strong (P >

99.9%) correlation is present between NLR luminosity and total X-ray luminosity, which remains significant if the common redshift dependence is excluded (solid line in Fig. 4.8, right panel). On the other hand, for the objects with C > 0 from this group (40 sources) only a marginal (P = 94%) correlation is present once the common redhift dependence is excluded. This suggests that mainly in objects with Ca H&K break values C = 0 (i.e. objects dominated by the jet emission; see composites in Fig. 4.4) we observe the nuclear component as the dominant source of X-ray emission. Note that only this component is assumed to be directly related to the object’s radio jet power and so expected to scale with emission line luminosity

88 A New Classification Scheme for Blazars

Figure 4.9. The broad line region luminosity versus the extended radio luminosity at 151 MHz (left panel) and the total X-ray luminosity at 1 keV (right panel) for objects from the DXRBS and 2 Jy surveys. Symbols are the same as in Fig. 4.8. The upper and lower solid lines in the left panel represent the observed correlations for strong-lined radio-loud AGN with Ca H&K break valuesC= 0 andC >0 respectively. The solid line in the right panel reprents the observed correlation for strong-lined radio-loud AGN with Ca H&K break valuesC= 0.

(Rawlings and Saunders, 1991). In objects with Ca H&K break valuesC >0, on the other hand, the X-ray emission seems to be predominantly thermal in origin. Now, in order to assess if weak- and strong-lined radio-loud AGN differ in the strength of their nuclear X-ray emission, and so X-ray jet power, we would need to compare only objects viewed at relatively small angles. In the current sample only 4 weak-lined objects haveC = 0. For these I get a mean X-ray luminosity logL_x= 27.48±0.24 erg s⁻¹ Hz⁻¹, which is very similar to the mean value for the strong-lined objects withC= 0 of logL_x= 27.51±0.09 erg s⁻¹ Hz⁻¹. Therefore, also in the X-ray band it appears that the difference between weak- and strong-lined radio-loud AGN lies mainly in their emission line luminosities rather than their jet powers.

We now want to turn to the broad emission lines. Fig. 4.9 plots the BLR luminosity versus the extended radio luminosity at 151 MHz (left panel) and total

4.4 Weak- and Strong-Lined Radio-Loud AGN 89 X-ray luminosity at 1 keV (right panel) for the sample under study. Fig. 4.7, lower panel, has shown that a large scatter is expected in any logarithmic plot involving BLR luminosity, if orientation effects are not accounted for properly. Therefore, in order to do so I have split the weak- and strong-lined radio-loud AGN additionally into objects withC = 0 (4 and 40 sources) andC >0 (8 and 31 sources). I find that for both strong-lined radio-loud AGN withC = 0 andC >0 the BLR luminosity is significantly (P >99.9%) correlated with the extended radio luminosity at 151 MHz (solid lines in Fig. 4.9, left panel). Both these correlations remain very strong even if the common redshift dependence is excluded. The two correlations have similar slopes 0.44±0.11 and 0.36±0.09 for objects with C = 0 and C > 0 respectively.

Furthermore, these slopes are similar to the one of 0.58±0.08 for the correlation between NLR luminosity and extended radio power (Fig. 4.8, left panel), suggesting that both the NLR and BLR luminosities scale with radio jet power in a similar way.

The dispersion of the two observed correlations isσ= 0.45 and 0.38 for objects with C = 0 and C >0 respectively, and comparable to the dispersion of σ = 0.58 that is obtained for the correlation between NLR luminosity and extended radio power. Therefore, it seems that it is indeed crucial to first disentangle orientation effects in studies involving BLR luminosity. Comparing the weak- and strong-lined radio-loud AGN (now separately forC = 0 andC >0) I get similarly to the findings in Fig. 4.8, left panel, that the weak-lined objects do not form merely a continuation of the strong-lined ones to lower BLR luminosities and radio jet powers. However, in this case the number statistics are considerably reduced owing to the separation in Ca H&K break value. But, e.g., I get that weak- and strong-lined radio-loud AGN with C > 0 (for which a larger number is available) reach maximum BLR luminosities L_BLR ∼ 10^44.5 and ∼ 10^42.5 erg s⁻¹ respectively, a factor of ∼ 100 apart, whereas their mean extended radio powers differ only by a factor of ∼ 10 (logL₁₅₁ = 33.45±0.24 and 34.43±0.18 erg s⁻¹Hz⁻¹ respectively). Therefore, this finding also suggests that the difference between weak- and strong-lined radio-loud AGN is mainly in emission line luminosity.

Considering the BLR luminosity versus the total X-ray luminosity (Fig. 4.9, right panel) I find that, once the common redshift dependence is excluded, a significant correlation is only present for strong-lined radio-loud AGN with Ca H&K break values C = 0 (solid line), but not for those with C > 0. Therefore, the BLR luminosity, similarly to the NLR luminosity, seems to correlate with the nuclear X-ray emission only.

All DXRBS sources for which an optical spectrum was not available have been

90 A New Classification Scheme for Blazars excluded from this analysis. Out of these, 37 have a redshiftz <0.8, the maximum value for which the [O III] emission line can be observed in the optical range, and so could have been in principle included. For these I get mean values of logL151 = 33.81±0.18 erg s⁻¹ Hz⁻¹ and logL_x= 26.50±0.20 erg s⁻¹ Hz⁻¹ respectively, well within the luminosity ranges of the objects included. Therefore, the exclusion of these sources is not expected to have biased the current results.