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4.4.1. Emission line properties

BLLs have been defined as blazars with rest-frame emission line equivalent widths smaller than 5 Å (Morris et al. 1991; Stickel et al. 1991). Many authors have criticized this definition as being arbitrary. [69] have remarked that, when emission lines are detected in BLLs, they fall into one of two categories: they are broad, but weak, similar (except for their equivalent width) to those seen in normal QSOs, or they are weak and narrow such as [O II]lambda3727, Halpha, etc; they have suggested that objects in the first category are HPQs rather than genuine BLLs. The distribution of the Mg II EW in blazars is not bimodal (Mg II is the line most often observed in BLLs, see tables 3 and 4); there is continuity between objects with EW > 5 Å and those with EW < 5 Å ([370]); moreover there are objects that cross the boundary depending on the strength of their variable continuum ([11]).

In optically selected and lobe-dominated radio loud QSOs, i.e. in objects with no Doppler boosting, the luminosity of Hbeta is closely proportional to the observed continuum luminosity with the median rest Hbeta EW equal to 80 Å  while the Hbeta luminosity varies by three orders of magnitudes ([293]); the median Mg II EW is ~ 35-50 Å  ([399]; [131]). HPQs are most probably normal radio QSOs in which an intrinsically weak nuclear continuum source is strongly beamed due to the alignment of the nuclear jet with the line of sight; when the enhancement of this jet component is such that its apparent luminosity is equal to the unbeamed luminosity of the QSO accretion disk, the emission line equivalent widths are reduced by a factor of 2; but we cannot exclude the possibility that the enhancement factor in some of these objects is five to ten times larger or more reducing the mean value of the Mg II EW to less than 5 Å; some of these objects would have no detectable broad emission lines and be called BLLs; however their extended radio luminosity would still be high.

We have seen above that, except for a single exception, none of the known FR I radio galaxies have broad emission lines implying that blazars which display broad emission lines, even with small EW, are HPQs rather than BLLs. On the other hand, many FR II radio galaxies have emission lines of very low excitation (the LERGs); they must be the parent population of a beamed sub-population which would be optically similar to BLLs but would have relatively high extended radio luminosity ([242]; [455]).

BLLs are dominated by a bright compact radio core; but maps made with a high dynamic range often show an extended structure. If BLLs are FR Is with their jet pointing towards us, these extended radio structures should have the low luminosity typical of FR Is. However, a number of objects classified as BLLs have extended radio luminosities above the FR I/FR II division and could be misclassified FR IIs, i.e. HPQs ([77]).

There are 35 known objects often classified as BLLs and in which one (or more) broad emission line (C IV, Mg II, Hbeta or Halpha) has been detected; they are listed in tables 3 and 4; the 5 GHz luminosity (col. 7) is computed from the published values of the extended flux densities assuming a spectral index alpha = 0.7. Many of these objects have a luminosity well above the FR I/FR II limit (~ 1032 erg s-1 Hz-1 at 5 GHz). The nine objects in table 4 show relatively strong (EW > 10 Å) broad emission lines; we shall consider them in what follows as genuine HPQs. It is very likely that all the 26 objects in table 3 are also misclassified HPQs (we shall provisionally call them below BL/HPQs).

Table 3. BLLs tentatively reclassified as HPQs (BL/HPQs). Col. 1: name, col.2: short position, col. 3: redshift, col. 4: HP indicates highly polarized QSOs, cols. 5 and 6: observed broad line and its rest EW, col. 7: 5 GHz luminosity of the extended component.
Name Position z EWrest log(L5GHz)
PKS 0139-09 0138-09 0.733 HP Mg II 25.4
3C 66.0A 0219+42 0.444? HP Mg II? 26.6
AO 0235+164 0235+16 0.940 HP Mg II <1.6-8 25.8
PKS 0537-441 0537-44 0.896 HP Mg II 1.2-10 26.5
PKS 0754+100 0754+10 0.66 HP Mg II? 24.9
PKS 0820+22 0820+22 0.951 HP Mg II 4.9 26.3
PKS 0823+033 0823+03 0.506 HP Mg II 5.1 24.3
0846+51W1 0846+51 1.860 - C IV -
OJ 287 0851+20 0.306 HP Hbeta 0.8 24.5
S5 0954+65 0954+65 0.367 HP Halpha 1.9 24.6
S5 1053+81 1053+81 0.706 - Mg II -
PKS 1144-379 1144-37 1.048 HP Mg II 2.5 -
3C 279 1253-05 0.538 HP Mg II 3.4 27.2
PKS 1335-127 1335-12 0.539 HP Mg II 5.2-9.3 -
PKS 1514+197 1514+19 1.07 HP C III 24.7
4C 14.60 1538+14 0.605 HP Mg II 0.8 26.0
B2 1722+40 1722+40 1.049 - Mg II 4.4 -
OT 081 1749+09 0.320 HP Hbeta 2.6-8 -
S5 1803+78 1803+78 0.684 - Mg II 2.9 25.8
4C 56.27 1823+56 0.664 HP Hbeta 4.1 26.6
OV-236 1921-29 0.352 HP Mg II 5.2 -
PKS 2029+121 2029+12 1.215 - Mg II 4.9-8 -
PKS 2032+107 2032+10 0.601 HP Mg II 25.5
PKS 2131-021 2131-02 1.285 HP Mg II? 26.5
BL Lac 2200+42 0.068 HP Halpha 5.6-7.5 23.5
PKS 2240-260 2240-26 0.774 HP Mg II 1.1 26.6

Table 4. BLLs with strong broad emission lines (EW > 10Å), reclassified as HPQs. Col. 1: name, col. 2: short position, col. 3: redshift, col. 4: HP indicates highly polarized QSOs, cols. 5 and 6: observed broad line and its rest EW, col. 7: 5 GHz luminosity of the extended component.
Name Position z EWrest log(L5GHz)
PKS 0057-338 0057-33 0.875 - Mg II 9.8 -
PKS 0215+015 0215+01 1.721 HP C IV 13.2 26.7
PKS 0256+075 0256+07 0.893 HP Mg II 24.1 26.4
PKS 0306+102 0306+10 0.863 - Mg II 35.0 -
PKS 0521-36 0521-36 0.055 HP Halpha 26.2 25.7
B2 1308+32 1308+32 0.997 HP Mg II 19.0 25.8
3C 446 2223-05 1.404 HP Mg II 16.2 -
3C 454.3 2251+15 0.859 HP Mg II 20.0 27.1
S5 2353+81 2353+81 1.334 - Mg II 27.0 -

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