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.  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]3727, H, 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 (); moreover there are objects that cross the boundary depending on the strength of their variable continuum ().
In optically selected and lobe-dominated radio loud QSOs, i.e. in objects with no Doppler boosting, the luminosity of H is closely proportional to the observed continuum luminosity with the median rest H EW equal to 80 Å while the H luminosity varies by three orders of magnitudes (); the median Mg II EW is ~ 35-50 Å (; ). 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 (; ).
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 ().
There are 35 known objects often classified as BLLs and in which one (or more) broad emission line (C IV, Mg II, H or H) 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 = 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).
|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|
|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||-|
|S5 1803+78||1803+78||0.684||-||Mg II||2.9||25.8|
|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|
|PKS 2240-260||2240-26||0.774||HP||Mg II||1.1||26.6|
|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||-|
|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||-|