The Properties of the Radio-Selected 1 J[CLC]y[/CLC] Sample of BL Lacertae Objects

Abstract

We present new optical and near-IR spectroscopy, as well as new high dynamic range, arcsecond-resolution VLA radio maps, of BL Lacertae objects from the complete radio-selected jansky (1 Jy) sample (RBLs) for which such data were not previously available. Redshift information is now available for all but six of the 37 BL Lac objects in the 1 Jy sample. Of the 31 with redshift information, four redshifts are only minimum values based on absorption lines, and four other objects have uncertain redshifts based on the detection of only a single emission line. Unlike BL Lac objects from the complete X-ray–selected Einstein Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but moderately luminous emission lines. The emission-line luminosities of RBLs are several orders of magnitude lower than flat-spectrum radio quasars (FSRQs); however, there is significant overlap in the luminosity distributions of the two classes. All but one object in the 1 Jy sample has now been observed with the VLA, and extended flux was detected for all but three of the observed objects. Whereas nearly all XBLs have extended power levels consistent with FR 1s, more than half of the RBLs have extended radio power levels too luminous to be beamed FR 1 radio galaxies. In fact, we find evidence for and examples of three distinct mechanisms for creating the BL Lac phenomenon in the 1 Jy sample: beamed FR 1s, beamed FR 2s, and possibly a few gravitationally lensed quasars. The V/Vmax value determined for the 1 Jy sample is 0.614 ± 0.047, which is markedly different from the negative evolution seen in the EMSS and other XBL samples. A correlation between logarithmic X-ray–to–radio flux ratio and V/Vmax value is observed across the EMSS and 1 Jy samples from negative evolution in the more extreme XBLs to positive evolution in the more extreme RBLs. There is evidence that the selection criteria chosen by Stickel et al. eliminates some BL Lac objects from the 1 Jy sample, although how many is unknown. In addition, several objects currently in the sample have exhibited strong emission lines in one or more epochs, suggesting that they should be reclassified as FSRQs. However, these selection effects cannot account for the observed discrepancy in XBL and RBL properties. From these observational properties, we conclude that RBLs and XBLs cannot be related by viewing angle alone, and that RBLs are more closely related to FSRQs.