The BL Lac catalogue assembles sources selected in different bands with different flux limits. Therefore, detailed statistical analysis should be restricted to appropriate subsamples and not to the whole collection of objects, unless all the biases due to the different selection processes can be fully taken into account. We can nevertheless have an overview of some general properties like the distribution on the plane of the sky and the redshift distribution.
Figure 1. The Aitoff projection in celestial (i.e. equatorial) coordinates of all BL Lacs in the catalogue. Note the strong bias in favour of northern declinations.
Figure 1 shows the Aitoff projection in equatorial coordinates of the BL Lacs in the catalogue. A bias in favour of northern declinations is clearly present: out of 233 objects, 178 (i.e. 76 per cent) have > 0° and there are no known BL Lacs with < -53°. This reflects the fact that most major surveys have been done in the northern hemisphere, which has a direct influence on radio samples and an indirect influence on serendipitous X-ray samples.
Figure 2. Redshift distribution for all BL Lacs in the catalogue. The hatched area indicates BL Lac candidates.
Only 115 objects (or about 50 per cent of the catalogue), excluding lower limits, have redshifts, which shows how difficult it is to extract this important information from BL Lac spectra. Figure 2 shows that the redshift distribution for the catalogue peaks at z ~ 0.1 - 0.2. Note that, apart from a BL Lac candidate at z = 1.715, the most distant confirmed object reaches z = 1.215 and only four objects (plus two lower limits) have z > 1. This is in marked contrast to the redshift distribution of quasars in the Hewitt & Burbidge (1993) catalogue (see their fig. 3) and is unlikely to be due to selection effects inherent to the different sample. The redshift distributions of BL Lacs and quasars, in fact, are also quite different in complete samples: in the 1-Jy catalogue, for example (Stickel et al. 1994), zmax ~ 3.8 for quasars but only ~ 1.2 for BL Lacs. This is probably related to the small (possibly absent or even negative) cosmological evolution displayed by BL Lacs (see e.g. the discussion in Padovani & Giommi 1995a).
The most ``popular'' BL Lac is Mrk 421 alias S4 1101+364 alias PG 1101+385 alias 1H 1104+382 alias 1ES1101+384 alias GRO J1106+38, as it belongs to five samples: S4, PG, HEAO-1, Slew, and GRO. It is closely followed by three other BL Lacs which belong to four samples: PG 1218+304 alias EXO1218.8+3027 alias 1H 1219+301 alias 1ES1218+304 (PG, EXOSAT, HEAO-1, and Slew surveys); Mrk 501 alias S4 1652+398 alias 1H 1651+398 alias 1ES1652+398 (1-Jy, S4, HEAO-1, and Slew surveys); and 3C 371 alias S4 1807+698 alias 1H 1803+696 alias 1ES1807+698 (1-Jy, S4, HEAO-1, and Slew surveys).
As anticipated in the Introduction, a first use of this catalogue has been the extraction of all BL Lacs from the WGA catalogue to study their X-ray properties. Those results will be presented elsewhere (Padovani & Giommi 1995b).
ASCII and TeX versions of the catalogue can be obtained on NCSA MOSAIC at the following URL: http://itovf2.roma2.infn.it/padovani/catalogue.html. We welcome comments, suggestions, corrections and additions, all of which should be addressed to the first author (electronic mail: email@example.com).
We thank Richard Green for confirming the identification of Mrk 421 with PG 1101+385, Eric Perlman for providing us with the latest Slew survey BL Lac list in advance of publication, Joe Pesce for useful comments, and Manfred Stickel for information about the S5 BL Lacs. This research has made use of the BROWSE program developed by the ESA/EXOSAT Observatory and by NASA/HEASARC, and of the on-line services provided by the European Space Information System (ESIS), and by the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.