Optical/UV Classes of Radio Sources
This paper concerns radio loud active galactic nuclei (AGN). "Radio loud" is sometimes defined by an absolute radio luminosity cutoff, and sometimes (less usefully) by a ratio of radio to optical luminosity. The nomenclature is mutifaceted, complex, and very confusing for a newcomer. We will divide the AGN into two broad classes, which correspond to the two popular and persuasive engine types. The presence of an optical/UV continuum of the type called the Big Blue Bump will be called "thermal" because there is a consensus that this is thermal radiation from a copious opaque and probably usually thin accretion flow. This includes the radio loud Quasars, the broad Broad Line Radio Galaxies, and the objects that have similar accretion flows hidden from the line of sight. (Some papers define the Big Blue Bump as the excess over a notional power law extending from the near-IR to the far-UV or X-ray, but that is not the most common usage, or the present usage.) The Big Blue Bump is virtually always accompanied by conspicuous broad permitted emission lines from regions collectively called the Broad Line Region. This combination is called a Type 1 spectrum.
By contrast, a radio loud AGN which lacks visible broad lines, is called a Narrow Line Radio Galaxy ("Type 2" optical spectrum). The narrow-line spectra of all radio types vary enormously from optically weak Low Ionization Galaxies - sometimes loosely called Low Excitation Galaxies - like M87, to very powerful High Ionization Galaxies like Cygnus A. The former are turning out to be almost all "nonthermal" radio galaxies, lacking a powerful Big Blue Bump and Broad Line Region, even a hidden one. 1
If the Big Blue Bump is directly visible in the total-flux spectrum, the object is called a radio loud Quasar, 2 or if low in optical luminosity (e.g. M(V) > -23 for H0 = 50 km sec-1 Mpc-1, as adopted for the older Veron-Cetty and Veron catalogs), it may be called a Broad Line Radio Galaxy. 3, 4 In fact when it's clear from context, "Quasars" will be taken to include Broad Line Radio Galaxies.
For radio-bright objects at redshifts of larger than a few tenths, the presence of a (directly visible or hidden) optical/UV Big Blue Bump is general - except in those rare objects whose optical/UV spectrum is overwhelmed by beamed synchrotron emission from the bases of favorably oriented relativistic jets ("Blazars"). For objects with a large contribution to the optical/UV continuum by highly variable, highly polarized beamed synchrotron radiation, the general term is Blazars, defined in Stein 1978. Blazars are defined as the union of two classes: 1) objects in which a Big Blue Bump/Broad Line Region is still discernable against a strong synchrotron component (Optically Violently Variable Quasars, also known as Highly Polarized Quasars) with 1960s-1970s technology, and 2) objects with a pure synchrotron continuum in those old spectra, and little or no detectable line emission or absorption (BL Lac Objects). However, it's been known since the 1970s at least that many historically defined "BL Lacs" show emission lines, both narrow and broad, especially (but not necessarily) when observed in low states. For example, BL Lac itself has weak narrow emission lines, and stellar absorption lines (Miller 1981); now we know that broad lines are often visible as well (Vermeulen et al 1995). In fact it is well known that many highly polarized, violently variable Quasars are indistinguishable from BL Lacs when in high states (Miller and French 1978; see also Miller 1981).
None of these, "BL Lac" nor "High Polarization Quasar," nor "Optically Violently Variable Quasar" is very well defined, and many studies have been damaged by blindly using these historical categories - sometimes just from catalog classifications, or by trying to mimic them with equivalent width cutoffs, which result in classifications changing with time (Antonucci et al 1987, 2002a)! It is also still often incorrectly asserted that the parent population (equivalent objects at more than a few degrees inclination) for "BL Lacs" is FR I double radio sources 5 (the lower-luminosity edge-darkened ones). This is manifestly not the case (see references to maps of diffuse radio emission in Antonucci 2002a, going back for decades). Yet people still write about FR II radio emission in BL Lacs as a "problem" for the unified model. (It is well known that parents of optically-defined "BL Lacs" can be of either FR type, e.g. Kollgaard et al 1992.) This has invalidated studies of cosmological evolution, among other things (e.g., Ostriker and Vietri 1985, 1990.) See Jackson and Wall 1999 for a well-informed and sensible discussion.
Great care is required in classifying AGN, and the price of carelessness is
spurious results. For example, historically
3CR382, 3CR390.3 and similar objects
were called Broad Line Radio Galaxies (Type 1 optical spectrum), and this is
still reasonable. But the same was done for 3CR234, because in fact the
broad H-
line is visible
in the total-flux spectrum. This wasn't
an error in its historical context, but we now know that the broad
lines and Big Blue Bump are seen only by reflection
(Antonucci 1982,
1984;
Tran et al 1995;
Young et al 1998).
Therefore from the point of view of
unified schemes, such objects must be included with the Narrow Line
Radio Galaxies (Type 2 optical spectrum), just as we keep Seyfert 2s (with
hidden Big Blue Bump/Broad Line Region) separate from the Seyfert 1s. It's
be great if the multi-dimensional classification scheme could be shown in a
drawing, but a confusing "tesseract" would be needed
(Blandford 1993)
- or more likely something worse.
1 Note that there is only a little evidence yet that the thermal and nonthermal objects are bimodal in any property, and such isn't necessarily expected theoretically. Back.
2 Recall that this paper is largely restricted to radio loud AGN. Back.
3 It is sometimes argued that at these low luminosities, there are some relatively subtle differences with respect to the Quasars, e.g., van Bemmel and Barthel 2001, but such a distinction will not be made here. Back.
4 For redshifts above a few tenths, the first two words of "Narrow Line Radio Galaxy" are often dropped, because the broad line objects are unambiguously called Quasars. Back.
5 FR I and II radio sources are discussed in Sec. 1.3. Back.