3.1. Radio Properties
These lower-luminosity ( 2 × 1032 erg/s Hz-1 at 1.5G Hz) big radio doubles generally have fairly symmetric twin jets on > 1 kpc scale, and the lobes are edge-darkened with no terminal hotspots. Much of the VLBI data on FR Is (and much of it on low-luminosity FR IIs) come from the group behind these references: Giovannini et al 2001, 2005; T. Venturi, pc, 2010. The data, though somewhat sparse on speeds especially, are consistent with the assertion that FR I jets start out relativistic, with the FR Is being a little slower than the low-luminosity FR IIs. The fraction of sources studied in the isotropically selected 2005 sample which are visibly two-sided on VLBI maps is ~ 30%, vs. 5-10% in earlier core-flux-selected samples. Somewhat of an update was provided by Liuzzo et al (2009), with single-epoch data on low-frequency selected FR Is, with aggregate results consistent with a single unified (beam) model.
The overall statistics on the depolarization asymmetry, another powerful constraint on the inclination distribution, show that the effect is weaker than for Quasars, and probably consistent with an isotropic distribution, though data are scarce. For example, Morganti et al (1997) looked at this for an FR I sample, and found that the depolarization asymmetry is usually weak. Garrington et al (1996) had come to a similar conclusion, noting however that strongly one-sided jet sources have strong depolarization asymmetry. Capetti et al (1995c) found a strong depolarization asymmetry in 2 out of 3 intermediate radio luminosity (between FR I and FR II) radio galaxies. I think that more depolarization work should be done, not just for the AGN field but for understanding galaxy and cluster hot gas atmospheres, where the depolarization presumably takes place.
Finally I'm getting to a topic that is a little bit controversial. It's not very controversial in that everyone seems to agree that most FR Is have predominantly nonthermal radio/infrared/optical and X-ray nuclei, e.g. Mûller et al 2004. But it is important not to overgeneralize, and assert a direct connection between FR I morphology and a nonthermal engine. 30 I listed a few exceptions in Antonucci 2002a, b, going back to the first FR I Quasar, reported in 1984 by Gower and Hutchings. (I don't know why the more common FR I Broad Line Radio Galaxies don't impress people equally, but some extra cachet seems to attach to those of Quasar optical/UV luminosity.)
Recently there have been several papers modeling individual FR I nuclei as purely nonthermal, which I think have been contradicted by subsequent Spitzer spectra. For example, consider NGC 6251 31, analyzed by Chiaberge et al (2003), for which the spectral index over most of the infrared is inferred to be ~ -0.6. Leipski et al (2009) shows that the mid-IR spectrum exceeds their predictions, and shows strong dust emission features. Our conclusion from the SED is that there is hot dust with at least as much flux as that due to synchrotron radiation. The aperture was ~ 4" but that may not matter much because dust emitting at a few microns must be near a somewhat powerful optical/UV source, i.e., the nucleus. It'd be easy to check from the ground. Thus the substantial near- and mid-IR dust emission may signal a hidden thermal optical/UV also. See however Gliozzi et al 2008, who shows that the X-rays are likely to be dominated by the jet in NGC 6251. I think a problem with most published decomposition of radio galaxy infrared spectra is that they require slopes flatter than those of Blazars, opposite to the beaming prediction.
A Spitzer IRS spectrum for BL Lac is shown in Fig. 2 of Leipski et al 2009: the slope is ~ -0.8 between 5 and 30µ, considerably steeper than the synchrotron slopes in most published nonthermal models for radio galaxies. What's more, Impey et al (1988) find that "The spectra of Blazars steepen continuously between 109-1015 Hz...the [frequency] at which the energy distribution turns down in ~ 2 × 1011 Hz with a very narrow range of spectral indices. Half of the Blazars with less than 1011 L show evidence for thermal infrared components..." to which I'd add: which should be much more conspicuous and thus more widespread in the high-inclination objects (radio galaxies). Moreover, "The average Blazar spectrum is flat ( ~ 0) at 109 Hz and steepens continuously to ~ -1.5 at 1015 Hz. Table 4 shows that the infrared slopes from 3 × 1012 Hz (100µ) to 3 × 1014 Hz (1µ) are all steeper than 1." For the SEDs of FR I synchrotron components, with lots of infrared Spitzer data, and both with and without dust bumps, see Leipski et al 2009: in our models, which feature relatively low synchrotron contributions throughout the infrared, the slopes are steeper than those found by other investigators and thus more reasonable in my opinion.
There are at least two ways around this objection to the required flatness of the synchrotron components in some of the published infrared decompositions. First, the emission from high-inclination may be dominated by a slow-moving component which is for some reason intrinsically flatter, and not directly related to the strong beamed component (e.g. Chiaberge et al 2000). Also, the Blazar samples aren't necessarily matched to the lobe-dominated radio galaxy samples and this could conceivably make a difference. They do however include many objects with FR I diffuse radio power.
Van Bemmel et al (2004) account for most of the nonstellar radiation from 3CR270 (= NGC 4261) with a nonthermal model, but find some evidence for a weak thermal component. Our Spitzer data show a big dust bump, which covers 3µ-100µ, and dominates the infrared energetically, at least as observed in the 4" Spitzer aperture. Please see Fig. 9 of Leipski et al 2009 for our spectral decomposition, and the location of the synchrotron component. The Big Blue Bump is extremely well correlated with the Broad Line Region in AGN, and I consider the possible detection of broad polarized H- in this object by Barth et al (1999) well worth following up. Again skipping ahead to the X-ray, Zezas et al (2005) conclude that 3CR270 is a heavily absorbed nucleus, NH ~ 8 × 1022 cm-2, far higher than most FR Is (see Figure 10 here; also Balmaverde et al 2006). Synchrotron is thought by Zezas et al (2005) to contribute only ~ 10% of the X-ray flux.
The detailed discussion of Cen A in Whysong and Antonucci 2004 still represents our views on this controversial and somewhat complicated case. We think it contains a hidden Big Blue Bump/Broad Line Region. Optical polarization imaging is relevant for this FR I radio galaxy. Capetti et al (2007) have measured the percent polarization of the HST nuclear sources at ~ 6060Å in several FR I galaxies. Restricting to those with PA errors 10° (the error functions have strong tails, unlike the Gaussian function), the seven remaining objects are a few percent polarized at random-looking angles. Cen A has a similarly puzzling optical (R/I band) polarization, influenced greatly by a foreground dust lane (Schreier et al 1996).
In the near-IR K band however, one can sometimes see through kpc-scale dust lanes of modest optical depth to the nuclear occultation/reflection region (Antonucci and Barvainis 1990; Whysong and Antonucci 2004; see also Bailey et al 1986). Packham et al (1996) report on both the near-IR polarization and the millimeter polarization (which turns out to be crucial): the polarization of the nucleus after various corrections is given as an impressive 17% ("in the near-IR"), and exactly perpendicular to the inner radio axis. This is expected for hidden thermal AGN rather than for Blazars. (Refined values can be found in Capetti et al 2000.) Packham et al remark that the millimeter polarization, given simply as "zero," is "not...consistent...with that of BL Lacs." As noted, we believe that near-IR observations often see through the dust lanes, enabling us to see this very high polarization exactly perpendicular to the radio jet, as we demonstrated with the radio galaxy 3C223.1 (Antonucci and Barvainis 1990; we used a 1-channel polarimeter [!] but our measurement was accurately confirmed with a modern instrument).
The mere fact that the PA is constant in time for each near-IR observation of Cen A is unlike BL Lacs (or compact synchrotron sources in general), as is the perpendicular relation to the radio jet. We also think that the spatially resolved azimuthal off nuclear near-IR polarization (Capetti et al 2000) is most consistent with scattering from a normal Type 1 nucleus.
For Cen A, we mention the X-ray spectrum here (Markowitz et al 2007). The superb Suzaku spectrum shows a column above 1023 cm-2 for two separate components, and many narrow fluorescent lines, including Fe K-, like a Seyfert 2.
Going back to the mid-IR data on FR Is and FR IIs generally, an imaging survey of nearby objects at 12µ by van der Wolk et al (2010) revealed results which are generally understandable and consistent with other arguments: the broad line objects, all FR IIs, were easily detected at 7 mJy sensitivity (they quote 10 !), as well as most of the High-Ionization Narrow Line Objects (also FR IIs). The low-excitation galaxies of both types were not detected.
Spitzer is much more sensitive than any ground-based instrument. The current state of the mid-IR art survey of FR Is from the IRS spectrograph is described in Leipski et al 2009. Here's where there is a little more controversy. We observed 25 FR I radio galaxies, and carefully removed the star formation contributions as well as possible using the PAH features, and also removed old stellar populations using the Rayleigh-Jeans tail of the starlight, and using the AGB star features at longer wavelengths. We reached the following conclusions for the 15 putative pure-synchrotron sources described in Chiaberge et al 1999. Of the 15 sources with "optical compact cores" from the Chiaberge group and others (see the "Optical" section below), we see four with the infrared dominated by contributions from the host galaxies. In another four of the galaxies with optical point sources (but probably no exposed Big Blue Bump/Broad Line Region), warm dust emission dominates, and is probably at least in part due to hidden nuclei, contrary to the conclusions from the optical papers. In seven cases, synchrotron radiation dominates the mid-IR. The comparison to the Chabarge et al core decompositions cannot be considered definitive however because of the larger Spitzer aperture.
Some information about the optical point sources was used above to provide context for the IR fluxes, but we must note here that these cores (Zirbel and Baum 1995, 2003; Verdoes Kleijn et al 2002; Chiaberge et al 1999, 2000) have been used to argue for synchrotron optical emission and no powerful hidden AGN or tori in most FR I radio galaxies; Baldi et al 2010 is closely related. Zirbel and Baum (1998, 2003) find that the low-luminosity radio galaxies with detected central compact optical cores are the ones with visible (single or highly one-sided) jets, and thus probably low inclinations. This is compatible with the jet idea for the optical cases.
There has also been a series of papers by the Chiaberge, Capetti group (Capetti et al 2005) on emission lines from low-luminosity radio galaxies, which is used to support the pure synchrotron model, but which can perhaps be interpreted differently. For example, the putative beamed continuum is said to provide sufficient ionizing photons for making the recombination lines, but subject to the assumption that the narrow line covering factor is as high as 0.3. This is in the rest frame of the putative ionization source, the relativistic jet. The solid angle is reduced by a factor of order the inverse square of the relativistic bulk gamma factor. 32 The cool radiating gas needs to lie very close to the jet direction, which is somewhat unexpected, though the gas mass requirement is said to be modest. I'm not aware of any Cloudy-type calculation showing that a jet spectrum would produce the suite of detected lines.
Capetti et al (2005) also made a factor of 5 correction downward to estimate the H- emission line flux from the measured flux of the blend with the [N II] doublet. The factor of 5 seems too high to me, and comes not from typical AGN behavior, but from a UGC sample of ordinary LINERs. Also, there was apparently no starlight subtraction before this factor was determined from spectroscopy (Noel-Storr et al 2003). (The effect of this can be estimated from formulae in Keel 1983.) Finally, they detected in most cases probable broad bases to the H- line, but not the forbidden lines. These lines are said to be "compatible with the broad lines seen in LINERs" by Ho et al (1997). Any Big Blue Bump accompanying these lines would probably be undetectable, at least with present data, so this is consistent with (but not proof of) the presence of a Big Blue Bump, albeit of low luminosity. Ho (1999, 2009) has argued however that for his LINERs with weak broad H-, the central engines are radiatively inefficient. My overall conclusion regarding the central compact optical cores is that they are indeed mostly synchrotron sources, but I don't share the same degree of confidence as the various authors.
Let us now return briefly to the question of broad emission lines in FR I galaxies, concentrating on AGN that can be observed with good contrast relative to the host galaxies. The most familiar object of this type is 3C120, with a fast superluminal VLBI source. Several others (Antonucci 2002a gives a brief compilation) are also highly core dominated, including BL Lac itself, in which the broad lines need to compete with the beamed radiation in order to be detected. This suggests that a Broad Line Region is sometimes visible in an FR I radio galaxy, when seen at low inclination. Falcke et al (1995) wrote a clever paper on this "missing FR I Quasar population."
This section will be brief because the results of many excellent studies are simple and consistent, within the noise and the limited number of sources analyzed, and the selection biases specific to each. Refer again to the present Fig. 10 (Hardcastle et al 2009) for strong evidence of very weak (ostensibly) accretion-related power Low-Ionization Galaxies, including both FR types. Other papers are generally very supportive of (and in some ways anticipated) Hardcastle et al 2009.
Some recent surveys with lots of FR I results: Donato et al 2004; Balmaverde et al 2006; Rinn et al 2005; Evans et al 2006; and Hardcastle et al 2006. Overall, the great majority of X-ray spectra of FR I radio galaxies suggest nonthermal emission. This finds strong independent support in that most of these objects (the current Fig. 10) differ from Cen A, and do not show the high absorption columns typical of hidden AGN.
30 It is easy to imagine that the few exceptions can be attributed to the grossly different timescales on which the core (~ 1 yr) and the lobes (~ 108 yr) are created. On the other hand, the host-dependence of the luminosity cutoff, strongly suggests environmental factors also play a role (Owen and Ledlow 1994). Back.
31 This is the first object with good evidence of a narrow line flux variation: Antonucci 1984, Fig. 1. Back.
32 The claimed linear flux and luminosity correlation between continuum and emission lines is not entirely expected since the former may be affected by beaming, while the latter is not. Back.