Presentation on theme: "A panchromatic review of thermal and non- thermal AGNs Robert R. J. Antonucci Department of Physics University of California, Santa Barbara Santa Barbara,"— Presentation transcript:
A panchromatic review of thermal and non- thermal AGNs Robert R. J. Antonucci Department of Physics University of California, Santa Barbara Santa Barbara, CA
SUMMARY Analyze the literature advocating the existence of "True Seyfert 2s," that is, Seyfert 2s that have no visible or hidden broad lines, Carefully explain nine errors frequently made in polarimetry papers along with six non-polarimetric arguments. I will argue that I don't find any of the published cases convincing (though there are probably some papers on this topic with which I'm not familiar.) For more details see review A panchromatic review of thermal and non-thermal AGNs on astro-ph.
OUTLINE Very Common Errors and Omissions in Papers Advocating True Seyfert 2s. I. Polarimetric evidence – 9 comments II. Other evidence – 4 comments Note: I will NOT discuss individual objects or papers, not even in the question period (though you can speak up), because I don't remember the facts for each one, and I have to think slowly and critically to respond to assertions by others!
Disclaimers: Some of what I will say is probably wrong! I'm critiquing. Unlike Fox News, I do not strive to be fair and balanced. Not all of my arguments apply to all objects or all papers!! Some of the points argue the plausibility of identification as common NLXGs, and some of big blue bumps/BLRs hidden by a nuclear torus. I'm not trying to present a single scenario for all. I don't really give a damn about whether or not there are True 2s! NOTE: A True Seyfert 2 is defined here to be an AGN with broad line EW (relative to the AGN continuum) much less that that of normal Seyfert 1s.
Polarimetric Arguments 1) Hidden BLRs revealed later Hidden broad lines are often detected at later epochs, or with better SNR. Even in their low states, I think the points below greatly weaken their candidacy. Note: One should NEVER say certain objects lack a hidden BLR, only that none has been detected so far. There is no evidence of bimodality – some with and some without hidden BLRs!
2) Lack of upper limits Few if any upper limits have been published for broad lines of candidates, in either flux or polarized flux!!! If someone gives does give EW upper limits, they must be relative to the AGN continuum, which is hardly detected in many cases.
3) Polarization is NOT due to nuclear scattering in most well-studied Seyferts!! Scrupulous removal of interstellar polarization in the Milky Way is necessary. Polarization is very often due to dust transmission in the host. Polarization may be due to large-scale dust scattering in the host.
4) Upper limits depend on assumed line widths For example, Stockton et al. claimed to put tight limits on BLs in Cygnus A in total flux, but they turned out to be wrong because the authors assumed much too narrow a line width.
5) "Secure non detections" of broad lines Several objects have non-detections of BLs in polarized light that are called "secure," though no upper limits are given. I'm not even sure that a non-detection can ever be secure.
6) Spectropolarimetry at H is inadequate H is often too weak to be detected, sometimes due to reddening. Early example: 3C 234. Undetected polarized H means almost nothing to me.
7) Near-IR polarimetry often required Often kpc-scale obscuration of the reflecting region blocks even H in polarized light... e.g., Cen A and Cyg A, for which the optical percentage polarization is 1% yet the near-IR percentage polarization is 20%!! You have to check for this.
8) Dilution of polarization by a starlight Old stars must be carefully removed. But also… The nuclear regions of most well-studied Seyfert 2s are dominated by starburst light! Only the broad lines themselves tell the scattering polarization. The percentage polarization is the polarized flux of the line divided by their total flux, yet generally the broad lines aren't detected in total flux in Seyfert 2s, by definition! True test: does the polarized flux look like a Seyfert 1?
9) It takes luck to find hidden broad line regions! From Tran et al. (2011): Could the [True 2s] be the hidden counterparts to the NLS1...? This can be ruled out by the simple observation that no emission lines of any kind, broad or narrow, are seen in the polarized flux spectra. But you need to be lucky to have a suitable mirror!
Non-polarimetric evidence for True Seyfert 2s. 1) Independent analysis by Stern & Laor (2012) Stern and Laor (2012) have shown proxy equivalent width limits by normalizing the BLs to the Hard X-ray... What fraction of Seyfert 1s in their sample have a value for this that's less than their upper limit then NGC 3147? Is it very small? No, it's 50%!!! And even these limits apply only to lines matching their velocity prescription, so they are really lower limits to upper limits!
2) Near-IR emission lines Candidates are almost always overwhelmed by dust emission and starlight… EW limits are only intelligible if given relative to the AGN continuum, which is usually undetected, especially in the near-IR.
3) The mid-IR calorimeter An excellent test for a hidden AGN is the mid-IR [warm] calorimeter, which indicates the luminosity of any visible or hidden AGN approximately but (in my opinion) robustly. But in the (heterogeneous) sample of Haas et al. (2007) the mid-IR bumps are just as strong relative to [O III] 5007 in the putative True 2s as in their other objects!!!
4) Are the True 2s even Seyfert 2s at all? Some candidates have optical spectra inadequate for classification. Though some liners do have visible or hidden BLRs, liners with no BLRs aren't surprising. Classification is crucially dependent on precise starlight subtraction. The best- observed NGC 3147 spectrum (Ho et al. 1985) shows no H emission line!! It can only be seen with precise starlight subtraction. Thus [O III] 5007 / H (and [N II] 6584 / H ) is biased upward because the Balmer emission lines sit in stellar absorption lines! This is a HUGE effect at low luminosities such as those of True 2s.
Additional Point: Isotropic selection is key for polarization and other surveys NGC1068 and NGC4151 have the same UV excess, but we're only seeing 1% of the nuclear UV in the former case, so that AGN comes from five magnitudes higher on the luminosity function!! Hidden BLR objects tend to have systematically larger optical flux and luminosity, and much better contrast against the host galaxy starlight. That's why their hidden BLRs were detected in the first place. Keel et al. (1994) is a fundamental paper because it was selected independently of orientation. Thus the Type 1s and the Type 2s can be compared legitimately. Here is a related and shocking widespread practice: Conclusions are routinely drawn from luminosity-luminosity plots: Very influential example that set this field back many years: Rawlings & Saunders 1991 Nature paper. Plot numbers of bars versus number of bookstores in all cities and towns: do drinkers like to read… or does reading drive you to drink?