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The Unification Model of Active Galaxies: Implications from Spectropolarimetric Surveys

The Unification Model of Active Galaxies: Implications from Spectropolarimetric Surveys. Hien D. Tran W. M. Keck Observatory. Optical Spectra of Various Types of AGNs. Bill Keel. Seyfert 1 & 2 Optical Spectra. Narrow forbidden lines: eg. [O III], [N II].

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The Unification Model of Active Galaxies: Implications from Spectropolarimetric Surveys

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  1. The Unification Model of Active Galaxies: Implications from Spectropolarimetric Surveys Hien D. Tran W. M. Keck Observatory

  2. Optical Spectra of Various Types of AGNs Bill Keel

  3. Seyfert 1 & 2 Optical Spectra Narrow forbidden lines: eg. [O III], [N II] Broad permitted lines: Ha, Hb Bill Keel Narrow lines only

  4. Hidden Sey1 Nucleus Inside a Sey2 Galaxy NGC 1068 Miller & Antonucci (1983) Antonucci & Miller (1985) Miller, Goodrich & Mathews (1991) Bill Keel

  5. NGC 1068 Hidden Broad-Line Region (HBLR)

  6. Mrk 348: Another “HBLR Sey2” HBLR = Hidden Broad-Line Region Typical Seyfert 2 (S2) spectrum Appears like a Seyfert 1 (S1)!

  7. AGN Unification Paradigm: Obscuring Torus Sey 1 view Sey 2 view Nuclear Engine + Broad-line region (BLR): broad Ha, Hb Obscuring torus Narrow-line region (NLR): [O III], [N II] Scattering e-, dust Urry & Padovani

  8. NGC 5252 Ionization Cones Morse et al. 1998

  9. Puzzling Differences between Sey1 & Sey2 • S2s more likely to be in found in hosts with enhanced star formation (Maiolino et al. 1995; Gu et al. 1998). • S2s tend to be in hosts with higher frequency of companions (de Robertis, Yee, & Hayhoe 1998; Dultzin-Hacyan et al. 1999). • S2 hosts tend to show richer dust features (Malkan, Gorjian, & Tam 1998).

  10. Spectropolarimetric Study of Seyfert Galaxies • General applicability of the Unification Model (UM)? • What determines the detectability of hidden broad lines? • Is there a connection to AGN power? • How are the HBLR S2s different from non-HBLR S2s? • Correlation of presence of hidden broad lines with observational properties? • Spectropolarimetric survey of complete samples of Seyfert 2s to look for hidden broad-line regions (HBLRs)

  11. Brief History of SpectropolarimetricSeyfert2 Surveys Young et al. (1996) • sample: 24 “warm” IRAS galaxies & selected Seyfert 2s • instrument: AAT 3.9-m Heisler, Lumsden, & Bailey (1997) • sample: 16 IRAS-selected Seyfert 2s, S60 > 5 Jy • instrument: AAT 3.9-m Moran et al. 2000 • sample: 38 objects from Ulvestad & Wilson (1989); distance-limited (cz < 4600 km s–1) • instrument: Keck 10-m (“snapshot” mode, ~20min per object) Lumsden et al. (2001) • sample: 24 IRAS-selected Seyfert 2s, S60 > 3 Jy, LFIR > 1010L¡, S60/S25 < 8.85 • instrument: AAT 3.9-m, WHT 4.2-m Tran (2001, 2003) • sample: 49 objects from the CfA and 12 mm samples • instrument: Lick 3-m & Palomar 5-m

  12. Summary of Surveys • HBLRs detected in  1/2 of Seyfert 2 population • Two possibilities: • HBLRs are there but hard to detect • Something wrong with the UM • Some explanations: • Only HBLR S2s are true counterparts to Seyfert 1s • Non-HBLR S2s are too weak to possess or sustain any BLRs • Broad-line variability • Non-HBLRs are narrow-line Seyfert 1s • Evolution • Obscuration / torus inclination • Scattering material

  13. Viewing Angle Hypothesis Miller & Goodrich (1990) C. Heisler (1997)

  14. Diagnostic Diagrams and Luminosity Distributions • Compare various properties among the HBLR, non-HBLR S2s and S1s, illustrating their similarities and differences. These properties include: AGN Strength: Optical: [O III] l5007 luminosity Infrared: Mid-IR 25mm luminosity (L25); IR color f25/f60 Radio: 20cm radio power (S20); S20/f60 X-ray: Hard X-ray (2-10 keV, HX) luminosity; HX/f60 Obscuration: Ha/Hb; NH; HX/[O III]; Fe Ka equivalent width [EW(Fe)] Star formation: Far-IR luminosity LFIR = L60 + L100

  15. [O III] Luminosity vs f25/f60 • HBLR Sey 2 • Non-HBLR Sey 2 • Sey 1 • HII, LINERs, Starburst stronger f25/f60 is the IR color  dust temp. around AGN [O III] Lum.  AGN strength HBLRs are more luminous and warmer than non-HBLRs weaker hotter cooler

  16. [O III] Luminosity S1 HBLR S2 Non-HBLR S2

  17. IR Color: f25/f60 S1 HBLR S2 Non-HBLR S2

  18. Radio Strength: S20cm/f60 vs f25/f60 • HBLR Sey 2 • Non-HBLR Sey 2 • Sey 1 • HII, LINERs, Starburst HBLR S2s are stronger and hotter than non-HBLR S2s. HBLR S2s are similar to S1s.

  19. Comparison between S1, HBLR S2 & non-HBLR S2 Mean, standard deviation (s) and K-S statistical test results for various properties of the S1, HBLR S2s and non-HBLRS2s in the CFA and 12mm samples. • Note: • L in solar luminosity, EW(Fe) in eV, NH in cm-2 • K-S Pnull is the probability for the null hypothesis that the two distributions are drawn at random from the same parent population.

  20. Comparison between S1s, HBLR S2s & non-HBLR S2s S1= Sey 1 S3 = HBLR Sey 2S2 = non-HBLR Sey 2

  21. Large-scale Properties of Seyfert Galaxies • De Robertis et al. 1998 environment study: • Very significant difference found in mean environment between S1s and S2s. • Sample contains few known HBLR S2s (NGC 4388). • Schmitt et al. 2001 study of a far-IR selected sample (Keel et al): • No statistically significant difference between S1s and S2s in terms of host galaxy morphology and freq. of companions. • However, sample was selected for warm IR color, and hence HBLR S2s. • 70% of S2s in this sample are HBLRs, compared to ~ 50% or less for others. • Clavel et al. 2000 ISO study: • Mid-IR ISO spectra of HBLR S2s are similar to S1s. • 7 mm continuum, PAH luminosities and EW. • Those of non-HBLR S2s are different (indistinguishable from SBs). • Smaller 7 mm conti. L and higher PAH EW, but similar PAH lum. • Malkan et al. 1998 HST snapshot survey: • S2s richer in dust than S1s. • However, when grouped into HBLR and non-HBLR S2s, differences in dust • morphology and incidence disappear. • non-HBLR S2: 55%, HBLR S2: 27%, S1: 23%

  22. Mid-IR ISO Spectra of Seyfert Gals S1 HBLR S2 PAH features Non-HBLR S2 From Clavel et al. 2000

  23. Two types of Seyfert 2s? • Not only are HBLR S2s stronger than non-HBLR S2s, they are comparable in a lot of ways to S1s. • Non-HBLR S2 properties, on the other hand, generally do not match well with those of HBLR S2s or S1s. Proposal: • HBLRs are visible largely because their AGNs are intrinsically more powerful • Many non-HBLR S2s maybe too weak to represent “real” hidden S1s, and HBLR S2s may be the only true counterparts to normal S1s.

  24. Mid-Infrared line ratios Wu et al. (2011)

  25. Obscuration Luminous S2s: More likely to find HBLRs in less obscured objects L [O III] > 1041 ergs/s L [O III] > 1041 ergs/s more obscured L [O III] < 1041 ergs/s L [O III] < 1041 ergs/s less obscured Wu et al. (2011) Obscuration plays a role for high-luminosity AGNs

  26. EddingtonRatios Marinucci et al. (2012) • Only objects with good quality data and MBH estimated from stellar velocity dispersion. • Clear separation between HBLRs and non-HBLRs, especially in Compton-thin sources. • Inclusion of Compton-thick sources blurs out the separation, but no HBLR falls below Lbol/Ledd threshold. • Theoretical models: BLR unable to form when AGN luminosities or accretion rates are too low to support outflows from accretion disks (Nicastro 2000; Elitzur & Shlosman 2006; Cao 2010)

  27. Two Key Questions • Non-detections real or due to limited depth of surveys? • Lower AGN luminosity of non-HBLRs leads to intrinsic differences?

  28. HBLR Detection: Flux Comparison • HBLR Sey 2 • Non-HBLR Sey 2 • HII, LINERs, Starburst • There is no separation in the observedfluxes for the most common AGN indicators between the two S2 types. • HBLR detection reaches to very low flux level, below those of many non-HBLR S2s. • Non-detections of HBLRs cannot simply all be due to the detection limit of the survey.

  29. Deep Keck Spectropolarimetric Survey • Same sample as Tran (2003) • CfA and 12μm samples • Target mainly non-HBLR Seyfert 2s • ~ 25 objects • LRIS + polarimeter • Typical exposure times: 80-160 min per object • Multiple epochs • 4 - 15 X deeper than previous surveys • Six new southern objects observed at CTIO

  30. Results of the Deep Keck Survey • Six new HBLRs • Non-HBLR  HBLR: Mrk 573, NGC 3892, NGC 5929, UGC 6100 (Keck) • ESO 541-IG12, NGC 1125 (CTIO) • Five new non-HBLRs • HBLR  non-HBLR: NGC 5347 (called “HBLR” by Moran et al.) • ESO 33-G2, ESO 253-G3, NGC 3147, NGC 4968

  31. New HBLRs: NGC 3982 & UGC 6100 NGC 3982 UGC 6100

  32. A non-HBLR: NGC 5347

  33. [O III] Luminosity – IR Color Diagram f25/f60 is the IR color  dust temp. around AGN stronger [O III] Lum.  AGN strength weaker warmer cooler

  34. Receding Torus: Evolution of AGN Obscuration • Obscured AGN fraction decreases with luminosity • Consistent with receding torus model: larger opening angle for higher luminosity AGN Hasinger (2008)

  35. Evolution? Yu & Hwang (2011) HBLR Non-HBLR Wu et al. (2011) solar 5x solar • Significant difference in [N II]/Ha ratio • Can be explained by an increase in nitrogen abundance in non-HBLR S2s • Stellar evolution  overabundance of N/O  evolutionary connection between HBLR and non-HBLR S2s

  36. Evolution? (2) S1s HBLR S2s Non-HBLR S2s Starbursts Wu et al. (2011)

  37. “Naked” View of Type 2 Seyferts • NGC 3147, NGC 4698, 1ES 1927+654 • Other candidates in Shi et al (2011) and Bianchi et al (2012) (IRAS 01428-0404, NGC 4594, Q2131-427 …) • All are Compton-thin, consistent with little or no intrinsic X-ray absorption above Galactic column density (~ 1020 - 1021 cm-2 ) • Rapid, persistent, and strong X-ray variability observed over 12 year time scale in 1ES 1927+654 • X-ray variability also observed in NGC 3147 • High hard X-ray to [O III] ratios (1-100) indicate little obscuration • Inferred nuclear optical extinction is less than ~ 1 mag. • All classified as type 2 AGNs (no broad lines observed)  contrary to expectation from the AGN unification model

  38. Spectropolarimetric Results Deep, repeated observations to probe weak lines. In each case, a small amount of polarization is detected but no polarized broad lines indicative of a hidden broad-line region are seen in the polarized flux spectra.

  39. Near-Infrared Spectroscopy Results The spectra are dominated by galactic starlight, and we do not detect any emission in Paβ or Brγ. No direct broad emission lines are present. • If typical broad lines were present, their non-detections would indicate an extinction of AV ~ 11-26 • Conclusions • If typical broad lines were present, their non-detections would indicate an extinction of AV ~ 11-26, inconsistent with the ``naked'' nature of these galaxies. • While the obscuration may be due to different material for X-ray and optical light, it appears that the BLRs in these objects are anemically small or absent, due to the weakness of their active central engines. • Conclusions • If typical broad lines were present, their non-detections would indicate an extinction of AV ~ 11-26, inconsistent with the ``naked'' nature of these galaxies. • While the obscuration may be due to different material for X-ray and optical light, it appears that the BLRs in these objects are anemically small or absent, due to the weakness of their active central engines. • Conclusions • If typical broad lines were present, their non-detections would indicate an extinction of AV ~ 11-26, inconsistent with the ``naked'' nature of these galaxies. • While the obscuration may be due to different material for X-ray and optical light, it appears that the BLRs in these objects are anemically small or absent, due to the weakness of their active central engines.

  40. Why don’t we see any broad lines, given the naked nature of these AGNs? • Misclassified Compton-thin AGNs? - High spatial resolution Chandra and XMM-Newton observations rule out confusion from external sources - Temporal variation in X-ray flux implies X-ray is likely not scattered • Variable broad emission lines? - Multi-epoch spectropolarimetric observations designed to search for variability failed to find any - Available spectra over timescales of years do not show any evidence of broad-line appearance • Hidden narrow-line Seyfert 1 (NLS1) galaxies? - No emission lines of any kind, broad or narrow, are seen in polarized flux spectra - No polarized FeII emission • X-ray unobscured, but optically highly obscured? - Narrow-line Balmer decrements are fairly normal, giving AV < 1.6 mag - Heavy obscuration in BLR itself? High AV/NH ratio?

  41. Most Likely Explanation • Low-powered AGNs with weak or absence of BLRs - All three objects are low-luminosity AGNs - Observed Eddington ratios are consistent with being below minimum threshold needed to support BLRs (L/LEdd ≲ 10-3) (e.g.: Nicastro 2000) More recent X-ray & optical analysis of NGC 3147 confirms it as a best candidate for a “true” S2 (Matt et al 2012, Bianchi et al 2012) However, NGC 4698 is likely Compton-thick (Bianchi et al 2012)

  42. Correlation with Broad Hα Luminosity Stern & Laor (2012) For NGC 3147, Bianchi et al (2012) found upper limit of broad Ha ~ two orders of magnitude smaller!

  43. Seyfert 2 Spectropolarimetry Some Surprises

  44. NGC 2110: A Hidden Double-Peaked Emitter Moran et al, Dec. 2005 February 2007 November 2007 FWHM ~ 15,000 km/s FWZI ~ 28,000 km/s

  45. Hidden (Polarized) Hα Profile Variability NGC 2110 NGC 5252 Timescales ≲ 1 yr (similar to dynamical timescales of accretion disks)

  46. HDPE: Observed Polarization Variability NGC 2110: Blue • continuum and broad-line polarization PAs are the same  similar polarization mechanism and geometry • polarization PA does not change with time • polarization PA (~70º) perpendicular to ionization cone axes •  scattering is the source of polarization NGC 5252: Green

  47. What Causes the Rapid Variations? • Variability timescales ≲ 1 yr  very compact scatterers • Discrete clouds ≲ 1 ly, NOT filled cones with large filling factor • Dramatic line profile changes in polarized flux • NOT due to “light echo” or “search light” effect • Non-changing polarization PA  changes in line-emitting flux, not scattering medium

  48. Constraining the Properties of the Scatterers • No “smearing” of line profile  Te≲ 106 K • Polarization PA ⊥ to cone axis  polar distribution • Scattering spherical “blob” model: f = Lsc/Lin ≈ σT ne 2r ΔΩ assume = σT ne 2r ~ 1 (optically thin) ΔΩ ≈ ¼ (2r/d)2 = r2/d2 2r ~ 1 ly f ~ 1%  ne ~ 107 cm-3; d ≲ 10 pc r τe d ΔΩ • Scatterers lie just outside the obscuring torus between BLR and NLR • Much more compact and close-in than previously thought

  49. Compact Scattering Region, Close to Nucleus • Line-emitting “ejectiles” from the nucleus • Ejection (bi-polar) outflow model • Simple: same material for both scatterer and line-emitting gas • Well-defined bi-cones • Radio hot spots or material entrained in the base of jets • Preference of DPEs in radio-loud AGNs • Problematic for accretion disk model as scattering angle must be small (< 15°) • Material from outskirts of obscuring torus itself • Clumpy obscuring torus model (Elitzur & Schlosman 2006; Nenkova et al 2008) • Properties similar to scattering clumps • Viewing angle (i ~ 65°) more consistent with extended cone morphology

  50. Unified Model of AGN is undoubtedly correct Orientation, evolution, luminosity, and obscuration may all play important roles Scatterers can be very compact, (≲ 1 ly), and located close (< 10 pc) to central nucleus, giving rise to variability of polarized light Summary

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