The Search for Type 2 Quasars

1. The Search for Type 2 Quasars Julian Krolik with: Reina Reyes, Michael Strauss, Ezequiel Treister, Nadia Zakamska

2. Radio-loud and Radio-quiet White et al. (2007): FIRST + SDSS

3. Obscured and Unobscured • Unobscured: • Strong, blue continuum in optical/UV • Broad emission lines in optical/UV • Strong X-ray continuum • Bright from IR through hard X-rays • Obscured: • Weak/no optical/UV continuum • Only narrow lines in optical/UV • X-rays absorbed or absent • Bright only in IR and sometimes hard X-rays

4. Obscuration Types United by Anisotropy radio jet axis NGC 1068 Antonucci & Miller (1985)

5. Additional Evidence in Nearby, Low-Luminosity AGN Ionization cones, as in NGC 5252 Morse et al. 1998

6. Soft X-ray absorption Distribution for obscured AGN selected by [OIII] flux: Risaliti et al. 1999 “Compton thick” means NH is only a lower bound

7. Digression: The Many Meanings of Compton Thick • NH much more than 1024 cm-2: no photons below the Klein-Nishina regime; possibly a weak electron-scattered continuum • NH around 1024 cm-2: photons leak through at and above 5—10 keV • NH much more than 1024 cm-2 and the far side of the obscuration can be seen: a spectrum due entirely to filtered Compton reflection

8. 1 ¡ F / º º “Warm” IR spectra Buchanan et al. 2006

9. Direct “imaging” via IR interferometry Jaffe et al. 2004

10. Does Anything Change with Increasing Luminosity? Unfortunately, type 2 quasars are hard to find: • Weak optical/UV continuum means color-based samples miss them • Absence of broad emission lines means grism/line-based samples miss them • Strong soft X-ray absorption makes soft X-ray surveys biassed against them

11. First Indication: Radio Samples In the 3CR, fobsc falls by ~2 over 4 dex in radio power (Lawrence 1991) But connection between LR and Lbol uncertain; And are radio-loud objects special?

12. IR Surveys Selecting on IR color* gives 40—50% obscured 8.0m – 4.5m Martinez-Sansigre et al. (2006) 5.8m -3.6m Lacy et al. (2006) *and X-ray or radio flux

13. IR Survey Biases/Limitations • Need another band to distinguish AGN candidates • Generic IR transfer models suggest the unobscured view is brighter: favors unobscured • Identification of intrinsically unobscured nuclei may be hampered by dust in the host galaxy: favors obscured • Relatively small sample sizes (~10 typically)

14. X-ray Surveys Deep Chandra and XMM surveys are dominated by AGN: strong, un-ionized soft X-ray absorption signals obscuration 50—70% of those selected at 4—7 keV are obscured obscured unobscured Wang et al. (2007): CDF-S

15. Many Obscured AGN Have Quasar Luminosities obscured quasars from the CDF-S: Tozzi et al. (2006)

16. A Trend in the Obscuration Ratio? Chandra selection-- red points: Hasinger, p.c., optical/X-ray types black points: Treister & Urry, optical types Integral selection finds a similar effect (Sazonov et al. 2007)

17. X-ray Survey Biases/Difficulties • At high redshift, moderate absorption is shifted to energies below the Chandra/XMM band: obscured can be mistaken for unobscured • Absorption itself reduces counts, especially at low energies: favors unobscured • Objects drop out completely when truly Compton thick: favors unobscured; IR+radio surveys find numerous examples • Optical identification difficult when faint: favors unobscured

18. Optical Surveys SDSS collects spectra from all galaxies with mi < 17; all point sources with non-stellar colors with mi < 19; FIRST, RASS sources,.. Search the database for everything with emission lines of high ionization, no broad components (Zakamska 2005): now > 900 obscured quasars known, 0.3 < z < 0.8

19. Confirmation with Spectropolarimetry Zakamska et al. (2005)

20. Optical Survey Biases/Difficulties • Limited in redshift range • To degree lines contribute to flux in selection bands, irregular sensitivity as function of redshift • Galaxy light can dilute line equivalent widths • Indirect connection between [OIII] luminosity and bolometric luminosity • For comparison to unobscured, must construct analogous [OIII]-based luminosity function

21. Accidental Reward:Best Possible Quasar Host Images Note: scattered quasar light can be a serious contaminant

22. SDSS-Based Luminosity Function • Based on 700 objects • Complicated selection function; LF is a lower limit • Type II/Type I ratio comparable to or greater than 1 Reyes et al. 2007, in preparation

23. f b o s c = L L ' ! b l I R o f 1 ¡ b o s c = L L b l I R o f ' b o s c = L L 1 + b l I R o An Indirect Approach: LIR/Lbol vs. Lbol Treister & K., in preparation

24. Sample Selection To eliminate possible evolutionary effects, choose a limited redshift range: 0.8 < z < 1.2 For high luminosities, need a wide-angle, bright survey: SDSS For low luminosities, need a pencil-beam, deep survey: GOODS+COSMOS

25. Determining Bolometric Luminosity All SDSS, GOODS, COSMOS objects have optical spectra— add GALEX photometry, interpolate, and integrate

26. Correlation

27. Summary There is now ample evidence that obscured quasars exist and are reasonably numerous--- But quantitative measures of their statistics are still in their infancy