Julian Krolik with: Reina Reyes, Michael Strauss,
Ezequiel Treister, Nadia Zakamska

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

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

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

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

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

Digression: The Many Meanings of Compton Thick
• N
H much more than 10 24 cm -2 : no photons below the Klein-Nishina regime; possibly a weak electron-scattered continuum
• N
H around 10 24 cm -2 : photons leak through at and above 5 —10 keV
• N
H much more than 10 24 cm -2 and the far side of the obscuration can be seen: a spectrum due entirely to filtered Compton reflection

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

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

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/linebased samples miss them
• Strong soft X-ray absorption makes soft X-ray surveys biassed against them

First Indication: Radio Samples
In the 3CR, f obsc
(Lawrence 1991) falls by ~2 over 4 dex in radio power
But connection between L
R and L bol uncertain;
And are radio-loud objects special?

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

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)

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
Wang et al. (2007): CDF-S unobscured

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

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)

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

Optical Surveys
SDSS collects spectra from all galaxies with m i
< 17; all point sources with non-stellar colors with m i
< 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

Confirmation with Spectropolarimetry
Zakamska et al. (2005)

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

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

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

An Indirect Approach: L
IR
/L bol vs. L bol
Treister & K., in preparation
L
I R
=L b ol
' f obsc
' f obsc
1 ¡ f obsc
!
L
I R
1 + L
=L
I R b ol
=L b ol

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

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

Correlation

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