The Search for Type 2 Quasars

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

Obscuration Types United by Anisotropy

NGC 1068

Antonucci & Miller (1985)

radio jet axis

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 NH is only a lower bound

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

Buchanan et al. 2006

“Warm” IR spectra

Fº / º ¡ 1

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/line-based

samples miss them• Strong soft X-ray absorption makes soft X-ray surveys

biassed against them

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?

IR Surveys

Selecting on IR color* gives

Lacy et al. (2006)

Martinez-Sansigre et al. (2006)

40—50% obscured

-

8.0 – 4.5

*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

Wang et al. (2007): CDF-S

50—70% of those selected at 4—7 keV are obscured

obscured

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 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

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 QuasarHost 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: LIR/Lbol vs. Lbol

L IR =Lbol 'f obsc

1¡ f obsc!

f obsc 'L IR =Lbol

1+ L IR =Lbol

Treister & K., in preparation

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