The Search for Type 2 Quasars

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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

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

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