The Search for Type 2 Quasars

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):
Obscured and Unobscured
•Strong, blue continuum in optical/UV
•Weak/no optical/UV continuum
•Broad emission lines in optical/UV
•Only narrow lines in optical/UV
•Strong X-ray continuum
•X-rays absorbed or absent
•Bright from IR through hard X-rays
•Bright only in IR and sometimes
hard X-rays
Obscuration Types United by Anisotropy
radio jet axis
NGC 1068
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
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
“Warm” IR spectra
Fº / º ¡
Buchanan et al. 2006
Direct “imaging” via IR
Jaffe et al. 2004
Does Anything Change with Increasing
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, 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
40—50% obscured
8.0m – 4.5m
Martinez-Sansigre et al. (2006)
5.8m -3.6m
Lacy et al. (2006)
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
Wang et al. (2007): CDF-S
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.
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
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 Quasar
Host Images
Note: scattered
quasar light can
be a serious
SDSS-Based Luminosity Function
•Based on 700 objects
•Complicated selection
function; LF is a lower
•Type II/Type I ratio
comparable to or greater
than 1
Reyes et al. 2007, in preparation
An Indirect Approach: LIR/Lbol vs. Lbol
Treister & K., in preparation
L I R =L b ol
f obsc
f obsc
1 ¡ f obsc
L I R =L bol
1 + L I R =L bol
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
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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