The Search for Type 2 Quasars

Report
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:
Obscured:
•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 ¡
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, 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)
*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
unobscured
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.
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 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: 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
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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