The Search for Type 2 Quasars Julian Krolik with: Reina Reyes, Michael Strauss, Ezequiel Treister,...
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Transcript of The Search for Type 2 Quasars Julian Krolik with: Reina Reyes, Michael Strauss, Ezequiel Treister,...
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The Search for Type 2 Quasars
Julian Krolik
with: Reina Reyes, Michael Strauss, Ezequiel Treister, Nadia Zakamska
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Radio-loud and Radio-quiet
White et al. (2007): FIRST + SDSS
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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
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Obscuration Types United by Anisotropy
NGC 1068
Antonucci & Miller (1985)
radio jet axis
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Additional Evidence in Nearby, Low-Luminosity AGN
Ionization cones, as in NGC 5252
Morse et al. 1998
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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
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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
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Buchanan et al. 2006
“Warm” IR spectra
Fº / º ¡ 1
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Direct “imaging” via IR interferometry
Jaffe et al. 2004
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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
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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?
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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
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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)
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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
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Many Obscured AGN Have Quasar Luminosities
obscured quasars
from the CDF-S: Tozzi et al. (2006)
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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)
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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
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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
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Confirmation with Spectropolarimetry
Zakamska et al. (2005)
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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
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Accidental Reward:Best Possible QuasarHost Images
Note: scattered quasar light can be a serious contaminant
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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
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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
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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
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Determining Bolometric Luminosity
All SDSS, GOODS, COSMOS objects have optical spectra—
add GALEX photometry, interpolate, and integrate
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Correlation
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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