Michael Strauss

Nearly Normal Active Galactic
Nuclei from z=0 to 6.4
(13 billion years of cosmic history in 12
minutes!)
Michael Strauss, Princeton University
- Quasar luminosity function
- Relation to Seyferts at low z
- What about obscured quasars?
- News at the highest redshifts

My collaborators on the work
shown here:
My former and current grad students: Xiaohui Fan, Lei Hao,
Nadia Zakamska, Joe Hennawi
Former Postdocs: Gordon Richards, Pat Hall
Julian Krolik, Tim Heckman, Bob Becker, Richard White,
Christy Tremonti, Guinevere Kauffmann.
And the whole SDSS collaboration, especially:
Jim Gunn, Jill Knapp, Robert Lupton, David Schlegel,
Sebastian Jester, Don Schneider, Dan Vanden Berk, Zeljko
Ivezic, and many others.

Using SDSS for AGN Studies
n A color-selected sample of quasars,
sensitive from z=0 to z>6. 90,000 quasars
and counting.
n A magnitude-limited sample of more than
500,000 galaxies, from which Seyferts can
be selected.
n Superb morphologies, photometry,
astrometry, and R=2000 photometrically
calibrated spectroscopy, 3800-9200 Å.

SDSS Quasar Sample
Courtesy Xiaohui Fan

The Luminosity Function from
the full SDSS
(here, a complete sample of
15,000 quasars)
n Can study luminous
quasars over the full range
of redshift with a single
sample (note that we do not
go to very low
luminosities…)
luminosities
n Substantial (and
quantifiable)
incompleteness at z=2.7,
3.5.
Richards et al. 2005c

Systematic flattening of the
LF above z~3

Comoving number
density peaks
between z=2 =2 and 3
LF slope increases at
z=3 =3 and above.

n 5645 quasars
with g < 21.85
selected from
SDSS imaging,
observed with
2dF at AAT. UV
excess sources,
almost all with z
< 2.5. Best-fit
slope b is -1.45 at
faint end.
A crucial question: the shape and
slope of the LF at the faint end… end
Richards et al. 2005a
MNRAS

The nature of low-luminosity
quasars
n As we push to lower luminosities still (at low
redshift), redshift),
emission from the host galaxy becomes
an ever-larger fraction of the total luminosity.
n Here we select both Type I and Type II Seyfert
galaxies from the SDSS main galaxy sample.
Lei Hao 2003, PhD Thesis; Hao et al. 2005ab in AJ

Host galaxies can contribute substantial light to the spectra,
which needs to be subtracted. Lei Hao, PhD Thesis, 2003

Ratios of various emission lines allow us to distinguish active nuclei
from star formation for Seyfert II galaxies (see also Kauffmann et al.)

Seyfert I galaxies are recognized by their broad permitted emission
lines. The distribution of line widths is strongly bimodal.

The relative number of Type I and Type II galaxies
is a function of luminosity

The active galaxy luminosity function fits nicely onto the higherredshift
quasar luminosity function.

n Very few high-luminosity Type II quasars are known.
Essentially all high-luminosity objects show broad
lines.
n If the unification model is correct, then Type II
quasars must exist at high luminosity.
Characteristics:
n Characteristics:
n Weak or absent non-thermal continuum
n Strong narrow high-ionization emission lines
n So we went to look for such objects in SDSS!
(Nadia ( Nadia Zakamska, Zakamska,
PhD Thesis, 2005)
2005

Type II Quasar Candidates

How do we determine intrinsic
luminosity?
Look in mid-IR?
Look in hard
X-rays?
Optically:
Use [OIII] 5007
We have carried out a
comprehensive follow-up
program:
X-ray (XMM, Chandra)
HST images
Far-IR photometry,
mid-IR spectra (Spitzer)
Spectropolarimetry (MMT)
Near-infrared spectroscopy
(Gemini)

A spectacular result:
20% polarization in
one object! This is
unambiguously an
obscured source!
(And we now have 11 more
objects with spectropolarimetry,
8 are strongly polarized).
Zakamska et al. 2005
Broad polarized Hb
Schmidt, Smith et al 2004

• Multiband continuum
imaging with HST resolves
the scattering region in
three cases.
• In each of these, the
scattering region is
perpendicular to the
direction of polarization, as
expected.
• The physical extent of the
scattering region (many
kpc) and the weakness of
any accompanying Balmer
emission rules out electron
scattering: dust scattering
seems to dominate.

• Very few objects
are detected in soft
X-rays in ROSAT,
consistent with
absorption by H
gas.
• Six objects have
Chandra or XMM
hard X-ray
detections,
consistent with
implied
luminosities, and
with column
densities of > few
¥ 10 22 cm -2
Ptak et al. 2005

A montage of HST images of SDSSSDSSselected obscured quasars
(Zakamska et al.)

Optical
3-8 mm
How luminous are these
systems? Spitzer
observations
10 44-46 erg/sec in
Far-IR

These objects have IRAC colors
consistent with, or perhaps redder
than, IRAC-selected AGN…
First IRS spectroscopy shows
pure power-law: no spectral
features! (where is 10mm Si?)

Conclusions
ß SDSS is a tremendously powerful tool for studying the optical
properties of AGN at z=0 to z=6.5.
ß Luminous quasars peaked in comoving space density at z~2.5.
Luminosity function slope change is qualitatively consistent
with ‘cosmic downsizing’ picture.
ß At low redshifts, AGN luminosity function can be pushed
down to very low luminosities. Continuity with classical
quasars is work in progress.
ß Very luminous Type II quasars can be selected from the
SDSS spectroscopic sample, and studied in detail.
ß Ask me about the latest results on quasars at high (z~6)
quasars…

19 quasars with z>5.7

Composite Spectrum at low and high
redshift
Quasar spectra at high redshift are completely normal!
Fan et al. 2004