Identification of the most heavily obscured AGN

22 Pages·2010·7.85 MB·English

by  David Alexander

Identification of the most heavily
obscured AGN
David M Alexander (Durham)
Obscured AGNs are the numerically dominant AGN population (3-10x)
Why care about obscured AGNs?
The growth of black holes The black-hole-host connection
M /M ~10-3
s
bh sph
c
i
h
p
a
Can ultimately provide best constraints on:
r
g
o
m
(1) AGN duty cycle
e
d

(2) Full range of denvironments that drive
n
the growath of black holes

s
c
(3) Black-hole growith efficyiency parameter
s t
(black-hole masys densitviy-AGN accretion)
h i
t
p c
(4) Host-galaxoy-blacka hole growth
Black Hole r
t N
s
G
A
A

:
n
o
i
t
Accretion Disk c
A
Action: Star formation
Most rapidly growing black holes may be most obscured
More obscuration at high redshift: increased gas fraction in high-z galaxies
(e.g., Daddi et al. 2008, 2010; Tacconi et al. 2009)
But AGN identification is a multi-scale, multi-
component, multi-wavelength problem
Problems facing obscured AGN searches
Black Hole
Problem: obscuration
C
changes the observed
e
n
t AGN signatures
r
a
l

e
n
g
Accretion Disk
i
n
e u s
o r
y t
s t
u
D
d
n
a
Problem: accretion u s Dusty starburst
o
e
s
disk is spatially G a
unresolved
Problem: host galaxy
can dilute/extinguish
AGN signatures
AGN Properties Altered by Dust and Gas
X-ray absorption Local Universe
n
k
Rest 0.8-24 keV hi c
i
(z~0.5-2 X-ray t h
n t
energies)
o n
t o
p
t
m p
m
o
C o
C
NGC1068
NGC6240
Compton-thick AGNs: likely 50% of
local AGN population but <50 have
been robustly ID’d to date
(~10-9th of cosmic population)!
Challenge: host-galaxy extinction
Spot the
difference!
AGNs at d<15 Mpc: Goulding & Alexander (2009)
Majority of small growing black holes are optically unidentified
Brandl et al. (2006)
AGNs at d<15 Mpc: Goulding et al. (2010)
Ignoring the most heavily obscured AGNs
could miss the majority of the small growing
black holes in the local Universe
Host-galaxy absorption can reach N ~1023-1024 cm-2 in some cases locally (e.g.,
H
NGC4945) and may be ubiquitious at high redshift (submm galaxies at z~2?)
Can we make significant progress in
finding all of the obscured AGNs in
the distant Universe?
X-ray Surveys: Penetrating Probe of AGN activity
Penetrate large
gas columns
Murray et al. (2005)
5 ks Bootes
High (quasar) luminosities
Moderate (Seyfert) luminosities
Low luminosities
2 Ms CDF-N & CDF-S
Just CDF-N sources
Alexander et al. (2003);
Luo et al. (2008)
Detection of even low-luminosity AGN out to high redshift
But even the deepest surveys are missing many AGNs
Tozzi et al. (2006)
Worsley et al. (2005)
X-ray background is
not fully resolved at >6 keV
Tozzi et al. (2006)
k
c
i
h
t

n Few heavily obscured AGNs
o
t
p (N >3x1023 cm-2) have been
m H
detected (identified), including most
o
Modelled fraction of C
of the Compton-thick AGNs
L >1041 erg/s AGNs
X
in 1Ms CDF-S