The merger-AGN connection since z~1:
causal or circumstantial?
Mauricio Cisternas
MPIA, Heidelberg
COSMOS Meeting
IfA, 09/06/2010
+ K. Jahnke, K. Inskip, A. Robaina (MPIA)
T. Lisker, J. Kartaltepe, A. Koekemoer, M. Scodeggio, J. Trump, K. Sheth
Co-evolution
• BH mass: built up during a quasar phase
• But, what triggers a quasar?
Step 1
Step 2
Major mergers
Minor mergers
Large scale bars
Nuclear bars
ISM turbulence
…
???
MBH/Msun
• Scaling relations: “proof” for the tied growth
of galaxies and their supermassive black
holes
Step 3
M* /Msun
z=0 Häring & Rix (2004)
Appealing scenario: major mergers
Since the 80’s,
observations have found:
BUT:
Are those representative
samples of QSOs?
• quasars with close
companions
High frequency of mergers
compared to what?
• post merger features on
their host galaxies
Signatures heavily
dependant on
bandpass, image depth
• “High frequency of
mergers”
COSMOS AGN
Our approach
We study the distortions of a sample of AGN
host galaxies. But, what makes us
special?
1)
The Data
•
~2000 X-ray sources detected with
XMM and Chandra
•
Classification as type-1/2 from
spectroscopic surveys and SED fitting
•
Optical counterparts: HST/ACS
•
Solid sample of 140 type-1/2
(IAB<24, 0.3< z <1)
Our approach
2) Comparison Sample
• The key measurement: not just the
merger fraction of the AGN hosts, but the
enhancement of merging over the
“background level”
• ~10 inactive galaxies per active galaxy
• Compiled from the same dataset
• Matched in redshift and brightness
(including special treatment for the type-1
AGN)
Active Galaxy
Control Sample
Smooth
Our approach
3) Visual Classification
• No definitive way to identify mergers
automatically...
…then let’s do it by eye (& brain)!
Mildly distorted
• Basically:
– Hubble type
– Distortion level
• Consistency:
– We use 10 independent classifiers
(people)
– We classify blindly: mixing the AGN
hosts with the inactive galaxies
Strongly distorted
4
2
3
5
1
6
7
9
10
8
14
11
12
13
The Result
Mean difference between the
distortion fractions: 2.4% ± 3.6%
• This means: No enhancement in the merger fraction of AGN host
galaxies over the background level
The Result
The K-S test can’t distinct between the 2 sets of measurements
“AGN do not prefer to live in
merging systems”
This result allows for 2 possible interpretations:
1.
There is a significant time-lag between merging and AGN triggering…
AGN lifetime:
Merger timescale:
time
2.
… or major merging is not the main fueling mechanism
Clues from the Hubble sequence
• Hubble-type classification: ~60% of AGN
hosted by galaxies with a significant disk
• Since z~1:
– Methods that do not involve destruction
of the disk dominate
– Minor mergers, accretion of surrounding
gas, bar instabilities, nuclear bars, SN
explosions, …
– Tied growth of BHs and their host
galaxies? not so much
(Preprint coming soon)
Quasar-host galaxy decomposition with GALFIT
(originals)
z=0.67
z=0.74
z=0.91
(models)
(host galaxies)
Comparison sample: creating mock AGN
Procedure:
i) For each type-1 AGN, we select 10
inactive galaxies that match in
redshift and magnitude
+
(inactive galaxy)
(star)
=
ii) Using the Host/Nucleus flux
relation for a given AGN, we
search for a star that fits that
ratio against the inactive galaxy
(mock AGN)
iii) By adding the star on top of the
galaxy, we create a mock AGN
iv) We treat our mock AGN exactly
the same way as the original
ones, which yields to a set of
galaxies with the same
conditions than our hosts
(galaxy + residuals)
To recap…
• AGN host galaxies show virtually the same frequency of distortions
than inactive galaxies
• Large fraction of disks on our AGN sample implies alternative fueling
methods not caused by recent major mergers
• Since z~1, merging and quasar activity disconnect
• Preprint coming soon…
Some advertising
www.mpia.de/coevolution
Just today at astro-ph:
“The non-causal origin of the black holegalaxy scaling relations”
K. Jahnke & A. Maccio (arXiv:1006.0482)