Co-evolution of AGN and their host galaxies
Piero Ranalli – IAASARS, NOA
Winter school of astrophysics, Athens 2014
AGN reside in host galaxies
normal spiral galaxy
AGN in spiral galaxy
quasar
from Kaufman
early
late
Feedback
“the black hole at the center of a galaxy bulge is no mere
ornament but may play a major role in determining the
fnal stellar mass of the bulge. The process by which this
occurs is known as AGN (active galactic nuclei) feedback,
and it takes place through an interaction between the
energy and radiation generated by accretion onto the
massive black hole (the active nucleus) and the gas in the
host galaxy”
(Fabian 2012)
bulge
spiral galaxy
“spheroids”
bulge
elliptical galaxy
spiral galaxy
Black holes are present also in quiescent galaxies
(probably, in all galaxies)
Why co-evolution?
●
the cosmic star formation history and the black hole accretion history
track each other, with a ratio of ~ 1000:1...
(Marconi et al.)
Why co-evolution?
●
●
the cosmic star formation history and the black hole accretion history
track each other, with a ratio of ~ 1000:1...
...which corresponds to the mass ratio between galaxy bulges and black holes
(Marconi et al.)
(McConnell & Ma 2013)
Why co-evolution?
●
black hole mass is also well correlated with the width of the statistical
distribution of the stellar orbits in a bulge, or in an elliptical galaxy
(McConnell & Ma 2013)
Why co-evolution?
●
average luminosity of both galaxies and AGN decreases with cosmic time
(“downsizing”): massive galaxies form most stars early, less massive later
massive black holes grow early, less massive later
(Hasinger et al. 2005)
Questions:
1. At what stage in their lives do galaxies feed their black holes?
What are the physical properties of galaxies where accretion is favoured? Are
these galaxies transitioning from one evolutionary stage to another, or are they
representatives of a general, stable phase? How many diferent, separate AGN
host galaxy populations are there, i.e. how many pathways are there to black hole
accretion?
2. What efect does black hole growth have on the evolutionary trajectory
of galaxies?
This question must be answered separately for each AGN host galaxy population:
does the energy liberated by the accretion phase actually impact the host galaxy,
or does it dissipate without consequences? How does the energy couple to the gas
in the host galaxy, and how does this depend on accretion mode (radiative vs.
kinetic feedback)?
(Schawinski 2012)
Questions:
1. At what stage in their lives do galaxies feed their black holes?
What are the physical properties of galaxies where accretion is favoured? Are
these galaxies transitioning from one evolutionary stage to another, or are they
representatives of a general, stable phase? How many diferent, separate AGN
host galaxy populations are there, i.e. how many pathways are there to black hole
accretion?
2. What efect does black hole growth have on the evolutionary trajectory
of galaxies?
This question must be answered separately for each AGN host galaxy population:
does the energy liberated by the accretion phase actually impact the host galaxy,
or does it dissipate without consequences? How does the energy couple to the gas
in the host galaxy, and how does this depend on accretion mode (radiative vs.
kinetic feedback)?
(Shawinski 2012)
there are observations of quasars at z~6, when the universe
was 900 Myr old ⇒ whatever process, it must start early
Mass-colour diagram for galaxies
U-B
early spectral type
late spectral type
7
8
9
log M
10
11
12
Pozzetti et al. 2010
(from Mo, van den Bosch & White)
Galaxy evolution
(from Mo, van den Bosch & White)
Galaxy evolution
A very basic picture of (late type) galaxy formation
(from Kaufmann)
(video)
Why the blue/red bimodality?
When gas cools, discs grow,
star formation happens,
and galaxies have blue colours.
Mergers can (and do) transform a pair of spirals into an elliptical,
triggering star formation in the process. But why are ellipticals red?
In red galaxies, something has stopped (“quenched”) star formation
(making them “red and dead”). There are a few possibilities...
build-up of the
red sequence
(Faber et al. 2007)
Supernovae from a starburst episode can produce hot winds, which sweep the gas.
Mass-colour diagram for galaxies
AGN are found in the “green valley”, suggesting they may be a
transition state between late/blue/star forming and early/red/dead.
Are AGN responsible for star formation quenching?
Feedback
Are AGN responsible for star formation quenching?
Or are AGN responsible for preventing further gas cooling and star formation?
(Large quantities of hot gas are present
in giant ellipticals and groups. Often,
cavities are present which may have
been infated by AGN jets.)
Chandra X-ray image of the HCG62 group,
with radio contours
Two types of feedback:
radiative/quasar/wind mode
“operates, or operated, in a typical bulge when the accreting black hole was close to the
Eddington limit. It is most concerned with pushing cold gas about” (Fabian 2012)
kinetic/radio/maintenance mode
“operates when the galaxy has a hot halo (or is at the center of a group or cluster of
galaxies) and the accreting black hole has powerful jets. At the present epoch, it tends to
occur at a lower Eddington fraction and in more massive galaxies and involves hot gas.”
(Fabian 2012)
Alexander & Hickox 2011
Two types of feedback:
radiative/quasar/wind mode
“operates, or operated, in a typical bulge when the accreting black hole was close to the
Eddington limit. It is most concerned with pushing cold gas about” (Fabian 2012)
A quasar accreting at the Eddington limit can prevent further accretion if
(Silk & Rees 1998: based on energy transfer. Predicts reasonable
slope of M-σ relationship but wrong normalization)
(King 2003: based on momentum transfer. Predicts reasonable slope
and normalization)
Two types of feedback:
kinetic/radio/maintenance mode
“operates when the galaxy has a hot halo (or is at the center of a group or cluster of
galaxies) and the accreting black hole has powerful jets. At the present epoch, it tends to
occur at a lower Eddington fraction and in more massive galaxies and involves hot gas.”
(Fabian 2012)
Hot gas in giant ellipticals and clusters should cool very fast, with mass cooling rate
~ 10-103 M�/yr, and form stars with the cooled gas, but this is not happening
Major-merger scenario:
originally proposed by Sanders et al. 1988;
cartoon by Alexander & Hickox 2011
quasar-mode feedback operates here
radio-mode feedback keeps the
galaxy red
Hopkins' “cosmic cycle”
“mergers between gas-rich galaxies drive nuclear infows of gas, producing
starbursts and feeding the buried growth of supermassive black holes ( BHs)
until feedback expels gas and renders a briefy visible optical quasar”
Hopkins et al. 2006
Hopkins' “cosmic cycle”
“mergers between gas-rich galaxies drive nuclear infows of gas, producing
starbursts and feeding the buried growth of supermassive black holes ( BHs)
until feedback expels gas and renders a briefy visible optical quasar”
Hopkins et al. 2006
a consequence of the merger idea is that an elliptical may acquire
new gas which cools and forms a disc around it, thus transforming
itself in a disc+bulge system
Hopkins' “cosmic cycle”
“mergers between gas-rich galaxies drive nuclear infows of gas, producing
starbursts and feeding the buried growth of supermassive black holes ( BHs)
until feedback expels gas and renders a briefy visible optical quasar”
Hopkins et al. 2006
another consequence is that one can defne a duty cycle
for AGN, because activity can be a recurring phenomenon
A completely diferent approach: mass averaging
Kormendy & Ho 2013
mass averaging may be as important
as quasar-mode feedback, but
relative importance is not yet known
Do Seyfert galaxies ft in these scenarios?
Kim et al. 2013
Seyferts mostly reside in massive spirals, which make up ~90% of the local
AGN host galaxies
what said for ellipticals, applies to bulges
any merger they experienced has not been catastrophic (or they have
recovered a disc afterwards)
the “green valley” colours may be explained with a low specifc star formation
rate
accretion is nowadays probably happening stochastically, via secular
processes
(Kormendy & Kennicutt 2004)
Do Seyfert galaxies ft in these scenarios?
Kim et al. 2013
Seyferts mostly reside in massive spirals, which make up ~90% of the local
AGN host galaxies
what said for ellipticals, applies to bulges
...which are present in early spirals, i.e. Sa, Sb types
Late type spirals don't have proper bulges!
and pseudobulges do not follow the M-σ relation
“peanut shaped” edge-on
profle of a pseudobulge
A picture in development
●
●
●
●
●
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big black holes grow at z~2-4 (“quasar era”) by radiatively efcient gas accretion;
M-σ is probably already in place (maybe with slightly diferent parameters)
spheroids (ellipticals and bulges) form via mergers
quasar-mode feedback quenches star formation
at z=0, most star formation and black hole happens in low-mass galaxies
(“downsizing”) which lack bulges
accretion is nowadays mostly episodic and secular
radio-mode feedback keeps gas in ellipticals hot
and prevents further star formation
All of the above still being investigated...