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The Riddle of Cooling Flows Meeting Summary? Joel N. Bregman. Thanks to the organizers for a great meeting. The basic cooling flow picture what parts are ok what parts need to be changed What would an outsider think? X-ray peak is on a cD galaxy (seems like it must be involved)

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The Riddle of Cooling Flows

Meeting Summary?

Joel N. Bregman


The basic cooling flow picture

what parts are ok

what parts need to be changed

What would an outsider think?

X-ray peak is on a cD galaxy

(seems like it must be involved)

dT/dr is similar from cluster to cluster

Metallicity and entropy gradients

azimuthal properties (T)

Occam’s razor: keep it simple

A counterpart to Occam’s razor

minimize Trips to the Tooth Fairy (TTF)


The Local Trend in Cooling Flows

The “Classic” Picture

Gas cools in the center of a cluster at 100 Msolar/yr

Gas either accumulates or forms into low mass stars

Evidence Against

missing intermediate temperature X-ray lines (OVII, FeXVII)

little evidence for the very high rate of star formation

Save the Picture (little enthusiasm expressed)

have the gas “jump” over the 1 keV range

(OVII, Fe XVII not produced)

(i.e., mixing with cold gas lost from stars)

might gain support if OVI seen in several clusters

The consensus view:

Reduce Mdot by heating the gas as it tries to cool


Conduction and Warming Flows

Energy budget considerations

plenty of energy (infinite thermal bath)

Energy transfer rate

good enough for the hot clusters

a bit low for cool clusters

Big Problem: how is the conduction so “smart” (major TTF)

conduction coefficient needs to be 19% of κSpitzer for

one cluster, 41% for another, etc.

otherwise, observed dT/dr would be wiped out


Possible Solution

add in the stellar mass loss from stars in cD

this gas is at 1 keV after themalization (provides Tmin)

the gas is flowing outward into the cluster

T will rise from Tmin to Tcluster in all cases,

but the shape varies with κSpitzer


not sensitive to κSpitzer as long as it is sufficiently large

produces a significant dT/dr that would be similar from

cluster to cluster

don’t get the intermediate temperature cooling lines



what about all that CO?

It’s just mass lost from a galaxy

how about that AGN activity?

It’s not the dominant heat source

is conduction ruled out by cool fronts?

Not if they are magnetically separate

can this work in cooler clusters

Maybe not, but Mdot is not large

didn’t Brighenti and Mathew show this doesn’t work?

Comments: Thomas; Brighenti; Sparks; Soker, …..


Heating by AGNs

Low power radio lobes create acoustic noise that is damped

Energy Budget

seems inadequate for most systems (by 10x)

poster by Birzan-Rafferty et al.

Can the energy be distributed azimuthally? (1 TTF)

How did the AGN get to be so smart?

dT/dr so similar from system to system

it puts just the right amount of energy into the system

Goldilocks: not too hot, not too cold; just right


Feedback? (1 TTF)

Feed the AGN with cooling gas

If the gas cools too much, the AGN gets fed more until

the heating shuts off cooling

Requires a good match in the timescales

Feeding rate must adjust faster than the energy generation rate.

Not obvious that this requirement is met.

If hot gas has angular momentum, it doesn’t reach the

center once it cools.

Feeding may be by stars scattered into the AGN, not by

the hot gas (no feedback in this case).

If feeding were by cooled gas, do you get the observed

number distribution with redishift?


If the lobe is heating the surroundings, it should be hotter close to the lobe than further away.

Observations can test this.

Discussion: Blanton; Nulsen; Böhringer; Begelman, …..


Does an AGN initiate star formation or are they both part of one phenomenon related to a disturbance, possibly by a passing galaxy?

There seems to be an association between these events in a galaxy cluster:

CO emission

Hα emitting gas

star formation

AGN (radio lobe) activity

“classic” X-ray cooling flow structure

One interpretation of this is within the Cooling Flow paradigm (at reduced Mdot):

Cooling Flow → Cooled Gas (Hα + CO) → star formation (possibly triggered by AGN)


But this can fit into the anti-Cooling Flow picture as well:

Late-type galaxy passes by cluster core

loses its cold ISM (CO)

shock and disturbance in this gas triggers

star formation (Hα)

disturbance with central galaxy ignites the AGN

In this second picture, X-ray emission has little to do with this

(thermal exchange with Hα gas)

Discussion: Crawford; Baum; McNamara; Edge; Salom, …


Additional Observations I Would Like to See

Mass of cold gas in clusters

CO to H2 conversion is problematic

dust masses (decent measure of gas mass)

Give HI another try (GBT)

Mass of cooling gas in clusters

is OVI emission common? (5 more CFs scheduled)

In-depth studies of the few best objects.

A test of the heating picture from AGNs.

Observations That We Don’t Need More Of

another X-ray spectrum (RGS) showing the same thing as the first 13 objects

> 50 clusters already observed with XMM


Additional Theoretical Calculations I’d Like to See

Calculations from first principles (minimize TTF).

Models involving AGN heating

Predictions, that if not verified, invalidates model.

Location and magnitude of the heating.

Azimuthal distribtuion.

Apparent conflict with average KE too low.

Distribution of quantities (dT/dr)

Further developments of magnetic heating.

(it’s important in the Sun)

Continued calculations with conduction (ongoing).


Final Items I hope will happen

A resolution to the Cooling Flow mystery by the next meeting

“Cooling Flows Reloaded”

A little more “love” from the rest of the community.