Cooling flowAdriana Gargiulo

Seminario Corso di astrofisica delle

alte energie

…the fundamental parameter… Cooling time:

ICM energy

Energy loss due to radiation in X rays

• nH proton density• value of cooling function at temperature T

tcool ≈ nH-1 !!!

tcool < tHubble cooling happens

NOTE:

Observational evidences for cooling flows - Imaging

Surface brightness (SB) strongly peacked at the center.

Since SB depends upon the square of gas density

very short tcool

Telescopes +

Copernicus

Rcool radius at which tcool = tH.

P(r ≥ rcool) weight of overlying gas where cooling is not important.

P(r < rcool) cooling reduces the gas temperature: to maintain the pressure the gas density must rise

Observational evidences for cooling flows

Rcool

The gas must FLOW inward

In absence of a suitable fine – tuned heating source, the cooling and condensation of the gas in the central region is a straight-forward consequence of the basic energy equation of the hot gas.

• Silk (1976)• Fabian & Nulsen (1977)• Cowie & Binney (1977)• Mathews & Bregman (1978)• Fabian et al. (1984)• Fabian (1994)

Observational evidences for cooling flows

70 % - 80 % of clusters have a cooling flow: common and long - living

Observational evidences for cooling flows - Spectra

Independently:Strong support from spectroscopic observations

Einstein Observatory Solid State Spectrometer +Focal Plane Cristal Spectrometer

The spectra show the existence of low temperature phases in addition to the hotter temperature gas.

White et al. 1994Allen et al. 1994

Strong cooling flow found from images

where not confirmed by

spectra…BUT…A478 SSS data

show strong X rays absorption.

Observational evidences for cooling flows - Spectra

Allen et al. 1993

Imaging vs spectraTwo different kind of observation lead to the same result

Ms = mass deposition rate computed from spectra analysis

MI = mass deposition rate computed from image analysis

.

.

White et al. 1991

The “cooling flow problem”

X vs Optical

X : large cooling rates of the KeV gas in the centers of clusters (tens to hundreds of solar masses per

year).

Optical: small star formation rates observed in central cluster galaxies (few to several tens of solar

masses per year).

“Only a small fraction of the cooled gas can form stars with a normal IMF: most must remain dark” Fabian, 1994.

Observational evidences against

The surface brightness is not as peaked as would be expected if all the cooling gas were to reach the center:

• Mass Dropout: a fraction of the gas cools out of the flow, at large radii, before reaching the center and some continues to flow inward most of the cooling gas never makes it to the center M(r) proportional to r.

• The gas is heated in someway…

.

The gas must be inhomogeneous

Inhomogeneous model (Nulsen ‘86)

Each radial zone in the cooling flow region comprises different plasma phases covering a wide range of T,

The gas comprising different temperature phases features an inflow in which all phases move with the same flow speed <v> << vs, forming a comoving flow

There is no energy exchange between the different phases, between material at different radii, and no heating.

XMM – Newton & Chandra

To further test the cooling flow picture most detailed X-ray spectroscopic observations

ASTRO E launch accident

XMM – Newton (2001)

Spectral signatures of different temperature phases range from the virial temperature Tvir to a limiting temperature Tlow (Tvir/3) , which is still above the

“drop out”temperature where the gas would cease to emit significant X-ray radiation

Unpreceded detailed spectroscopic diagnostics of the central regions of clusters

Evidence of failure of inhomogeneous standard cooling flow model

Fe L seriesThe spectroscopic signatures sensitive in the temperature range of cooling flow are the emission lines from the complex of iron L series

ions that have their ionization potentials in the temperature range near and below cluster virial temperature.

Fractional abundance of a given ion plotted against temperature in KeV (Arnaud & Raymond, 1992) . The fractional abundance is multiplied by the abundance

of that element relative to hydrogen in the solar neighborhood.

Fe-L series

The Fe-L line complex in X ray spectra as a function of the plasma temperature for a metallicity value of 0.7 solar.

The energy change is caused by the fact that with decreasing temperature the degree of ionization of the Fe ions also decrease. H. Bohringer et al., 2002

Spectra model

Spectral prediction for an inhomogeneous flow based on:

Peterson et al. 2003

Comparison between the model and the spectrum of Abell 1835. Notably absent in the data are the Fe XVII lines. The plasma appears to match the cooling flow model between 3 KeV and the maximum cluster temperature of 8 KeV but not

below 3 KeV.

Spectra model

Spectra model

Model where the emission below 3 KeV is suppressed.

…other examples

The cooling paradox

Does the gas cool?

• The gas is radiatively cooling, but for some reason it evades detection.

• The gas is being heated in some way so that very little gas cools.

Cool coresWhat happens to the gas which should be cooling on very

short timescales?

Two classes of solutions have been proposed:

The cooler gas is there but

it is somehow hidden

The gas is prevented from cooling below a certain temperature by

some form of heating. Different classes of mechanisms have been considered:

Turbolence, shock, mergingHeating from SN

ConductionHeating from the central AGN

Properties of a successful heating model

• Provide sufficient heating to balance the cooling flow losses (1043 – 1044 erg s-1)

• be fine-tuned: mass deposition triggers the heating process and the heating process reduce the mass deposition

• Provide a global heating effect: local energy deposition would result in local heating while the mass deposition can still go on in the less well-heated regions.

The heating source have to:

Heating from AGNThe vast majority of cooling flow clusters contain powerful

radio sources associated with central cD galaxies.

Spectacular anti-correlation between decrements in the X-ray emission and extended radio emission.

Chandra results: Holes in the X – ray surface brightness are seen to coincide with some radio lobes bubbles of relativistic plasma blown by AGN

The first cooling flow cluster with a central radio source observed by Chandra was Hydra A.

Heating from AGN

Radio source / X ray gas interaction (Mc Namara et al. 2000)

Cooling time at center: 6 x 108 yrDiameter of cavities evacuated by the radio source: 25 kpc

Radio lobes inflated by jets of central AGN appear to be making their way pushing aside the X –ray emitting plasma.

X ray / Radio interaction

Is the energy deposition into the ICM from the radio sources sufficient to account for the lack of gas seen at very low

temperatures in cooling flow clusters?

TOTAL ENERGY OUTPUT OF A RADIO SOURCE

Churazov et al. 2002

Internal energy of the bubble Work done to expand the bubble

V = volume bubbles P = pressure of X ray bright shell surrounding the bubbles

Radio sources have a profound effect on the X – ray emitting ICM

PdVPVE rad

1

1

Compare energy input rate with luminosity of cooling gas

Hidra A

Erad = 2.7 x 1044 erg s-1 Lcool = 3 x 1044 erg s-1

In many systems the amount of energy is comparable to the amount required to offset cooling .

X ray / Radio interaction

Black hole mass = 3 x 109 Msol

Mass accretion rate = 0.01 Msol yr-1

Energy output = 7 x 1043 erg s-1

Accretion radius (vkep = vs) = 50 pc

Self – regulation mechanismThe most simple physical situation would be given if simple Bondi type of accretion from

the inner cooling core region would roughly provide the order of magnitude of power output that is observed and required

Spherical accretion on to the black hole: 30

225.2

s

BH

c

mG

How this energy is distributed on the right spatial scale ?

About 40% of the energy is transferred by the PdV work done on ambient medium. Since, on

average, the bubbles expand subsonically this energy will be converted into sound waves and

in low amplitude shock waves.

Ripples in the gas interpreted as due to sound waves generated by the cyclical bubbling of the central radio source (Fabian et al. 2003)

The gas directly bounding the bubbles seems colder the energy is not

deposited directly in the boundary of the bubbles, as it would be expected for

supersonic expansion.

Dissipation of sound waves, if ICM is viscous, may produce diffuse heating.

…some problems• For Hydra A and Abell 2052 the radio source is depositing enough

energy into the ICM to offset the cooling gas, but…• For Abell 262 the radio source power is more than an order of

magnitude lower of that required to offset the cooling luminosity!!!• Dimension of cool cores vs accretion disk…???

Current efforts are concentrated on finding plausible heating sources to balance the cooling flow.

…Grazie

Bibliografia

• Fabian “Cooling flows in clusters of galaxies” ARA&A 32, 277-318, 1994.

• Bohringer et al. “The new emerging model for the structure of cooling cores in clusters of galaxies” A&A 382, 804-820, 2002

• Mathews & Brighenti “Hot gas in and around Elliptical galaxies” ARA&A 41, 191-239, 2003.

Deprojections analysis of X ray imaging

Starting point: surface brightness.

Goal: deriving a temperature T appropriate to the count rate per unit volume (C) produced in the Einstein detector at the local pressure.

Method: counts rate are accumulated in concentric annuli. Counts from the outer

annulus were used as

“background”. Counts

from inner annuli are

assumed to originate

from spherical shells.

Fabian et al. 1981

Deprojections analysis of X ray imaging

Q(E) = effective area of High Resolution Image (HRI)

demissivity of the gas in the band E – E + dE

NH (E) = optical depth

D = distance to the cluster

P = outer pression

)(4

1)(exp)(

),(

4

12

2

22

2

2Tv

T

P

DdEENEQ

E

ET

T

P

DC H

Estimate of mass deposition ratefrom imaging

From the deprojection analysis temperature profile T(r)

estimate of mass deposition rate

Lcool = kTm

M

.

2

5T = temperature at rcool

Radiation of thermal energy = kTm

M

.

2

3+ PdV work = kT

m

M

.

Estimates of mass deposition rate from X spectra

A volume V of gas at density n cooling at constant pressure from T to T – dT emits a luminosity :

mean molecular weight of the gas.

The luminosity of the spectrum at each frequency is:

is the emissivity at frequency

kdTm

MdVTnndL

HHecool

.

2

5)(

Integreting:

where:

emissivity of cooling gas in a single spectral line (Canizares et al. 1988).

Fit of the spectrum mass deposition rate

Estimates of mass from X spectra

TMAX

Hcool dT

TM

m

kL

0

.

)(2

5)(

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