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A Review of the Observations: Cluster Cooling Flows 2003. Megan Donahue STScI and Michigan State University. The Riddle of Cooling Flows in Galaxies and Clusters of Galaxies. The Origins of “Cooling Flows”.

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A review of the observations cluster cooling flows 2003

A Review of the Observations:Cluster Cooling Flows 2003

Megan Donahue

STScI and

Michigan State University

The Riddle of Cooling Flows in Galaxies and Clusters of Galaxies

The origins of cooling flows
The Origins of “Cooling Flows”

  • Clusters discovered to be extended X-ray sources (Gursky et al. 1971, UHURU; Kellogg et al. 1972; Forman et al. 1972)

  • Thermal emission was the natural interpretation (Felten et al. 1966; Lea et al 1973; Lea 1975) given the spectrum (Davidsen et al. 1975; Gorenstein et al. 1973.) and extent.

  • Intracluster Fe detected: hot gas (Mitchell et al. 1976, Ariel V; Serlemitsos et al. 1976 OSO8)

  • Cooling times in many clusters very short. (Cowie & Binney 1977; Fabian & Nulsen 1977; Mathews & Bregman 1978; Cowie, Fabian, & Nulsen 1980)

  • Unusual optical nebulae associated with cluster cooling flows. (Hu, Cowie & Wang 1985; Heckman et al 1989;Fabian, Nulsen & Canizares 1984)

Ngc 1275 baade minkowski 1954
NGC 1275 Baade & Minkowski 1954

See also: Hubble & Humason 1931

WIYN image



NGC 1275

Hubble HeritageMay 2003

Trouble with cooling flows
Trouble with “Cooling Flows”?

  • Cluster “cooling flows” were defined by an ICM cooling time shorter than a Hubble time.

  • If no heating counteracts the cooling, the mass cooling rates implied ~ 100-1000s solar masses per year in the centers of clusters.

  • Claim of soft X-ray absorption (White et al. 1991)

  • By the early 90s: the search was on.

Search for cool baryonic matter
Search for cool baryonic matter

  • Ionized gas: 104-7 solar masses: Ha (Fabian & Nulsen 1977; Ford & Butcher 1979; Heckman 1981; Hu, Cowie, & Wang 1985; Heckman et al. 1989; Donahue, Stocke, & Gioia 1992; Crawford & Fabian 1992)

  • HI – limited to an occasional faint absorption line (21 cm, Lyman alpha) (Taylor et al 1999; Jaffe 1990; Laor 1997; McNamara, Bregman & O’Connell 1990; Dwarakanath, van Gorkhom, Owen 1994)

  • Metal line limits (Miller, Bregman, Knezek 2002)

H alpha curious correlations
H-alpha: Curious correlations?

  • Only in clusters with short cooling times (Hu, Cowie & Wang 1985)

  • Correlated with X-ray mass cooling rates (Heckman et al 1989)

H a nii sii oi oii
Ha, [NII], [SII], [OI], [OII]…

Too bright for simple cooling (FNC 1984; Heckman et al 1989)

Hot stars (Johnstone et al. ; Voit & Donahue 1997)

  • Blue light (McNamara et al.)

  • Diluted stellar absorption lines (Cardiel et al.)

    Strong shocks not consistent with the data (molecular lines, X-ray imaging, temperature diagnostics)

Search for cool baryonic matter1
Search for cool baryonic matter

  • H2 – vibrationally excited, correlated with Ha (Elston & Maloney 1994; Jaffe & Bremer 1997 ) & extended (HST; Donahue et al. 2000; Falcke et al 2003)

  • CO upper limits (Bregman & Hogg 1988; McNamara & Jaffe 1994; O’Dea et al. 1994; Braine & Dupraz 1994; Antonucii & Barvainis 1994)

  • CO detections 109-1011.5 solar masses in some BCGs (Edge 2001; Salome’ & Combes 2003)

Cool baryonic matter
“Cool” baryonic matter

  • Not in every “CF”; compact relative to the cooling region; low mass.

  • Nebulae extended, correlated: what’s heating them? Stars? Conduction from the hot phase?

  • What is their origin: condensation or cannibalism?

Abell 2597
Abell 2597

McNamara et al. 2002

Donahue et al. 2001

X ray and fuv emission lines
X-ray and FUV Emission Lines

  • Individual X-ray emission lines can measure the cooling rate.

    • DL a [ dM/dt ] DT (Cowie et al. 1980) in a steady cooling flow

    • Early high resolution spectra (Canizares, Markert & Donahue 1988; Mushotzky & Szymkowiak 1988)

    • Recent XMM spectra (Peterson et al. 2003; Peterson et al. 2001; Kaastra et al. 2001; Tamura et al. 2001.)

Xmm spectroscopy
XMM spectroscopy

  • Peterson, et al. 2003

  • FeXVII and other lines from 1 keV gas not present.

  • Two-temperature or “truncated” cooling flow (at ~T/3 - T/2)

Fexvii in perseus then and now
FeXVII in Perseus: then and now

  • XMM/RGS 60 cm2

  • FPCS1 cm2

Cooling rates are reduced
Cooling rates are reduced

  • Chandra grating spectra results are consistent with XMM. (e.g. Hicks et al. 2002; Wise 2003)

  • Chandra low resolution spectra also implies lowered dM/dt. (e.g. Lewis et al. 2002; McNamara et al. 2000)

  • FUSE OVI limits and faint detections record very low dM/dt (10%). (Oergerle et al. 2002)

  • Are they consistent with the baryon limits and detections of gas at cooler temperatures? (Close!)

  • But are we asking the right question anymore?

New name cool core clusters
New Name: Cool core clusters?

  • In a steady cooling flow: DL ~ DT . dM/dt

  • XMM and Chandra spectra show that gas cooler than Tx/3 is not there in quantities consistent with DT . dM/dt

  • Cluster gas does not simply cool and flow toward the center: the simple cooling flow model is wrong

  • The correlations with M-dot should be revisited!

    • But maybe call it the “Cooling Term” instead.

Some gas still cools
Some gas still cools…

  • Temperature gradients in the core are confirmed by Chandra and XMM.

  • The virial temperatures are hotter than the core temperatures.

  • Gas cooler than ~1 keV not seen or is very faint.

  • The minimum core temperature seems to be related to the ICM temperature:

    Tcore = Tx/3 - Tx/2

  • The cooling time is still short.

Temperature gradients
Temperature Gradients

Horner, Donahue, & Voit 2003

Entropy s t n 2 3 gradients
Entropy (S=T/n2/3) Gradients

Cool cores mean something more curious correlations
Cool cores mean something …more curious correlations

  • “Cooling flows” center on BCG

  • While “cooling flows” often have…

    • Optical emission line nebulae consistent with star formation but not strong shocks

    • Molecular gas (both warm and cold) & dust

    • Excess blue light and hot star absorption features

    • Central radio sources = AGN

      …Clusters with tc > tH don’t have these things

Clues from morphology
Clues from morphology?

  • Central radio sources

    • Some are powerful, most are relatively weak (some are faint to non-existent)

    • Radio power correlates with cooling rate; M87’s cool X-ray filament & radio source have similar structure

    • Radio structure anti-correlated with:

      • X-ray emission (cavities)

      • Ha and infrared H2 emission


  • Single phase?

  • 2nd temperature associated with filament (~1 keV)

  • Radio source & cavities

    (Young et al. 2002; Matsushita et al. 2002; Molendi 2002

Young et al. 2002

Abell 1795
Abell 1795

CXC/SAO/Fabian et al. 2001

Institute for Astronomy/Cowie

Clues from dust
Clues from dust?

  • Dust has a short lifetime in hot dense gas (Draine & Salpeter 1979; Dwek & Arendt 1992).

  • Existence of dusty optical filaments with Galactic extinction properties suggests the origin of the filaments is not the ICM (Donahue & Voit 1993; Sparks et al. 1989; Voit & Donahue 1997).

  • If the cool gas is dusty, could it have been cannibalized from other galaxies? Or does it condense from an dust-enriched ICM (See limits from extinction (Maoz 1995), ISOPHOT emission (Stickel et al. 2002)

  • ICM could be contaminated with dust via winds, stripping; but with a small covering factor?

Physics of structure formation
Physics of structure formation?

  • Assembly of a cD: cannibalism, mergers

  • Effect of an AGN on structure formation

  • Effect of star formation on galaxy formation

  • Signature of hierarchical assembly on clusters: contribution of in-falling groups, galaxies? Time dependence?

Spectral emission measure
Spectral Emission Measure

  • Spectral luminosity dL/dT  dM/dT * Ta

    • a = 0 expected

    • a =1-2 observed: what is the physical explanation?

  • Is there really a relationship to the virial temperature? (Good to confirm!)

Peterson, et al. 2003

Entropy profile
Entropy Profile

The distribution of entropy with radius and time is a prediction of the heating process: convection, conduction, gravitational processes have different effects.

Kaiser & Binney 2003:

Episodic heating

Questions for discussion this week
Questions for discussion this week

  • Is “M-dot” a valid cluster core property?

  • Are there model-independent quantities that can be used to describe the spectral and imaging characteristics of cluster cores?

  • How does gas cool from the hot phase?

Questions for the committed
Questions for the committed

  • Do the X-ray spectra consistently suggest that T_low scales with the virial temperature?

  • How are metallicity gradients affected by convective processes?

  • What heats the optical and infrared nebulae? How does that mechanism affect the hot gas?

  • If temperature gradients exist in most of the cool cores, how quickly must the cool gas be replenished if conductive evaporation is important?

  • What are the roles of stars and AGN?


  • New observations from XMM and Chandra have ruled out the steady cooling flow model.

  • Time to divorce the model from the observations: cool cores, high central densities…

  • Observations: dT/dr, spectra of T/3 but not T/2 gas, entropy gradients, correlations of radio, IR, and optical phenomena with short cooling times.

  • The same observations have introduced a new problem: what are the heating terms in the energy equation? (Alternative: is it possible to cool the gas but not allow it to radiate X-rays and FUV?)