Determining the temperature
structure of the hot plasma halo
around M87 with XMM-Newton
May 2006
Aurora Simionescu
Fakult¨
at f¨
ur Physik der
Ludwig-Maximillians Universit¨
at
M¨
unchen
Contents
1
Introduction
5
1.1
Clusters of galaxies . . . . . . . . . . . . . . . . . . . . . . . .
5
1.2
The cooling flow problem . . . . . . . . . . . . . . . . . . . . .
6
1.2.1
Cooling flows before XMM-Newton and Chandra . . .
6
1.2.2
The fall of the cooling flow model . . . . . . . . . . . .
8
1.2.3
Emerging new models for cooling core clusters . . . . .
8
1.3
Previous observations of M87 . . . . . . . . . . . . . . . . . . 11
1.4
Scientific goals of this work . . . . . . . . . . . . . . . . . . . . 16
2
Observational details
19
2.1
XMM-Newton . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2
The present observation of M87 . . . . . . . . . . . . . . . . . 24
3
Data analysis methods
25
3.1
X-ray brightness profile . . . . . . . . . . . . . . . . . . . . . . 27
3.2
Temperature profile . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.1
Determining the temperature from color maps . . . . . 32
3.2.2
Determining the temperature from spectral fitting . . . 35
4
Results and interpretation
41
5
Summary and Outlook
51
3
Chapter 1
Introduction
The launch of the advanced X-ray observatories Chandra and XMM-Newton
revolutionized our view of the X-ray sky providing valuable new and more
detailed data which answered some scientific questions and raised many more,
shaking some of the main physical models representing our understanding of
X-ray astrophysics. Among the most disputed problems posed by the new
observation capabilities of these two satellites is the so-called "cooling-flow
problem" in galaxy clusters. XMM-Newton and Chandra data proved that
the hot gas at the center of many galaxy clusters does not cool below a
certain threshold temperature although its cooling time is significantly lower
than the age of the Universe, thus challenging scientists to find a source of
heating which would be so well-tuned as to give the gas just the right amount
of energy to prevent cooling below the observed limit. Known as the central
dominant galaxy of the closest cooling-core cluster, as well as the first and one
of the brightest extragalactic X-ray sources ever discovered, M87 is a perfect
object for detailed studies which may indicate the origin and properties of the
inferred heat input halting the cooling flow. The aim of the current work is to
analyse the most recent, deepest observation of M87 with XMM-Newton to
date and produce detailed, accurate and objective temperature maps which
may enable us to better understand the heating mechanism as well as the
physics of the intracluster medium in the vicinity of the central galaxy in
general.
1.1
Clusters of galaxies
Contrary to what the name suggests, clusters of galaxies are not merely col-
lections of galaxies grouped close together; they are well-defined, connected
structural entities. In fact, only roughly 2% of the total mass of such a
5
6
CHAPTER 1.
INTRODUCTION
cluster is contained in galaxies while 11% consists of hot intracluster gas
and 87% of the mass is in dark matter. The hot, highly ionized intracluster
medium (ICM) has temperatures of the order of tens of millions of degrees,
radiating therefore primarily in the X-ray domain. Images of galaxy clusters
in this wavelength regime provide a very different picture from that seen in
the optical range, since the intracluster gas fills the whole cluster volume and
therefore the diffuse X-ray emission from the ICM traces the cluster structure
contiguously. X-ray imaging information and X-ray spectra are thus crucial
elements in understanding the astrophysics of galaxy clusters.
1.2
The cooling flow problem
By emitting X-rays, the intracluster gas loses energy and therefore should
cool if no other source of heating is available. The intensity radiated in
the process of thermal bremsstrahlung can be derived to be approximately
proportional to the square of the gas density and the square root of the gas
temperature. Therefore, the cooling is largest in the central parts of clusters
where the density is high. Observations of clusters with the Uhuru satellite
first showed that the mean cooling time of the gas in some cluster cores which
show a very peaked density profile is close to the Hubble time (Lea et al
1973 [30]), leading scientists to develop the cooling flow model in an attempt
to describe the effects of significant cooling of the central gas (Fabian and
Nulsen 1977 [19], Mathews and Bregman 1978 [33]). Later observations with
Einstein and EXOSAT set the estimated percentage of clusters which host a
cooling core to a significant 70-80% [15], which emphasizes the importance
of understanding cooling flows in cluster astrophysics.
1.2.1
Cooling flows before XMM-Newton and Chan-
dra
The cooling flow model, based on early observations mainly with the Uhuru
and Einstein observatories, states that upon cooling (which takes place within
the so-called cooling radius where the cooling time is equal to or less than the
Hubble time) the gas condenses to cold blobs which sink or "flow" through
the remaining hotter atmosphere to the very center of the cluster which is
often associated with a central dominant galaxy. The cooling radius is small
compared to the extent of the entire cluster such that the gas loss through
cooling is negligible compared to the total amount of gas in the ICM, however
the cooling radius can be as large as some hundred kiloparsecs, allowing
the cooling region to be resolved by X-ray satellites. It was inferred that
1.2.
THE COOLING FLOW PROBLEM
7
through the cooling flow, the central dominant (cD) galaxy is still growing
at the present time. However, no significant signatures of star-formation were
observed in cD's, therefore it was assumed that either the gas from the ICM
cools into optically thick objects [16] or that mostly small stars are produced
which show much less observational evidence of star formation, which would
imply a modified stellar initial mass function in the central galaxy [20].
The cooling flow model implies that each radial zone in the cooling flow
region comprises different plasma phases (the cooling blobs), covering a wide
range of temperatures, and that there is no energy exchange between these
different phases, otherwise the blobs would be mixed with the hotter ICM
before condensing to the center of the cluster. Consequently, heat conduction
must be suppressed, which was presumed to be due to cluster-scale magnetic
fields.
According to the cooling flow model, the mass deposition rate - that is,
the mass of cool gas which sinks to the core per unit time - can be calculated
from the relation
L
cool
=
5
2
M
µm
kT
where L
cool
is the luminosity associated with the cooling region, usually of
the order of 10% of the total luminosity, and T is the temperature of the
gas at the cooling radius. In deriving this formula, it was assumed that the
luminosity associated with the cooling region is due to the thermal emission
from the cooling gas and the PdV work done on the compressed cooling gas.
Typical mass deposition rates calculated from this formula are in the range
of hundreds of solar masses per year [16].
Observational evidence for the cooling flow model came mostly from the
X-ray domain. Strongly peaked surface brightness profiles confirmed by ever
higher resolution detectors with the launch of ROSAT indicated very short
cooling times. Low temperature components were detected with the Solid
State Spectrometer onboard the Einstein satellite in the Perseus and Virgo
clusters (Canizares et al 1979 [12], Lea et al 1982 [29]). Further evidence
from spectral data was the correlation between the X-ray luminosity, X-ray
emission-weighed temperature, metallicity and inferred mass deposition rate,
such that strong cooling flows were shown to have a systematically higher
luminosity and metallicity at a given temperature compared to weaker ones
(Fabian et al 1994 [17]).
However convincing the X-ray data, no other wavelength regime showed
strong evidence in favor of the cooling flow model. The cD's in cooling
cores were found to show anomalies in the optical band, such as the emission
of diffuse blue light (McNamara and O'Connell 1989 [36]), and in the radio
8
CHAPTER 1.
INTRODUCTION
band, such as high Faraday rotation or depolarization (Owen et al 1990 [42]),
but the strength of these signatures implied mass deposition rates one to
two orders of magnitude smaller than those calculated from the cooling flow
model. Many scientists were thus still skeptical of the cooling core model and
it was the launch of Chandra and XMM-Newton that proved them entitled
to their skepticism.
1.2.2
The fall of the cooling flow model
High-resolution spectra of cooling core clusters obtained with the Reflecting
Grating Spectrometer (RGS) onboard XMM-Newton showed a different pic-
ture from what the cooling flow model had predicted. The temperature of
the ICM did show a decrease at the center of cooling core clusters, but no ev-
idence of gas cooling below a certain threshold, usually one half to one third
of the virial temperature, was found. The work of Peterson et al [44],[45]
clearly showed that the cooling flow model also overpredicted the line emis-
sion from low-temperature ions, thus giving a very different spectrum to what
the XMM RGS measured. For example, in clusters with a virial temperature
of 7-10 keV the predicted Fe XVII-XXII lines were not observed; for virial
temperatures of 4-7 keV, Fe XVII-XXI were also not observed while for clus-
ters with temperatures of 2-4 keV the Fe XVII-XVIII lines were much weaker
than the predictions of the cooling flow model. A visual example of this is
given in Figure 1.1.
Some solutions were proposed to reconcile these findings with the cooling
flow model, among which the presence of a cool absorber along the line of
sight which would account for the deficiency in soft X-ray detection, or an
inhomogeneous distribution of the metal abundances in the ICM resulting
in the suppression of low-temperature line emission (Fabian et al 2001 [18]).
All of these explanations however were refuted by the available data.
1.2.3
Emerging new models for cooling core clusters
Since the cooling flow model could not explain the spectra obtained with
the most recent X-ray satellites, new perspectives came to the attention of
astrophysicists. Based on detailed spectra, especially those obtained from
M87 at the center of the nearby Virgo cluster, it was inferred that the mass
deposition rates must in fact be at least an order of magnitude less than the
value predicted by the cooling flow model. For this to be possible, an extra
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