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OCR for page 112
Cluster Research with X-Ray Observations
RICCARDO GIACCONT AND RICHARD BURG
Space Telescope Science Institute
ABSTRACT
Past X-ray surveys have shown that clusters of galaxies contain hot
gas. Observations of this hot gas yield measurements of the fundamental
properties of clusters. Results from a recent study of me X-ray luminosity
function of local Abell clusters are described. Future surveys are discussed
and the potential for studying the evolution of clusters is analyzed.
INTRODUCTION
The systematic study of clusters began with the surveys of Abell (1958)
and Zwicly e! al. (1968) who each created well-defined catalogues according
to specific definitions of the object class. In particular Abell defined clusters
as overdensities of gal~es within a fixed physical radius around a center,
classifying such objects as a function of their apparent magnitude (distance)
and of their overdensity ("richness"~.
The first X-ray survey of the sly by the UHURU X-ray satellite
showed that "rich" nearby clusters were powerful X-ray sources (Gursly et
al. 1971; Kellogg et al. 1972~. Subsequent spectroscopic studies detected
X-ray emission lines of highly ionized iron and demonstrated that the X-ray
emission was produced by thermal radiation of a hot gas with temperatures
in the range of 30 to 100 million degrees (Mitchell et al. 1976; Serlemitsos
et al. 1977~.
With the launch of the HEAO1 and the Einstenl Observatories, surveys
of significant samples of nearby clusters demonstrated that as a class,
clusters of galaxies are bright X-ray sources with luminosities between 1042
112
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HIGH-ENERGY ASTROPHYSICS
113
and 1~5 ergs/see (Johnson et at. 1~3; Abramopoulos and Ku 19~; and
Jones and Forman 1984~. The increased sensitivity of the Einstein imaging
detectors also provided the capability to study clusters at large redshifts (z
> 0.5) (Henry e! al. 1979~.
The general problem one wishes to attack by means of X-ray obser-
vations is the study of the formation and dynamic evolution of structures
consisting of gravitationally bound galaxies. It has been pointed out by sev-
eral authors (Kaiser 1986; Shaeffer and Sink 1988) that X-ray observations
of such systems may offer important advantages with respect to studies
other wavelength domains, particulars at early epochs of the universe. In
Table 1 we list the fundamental properties of clusters that can be measured
in X-ray surveys along with a brief description of the measurement. We
also include, for comparison, the analogous measurement in the optical
In order to be efficiently detected in X-rays, such systems can be
empirically defined as having the following properties:
1. They must contain sufficient intergalactic gas (typically 1/10 of the
cluster mass).
The gas must have been heated to X-ray emitting temperatures
typically larger than those corresponding to escape velocity from
a single galaxy. It should be noted that the efficiency of X-ray
emission depends on metallici~y.
The gas must be centrally concentrated in the cluster, (Lo ~
p2), although not more so than the galaxies in nearby observed
systems.
Such properties have been shown to exist in Abell-~e clusters, as
well as In much poorer systems such as cD groups (Kriss et al. 1980~. Thus
the class of X-ray luminous clusters of galaxies which may be retrieved In
future sensitive X-ray surveys in a sufficiently soft X-ray band (for example
0.1-2 Key), will include both optically defined classes of rich clusters
(such as Abell or Zwicky) as well as poorer clusters or any gravitationally
bound system of galaxies containing high-temperature gas. The Eu~steu'
ObseIvatory Medium Sensitivity SuIvey which uses the IPC data (0.35 to
3.5 keV) has in fact detected a number of optically poor X-ray emitting
clusters (Gioia et al. 19~.
X-RAY LUMINOSITY [IJNC1ION
We would like to briefly summarize some of the recent work by Forman,
Jones, and ourselves on the X-ray luminosity function of Abell cluster as
an introduction to the subject.
The earliest determinations of the X-ray luminosity function for Abell-
like clusters of galaxies were based on the UHURU and Ariel surveys.
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114
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HIGH-ENERGY ASTROPHYSICS
115
Schwartz used a sample of 6 clusters and McHardy a sample of 20 clusters
to derive luminosity functions. Extensions of these first attempts included
the analysis of HEAO1-A2 data (Piccinotti et ale 1982) with samples of
30 clusters. More recent surveys with HEAO1-A2 were based on 128
detected clusters (Johnson e! al. 1983~. Finally, Abramopoulos and Ku
(1983) used Eu~steu~ imaging observations of 74 nearby clusters with z <
0.27. It should be noted that while the UHURU, Ariel and HEAO1 surveys
were X-ray flux limited surveys which, in principle, could have studied the
X-ray luminosity of a more general cluster population, they were severely
biased by their sensitivity and energy range to He detection of Abell-like
rich, high-temperature clusters, although some X-ray emitting groups were
observed (Schwarz et al. 1980~.
These determinations of the X-ray luminosity function of Abell-like
clusters were based on relatively small samples and had intrinsic limitations
or deficiencies. In particular, previous determinations of the Abell-like clus-
ter X-ray luminosity function did not take into account the incompleteness
for richness O clusters (Abramopolos and Ku 1983), did not include richness
O clusters (Kowalski et al. 1983), or were not sensitive to low temperatures
because of their effective energy band (Piccinotti et al. 1982; Kowalski et
al. 1983~. The last limitation could be quite important in attempting to
understand the low end of the X-ray luminosity function since there ap-
pears to be (at least for R > 1) a correlation between X-ray luminosity and
temperature ~Iusho~zky 1988~.
In our recent work we have investigated the statistical properties of
the 226 Abell clusters with z < 0.15 observed by the Einstein Observatory
(Burg et al. 1990, hereafter referred to as BFGJ). This sample is taken from
the larger compilation of Einstein cluster observations analyzed by Jones
and Forman. We show that this set of clusters form, for the purpose of
this work an unbiased sample of Abell clusters that spans richness classes
O to 2. We use the Einstein sample to derive an X-ray lum~nositr function
which is free of some of the problems which beset previous analyses.
The main advantages of this determination are: the ability to detect low-
temperature clusters because of the energy band (0.5 to 4.5 keV) (in
common with the Abramopoulos and Ku surveys; the larger sample which
allows us to adopt stringent criteria to insure completeness and allows us to
determine the X-ray luminosity function for different richness classes; and
the higher sensitivity which allows us to explore the low-luminosity end of
the luminosity function.
The redshift limit of 0.15 was chosen for our sample since within this
range the Atoll richness classification is distance independent Furthermore
we have established that for redshifts < 0.15 there is no correlation between
redshift and X-ray luminosity (see Figure 1~. Thus this subset of the Abell
catalog can indeed be considered a proper sample to derive the shape of
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116
o
o
o
o
o
o
o
a'
Y o _
U. _ ~
-
x
J g
O _
U.
AMERICAN AND SOVIET PERSPECTIVES
o
o
-
~ V
0.01 0.05 0.1 o.5
redshift
FIGURE 1 Scatter diagram of X-ray luminosity versus redshift for richness 1 clusters.
the X-ray luminosity function by richness class since the entire range of
X-ray luminosity can be observed throughout the chosen volume.
The IPC fluxes from the compilation of Jones and Fran (1990) have
been obtained by integration over a region of 1 Mpc radius centered on the
X-ray determined cluster center. These fluxes are computed in the 0.5~.5
keV (observed) band, from the observed counting rates, using the hydrogen
column density and either the observed or estimated gas temperature.
The estimated gas temperature is computed using the observed lummosi~r
temperature relation, which we have redenved for our sample and which is
given by Lee or T5/2 (Mushotzky 1988).
The luminosity at the source is computed utilizing the measured X-
ray flux and the measured or estimated redshifL K-corrections have been
computed using Me Raymond-Smith model, assuming 0.5 solar metallici~,
and they are of order 20% over the redshift and temperature range of the
sample (Burg and Giacconi l990~. The method used for computing the
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HIGH-ENERGY ASTROPHYSICS
117
luminosity function is dictated by the criteria used in selecting the sample.
In this work the undertring sample is defined by optical properties (he. the
number of galaxies ~ an Abell radius) and not by X-ray properties. Thus
the computed luminosity function is a bi-vanate function of both Lo and
cluster richness R.
The sample is volume limited in the sense that the Abell sample
(with the same redshift cutoff is volume limited and with the same in-
completeness problems. Therefore each cluster contributes 1/V to the
luminosity function This is different from the methodology used for an
X-ray flux limited survey where each cluster would contribute
VmaX(s km, L=)
1b calculate the cumulative luminosity function, we use the Kaplan-
Meier product limit estimate method (Cox and Oakes 1984; Schmitt 1985;
Feigelson and Nelson 1985~. This is equivalent to the techniques developed
by Avoi et al. (1980~. Specifically, the following probability is calculated:
( ~) 0` N < ' ~
This is the unnormalized cumulative luminosity function and is formally
the minimum likelihood estimate of the luminosity function The results
are shown in Figure 2a.
Since our X-ray sample Is not an independently complete sample, we
must rep on the understanding of the completeness characteristics of the
Abell sample to derive the normalization. The Abell Catalogue is known
to be incomplete for richness class 0. Abell recognized this and did not
include the richness class 0 objects in his "statistical" sample. Later work
by Bahcall (197~, see also Lucy 1983), based on analysis of the multiplicity
function (the number of clusters per unit volume versus nchness) has shown
that richness 0 clusters are incomplete by a factor of > 3. Quantitatively
we fit Schechter functions to the data with the results shown in Figure 2b.
It must be stressed that the normalization does not affect the shape of the
luminosity function.
Some aspects of our results are immediately apparent:
a. The shape of the luminosity function is similar for each richness
class, although there is a change in scale.
b. Richer clusters are systematically brighter in X-rays. The value
of Lid, the characteristic luminosity for each richness class is
roughly proportional to (N*,a')~' (where N* is the characteristic
number of galaxies per richness class, Lucy 1983), by = 1.6 ~ 0.4.
