Hot tenuous plasmas with temperatures in the range 1,000,000 to 3.10 to the 8th power K and densities of .000001 to .001 per cu cm are considered. The dominant radiation processes are thermal bremsstrahlung and collisional line excitation and all except the very hottest objects will have observable X-ray emissions lines. The most important unifying point is the angular scales involved. For the interstellar gas, structure ranging in size from about 10 ft to tens of degrees is expected

Cowie, L. L.

English

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PFZECXDING PAGE BLANK NOT FILMED 
INTERSTELLAR, INTRACLUSTER AND SUPERCLUSTER GAS 
Lennox L. Cowie 
Physics Department, Center for Space Research, and 
Center for Theoretical Physics 
Massachusetts Institute of Technology 
Cambridge, Massachusetts 02139 
Talk Presented at: 
Symposium on X-Ray Astronomy and Spectroscopy 
Goddard Space Flight Center, October 1981 
https://ntrs.nasa.gov/search.jsp?R=19820018184 2020-03-21T07:36:37+00:00Z
120 
I. INTRODUCTION 
The topics which this talk has to cover range in size from 
about 100 Pc to 100 MegaPc but in many respects are remarkably 
similar. In all cases we are considering hot tenuous plasmas 
with tebnperatures in the range lo6 + 3.10e OK and densities of 
lo-' + lo-' ~m'~. The dominant radiation processes are thermal 
bremstrahlung and collisional line excitation and all except the 
very hottest objects will have observable X-ray emission lines. How- 
ever the most important unifying point is the anaular scales involved. 
For the interstellar gas one may expect structure ranqing ir? size 
from about 10' (shadowing by IS clouds; Fried e t  a l .  1980: to 
tens of degrees (nearby evolved supernova remnants (Nousek e t  a l .  
1981). Nearby (distance ~ 1 2 0  M Pc) irregular clusters will have 
structure on qalaxy scalelengths of a few minutes, which is similar 
to the core sizes of distant. more regular clusters (e.9. Fvrman 
and Jones 1982). The cores of very distant clusters at z=l 
will be resolvable at 30'' -c 1' [ e . g .  Henry e t  G T Z .  19791. 
Superclusters will have sizes of degrees [e.g. Murray e t  a l .  19781. 
Thus there are an enormously wide range of problems which 
an instrument with a spatial resolution of $ *  -c 1' and a field of 
view of degrees could tackle. Though there are problems in these 
areas which such an instrument could not cover,* I think it is 
fairly clear that what we need most is an instrument of this 
general type. 
*Examples are t!-.e spatial mapping of cooling flows in clusters 
(Fabian c t ,  J I .  1981) or the mapping of gas around galaxies in 
distant irregular clusters. 
121 
Of the three classes of problems listed in the title the 
only one on which we have really excellent X-ray data is the 
clusters [e.g. Forman and Jones 19821. We have a good deal of 
information on X-ray emission from the interstellar gas both from 
rocket flights and the HEAO 1 and SAS C experiments [e.g. Fried 
et aZ. 19801. Finally we have very little solid information of 
any sort on the superclusters. Despite this diversity in 
existing understanding, the way to the future in all three areas 
must be through larger collecting areas.* In the rest of the 
talk it will be seen that there are many problems where a highly 
s p e c t r a l  resolution [R n- 100-1000] would be useful or even essen- 
tial. However, we could make a great deal of progress with an 
instrument of moderate resolution (R \r 10) provided ;it? could 
obtain the necessary increase in sensitivity. 
A rather incomplete list of important problems in these 
fields is as follows: 
Interstellar Cas: 
Mapping at moderate ( 1 ' )  spatial Morphology of individual structures in 
resolution: the IS gas 
Separation of stellar and gaseous con- 
tributions to the soft X-ray background 
Interstellar cloud shadowing 
Structure o f  the galactic halo 
(High Spectral Resolution 
Studies : 1 
Chemical composition of the galactic halo 
X-ray absorption line studies of the disk 
and halo 
Spectroscopy of individual structures 
*The necessity for large area applies irrespective of the spectral 
resolution. 
122 
Clusters : 
Mapping at 1' resolution: 
(High spectral resolution 
studies: 1 
Superclusters: 
(Figh spectral resolution 
studies:) 
Samples of distant clusters/c:uster evolution 
Outer halos of rich homogeneous clusters 
Interaction of galaxies and cluster gas 
in irregular clusters 
Morphology of gas in clusters 
Spectroscopy of the centers of radiatively 
cooling clusters 
Abundance gradients 
Primordial material in irregular clusters 
retection and mapping of hot gas 
Interaction bekween clusters and superclusters 
Chemical enrichment in intercluster qas 
I've split the .'.ist into experiments which could be performed 
without high spectral resolution and those in which it is essen- 
! 
tial. I think it's worth noting that roughly 504, of the experiments I 
require hiat spectral resolution as well as the imaging capability. 
Clearly if both sensitivity and high spectral resolution could be 
obtained this would he ideal. 
t 
f 
I now want to consider a few of these topics in more detdil. ! 
t 
The choice is somewhat arbitrary and I have avoided topics which i 1 are touched on elsewhere (in particular in the talks of Fabian 
and of Shull). i 
I I. SUFZRCLUSTER GAS 
It seems increasingly probable that the general intergalactic 
gas lies at temperatures less than lo6 O K  and probably cannot be 
detected by its X-ray emission (e.g. Sargent e t  a ? .  1979). ks 
123 I 
yet the arguments and observations of this point are in no way 
compelling but it seems worthwhile to focus our attempts to 
detect the IGM in the X-rays on those regions where the heat 
input is most likely to be concentrated. 
The best location for this is probably in the intercluster 
regions of the superclusters. Bookbinder st a t .  (1980) have argued 
that galaxies of present visual luminosity L, = L release 
about lOb1.’ ergs into the intergalactic gas at temperatures 
around lOgoo K in their early stages of evolution. 
0 
Such material 
will not be bound to any cluster of which the galaxy is a member. 
At this stage the cluster would then possess a hot outflowing 
wind. * 
Assuming that the net visual luminosity of a typical super- 
cluster is around lOI4 Lo, the total energy release would amount 
to about 2x106’ ergs. If this occurred at z=5, allowing for 
adiabatic expansion losses, it would heat a gas of density 
= 9.6~10-~Rh~ ~m’~, (h = Ho/lOO km s-l Mpc-l), filling a radius ”e 
of 20 Mpc to a temperature of ~2xlO~R”h‘~ OK.** 
itlass of gas in t h e  superc lus ter  is 1316Ch2 ;-la while the galaxies 
will have released about 5 ~ 1 0 ~ ‘  Ma of processed gas or about lOI4 
The total 
*As was first pointed out by Ostriker (1979), this has two very 
attractive features. Firstly it drives out any primordial gas 
which would like to fall into the cluster. Secondly, by removing 
very metal enriched qas from the cluster it can solve various 
abundance problems of the type discussed by deYoung (1978). 
* *A  complete self consistent calculation would follow the evolution 
of the blast wave generated by the overlapping cluster winds and 
calculate the temperature from this (Ostriker and Cowie 1981). 
The result is very close to the quick estimate given above and 
the present radius of the blast wave is around 20 Mpc for  R=l. 
I 
i 
124 
Mo of metals (Bookbinder et a l .  1980) .  The metallicity may 
therefore be a substantial fraction of cosmic particularly if 
R is less than 1. 
Assuming that the emissivity is around n2 ergs cm3 s-’, e 
a temperature of about 200 ev-1 Kev, as is suggested by the above 
calculation, the total luminosity of the supercluster would be 
around lo4’ Q2h4 ergs s-’ and its surface brightness about 5 ~ 1 0 ’ ~  
R2h4 ergs/cm’* s-l ster” or a fraction of the diffuse X-ray 
background at these energies. 
In general terms and independent of this rpecific model it 
is clear that the most important requirement for a supercluster 
search is the ability to exclude member clusters and other X-ray 
objects from the field and to consider only the intercluster com- 
ponent. This requires an angular resolution of at least a 
fraction of a degree. The experiment must also be capable of 
imaging the whole supercluster which would generally mean a FOV 
of several degrees for relatively nearby superclusters. 
For a supercluster of radius 3 O  at a distance of 500 Mpc we 
can estimate that a 30 detection above fluctuations in the back- 
ground would crudely require a source with 10% of the diffuse X- 
ray flux in the same region (Levine e t  a t .  1977,  Schwarz 1 9 8 0 ) .  
This would imply that a sensitive experiment could reach surface 
brightness limits of about 2x10’’ ergs cm’2 sec” ster” (Fried e t  aZ. 
1980) and constrain R < 1 or so for an individual supercluster. 
Clearly a survey of a large number of superclusters could provide 
very interesting constraints on the IGM density. 
125 
A spectroscopic instrument, measuring the relative line 
strengths, could also constrain the metallicities of the gas. 
This could be of crucial importance, if and when the superclusters 
were detected, in distinguishing between cluster heating of the 
intracluster gas as discussed above and alternative heating 
mechanisms such as gravitational in-fall. The history of cluster 
X-ray emission studies should remind us how important such 
information can be. 
111. CLUSTER X-RAY SOURCES 
I want to say relatively little about this topic, 
since the problems are so well known to everyone. 
be found in the reviews of Cowie (1981) and Forman and Jones 
(1982). Briefly, a large are3 imaging instrument is needed to 
study distant clusters and to ex:.end the Einstein results of Henry 
and coworkers (e.g. Henry e t  al. 1979). Realistically there is 
little hope of obtaining cosmological information from this type 
of measurement but a very large sample of distant clusters may 
allow us to obtain t h e  evolution of the average X-ray luminosity 
and gas core radius f : r  inlividual classes of cluster. Hopefully 
we may at least determine if there has been any evolution of the 
potential of the cluster over recent ( z  5 1) times. However, 
one should contrast the results of Cowie and Perrenod (19791 
with those of Perrenod (1977) to see how small the expected dif- 
ferences are. 
Discussion can 
A second direct application of a large area imaging detector 
would be the study of gas lossage by galaxies in irregular 
126 
clusters (e.g. Fabian e t  al. 1980). Pushing existing results 
farther back in z would probably help us understand the nature 
of galaxy stripping in clusters, gas evolution in galaxies and 
cluster galaxy evolutionary effects such as the color changes 
described by Butcher and Oemler (1978). Models for the formation 
of cluster atmospheres such as those by Norman and Silk (1979) 
or Sarazin (1979) make detailed and testable predictions of the 
evolution of the gas distribution in clusters which could be 
checked. 
There are a number of additional problems which a spectro- 
scopic instrument could tackle. Perhaps the most interesting 
would be an extension of the fascinating Einstein SSS and FPCS 
results (e.g. Canizares e t  at. 198U, Mushotzky e t  aZ. i981) 
on the cooling central cores of clusters out to more distant 
clusters. Refinement of the cluster metallicities which can be 
obtained in this way could be of primary importance in under- 
standing galaxy evolution within the clusters and cluster gas 
evolution. 
IV . INTERSTELLAR GAS 
In this section I want to consider two very specific experi- 
ments, one of which illustrates what could be done with a large 
area imaging detector and one of which would additionally require 
high resolving power. 
a. Cloud Shadowing of the Soft X-Ray Background 
The great majority of the soft X-ray background appecrs to 
be truly diffuse, arising primarily from a local hot spot in the 
127 
galactic disk and from. a hot gaseous galactic halo (Marshall and 
Clark 1981). About 20-30% at 1 Kev and substantially less at 
200 eV arises from dM stars (Rossner e t  aZ. 1981) with about 10 
stars in each square degree contributing most of the flux. 
The fractional contributions from Pop 11 stars to the halo emis- 
sion may still constitute a problem. However if the soft X-ray 
background could be imaged on a scale of minutes, fluctuation 
analyses of the type given by Levine e t  a l .  (1977) at larger 
scales should allow us to obtain a stringent limit to the stellar 
contribution. Imaging of the X-ray background at such scales is 
a demaneing problem however, since the high galactic latitude 
photon arrival rate in the 200 eV-1 Kev range is only around 
5~10'~ ph cm'2 sec" (a')-'. 
with long exposure and a low background are necessary. 
Therefore both a large area detector 
As Fried e t  al. (1980) have recently pointed out, typical 
diffuse interstellar clouds with column densities of 3 ~ 1 0 ~ '  cm* 
or more are opaque to radiation in the 200 eV energy range. The 
local hot gas behind such regions and, more importantly, the hot 
halo gas or halo star contribution will be substantially shielded 
by such regions. A typical line of sight ai. high latitudes is 
most likely to pass through exactly one cloud but has a finite 
possibility of passing through none or more than one (e.9. Spitzer 
1977). Since the cloud sizes are large compared to arcmins (at 
100 pc a 1 pc radius cloud has an angular radius of 30'1, indi- 
vidual clouds will appear as large shadowed regions on the diffuse 
X-ray background at low energy. Since we may also estimate the 
distance from the amount of missing soft X-rays this type of 
128 
measurement could determine the size spectrum of interstellar 
clouds a quantity of ctxial importance to theories of the ISM 
(McKee and Ostriker 1977). It is worth commenting that this 
quantity is remarkably difficult to determine by other methods. 
b. X-Ray Absorption Lines from the Hot ISM 
The possibility of making X-ray absorption line studies, 
analogous to the optical and W absorption line studies which 
have been so important in stvdying the interstellar medium, has 
only recently begun tn be considered. The rea.;ons for this are 
clear - the instrument requiremerts are quite severe both as 
regards effective w e a  and spectral resolutlon - but the poten- 
tial for studying the intergalactic medium (Shapiro and Bahcall 
1980) and the hot interstellar meJium and the galactic halo (York 
and Cowie 1981) are enormous. The reason for this is that the 
absorption lines directly measure column densities of a given 
ionization stage and remain sensitive to low density gas in 
contrast to the emission lines. 
In Table I, I have summarizec? from York and Cowie the strongest 
available lines together with the expected equivalent widths €or 
a 1018=s cm'2 column density of hot gas at the optimal tempera- 
ture. This value is representative of the hot ISM (e.g. McKee 
and Ostriker 1977). 
It can be seen from Table I that a resolution of at least 
R a 100 is required to discriminate neighboring strong lines. In 
addition, one may simply estimate on the basis of photon statistics 
the required instrument parameters. The necessary effective area 
! 
129 
Table 1 (adaptel from York and Cowie 1981). 
I
I 
' Species 
 
c v  
c VI 
0 IV 
o v  
0 VI1 
0 VI11 
Si VI11 
Line 
Wavelength 
eft, 
40.27,40.73,41.4 
33.7c 
22.86 
22.52 
21.8,21.6 
19.0 
61.0 
'Logarithmic Temp. 
at Which Species 
is Dominant 
Ionization Stage 
5.7 
6.0 
5.4 
5.4 
e o  
6.3 
6.0 
f 
(Oscillator 
Strength) 
0.65 
0 .42  
0.45 
0.62 
0.69 
0.42 
1.2 
A is given in terms of the sourc :;=tensity I by 
3 x 10'' 
1.6x10-' 
I. 6x10' 
3 x 10" 
3 x io-' 
1.6x10-' 
10- 
[p$)lz J (kev/ (cm' s kev) I .  A(cm2)Rt(hrs) = 3 ~ 1 0 ~  n2/ 1 1 
For a source with I = 5 kev cm'2 s-l kev" at 500 eV, such as the Crab 
(Charles e t  a l .  1979), a 3a detection of a line with WE/E = 3x10"' 
in an hour's exposure would require A(cm')R = 6000. 
terms this is a factor of 30 or so over the FPCS detectors aboard 
Einstein (Giacconi e t  a l .  1979). 
In rough 
There are a number of problems with this technique, of course, 
not least that of finding sufficiently string background sources 
with well defined continua in the rieighborhood of the absorption 
lines. in s3me cases (e.g. ScoX1) there may be relatively narrow 
emission lines which will confuse the absorption line studies. 
Synchrotron sources such as the Crab are ideal, of course. 
! 
1 
130 I 
I 
v. SUMMARY 
Clearly this has been an idiosyncratic apd personal view of 
a complex range of topics, but I think two pints are very clear. 
Firstly, a large area imaging instrument could cover a great deal 
of ground with or without a spectroscopic capability. However, 
high resolution spectral capability is highly desirable and would 
greatly enhance the power of such an instrument. Either class 
of instrument would be highly flexible, capable of dealing with 
a wide range of problems and of great general utility. 
ACKNOWLEDGEMENTS 
I would like to thank J. Grindlay and A.C. Fabian, who pointed 
out omissions in the original presentation. This work was supported 
by NASA grants NAGW-208, NSG-7643 and NGL-22-009-638. 
I 
131 i .  
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Interstellar, intracluster and supercluster gas

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