4.3.3. Lower energy lines
In addition to the 7 keV Fe line complex, the X-ray spectrum of a solar
abundance low density plasma contains a large number of lower energy lines
(Sarazin and Bahcall,
1977;
The SSS has detected the K-lines from Mg, Si, and S and the L-lines from Fe
in the spectra of M87/Virgo, Perseus, A496, and A576
(Mushotzky, 1980,
1984;
Mushotzky et al.,
1981;
Lea et al., 1982;
Nulsen et al.,
1982;
Rothenflug et
al., 1984).
Figure 14 shows the SSS spectrum from Virgo. In general, line emission
from both the helium-like and hydrogenic ions of Si and S is seen,
indicating that
the emission occurs at relatively low temperatures Tg
2 ×
107 K. The
observations in M87/Virgo are consistent with nearly solar abundances of
Si, S, and Mg, while the observation in A576 may require lower abundances.
Observations of SSS spectra away from the center of M87/Virgo show that the
heavy element abundances are roughly constant throughout the gas
(Lea et al.,
1982).
Figure 14. The moderate resolution X-ray
spectrum of the M87/Virgo cluster
taken with the Solid State Spectrometer on the Einstein satellite
(Lea et al.,
1982).
The spectral lines of Mg, Si, S, and the lower energy (L-shell)
lines of Fe are marked.
In Perseus, the SSS observations of the Fe L lines
imply that the iron abundance is about one-half of the solar value
(Mushotzky et
al., 1981),
which agrees with the abundance derived from the 7 keV Fe line in
proportional counter
spectra. However, the SSS observations were made with a very small
( 6
arc min) aperture centered on the cluster center, while the proportional
counter observations determine the spectrum of the entire cluster. The
approximate agreement of the two abundances suggests that the iron is well
mixed throughout the cluster, and not just concentrated in the cluster
core.
In general, the SSS spectral line observations seem to imply that many
clusters contain cooler gas than was required to explain their continuum
spectra. Because the SSS has a small field of view and was centered on the
cluster center
in these observations, this cool gas must be concentrated at the center
of the cluster; in the case of the Virgo and Perseus clusters, the center
coincides with
the central dominant galaxies M87 and NGC1275. In Perseus, this cool gas has a
cooling time (see Section 5.3.1) of
less than 2 × 109 yr, which is considerably
less than the probable age of the cluster. It therefore seems likely that
the cool gas observed is part of a steady-state cooling flow (see
Section 5.7 for a discussion of the theory
of such flows). From the observed line intensities,
Mushotzky et al.
(1981)
determined that about 300
M per
year of gas must currently be cooling onto NGC1275 in the Perseus cluster,
and Nulsen et al.
(1982)
found that about 200
M per
year must be accreting onto the cD in A496. These rates assume that the
gas is not being heated.
The FPCS has also provided strong evidence for the cooling and accretion of
gas onto M87 in the Virgo cluster and NGC1275 in the Perseus cluster
(Canizares et
al., 1979,
1982;
Canizares, 1981).
In M87, the FPCS has detected
the O+7 K
line, as well as blends of the Fe+(16-23) L lines and the
Ne+9 K
line. Figure 15 shows the FPCS detection of the
O+7
K line in M87. The ratio
of the abundance of oxygen to iron is apparently 3 - 5 times higher than
the solar ratio. The relative strengths of the various Fe L line blends
cannot result from
gas at any single temperature. Apparently, a range of temperatures is
necessary, with the X-ray luminosity originating from gas in any range of
temperature dTg being roughly proportional to
dTg. This is just what is
predicted if the cool gas results from the cooling and accretion of
hotter gas onto the center of M87
(Cowie, 1981).
Canizares et al.
(1979,
1982)
and Canizares (1981)
show that the spectra are consistent with radiative
accretion at a rate of
3 - 10M
per year. Similar results were found
for the Perseus cluster, except that the required accretion
rate is very large 300
M per
year (Canizares, 1981),
in agreement with the results
from the SSS. The SSS spectra of about a half dozen other clusters also
show evidence for such accretion flow, with rates between those of the
M87/Virgo cluster and the Perseus cluster
(Fabian et al.,
1981b;
Mushotzky, 1984).
Figure 15. The very high resolution X-ray
spectrum of the M87/Virgo cluster, showing the O VIII K line, from
Canizares et al.
(1979)
using the Focal Plane Crystal Spectrometer on the Einstein
satellite.
Thus the two primary observational results of the Einstein
spectrometers are these: first, the intracluster gas contains the heavy
elements oxygen, magnesium,
silicon, and sulfur, as well as iron; second, gas is cooling and being
accreted onto
central dominant galaxies in many clusters at rather high rates (3 - 400
M / yr).
Such accretion had been predicted by
Cowie and Binney (1977),
Fabian and Nulsen
(1977),
and Mathews and Bregman
(1978);
models for these cooling flows are discussed in
Section 5.7. The rates of cooling are so high
that if they have persisted for the age of the cluster, the entire mass
of the inner portions of the central dominant galaxies might be due to
accretion. Models for central dominant galaxies based on this idea have been
given by
Fabian et al.
(1982a)
and Sarazin and
O'Connell (1983),
who argue that the majority of the accreted gas is converted into low mass
stars.
X-ray line observations have established that the primary emission
mechanism of X-ray clusters is thermal emission from hot, diffuse
intracluster gas. They have also shown that at least part of that gas
has been ejected from stars and presumably from galaxies. Apparently,
some of this intracluster gas is
now completing the cycle and returning to the central galaxies, and possibly
being formed into stars!