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; Figure 34 below). These include the K lines of the common elements lighter than iron, such as C, N, O, Ne, Mg, Si, S, Ar, and Ca, as well as the L lines of Fe and Ni. Although the Fe L line complex was tentatively identified in proportional counter spectra of the M87/Virgo cluster by Fabricant et al. (1978) and Lea et al. (1979), the capability for detecting these lines was greatly increased with the launch of the Einstein X-ray observatory with its moderate resolution Solid State Spectrometer (SSS) and its high resolution Focal Plane Crystal Spectrometer (FPCS); see Giacconi et al. (1979) for a description of the satellite and its capabilities.

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 T_g 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 dT_g being roughly proportional to dT_g. 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!