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Central galaxies in rich clusters are the sites of cluster cooling flows, with large masses of gas cooling through part of the X-ray band. Many of these galaxies host powerful radio sources. These sources can displace and compress the X-ray gas leading to enhanced cooling and star formation. We observed the bright cooling flow Abell 2626 with a strangely distorted central radio source. We wished to understand the interaction of radio and X-ray thermal plasma, and to determine the dynamical nature of this cluster. One aim was to constrain the source of additional pressure in radio "holes" in the X-ray emission needed to support overlying shells of X-ray gas. We also aimed to study the problem of the lack of kT < 1-2 keV gas in cooling flows by searching for abundance inhomogeneities, heating from the radio source, and excess absorption. We also have a Chandra observation of this cluster. There were problems with the pipeline processing of this data due to a telemetry dropout. We are publishing the Chandra and XMM data together. Delays with the Chandra data have slowed up the publication. At the center of the cluster, there is a complex interaction of the odd, Z-shaped radio source, and the X-ray plasma. However, there are no clear radio bubbles. Also, the cluster SO galaxy IC 5337, which is projected 1.5 arcmin west of the cluster center, has unusual tail-like structures in both the radio and X-ray. It appears to be falling into the cluster center. There is a hot, probably shocked region of gas to the southwest, which is apparently due to the merger of a subcluster in this part of the system. There is also a merging subcluster to the northeast. The axes of these two mergers agrees with a supercluster filament structure.

Modern stellar evolutionary tracks are used to calculate the evolution of a very large number of massive binary star systems (M(sub tot) greater than or = 15 solar mass) which cover a wide range of total masses, mass ratios, and starting separations. Each binary is evolved accounting for mass and angular momentum loss through the supernova of the primary to the X-ray binary phase. Using the observed rate of star formation in our Galaxy and the properties of massive binaries, we calculate the expected high-mass X-ray binary (HMXRB) population in the Galaxy. We test various massive binary evolutionary scenarios by comparing the resulting HMXRB predictions with the X-ray observations. A major goal of this study is the determination of the fraction of matter lost from the system during the Roche lobe overflow phase. Curiously, we find that the total numbers of observable HMXRBs are nearly independent of this assumed mass-loss fraction, with any of the values tested here giving acceptable agreement between predicted and observed numbers. However, comparison of the period distribution of our HMXRB models with the observed period distribution does reveal a distinction among the various models. As a result of this comparison, we conclude that approximately 70% of the overflow matter is lost from a massive binary system during mass transfer in the Roche lobe overflow phase. We compare models constructed assuming that all X-ray emission is due to accretion onto the compact object from the donor star's wind with models that incorporate a simplified disk accretion scheme. By comparing the results of these models with observations, we conclude that the formation of disks in HMXRBs must be relatively common. We also calculate the rate of formation of double degenerate binaries, high velocity detached compact objects, and Thorne-Zytkow objects.

Recently Miyaji et al. (1993) detected excess soft X-ray absorption in the Broad Band X Ray Telescope (BBXRT) spectrum of the cluster A2256. The absorption was due to the K-edge of oxygen; from the observed edge energy and the redshift of the cluster, Miyaji et al. argued that the primary absorber was oxygen in an ionization state between O(+2) and O(+4) at a temperature in the range 0.5-5 x 10(exp 5) K. Such gas should produce strong UV and optical line emission. We predict the strongest UV and optical line fluxes for this X-ray absorbing gas. The strongest UV lines include Lyman-alpha, C III lambda-1909, C IV lambda-1549, O IV lambda-1401, O V lambda-1218, O VI lambda-1034, Si III lambda-1207, and Si IV lambda-1397 lines. These lines should be detectable with the International Ultraviolet Explorer, the Hubble Space Telescope, or the Hopkins Ultraviolet Telescope. In the optical the lines are weaker, but the (O II) lambda lambda-3726, 3729, (O III) lambda lambda-4959, 5007, (N II) lambda lambda-6567, 6584, (Ne III) lambda lambda-3918, 3969, (S II) lambda-6720, and (S III) lambda-9320 lines should be detected with moderate exposures if the warm absorber is near the lower limit of the allowed temperature range, T less than or approximately equal to 1.5 x 10(exp 5) K. In this limit the H-alpha and (S III) lambda-3720 lines will require longer exposure times, but should be detectable as well.

We have devised a method to do nonequilibrium ionization calculations on the results of two-dimensional hydrodynamical simulations, based on the algorithm of Hughes & Helfand (1985). We have calculated the ionization structure and X-ray emission for a two-dimensional numerical hydrodynamical simulation for the remnant of Kepler's supernova (SN); the hydrodynamical model was presented in a previous paper. In this model, the progenitor of Kepler's SN is assumed to have been a massive runaway star ejected from the Galactic plane. In its red supergiant stage, its dense stellar wind was distorted and compressed into a bow shock by the ram pressure of the tenuous interstellar medium. The subsequent interaction of the supernova ejecta with this asymmetric circumstellar matter produced a strongly asymmetric supernova remnant (SNR). In this paper, we present calculated X-ray spectra for this hydrodynamical model. A comparison with observations implies a moderate overabundance of Fe in Kepler's SNR (only 50% larger than its cosmic value), in contrast to a large (6 to 15) Fe overabundance derived previously. However, we confirm earlier conclusions that Si and S abundances are 2 to 3 times solar. These modest enhancements of Si, S, and Fe may be attributed either to heavy-element enriched SN ejecta or to the initial chemical abundances of the SN progenitor, which originated in the metal-rich inner Galaxy. The comparison of our models with the observed spectra confirm theoretical predictions that moderate electron heating occurs at strong collisionless shock fronts, with the implied electron/mean temperature ratio of approximately 0.5.

In view of the expected collision of comet Shoemaker-Levy 9 (1993e) with Jupiter in 1994 July, we calculate basic properties of the initial interaction for a simplified Jovian atmosphere. The comet is expected to impact Jupiter at 60 km/sec and at an angle of 45 deg to the zenith. The shock wave generated by the bolide should be optically thick once it has penetrated to an atmospheric density approximately 10(exp -6) gr/cm(exp 3), and we calculate the post-shock conditions assuming local thermodynamic equilibrium (LTE) for shock velocities v(sub sh) in the range 10 to 60 km/sec and preshock densities rho(sub a) = 10(exp -6) to 10(exp -2) gr/cm(exp 3). Our shock calculations include molecular hydrogen, atomic hydrogen, ionized hydrogen, neutral helium, and singly ionized helium. Even at the highest shock velocity, the gas is only partially ionized and the postshock temperature rises with preshock density in order to maintain the ionization. The value of the effective shock adiabatic index gamma(sub sh) varies from 1.17 (at low v(sub sh) and rho(sub a) to 1.40 (at high v(sub sh) and rho(sub a). The ablation rate is limited by the radiative flux that reaches the bolide surface. We argue that the ablated gas does not efficiently transfer its kinetic energy to the atmosphere, and it ultimately slows in a similar fashion to the comet material. As the bolide initially falls through the atmosphere, the character of the shock emission changes. At rho(sub a) approximately 10(exp -8) gr/cm (exp 3), the gas is optically thin and we expect line emission; in the optical spectrum, Balmer emission is expected from the shocked atmosphere and low-ionization metal lines from ablated cometary material. At rho(sub a) approximately 10(exp -6) gr/m(exp 3), the shocked gas is optically thick and the shock front near the bolide produces a blackbody spectrum. The temperature is favorable for ultraviolet (1000 to 3000 A) emission and the luminosity may be approximately 5 x 10(exp 23) ergs/sec for approximately 0.6 sec for a bolide 1 km in radius. At rho(sub a) approximately 10(exp -4) gr/cm(exp 3), the bolide has passed below the ultraviolet photosphere. The shock front emits considerable ionizing radiation, but it is absorbed in a narrow preshock region. The bolometric correction for the optical luminosity is large and we expect a 3000 to 8000 A luminosity of approximately 3 x 10(exp 23) ergs/sec for approximately 1 sec. The optical emission is strongly peaked in the vicinity of the bolide. The bolide does have a somewhat less luminous, optically thick trail extending greater than or equal to 10 km, but the radiation is characterized by a temperature of 4000 to 5000 K. From the fragmentation model of Chyba, Thomas, & Zahnle (1993), the bolide deposits most of its kinetic energy at rho(sub a) approximately 10(exp -3) gr/cm(exp 3) and this is the effective explosion site. The shock wave from such an explosion can move up about one density scale height. We examine the breakout of the shock front from the Jovian atmosphere and find that the shock acceleration in the decreasing density region is slow, so that the energy flux in the shock front is small. Higher velocities might be generated by shock acceleration along the channel left by the bolide if the shock motion can occur before the channel closes off as a result of radiative cooling. Hot gas created by the explosion ultimately rises due to buoyancy on a timescale of a minute. The luminosity is highest when the bubble first rises into the optically thin part of the atmosphere and may be approximately 1 x 10(exp 25) ergs/sec in the near-infrared. Roughly 1% of the initial bolide energy may be radiated in this way; the rest of the energy is lost to sound waves from the initial explosion and to work done by the bubble on the surrounding atmosphere.

Calculations are carried out of optically thin X-ray spectra for a set of theoretical cluster cooling flow models including homogeneous models and flows with strong gas loss. Integrated X-ray properties for the models are presented, together with surface brightness profiles, aperture luminosities, and line profiles for a representative set of X-ray lines. It is found that models with gas loss have lower total X-ray luminosities than homogeneous models by a factor of 1.35 or less.

The correctness of the view that the Fanaroff-Riley type I (F-R I) radio galaxies and the BL Lac objects are intrinsically the same and that the strong emission seen in BL Lac may be the result of beaming of radiation from the galactic nucleus is examined. Expressions are derived for the surface brightness of scattered beams of polarized or unpolarized radiation, and the expected scattered light surface is calculated assuming the beams have spectra and fluxes consistent with those observed for BL Lac objects. The models are compared to recent observations of lobes of excess, polarized blue light in Cy-A, and observations of lobes of UV light in the central regions of the cD galaxies in A1795 and A2597. It was found that the models predict scattered surface brightnesses in reasonable agreement with those observed. If these lobes are indeed scattered light, this would indicate that F-R I radio galaxies do possess beams of radiation with apparent luminosities similar to BL Lac objects and that the radiation is actively beamed and not merely obscured by a torus of dust around the nucleus.

We have calculated the emergent x-ray properties for a set of spherically symmetric, steady-state cluster cooling flow models including the effects of radiative transfer. Opacity due to resonant x-ray lines, photoelectric absorption, and electron scattering have been included in these calculations, and homogeneous and inhomogeneous gas distributions were considered. The effects of photoionization opacity are small for both types of models. In contrast, resonant line optical depths can be quite high in both homogeneous and inhomogeneous models. The presence of turbulence in the gas can significantly lower the line opacity. We find that integrated x-ray spectra for the flow cooling now are only slightly affected by radiative transfer effects. However x-ray line surface brightness profiles can be dramatically affected by radiative transfer. Line profiles are also strongly affected by transfer effects. The combined effects of opacity and inflow cause many of the lines in optically thick models to be asymmetrical.

Observations of Kepler's SNR have revealed a strong interaction with the ambient medium, far in excess of that expected at a distance of about 600 pc away from the Galactic plane where Kepler's SNR is located. This has been interpreted as a result of the interaction of supernova ejecta with the dense circumstellar medium (CSM). Based on the bow-shock model of Bandiera (1985), we study the dynamics of this interaction. The CSM distribution consists of an undisturbed stellar wind of a moving supernova progenitor and a dense shell formed in its interaction with a tenuous interstellar medium. Supernova ejecta drive a blast wave through the stellar wind which splits into the transmitted and reflected shocks upon hitting this bow-shock shell. We identify the transmitted shock with the nonradiative, Balmer-dominated shocks found recently in Kepler's SNR. The transmitted shock most probably penetrated the shell in the vicinity of the stagnation point.

We present a new high-resolution X-ray image of the 2A 0335 + 096 cluster of galaxies obtained with the High Resolution Imager (HRI) aboard the ROSAT satellite. The presence of dense gas having a very short cooling time in the central regions confirms its earlier identification as a cooling flow. The X-ray emission from the central regions of the cooling flow shows a great deal of filamentary structure. Using the crude spectral resolution of the HRI, we show that these filaments are the result of excess emission, rather than foreground X-ray absorption. Although there are uncertainties in the pointing, many of the X-ray features in the cooling flow region correspond to features in H-alpha optical line emission. This suggests that the optical emission line gas has resulted directly from the cooling of X-ray-emitting gas. The filament material cannot be in hydrostatic equilibrium, and it is likely that other forces such as rotation, turbulence, and magnetic fields influence the dynamical state of the gas.

The electron-scattered surface brightness profiles are calculated for a strong central source embedded in a cluster cooling flow, and the profiles are used to discuss the physical state of the gas in the flow and the behavior of the central source. The effects of temporal variations in the luminosity of the central source are calculated for a burst model and a source whose luminosity increases linearly with time. If the emission from the central AGN is polarized, the surface brightness profiles have cusps along the direction of the projection of the normal to the polarization vector on the sky. The 2D surface brightness profiles produced by beamed radiation are calculated.

High-resolution X-ray observations of the cluster A2029 are presented which confirm the presence of a cooling flow, despite the lack of optical line emission or evidence for recent star formation. The cooling rate and radius are about 370 solar mass/yr and 230 kpc, respectively. Emission from the inner cooling flow is dominated by a number of X-ray-emitting filaments. This may be the first case where such inhomogeneities are clearly resolved. The filaments are theorized to be supported in part by magnetic fields and may be connected with the filaments of very strong Faraday rotation seen in several nearly cooling flows.

Luminosities, surface brightnesses, and line profiles of the coronal lines predicted by standard models for cooling flows in clusters of galaxies and in individual early-type galaxies are presented. The coronal line luminosities of cooling flows depend primarily on the total rate at which gas is cooling. In homogeneous cooling flow models (HMs) in which all of the gas flows into the central regions, the coronal lines are very centrally concentrated, coming mainly from a limb-brightened shell lying just within the sonic radius of the inflow. For HMs, the central surface brightnesses of the stronger coronal lines from nearby clusters with large cooling rates are expected to be about 6 x 10 to the -17th ergs/sq cm/s/sq arcsec. For inhomogeneous models (IMs) in which gas cools out of the flow at large radii, the surface brightness profiles of the coronal lines are more extended, and the central surface brightness is much lower. The integrated line profiles of the HMs are very wide and flat and the central line profiles are double-peaked. The IMs have nearly Gaussian line profiles.

It is suggested that nonlinear perturbations which lead to thermal instabilities in cooling flows might start as blobs of interstellar gas which are stipped out of cluster galaxies. Assuming that most of the gas produced by stellar mass loss in cluster galaxies is stripped from the galaxies, the total rate of such stripping is roughly 100 solar masses/yr, which is similar to the rates of cooling in cluster cooling flows. It is possible that a substantial portion of the cooling gas originates as blobs of interstellar gas stripped from galaxies. The magnetic fields within and outside of the low-entropy perturbations may help to maintain their identities by suppressing both thermal conduction and Kelvin-Helmholtz instabilities. These density fluctuations may disrupt the propagation of radio jets through the intracluster gas, which may be one mechanism for producing wideangle-tail radio galaxies.

Possible causes for the large dispersion in the X-ray luminosities (Lx) of early-type galaxies are discussed. It is shown that early-type galaxies with relatively low values of Lx for a given optical luminosity L(B) have about 50 percent more neighbors than galaxies with relatively high Lx, suggesting that low-Lx galaxies tend to be in denser environments. It is concluded that the large dispersion in X-ray luminosity, for a given L(B) might be caused either by ram-pressure stripping of the gaseous halos or by mergers.

A detailed model was developed for Kepler's supernova remnant (SNR). Observations of the SNR revealed a strong interaction with the surrounding circumstellar medium, which was studied through both analytical and numerical calculations. Effects were studied of electron thermal conduction on the structure of radiative interstellar shock waves. An explanation is sought for the observed line emission from metal rich ejecta in SNR, incorporating atomic data. Light echoes around SN 1987A was also studied. Analysis of infrared and scattered circumstellar light echoes was accomplished with early observations to set limits on the mass of circumstellar dust. Work was completed on the emission from heavy element gas ejected in the supernova explosion of massive stars. It was assumed that a radioactive energy source was present and calculated the detailed heating and ionization of the gas. The evolution was studied of SNR in the very high pressure environment of a starburst galaxy.

A calculation has been made of the emission from gas at temperature of about 10 to the 6th K due to optical coronal lines that result from fine structure transitions within the ground states of highly ionized atoms. These lines include the Forbidden Fe X 6374-A line, the Forbidden Fe XIV 5303-A line, and many other lines from stages of ionization of S, Ca, Fe, and Ni. The calculations are valid in the limit of very low density, and apply to the diffuse gas found in supernova remnants, the interstellar medium in galaxies, and intergalactic gas. It is found that the amount of optical coronal emission produced by a given amount of cooling gas is largely unaffected by changes in element abundances (provided the proportions of heavy elements remain the same).

Cluster cooling flows have electron-scattering optical depths which are estimated to be about 0.01. Many of the cluster-dominant galaxies centered in cluster cooling flows have active nuclei, and about 1 percent of the luminosity from the active nuclei should appear as diffuse, scattered radiation. It is shown here that detection of this scattered radiation at radio wavelengths can provide a measure of the distance to the cluster that is independent of the Hubble constant. Observations of the scattered surface brightness profile can also be used to test for clumpiness in the gas, for variations in the central radio source on time scales less than 100,000 yr, and for the degree of beaming of radiation from the central source.

The interaction of a moving planetary nebula (PN) with the interstellar medium is considered. The PN shell is compressed first in the direction of the stellar motion. This produces a dipole asymmetry in the surface brightness of the nebula, typically at a nebular density of about 40/cu cm if the nebula is located in the Galactic plane. In the later stages of the interaction, this part of the shell is significantly decelerated with respect to the central star, and the PN becomes strongly asymmetric in shape. This distortion and the subsequent stripping of the nebular gas away from the central star typically occurs at a low nebular density of about 6/cu cm. The morphology of PNs with central stars whose proper motions exceed 0.015 arcsec/yr was examined, and it was found that many of the extended nebulae are interacting with the interstellar medium (ISM). The sample doubles the number of known PNs interacting with the ISM. The morphology of nearby PNs was examined, and a number of strongly asymmetric nebuale were found.

The x ray luminosities of early-type galaxies are correlated with their optical (e.g., blue) luminosities (L sub X approx. L sub B exp 1.6), but the x ray luminosities exhibit considerable scatter for a given optical luminosity L sub B. This dispersion in x ray luminosity is much greater than the dispersion of other properties of early-type galaxies (for a given L sub B), such as luminosity scale-length, velocity dispersion, color, and metallicity. Here, researchers consider several possible sources for the dispersion in x ray luminosity. Some of the scatter in x ray luminosity may result from stellar population variations between galaxies with similar L sub B. Since the x ray emitting gas is from accumulated stellar mass loss, the L sub X dispersion may be due to variations in integrated stellar mass loss rates. Another possible cause of the L sub X dispersion may be variations in the amount of cool material in the galaxies; cool gas may act as an energy sink for the hot gas. Infrared emission may be used to trace such cool material, so researchers look for a correlation between the infrared emission and the x ray emission of early-type galaxies at fixed L sub B. Velocity dispersion variations between galaxies of similar L sub B may also contribute to the L sub X dispersion. The most likely a priori source of the dispersion in L sub X is probably the varying amount of ram-pressure stripping in a range of galaxy environments. The hot gaseous halos of early-type galaxies can be stripped in encounters with other galaxies or with ambient cluster gas if the intracluster gas is sufficiently dense. Researchers find that the most likely cause of dispersion in the x ray properties of early type galaxies is probably the ram-pressure stripping of gaseous halos from galaxies. For a sample of 81 early-type galaxies with x ray luminosities or upper limits derived from Einstein Observatory observations (CFT) researchers calculated the cumulative distribution of angular distances between the x ray sample members and bright galaxies from the Revised Shapley - Ames catalog. Collectively, galaxies with low x ray luminosities (for a given L sub B) tend to be in denser environments than galaxies with higher x ray luminosities.

Analyses of the x ray surface brightness profiles of cluster cooling flows suggest that the mass flow rate decreases towards the center of the cluster. It is often suggested that this decrease results from thermal instabilities, in which denser blobs of gas cool rapidly and drop below x ray emitting temperatures. If the seeds for the thermal instabilities are entropy perturbations, these perturbations must enter the flow already in the nonlinear regime. Otherwise, the blobs would take too long to cool. Here, researchers suggest that such nonlinear perturbations might start as blobs of interstellar gas which are stripped out of cluster galaxies. Assuming that most of the gas produced by stellar mass loss in cluster galaxies is stripped from the galaxies, the total rate of such stripping is roughly M sub Interstellar Matter (ISM) approx. 100 solar mass yr(-1). It is interesting that the typical rates of cooling in cluster cooling flows are M sub cool approx. 100 solar mass yr(-1). Thus, it is possible that a substantial portion of the cooling gas originates as blobs of interstellar gas stripped from galaxies. The magnetic fields within and outside of the low entropy perturbations can help to maintain their identities, both by suppressing thermal conduction and through the dynamical effects of magnetic tension. One significant question concerning this scenario is: Why are cooling flows seen only in a fraction of clusters, although one would expect gas stripping to be very common. It may be that the density perturbations only survive and cool efficiently in clusters with a very high intracluster gas density and with the focusing effect of a central dominant galaxy. Inhomogeneities in the intracluster medium caused by the stripping of interstellar gas from galaxies can have a number of other effects on clusters. For example, these density fluctuations may disrupt the propagation of radio jets through the intracluster gas, and this may be one mechanism for producing Wide-Angle-Tail radio galaxies.

X-ray line observations of clusters of galaxies have shown that the X-ray emission in clusters is mainly thermal emission from hot diffuse gas, and that much of this gas has come out of stars, probably having been ejected from galaxies in the cluster. Future high resolution observations should allow us to determine the physical state of the gas. X-ray line measurements and abundance determinations can lead to strong constraints on the origin of the intracluster gas, and on the chemical evolution and history of galaxies. Some of the stronger resonant X-ray lines may be observable as absorption lines against a background quasar. Such X-ray absorption line measurement can be used to directly derive distances to clusters, using a technique similar to (and possibly complementary to) that the well-known method using the Zel'dovich-Syunyaev effect.

An investigation is made of the dynamical effects of the intracluster magnetic field, whose radial inflow and shear can produce a dramatic increase in the field's strength while rendering it more radial, with cooling flows. It is found that field reconnection is the most likely dominant-loss mechanism, so that buoyancy effects are probably not important. Attention is given to the effect of the magnetic field on thermal instabilities. The most important observable effect of the magnetic field in cooling flows will probably be very strong Faraday rotation of the polarization of radio sources within or behind the cooling flow.

The X-ray properties of normal early-type galaxies and the limited theoretical understanding of the physics of the hot interstellar medium in these galaxies are reviewed. A number of simple arguments about the physical state of the gas are given. Steady-state cooling flow models for these galaxies are presented, and their time-dependent evolution is discussed. The X-ray emission found in early-type galaxies indicates that they contain significant amounts of hot interstellar gas, and that they are not the gas-poor systems they were previously thought to be. In the brighter X-ray galaxies, the amounts of hot gas observed are consistent with those expected given the present rates of stellar mass loss. The required rates of heating of the gas are consistent with those expected from the motions of gas-losing stars and supernovae. The X-ray observations are generally more consistent with a lower rate of Type I supernovae than was previously thought.

A grid of cooling flow models for the hot gas in normal elliptical galaxies is calculated, including the loss of gas due to inhomogeneous cooling. The loss process is modeled as a distributed sink for the gas with the rate of loss being proportional to the local cooling rate. The cooling flow models with gas loss have smaller sonic radii, smaller inflow rates in their central regions, lower densities, and higher temperatures than homogeneous models. The reduction in the amount of hot gas flowing into the center of the models brings the models into much better agreement with the observed X-ray surface brightness profiles of elliptical galaxies. However, there is a large dispersion in the observed X-ray luminosities of ellipticals, and this cannot be explained by variations in the efficiency of gas loss. The gas-loss models have X-ray surface brightness profiles which are much less centrally peaked than the no-gas-loss models.

The X-ray line emission excited by charge transfer between neutral hydrogen and the hydrogenic or fully stripped ions of heavy elements has been computed for a grid of nonequilibrium ionization models for supernova remnants. The effects of charge transfer excitation on the X-ray spectra of supernova remnants are discussed. The charge transfer process depends very sensitively upon the abundance of neutral hydrogen behind the shock. Consequently, a careful treatment of the ionization of hydrogen was required. The model line fluxes are compared with those excited by conventional collisional processes involving electrons. Results are presented in the form of contour plots for both the flux excited by charge transfer and the ratio of the charge transfer flux to that associated solely with collisional processes involving electrons. Charge transfer never contributes more than 10 percent of the flux of the strongest electron collisionally excited lines, and its contribution is typically 0.001 to 10 to the -5th for a shock moving into a medium composed of neutral hydrogen.

The stronger resonant X-ray lines from the intracluster gas should be observable as absorption lines in the spectrum of an X-ray emitting background quasar. The equivalent widths of the strongest absorption lines in a set of model clusters with typical parameters are calculated, and the strongest lines are found to have equivalent widths of several eV. By comparing the equivalent width of such a line and the flux from the same line seen in emission, the distance to the cluster can be directly determined, independent of the standard extragalactic distance scale. A method of analyzing the X-ray absorption and emission measurements is presented, and the effects of resonant scattering on the distances are discussed.

X-ray observations indicate that M87/Virgo and NGC 1275/Perseus have cooling flows that are associated with accretion rates of 20-30 and 300-500 solar masses/year, respectively. An assessment is made as to whether star formation is necessarily occurring in these cooling flows by calculating constant mass-flux models for all reasonable parameter space. No constant-mass-flux models that are consistent with all of the relevant observations are found; hence, it is concluded that mass is dropping out of these cooling flows.

Steady state, spherical, homogeneous cooling flow models are compared to the observed X-ray luminosities, spectra, and surface-brightness profiles of elliptical galaxies. Galaxies without heavy halos and SNe have temperatures which are too low and X-ray surface brightness profiles which are too rapidly decreasing with radius to fit the observations. Galaxies with heavy halos and SNe have luminosities which are too large except for the highest luminosity ellipticals. Galaxies without heavy halos but with SNe have temperatures which are probably too cool in the outer parts. Galaxies with heavy halos but without SNe, where the gas is heated by the motions of gas-losing stars and by infall and compression in a heavy halo potential, are favored. This suggests that elliptical galaxies have heavy halos of optically dark material.

Three-dimensional numerical hydrodynamic simulations are used to study the bending of radio jets. The simulations are compared with observations of jets in narrow-angle-tail radio sources. Two mechanisms for the observed bending are considered: direct bending of quasi-continuous jets by ram pressure from intergalactic gas and bending by pressure gradients in the interstellar gas of the host galaxy, the pressure gradients themselves being the result of ram pressure by intergalactic gas. It is shown that the pressure gradients are much less effective in bending jets, implying that the jets have roughly 30 times lower momentum fluxes if they are bent by this mechanism. Ram-pressure bending produces jets with 'kidney-shaped' cross sections; when observed from the side, these jets appear to have diffuse extensions on the downstream side. On the other hand, pressure-gradient bending causes the jets to be densest near their upstream side.

This paper considers what happens to radio jets which, propagating out from the nucleus of an elliptical galaxy, encounters a point at which the ambient gas pressure increases very rapidly. The pressure variation within the sonic radius of a cooling flow is calculated, and a semianalytical calculation of the stability of a jet which encounters a rapidly increasing pressure gradient is given. The results of numerical hydrodynamic simulations which test the results of the analytic calculations are shown. The case where the jet encounters the sonic region at an oblique angle is considered, and it is suggested that this may provide an explanation for jets which are bent on small scales. These results are applied to radio sources associated with central dominant cluster galaxies and with more normal elliptical galaxies.

Recent results on clusters are reviewed and discussed in relation to cooling flows. Mergers of subclusters are apparently common in clusters. Such mergers will tend to disrupt cooling flows. The recently observed luminous arcs in clusters may be shocks generated by a subcluster merger in a cluster with a cooling flow. The peaked galaxy distributions and radial orbits suggested by recent studies of clusters will lead to a larger number of rapidly moving galaxies passing through the cooling flow region, and this may affect the cooling flow. Various ways in which the cooling flow environment (which appears to be ubiquitous in the early-type galactic systems which form the hosts for strong radio sources) may affect the origin and structure of the radio emitting regions are discussed.

An extensive grid of steady state cooling flow models for the hot gas in elliptical galaxies is calculated. The effects of allowing inflow of surrounding gas and varying the supernova heating rate, the mass distribution of the galaxy, the rate of stellar mass loss, and the outer boundary pressure are studied. The X-ray luminosities, spectra, and surface brightness profiles of the galaxies are computed. In all of the models, the gas density varies approximately as r exp -3/2. In the models with heavy halos, the X-ray and optical surface brightnesses are nearly proportional to one another within individual galaxies, and the temperature profiles are roughly isothermal.

Many X-ray clusters of galaxies are observed to have cooling flows at their centers. Each of these cooling flows is depositing mass onto a central dominant galaxy at a rate of 10-400 solar masses/yr. With such large accretion rates it seems possible that these accreting galaxies are still being formed through ongoing star formation in their associated cooling flows. In this paper techniques are developed to determine directly the distributions of local star formation rate, mass, gas density, temperature, and velocity from cooling flow X-ray surface brightness data. These techniques take account of the potentially important X-ray emission from star-forming cooling condensations dropping out of the background flow. Surface brightness data with either good or poor energy resolution are considered separately.

A grid of numerical models is used here to study how accretion flows are affected by star formation. The observables associated with these models are investigated, presenting their X-ray surface brightness profiles. An X-ray diagnostic is searched for that, in the absence of spatially resolved X-ray spectral data, can be an unambiguous indicator for ongoing star formation in cooling flows. The spatial distributions of newly formed stars are presented and compared to profiles of the accreting galaxy in each model. Whether the accretion population can form the luminous parts of the accreting galaxy or whether it forms a dark component is considered.

The basic equations for X-ray cluster cooling flows, including the effects of star formation, are derived. It is confirmed that cooling flows are thermally unstable, and it is found that the fastest-growing linear perturbations in the flow are radial, comoving, and isobaric. A local approximation for the star formation rate is developed which allows analytic solutions to be found for both isobaric and gravity-dominated cooling flows. These solutions are used to show how star formation reduces the gas density, thereby lowering the cooling rate, which raises the gas temperature relative to the non-star-forming case. The analytic solutions are also used to estimate the spatial distribution of newly formed stars. It is found that star formation must proceed at a nearly maximal rate if isothermal mass profiles are to result.

The present data on the orientations of the tails with respect to the cluster centers of a sample of 70 narrow-angle-tail (NAT) radio sources in Abell clusters show the distribution of tail angles to be inconsistent with purely radial or circular orbits in all the samples, while being consistent with isotropic orbits in (1) the whole sample, (2) the sample of NATs far from the cluster center, and (3) the samples of morphologically regular Abell clusters. Evidence for very radial orbits is found, however, in the sample of NATs near the cluster center. If these results can be generalized to all cluster galaxies, then the presence of radial orbits near the center of Abell clusters suggests that violent relaxation may not have been fully effective even within the cores of the regular clusters.

Recently, X-ray observations have shown that elliptical galaxies generally contain large quantities of hot gas. Central dominant cluster ellipticals have even more gas, which they have accreted from the surrounding clusters. The mass distributions in these galaxies can be derived from the condition of hydrostatic equilibrium. M87, the best studied central dominant galaxy, has a massive, dark halo with a total mass of about 4 x 10 to the 12th solar masses within a radius of 300 kpc. The total mass-to-light ratio within this radius is at least 150 solar mass/solar luminosity. The X-ray observations of normal ellipticals also strongly suggest that they have heavy halos, although the distribution of the mass is much less certain than in M87.

X-ray measurements provide an excellent method to determine the amount and distribution of the dark matter in clusters. Unfortunately, accurate temperature profiles, necessary to this method, are currently not available. However, if the intracluster gas is assumed to have a monotonically decreasing temperature, one finds that the dark matter is strongly concentrated to the cluster center, and has a mass which only exceeds the known baryonic mass by a factor of about three. On a second topic, cooling flows are shown to be a very common feature of cluster central and normal elliptical galaxies. The cooling gas is probably ultimately converted into low mass stars.

Published observational data on the tail orientations (TOs) of 60 narrow-angle-tail (NAT) radio sources in Abell clusters of galaxies are analyzed statistically using a maximum-likelihood approach. The results are presented in a table, and it is found that the observed TO distributions in the whole sample and in subsamples of morphologically regular NATs and NATs with pericentric distances d greater than 500 kpc are consistent with isotropic orbits, whereas the TOs for NATs with d less than 500 kpc are consistent with highly radial orbits. If radial orbits were observed near the centers of other types of cluster galaxies as well, it could be inferred that violent relaxation during cluster formation was incomplete, and that clusters form by spherical collapse and secondary infall, as proposed by Gunn (1977).

The X-ray properties of clusters of galaxies and elliptical galaxies are reviewed, with an emphasis on cooling flows and their effects on the structure and energetics of radio sources. It is found that the radial profiles of the hot (10 to the 8th K) diffuse (n = about 0.001/cu cm) gas in clusters do not require heating of the intracluster medium after the formation of the cluster. Consideration is given to theoretical formulas for cooling rates, the possible reasons for the observed correlation between cooling flows and the presence of a dominant central galaxy, cooling-flow models of elliptical galaxies, and the significant shear in the velocity field of a cooling flow.