Cooling and Clusters: When Is Heating Needed?

There are (at least) two unsolved problems concerning the current state of the thermal gas in clusters of galaxies. The first is identifying the source of the heating which offsets cooling in the centers of clusters with short cooling times (the ``cooling flow'' problem). The second is understanding the mechanism which boosts the entropy in cluster and group gas. Since both of these problems involve an unknown source of heating it is tempting to identify them with the same process, particular since AGN heating is observed to be operating at some level in a sample of well-observed ``cooling flow'' clusters. Here we show, using numerical simulations of cluster formation, that much of the gas ending up in clusters cools at high redshift and so the heating is also needed at high-redshift, well before the cluster forms. This indicates that the same process operating to solve the cooling flow problem may not also resolve the cluster entropy problem.Comment: 10 pages, 5 figures, published in Philosophical Transactions A (Royal Society

Cosmological Blastwaves and the Intergalactic Medium

Winds from protogalactic starbursts and quasars can drive shocks that heat, ionize, and enrich the intergalactic medium. The Sedov-Taylor solution for point-like explosions adequately describes these blastwaves early in their development, but as the time since the explosion ($t - t_1$) approaches the age of the universe ($t$), cosmological effects begin to alter the blastwave's structure and growth rate. This paper presents an analytical solution for adiabatic blastwaves in an expanding universe, valid when the IGM is homogeneous and contains only a small fraction of the total mass density ($\Omega_{\rm IGM} << \Omega_0$). Using this analytical solution, we examine the role protogalactic explosions might play in determining the state of intergalactic gas at $z \sim 2 - 4$.Comment: 37 pages, 11 Postscript figures, LaTeX aaspp.sty file; to appear in Astrophysical Journal, in press (1996 July 10

On Nulling Interferometers and the Line-Emitting Regions of AGNs

The nulling interferometers proposed to study planets around other stars are generally well suited for studying small-scale structures surrounding other bright pointlike objects such as the nuclei of active galaxies. Conventional interferometric techniques will produce useful maps of optical/IR line and continuum emission within active galaxies on scales of 10 milliarcseconds (mas), but similar studies of broad-line regions will require baselines longer than those currently envisaged. Nevertheless, nulling interferometers currently under development will be able to constrain quasar velocity fields on milliarcsecond scales, as long as they are equipped with spectrographs capable of resolving lines several hundred km/s wide. This Letter describes how analyses of line emission leaking through the edges of the null in such an instrument can reveal the size, shape, and velocity field of nebular gas on the outskirts of a quasar broad-line region. If this technique proves effective, it could potentially be used to measure the mass function of quasar black holes throughout the universe.Comment: Latex, 9 pages, 2 figures, to appear in 1 October ApJ Letter

Confusion of Diffuse Objects in the X-ray Sky

Most of the baryons in the present-day universe are thought to reside in intergalactic space at temperatures of 10^5-10^7 K. X-ray emission from these baryons contributes a modest (~10%) fraction of the ~ 1 keV background whose prominence within the large-scale cosmic web depends on the amount of non-gravitational energy injected into intergalactic space by supernovae and AGNs. Here we show that the virialized regions of groups and clusters cover over a third of the sky, creating a source-confusion problem that may hinder X-ray searches for individual intercluster filaments and contaminate observations of distant groups.Comment: accepted to ApJ Letters, 7 pages, 3 figure

The baseline intracluster entropy profile from gravitational structure formation

The radial entropy profile of the hot gas in clusters of galaxies tends to follow a power law in radius outside of the cluster core. Here we present a simple formula giving both the normalization and slope for the power-law entropy profiles of clusters that form in the absence of non-gravitational processes such as radiative cooling and subsequent feedback. It is based on seventy-one clusters drawn from four separate cosmological simulations, two using smoothed-particle hydrodynamics (SPH) and two using adaptive-mesh refinement (AMR), and can be used as a baseline for assessing the impact of non-gravitational processes on the intracluster medium outside of cluster cores. All the simulations produce clusters with self-similar structure in which the normalization of the entropy profile scales linearly with cluster temperature, and these profiles are in excellent agreement outside of 0.2 r_200. Because the observed entropy profiles of clusters do not scale linearly with temperature, our models confirm that non-gravitational processes are necessary to break the self-similarity seen in the simulations. However, the core entropy levels found by the two codes used here significantly differ, with the AMR code producing nearly twice as much entropy at the centre of a cluster.Comment: Accepted to MNRAS, 8 pages, 9 figure