Chandra Detection of Massive Black Holes in Galactic Cooling Flows

Anticipating forthcoming observations with the Chandra X-ray telescope, we describe the continuation of interstellar cooling flows deep into the cores of elliptical galaxies. Interstellar gas within about r = 50 parsecs from the massive black hole is heated to T > 1 keV and should be visible unless thermal heating is diluted by non-thermal pressure. Since our flows are subsonic near the massive black holes, distributed cooling continues within 300 pc from the center. Dark, low mass stars formed in this region may be responsible for some of the mass attributed to central black holes.Comment: 6 pages with 3 figures; accepted by Astrophysical Journal Letter

Recent X-ray Observations and the Evolution of Hot Gas in Elliptical Galaxies: Evidence for Circumgalactic Gas

X-ray emitting gaseous halos, such as that in elliptical galaxies like NGC 4472, cannot have been produced solely from gas expelled from galactic stars. In traditional models for the evolution of hot interstellar gas (cooling flows) in ellipticals, the galaxies are assumed to have been cleared of gas by SNII-driven winds at some early time then gas is subsequently replenished by mass loss from an evolving population of old stars. To test this, we accurately determine the stellar and dark halo mass of NGC 4472 using hydrostatic equilibrium, then solve the standard time-dependent cooling flow equations to recover the observed hot gas temperature and density distributions when evolved to the present time. This procedure fails: the computed gas density gradient is too steep, the total gas mass is too low, and the gas temperatures are much too low. All variants on this basic procedure also fail: increasing the SNIa rate, using the mass dropout assumption, arbitrarily adjusting uncertain coefficients, etc. However, agreement is achieved if the galaxy is supplied with additional, spatially-extended hot gas early in its evolution. This old ``circumgalactic'' gas can be retained to the present time and may be related to cosmological ``secondary infall''.Comment: 15 pages in two-column AASTEX LaTeX including 1 table and 8 figures; abstract corrected in replacement; accepted by Astrophysical Journa

Cooling Flow Star Formation and the Apparent Stellar Ages of Elliptical Galaxies

Observational constraints and theoretical arguments indicate that cooled interstellar gas in bright elliptical galaxies forms into a young stellar population throughout the region within the half-light radius. The young population has a bottom-heavy, but optically luminous IMF extending to 1 - 2 M_sun. When the colors and spectral features of this young population are combined with those of the underlying old stellar population, the apparent ages are significantly reduced, similar to the relatively young apparent ages observed in many ellipticals. Galactic mergers are not required to resupply young stars. The sensitivity of continuous star formation to L_B and L_x/L_B is likely to account for the observed spread in apparent ages among elliptical galaxies. Local star formation is accompanied by enhanced stellar H_beta equivalent widths, stronger optical emission lines, enhanced thermal X-ray emission and lower apparent temperatures in the hot gas. The young stars should cause M/L to vary with galactic radius, perturbing the fundamental plane occupied by the old stars.Comment: 6 pages with 2 figures; accepted by Astrophysical Journal Letter

Where Do Cooling Flows Cool?

Although only about 5 percent of the total baryonic mass in luminous elliptical galaxies is in the form of cooled interstellar gas, it is concentrated within the optical effective radius r_e where it influences the local dynamical mass. The mass of cooled gas must be spatially distributed since it greatly exceeds the masses of central black holes. We explore here the proposition that a population of low mass, optically dark stars is created from the cooled gas. We consider a wide variety of radial distributions for the interstellar cooling, but only a few are consistent with observed X-ray surface brightness profiles. In a region of concentrated interstellar cooling, the X-ray emission can exceed that observed, suggesting the presence of additional support by magnetic stresses or non-thermal pressure. In general we find that the mass of cooled gas contributes significantly to stellar dynamical mass to light ratios which vary with galactic radius. If the stars formed from cooled interstellar gas are optically luminous, their influence on the the mass to light ratio would be reduced. The mass of cooled gas inside r_e is sensitive to the rate that old stars lose mass, which is nearly independent of the initial mass function of the old stellar population.Comment: 18 pages with 6 figures; accepted by Astrophysical Journa

Why Are Rotating Elliptical Galaxies Less Elliptical at X-ray Frequencies?

If mass and angular momentum were conserved in cooling flows associated with luminous, slowly rotating elliptical galaxies, the inflowing hot gas would spin up, resulting in disks of cold gas and X-ray images that are highly flattened along the equatorial plane out to several effective radii. Such X-ray flattening is not observed at the spatial resolution currently available to X-ray observations. Evidently mass and angular momentum are not in fact conserved. If cooling flows are depleted by localized radiative cooling at numerous sites distributed throughout the flows, then disks of cooled gas do not form and the X-ray images appear nearly circular. However, the distribution of young stars formed from the cooled gas is still somewhat flattened relative to the stellar light. X-ray images of galactic cooling flows can also be circularized by the turbulent diffusion of angular momentum away from the axis of rotation, but the effective viscosity of known processes -- stellar mass loss, supernovae, cooling site evolution, etc. -- is insufficient to appreciably circularize the X-ray images. Radial gradients in the interstellar iron abundance are unaffected by the expected level of interstellar turbulence since these gradients are continuously re-established by Type Ia supernovae.Comment: 17 pages with 6 figures; accepted by Astrophysical Journa

Self-Generated Magnetic Fields in Galactic Cooling Flows

Interstellar magnetic fields in elliptical galaxies are assumed to have their origin in stellar fields that accompany normal mass loss from an evolving population of old stars. The seed fields are amplified by interstellar turbulence driven by stellar mass loss and supernova events. These disordered fields are further amplified by time-dependent compression in the inward moving galactic cooling flow and are expected to dominate near the galactic core. Under favorable circumstances, fields similar in strength to those observed $B \sim 1-10~(r/10~kpc)^{-1.2}\mu$G can be generated solely from these natural galactic processes. In general the interstellar field throughout elliptical galaxies is determined by the outermost regions in the interstellar gas where the turbulent dynamo process can occur. Because of the long hydrodynamic flow times in galactic cooling flows, currently observed magnetic fields may result from periods of intense turbulent field amplification that occurred in the outer galaxy in the distant past. Particularly strong fields in ellipticals may result from ancient galactic mergers or shear turbulence introduced at the boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical Journa

Heated Cooling Flows

In conventional models of galactic and cluster cooling flows widespread cooling (mass dropout) is assumed to avoid accumulation of unacceptably large central masses. However, recent XMM observations have failed to find spectral evidence for locally cooling gas. This has revived the notion that cooling flows are heated by some process such as an intermittent, low-level AGN involving supermassive black holes in the central galaxy. To explore this hypothesis, we consider the gasdynamical consequences of galactic cooling flows heated by many different scenarios without specifying the detailed physics of the heating process. We are unable to find a single acceptable heated flow in reasonable agreement with well observed hot gas temperature and density profiles, even using finely tuned parameters. Idealized flows in which radiative cooling is perfectly balanced by global heating are grossly incompatible with observations. Flows heated by episodic central feedback generate quasi-cyclic changes in the hot gas density profile which are not supported by current observations. Paradoxically, centrally heated (or pressurized) cooling flows experience spontaneous non-linear compressions that result in spatially widespread cooling instabilities. Therefore, spectral evidence for cooling gas is difficult to avoid by central heating.Comment: 17 pages (emulateapj5) with 12 figures and Appendix; accepted by The Astrophysical Journa

Formation of Low Mass Stars in Elliptical Galaxy Cooling Flows

X-ray emission from hot (T = 10^7 K) interstellar gas in massive elliptical galaxies indicates that 10^{10} M_sun has cooled over a Hubble time, but optical and radio evidence for this cold gas is lacking. We provide detailed theoretical support for the hypothesis that this gas has formed into low luminosity stars. Within several kpc of the galactic center, interstellar gas first cools to T = 10^4 K where it is heated by stellar UV and emits the observed diffuse optical line emission. This cooling occurs at a large number (10^6) of isolated sites. After less than a solar mass of gas has accumulated (10^{-6} M_sun/yr) at a typical cooling site, a neutral (HI or H_2) core develops in the HII cloud where gas temperatures drop to T = 15 K and the ionization level (from thermal X-rays) is very low (x = 10^{-6}). We show that the maximum mass of cores that become gravitationally unstable is only about 2 M_sun. No star can exceed this mass. Fragmentation of collapsing cores produces a population of low mass stars with a bottom-heavy IMF and radial orbits. Gravitational collapse and ambipolar diffusion are rapid. The total mass of star-forming (dust-free) HI or H_2 cores in a typical bright elliptical is only 10^6 M_sun, below current observational thresholds.Comment: 23 pages in AASTEX LaTeX with 8 figures; accepted by Astrophysical Journa

Clocking the onset of bilayer coherence in a high-Tc{T}_{c} cuprate

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature TC. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of NdBa2Cu3O6+δ. The pseudogap correlations were disentangled from the response of laser-broken pairs by clocking their characteristic time scales. The onset of a superconducting precursor state was found at a temperature TONS>TC, consistent with the temperature scale identified via static optical spectroscopy. Furthermore, the temperature evolution of the coherent vibration of the Ba ion, strongly renormalized by the onset of superconductivity, revealed a pronounced anomaly at the same temperature TONS. The microscopic nature of such a precursor state is discussed in terms of preformed pairs and enhanced bilayer coherence

Confrontation of Intracluster and Interstellar Gas in Cluster-Centered Elliptical Galaxies: M87 in Virgo and NGC 4874 in Coma

X-ray observations of M87 in the Virgo cluster and NGC 4874 in Coma reveal that the gas temperature beyond about 50 kpc from these cD galaxies is comparable to the virial temperature of the cluster, 3 or 9 keV respectively, but within the optical galaxy the temperature drops to the galactic virial temperature, about 1 keV. We show that these steep thermal (and density) gradients follow naturally from the usual cooling inflow assumptions without recourse to thermal conductivity. To avoid unobserved central mass concentrations, most of the gas must radiatively cool (``dropout'') before it flows to the galactic core, i.e. the gas must be multiphase. Recent XMM spectra of M87 indicate single phase flow at every radius with no apparent radiative cooling to low temperatures. However, the X-ray spectral evidence for multiphase cooling beginning at lower temperatures near 1 keV (within about 10 kpc) may be less apparent and may have escaped detection. Finally, we show that the standard decomposition method used by X-ray observers to determine the mass flow dM/dt(r) may fail rather badly due to small deviations from perfectly steady state flow. When applied to our slightly non-steady computed flows, this decomposition (incorrectly) gives the usual result, dM/dt proportional to r, and overestimates the mass dropout dM/dt(r) at large radii.Comment: 12 pages (emulateapj5) with 10 figures; accepted by The Astrophysical Journa