The X-ray Evolution of Merging Galaxies

We present here the first study of the X-ray properties of an evolutionary sample of merging galaxies. Both ROSAT PSPC and HRI data are presented for a sample of eight interacting galaxy systems, each believed to involve a similar encounter between two spiral discs of approximately equal size. The mergers span a large range in age, from completely detached to fully merged systems. A great deal of interesting X-ray structure is seen, and the X-ray properties of each individual system are discussed in detail. Along the merging sequence, several trends are evident: in the case of several of the infrared bright systems, the diffuse emission is very extended, and appears to arise from material ejected from the galaxies. The onset of this process seems to occur very soon after the galaxies first encounter one another, and these ejections soon evolve into distorted flows. More massive extensions (perhaps involving up to 1e10 solar masses of hot gas) are seen at the `ultraluminous' peak of the interaction, as the galactic nuclei coalesce. The amplitude of the evolution of the X-ray emission through a merger is markedly different from that of the infrared and radio emission however, and this, we believe, may well be linked with the large extensions of hot gas observed. The late, relaxed remnants, appear relatively devoid of gas, and possess an X-ray halo very different from that of typical ellipticals, a problem for the `merger hypothesis', whereby the merger of two disc galaxies results in an elliptical galaxy. However, these systems are still relatively young in terms of total merger lifetime, and they may still have a few Gyr of evolution to go through, before they resemble typical elliptical galaxies.Comment: 30 pages, 15 figures, accepted by MNRA

The X-ray properties of the merging galaxy pair NGC 4038/9 - the Antennae

We report the results of an X-ray spectral imaging observation of the Antennae with the ROSAT PSPC. 55% of the soft X-ray flux from the system is resolved into discrete sources, including components identified with the galactic nuclei and large HII regions, whilst the remainder appears to be predominantly genuinely diffuse emission from gas at a temperature ~4x10^6 K. The morphology of the emission is unusual, combining a halo which envelopes the galactic discs, with what appears to be a distorted, but well-collimated bipolar outflow. We derive physical parameters for the hot gas in both diffuse components, which are of some interest, given that the Antennae probably represents an elliptical galaxy in the making.Comment: 15 pages plus 9 figures, uuencoded encapsulated postscript file. Accepted for publication in MNRA

The X-ray Evolution of Merging Galaxies

From a Chandra survey of nine interacting galaxy systems the evolution of X-ray emission during the merger process has been investigated. From comparing Lx/Lk and Lfir/Lb it is found that the X-ray luminosity peaks around 300 Myr before nuclear coalescence, even though we know that rapid and increasing star formation is still taking place at this time. It is likely that this drop in X-ray luminosity is a consequence of outflows breaking out of the galactic discs of these systems. At a time around 1 Gyr after coalescence, the merger-remnants in our sample are X-ray dim when compared to typical X-ray luminosities of mature elliptical galaxies. However, we do see evidence that these systems will start to resemble typical elliptical galaxies at a greater dynamical age, given the properties of the 3 Gyr system within our sample, indicating that halo regeneration will take place within low Lx merger-remnants.Comment: 4 pages, 1 figure, to appear in the Proceedings of the IAU Symposium No. 23

Fractionalization and confinement in the U(1) and Z2Z_2 gauge theories of strongly correlated systems

Recently, we have elucidated the physics of electron fractionalization in strongly interacting electron systems using a $Z_2$ gauge theory formulation. Here we discuss the connection with the earlier U(1) gauge theory approaches based on the slave boson mean field theory. In particular, we identify the relationship between the holons and Spinons of the slave-boson theory and the true physical excitations of the fractionalized phases that are readily described in the $Z_2$ approach.Comment: 4 page

Thermal metal in network models of a disordered two-dimensional superconductor

We study the universality class for localization which arises from models of non-interacting quasiparticles in disordered superconductors that have neither time-reversal nor spin-rotation symmetries. Two-dimensional systems in this category, which is known as class D, can display phases with three different types of quasiparticle dynamics: metallic, localized, or with a quantized (thermal) Hall conductance. Correspondingly, they can show a variety of delocalization transitions. We illustrate this behavior by investigating numerically the phase diagrams of network models with the appropriate symmetry, and for the first time show the appearance of the metallic phase.Comment: 5 pages, 3 figure

Density of quasiparticle states for a two-dimensional disordered system: Metallic, insulating, and critical behavior in the class D thermal quantum Hall effect

We investigate numerically the quasiparticle density of states $\varrho(E)$ for a two-dimensional, disordered superconductor in which both time-reversal and spin-rotation symmetry are broken. As a generic single-particle description of this class of systems (symmetry class D), we use the Cho-Fisher version of the network model. This has three phases: a thermal insulator, a thermal metal, and a quantized thermal Hall conductor. In the thermal metal we find a logarithmic divergence in $\varrho(E)$ as $E\to 0$, as predicted from sigma model calculations. Finite size effects lead to superimposed oscillations, as expected from random matrix theory. In the thermal insulator and quantized thermal Hall conductor, we find that $\varrho(E)$ is finite at E=0. At the plateau transition between these phases, $\varrho(E)$ decreases towards zero as $|E|$ is reduced, in line with the result $\varrho(E) \sim |E|\ln(1/|E|)$ derived from calculations for Dirac fermions with random mass.Comment: 8 pages, 8 figures, published versio

Stochastic polarization formation in exciton-polariton Bose-Einstein condensates

We demonstrate theoretically the spontaneous formation of a stochastic polarization in exciton-polariton Bose-Einstein condensates in planar microcavities under pulsed excitation. Below the threshold pumping intensity (dependent on the polariton life-time) the average polarization degree is close to zero, whilst above threshold the condensate acquires a polarization described by a (pseudospin) vector with random orientation, in general. We establish the link between second order coherence of the polariton condensate and the distribution function of its polarization. We examine also the mechanisms of polarization dephasing and relaxation.Comment: 4 pages, 3 figure

Scaling and Crossover Functions for the Conductance in the Directed Network Model of Edge States

We consider the directed network (DN) of edge states on the surface of a cylinder of length L and circumference C. By mapping it to a ferromagnetic superspin chain, and using a scaling analysis, we show its equivalence to a one-dimensional supersymmetric nonlinear sigma model in the scaling limit, for any value of the ratio L/C, except for short systems where L is less than of order C^{1/2}. For the sigma model, the universal crossover functions for the conductance and its variance have been determined previously. We also show that the DN model can be mapped directly onto the random matrix (Fokker-Planck) approach to disordered quasi-one-dimensional wires, which implies that the entire distribution of the conductance is the same as in the latter system, for any value of L/C in the same scaling limit. The results of Chalker and Dohmen are explained quantitatively.Comment: 10 pages, REVTeX, 2 eps figure

X-ray Constraints on Accretion and Starburst Processes in Galactic Nuclei I. Spectral Results

The results of a 0.4-10.0 keV ASCA spectral analysis of a sample of low-luminosity AGN (LLAGN; M51, NGC 3147, NGC 4258), low-ionization nuclear emission line regions (LINERs; NGC 3079, NGC 3310, NGC 3998, NGC 4579, NGC 4594) and starburst galaxies (M82, NGC 253, NGC 3628 and NGC 6946) are presented. In spite of the heterogeneous optical classifications of these galaxies, the X-ray spectra are fit well by a ``canonical'' model consisting of an optically-thin Raymond-Smith plasma ``soft'' component with T ~ 7 x 10^6 K and a ``hard'' component that can be modeled by either a power-law with a photon index ~ 1.7 or a thermal bremsstrahlung with T ~ 6 x 10^7 K. The soft-component 0.4-10 keV instrinsic luminosities tend to be on the order 10^39-40 ergs/s while the hard-component luminosities tend to be on the order of 10^40-41 ergs/s. The detection of line emission is discussed. An analysis of the short-term variability properties was given in Ptak et al. (1998) and detailed interpretation of these results will be given in Paper II. (abridged)Comment: Accepted for Jan. 99 issue of ApJS. 35 pages with embedded postscript figures. 8 large tables included externally as postscript file