Winds from Nuclear Starbursts: Old Truths and Recent Progress on Superwinds

I will discuss a few select aspects of the most common and best understood galactic-scale outflow -- starburst-driven superwinds, focusing on winds from nuclear starburst galaxies. I will show that modern observations, in particular in the soft and hard X-ray bands, complement and reinforce the existing paradigm of superwinds as flows collectively driven by multiple SNe. The properties of the diffuse X-ray emission from dwarf starburst galaxies, L_BOL ~ L_* starbursts in spiral galaxies, and ULIRGS, are all consistent with superwind activity. Where appropriate, I contrast the physics of starburst-driven winds with poorly collimated winds from AGN, and discuss what we know of the role of LLAGN and Seyfert nuclei in starburst superwind galaxies.Comment: To appear in the proceedings of IAU symposium 222: The Interplay among Black Holes, Stars and ISM in Galactic Nuclei, held in Gramado, Brazil, March 1-5 2004. Eds. Th. Storchi Bergmann, L.C. Ho & H.R. Schmitt. 6 pages, 1 figur

Starburst-driven galactic winds: I. Energetics and intrinsic X-ray emission

We have performed an extensive hydrodynamical parameter study of starburst-driven galactic winds, motivated by the latest observation data on the best-studied starburst galaxy M82. We study how the wind dynamics, morphology and X-ray emission depend on the host galaxy's ISM distribution, starburst star formation history and strength, and presence and distribution of mass-loading by dense clouds. We find that the soft X-ray emission from galactic winds comes from low filling factor (ff < 2 per cent) gas, which contains only a small fraction (f < 10 per cent) of the mass and energy of the wind, irrespective of whether the wind models are strongly mass-loaded or not. X-ray observations of galactic winds therefore do not directly probe the gas that contains the majority of the energy, mass or metal-enriched gas in the outflow. The soft X-ray emission comes from gas at a wide range different temperatures and densities. Estimates of the physical properties of the hot gas in starburst galaxies, based on fitting the standard simple spectral models to existing X-ray spectra, should therefore be treated with extreme suspicion. The majority of the thermal and kinetic energy of these winds is in a volume filling hot, T approx 10^7 K, component which is extremely difficult to probe observationally due to its low density and hence low emissivity. Most of the total energy is in the kinetic energy of this hot gas, a factor which must be taken into account when attempting to constrain wind energetics observationally. We also find that galactic winds are efficient at transporting large amounts of energy out of the host galaxy, in contrast to their inefficiency at transporting mass out of star-forming galaxies. (Abridged)Comment: Accepted for publication in MNRAS. Letter page size postscript available from http://adcam.pha.jhu.edu/~dks/dks_published.htm

Predicting X-ray emission from wind-blown bubbles - Limitations of fits to ROSAT spectra

Wind-blown bubbles, from those around massive O and Wolf-Rayet stars, to superbubbles around OB associations and galactic winds in starburst galaxies, have a dominant role in determining the structure of the Interstellar Medium. X-ray observations of these bubbles are particularly important as most of their volume is taken up with hot gas, 1E5 < T (K) < 1E8. However, it is difficult to compare X-ray observations, usually analysed in terms of single or two temperature spectral model fits, with theoretical models, as real bubbles do not have such simple temperature distributions. In this introduction to a series of papers detailing the observable X-ray properties of wind-blown bubbles, we describe our method with which we aim to solve this problem, analysing a simulation of a wind-blown bubble around a massive star. We model a wind of constant mass and energy injection rate, blowing into a uniform ISM, from which we calculate X-ray spectra as would be seen by the ROSAT PSPC. We compare the properties of the bubble as would be inferred from the ROSAT data with the true properties of the bubble in the simulation. We find standard spectral models yield inferred properties that deviate significantly from the true properties, even though the spectral fits are statistically acceptable, and give no indication that they do not represent to true spectral distribution. Our results suggest that in any case where the true source spectrum does not come from a simple single or two temperature distribution the "observed" X-ray properties cannot naively be used to infer the true properties.Comment: 14 pages, LaTeX with 13 eps figures, condensed abstract. MNRAS in pres

The Energetics and Mass-loss of Mrk33

We present ROSAT HRI X-ray data and optical imaging of the important dwarf starburst Markarian 33. We find an extended, complex, shell-like morphology in the X-ray emission, with an extent of 2.3 x 1.9kpc, coincident with the bright star-forming regions at the centre of the galaxy. The physical extent of this X-ray emission from Mrk 33 is very similar to the observed Halpha emission, and suggests that the bulk of the X-ray emission is coming from an expanding superbubble. We estimate the age and mass of Mrk 33's starburst to be 5.8 Myr and 6.9 x 10^{6} Msolar respectively with the energy injection rate in the central regions of the galaxy being 10^{41} erg/s, while the associated mass-loss rate from the star-forming regions is estimated to be 0.2 Msolar/yr. We suggest that the X-ray emission is predominantly powered by starburst type activity and argue that a blowout in the form of a galactic wind is the most likely fate for Mrk 33 resulting in the loss of most of the galaxy's metal-enriched material and a small fraction (<1 per cent) of the ISM.Comment: 13 pages, 6 figures, accepted for publication in MNRA

Thermalization and the chromo-Weibel instability

Despite the apparent success of ideal hydrodynamics in describing the elliptic flow data which have been produced at Brookhaven National Lab's Relativistic Heavy Ion Collider, one lingering question remains: is the use of ideal hydrodynamics at times t < 1 fm/c justified? In order to justify its use a method for rapidly producing isotropic thermal matter at RHIC energies is required. One of the chief obstacles to early isotropization/thermalization is the rapid longitudinal expansion of the matter during the earliest times after the initial nuclear impact. As a result of this expansion the parton distribution functions become locally anisotropic in momentum space. In contrast to locally isotropic plasmas anisotropic plasmas have a spectrum of soft unstable modes which are characterized by exponential growth of transverse chromo-magnetic/-electric fields at short times. This instability is the QCD analogue of the Weibel instability of QED. Parametrically the chromo-Weibel instability provides the fastest method for generation of soft background fields and dominates the short-time dynamics of the system.Comment: 8 pages, 4 figures, Invited plenary talk given at the 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14-20 Nov 200

Chromoelectric oscillations in a dynamically evolving anisotropic background

We study the oscillations of a uniform longitudinal chromoelectric field in a dynamically-evolving momentum-space anisotropic background in the weak field limit. Evolution equations for the background are derived by taking moments of the Boltzmann equation in two cases: (i) a fixed relaxation time and (ii) a relaxation time that is proportional to the local inverse transverse momentum scale of the plasma. The second case allows us to reproduce 2nd-order viscous hydrodynamical dynamics in the limit of small shear viscosity to entropy ratio. We then linearize the Boltzmann-Vlasov equation in a dynamically-evolving background and obtain an integro-differential evolution equation for the chromoelectric field. We present numerical solutions to this integro-differential equation for a variety of different initial conditions and shear viscosity to entropy density ratios. The dynamical equations obtained are novel in that they include a non-trivial time-dependent momentum-space anisotropic background and the effect of collisional damping for the first time.Comment: 10 pages, 4 figure