Scientific Research

Overview

Most of my research activity is focussed on furthering our [Us being the Astronomical community, or Humanity in general] understanding the (astro)physics of inter-stellar gas within, around and outside galaxies, and how the properties of this gas are affected by feedback processes associated with star formation. The gaseous structure of galaxies, their ongoing evolution with time, and their original formation, all depend to some extent on this physics.

Primarily I am interested in energy and mass feed back into the ISM by massive stars, via supernovae and stellar winds from massive stars, in particular in starburst galaxies where the collective effect of large numbers of massive stars drive galactic scale outflows (starburst-driven galactic winds, often referred to simply as superwinds). In addition, I have also studied stellar feedback on smaller scales, in superbubbles around indivdiual star clusters, and in wind-blown bubbles around single massive stars, and the hot gas in more "normal", Milky-Way-like, spiral galaxies.

Two views of the classic starburst superwind galaxy M82. Top panel: B, V and H-alpha emission (tracing T~1e4 K gas entrained into the wind). Bottom panel: Soft, medium and hard X-ray image of M82, shown on the same scale as the top panel. Extra-planar soft diffuse X-ray emission from a T ~ 4 million degree plasma clearly shows the presence of the wind, although this emission does not come from the volume filling wind fluid (which is only visible as diffuse hard X-ray in the central starburst region).

I specialize at both observational (primarily using X-ray observations) and at theoretical (largely via computational simulations) studies of these topics. Observationally I have concentrated on relatively detailed X-ray-based studies of the nearest and brightest starburst galaxies, often returning to study Messier 82 which is the brightest and best example of a superwind. Such observations tell us about the hot or very hot gas (1 million to 100 million degrees Kelvin) that ultimately drives superwinds. My theoretical work has primarily used high-resolution, multi-dimensional, grid-based hydrodynamical simulations of individual superbubbles or superwinds in order to understand their complex structure and the origin of the observed multi-wavelength emission from them.

Current Research Directions

The following questions illustrates some of the research problems I am working on solving at this time (2007).

• Physics and Observations of Starburst-Driven Superwinds:
  • Are observed starburst-driven superwinds mechanically or radiatively driven? Do the dominant driving mechanism vary with gas phase, or is the hot gas responsible for the acceleration of all the cooler phases? How can such alternatives be discriminated between with current and future observational data.
  • Can low-luminosity AGN drive poorly-collimated superwinds-like flows? If so, are these also mechanically-driven by the thermal and ram pressure of hot gas (in this case heated by a frustrated jet, for example)?
  • What fraction of the ubiquitous soft thermal X-ray emission in superwinds comes from the volume-filling energetic gas driving the outflow? What are the true metal abundances of this plasma, and is it metal enriched?
  • Do the shapes of the large-scale X-ray surface brightness profiles of superwind reflect the density profile of the initial halo medium the superwind propaated into?
  • Are there robust techniques to avoid parameter estimation bias when fitting the low resolution X-ray spectra of intrinsically multi-temperature hot plasmas?
  • How do the temperatures of the X-ray emitting phases (yes, there are multiple sources of X-ray emitting plasma in superwinds) relate to their velocity and specific energy? How likely is this plasma to escape into the IGM (especially with respect to the entrained warm neutral medium and warm ionized medium seen in Na I or Mg II absorption line and H-alpha/[N II] emission line studies of winds)?
  • What is the nature and lifetime of the cooler (non-X-ray-emitting) phases of ISM found within superwinds? How do their measured velocities relate to the (unmeasured) true wind fluid velocity? Do the cool phases like Na D or H-alpha truly meaure wind terminal velocity? Why do Na D and H-alpha velocities disagree for the best-studied superwinds? Physically, how to the shape of the WNM/WIM absorption-or-emission line profiles tell us about the kinematics and distribution of entrained gas in superwinds?
  • Is thermal conduction a significant process in the hot X-ray emitting phases of superwinds (or even normal galactic disks)? What role does it play in (a) altering the observable properties of the hot and coronal phases; or (b) decreasing or increasing the lifetime of embedded cold clouds?
  • Can superwinds completely eject some metal-enriched gas from their host galaxies into the IGM? How does this depend on starburst strength and galaxy mass, i.e. which galaxies dominate IGM enrichment?
  • What is the size of the region a superwind, from a starburst galaxy of a given mass and star formation rate, can effect (e.g. 10 kpc, 100 kpc, or 1 Mpc)? How is this gas-phase dependent?
  • Can all starburst galaxies drive superwinds, or is there a threshhold in star formation intensity (perhaps dependent on galaxy mass) that lies at a different level to the convential definition of a starburst?
  • Are observed winds young (i.e. their observed extent indicates the region out to which they have just propagated) or mature (they have reached a pseudo-steady state with the currently observed region being smaller than their true total extent)? How time-variable are the properties of superwinds?
  • Are there old superwinds (e.g. in post-starburst galaxies), or does the wind cease to be actively driven once the core-collapse SN rate drops? Can SN 1a associated with (but following) the star burst contribute to a starburst-driven superwind?

• The hot phases of the ISM in star forming galaxies of all types:
• How do the X-ray properties of star-forming galaxies (both starburst and "normal" non-starburst galaxies) vary with mass and star formation history?
• Do normal, non-starbursting, spiral galaxies have significant hot X-ray emitting halos, and if so what creates them? Over what spatial scale (3 kpc?, 30 kpc?, 300 kpc?) is most of the hot gas distributed in the halo of a normal spiral galaxy like the Milky Way?
• Is there a threshhold in star formation intensity (SNe/area) necessary to create a galactic fountain, or do all star forming galaxies vent hot gas into their halos (just some operate at levels that are too faint to detect)?
• Can the classic Weaver wind-blown bubble model also explain the X-ray properties of superbubbles, or are alternative models (e.g. internally mass-loaded instead of conductively-driven shell evaporation, or SNR/shell impacts by off-center SNe) required?

• Understanding Stellar Feedback, of all types, and applying it to cosmology:
• How efficient are the supernovae and stellar winds in real starburst galaxies at heating the ISM (what is the SN thermalization efficiency)? Is the hot, metal-enriched, gas in superwinds significantly diluted via mass-loading? How does the thermalization efficiency and mass loading vary with star formation intensity and galaxy mass? What is the specific energy of the metal-enriched gas in superwinds?
• What do the lessons learnt about SN feedback from observations and modelling nearby starbursts and superwinds tell us about how to (a) understand observations of starsbursts in the high redshift (z > 2) Universe, and (b) accurately implement feedback in cosmological simulations of galaxy formation and evolution?
• If metal enrichment of the IGM by superwinds is significant, is this of a level such that its effects on the mean galaxy metallicity (i.e. the galaxy M-Z or effective yiled-M relationships) is also significant?

Highlights of Previous Research

2-D (cylindrical coordinate system) hydrodynamical simulation of a superwind produced by a M82-like starburst event.

(to be added at a later date)

I maintain a moderately up-to-date list of scientific publications I am an author on, along with links to postscript or PDF versions of them, here.

Other Science Topics Resulting in Publication

I have also written or partipated in papers on X-ray binary populations in nearby galaxies, possible intermediate-mass black holes, the so-called Ultra-Luminous X-ray sources (ULXs) and hidden AGN.