Monday, August 20, 2007

FUSE observations of O VI in Superwinds


Following up from the previous obituary for the Far Ultraviolet Spectroscopic Explorer (FUSE) I thought I'd start to discuss one way in which FUSE advanced the scientific topic I study most: starburst driven superwinds.

FUSE observations of starburst galaxies and superwinds yielded many important results, although they generated as many questions as they answered. In particular the 1032 and 1038 Angstrom doublet of the O VI ion (five times ionized oxygen, O^5+). This coronal phase gas could be used to measure the velocity and amount of material at a temperature of about 300,000 K (3e5 K), gas far hotter than the 10,000 K gas seen in Hubble observations of superwinds (this latter gas phase is technically termed warm ionized gas to differentiate it from the hotter ionized phases detected with O VI or X-rays).

Although 3e5 K is still much cooler than the temperature of the X-ray emitting gas in superwinds (which has temperatures of several times 1e6 to several times 1e7 K)
, this is still the hottest gas for which the velocity in a superwind has been measured.

The standard theoretical model for superwinds predicts that the hotter the gas phase the higher its outflow velocity. This is because it is the gas thermal pressure and ram pressure-driven expansion of the hottest material (the merged supernova and stellar wind ejecta) that sweeps up and accelerates the cooler ambient gas clouds that are seen in optical observations of superwinds. As this acceleration process is imperfect the cooler material is never accelerated to the same velocity as the hot gas.

In other theoretical models for superwinds, for example radiation-pressure driven winds or cosmic-ray driven winds, this specific variation of outflow velocity with temperature is not expected. Thus measurements of the outflow velocity of different gas phases at different temperatures can be used as a test of our theories of how superwinds are created and how they work physically.

Initial observations with FUSE of the starbursting dwarf galaxy NGC 1705 (Heckman et al, 2001, ApJ, 554, 1021) suggested that this O VI-absorbing gas was flowing outward faster than the warm and neutral ionized gaseous media in line with expectations from the standard superwind model [and that the amount of O VI absorbing material was inconsistent with thermal condiction in the shell of a superbubble, a slightly different model for what was is happening in NGC 1705].

The image shown above is an optical image of NGC 1705 taken from the SINGG survey of Meurer et al 2006, ApJS, 165, 307. The red in this image is H-alpha emission, light from warm ionized hydrogen with a temperature of 8000 - 10000 Kelvin. The filaments, shell and arcs of ionized hydrogen cover a region about 1 kpc (about 3000 light years) in diameter.

Since those early FUSE results on NGC 1705 were published in 2001 many more FUSE observations of a variety of different starbursting galaxies were taken, most of them analyzed by colleagues here at Johns Hopkins such as Charles Hoopes and John Grimes. In a future post I'll summarize what the latest thinking is on O VI in starbursts and the issue of phase-dependent velocities in superwinds.

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