Hot Gas in Star-Forming Galaxies
Star Formation and the ISM at the Center of the Milky Way
The central region of the Milky Way galaxy allows us to study the impact of star formation on the ISM with a factor of 100 higher spatial resolution. The region exhibits an intriguing regime of star formation: the rate within the central 150 pc of the Galaxy is 0.04 Msun yr-1, several hundred times higher than the volume density of star formation in the Galactic disk, and a similar factor lower than that in starburst galaxies (Figer et al. 1999). This on-going star formation has dramatic manifestations in the Constellation-X bandpass.
Chimneys connecting the galactic plane to the halo in spiral galaxies provide a mechanism for hot plasma with enhanced abundances to escape the disk. Such plasma, the result of supernovae and the winds of massive stars, heat and enrich the halo. This figure shows a possible example of such a chimney in the Milky Way, with a trail of enhanced X-ray emission from a number of energetic regions in the plane to a plume into the lower halo. The color image shows the ROSAT All-Sky Survey data at ¾ keV where red indicates high intensity and blue indicates low. The three panes show model Constellation-X spectra, based on the ROSAT data, from three points in the chimney. The cooling of the plasma as it progresses out of the disk can clearly be seen by the shifting of the emission to lower energies. However, Constellation-X observations will allow a much deeper study than what can currently be achieved with a considerably greater sensitivity to changes in ionization structure and abundances. With this additional information greater insight into the Galactic halo environment can be achieved.
A hot, 108 K plasma is observed through hydrogen and helium-link Fe line emission (Muno et al. 2004 a,b). The high temperature of this plasma is surprising, because it is much hotter than that seen in supernova remnants. Moreover, any hydrogen with T~108 K would be too hot to be bound to the Galactic center, so if the plasma is indeed thermal, then it must be composed largely of helium and heavier ions (Belmont, et al. 2005). Constellation-X measurements of the elemental abundances could confirm this hypothesis. Alternatively, the plasma could be produced as a by-product of cosmic-ray acceleration (Dogiel et al. 2005). In this case, Constellation-X should observe radiative recombination continua from ions in the plasma and will be able to study the above spectral features on spatial scales of tens of arcseconds, determining where the plasma is accelerated.
Young star clusters at the Galactic center have been identified through the X-ray emission from shocks formed both when winds of binary Wolf-Rayet and O (WR/O) stars collide and when the collective winds encounter the surrounding ISM (Law and Yusef-Zadeh 2004). The Arches cluster is the brightest such object and exhibits diffuse line emission from neutral iron (Yusef-Zadeh et al. 2002). The presence of neutral iron suggests that the cluster wind is accelerating low-energy cosmic rays, which in turn bombard iron in nearby molecular clouds, causing fluorescence. Constellation-X has the right combination of spatial and spectral resolution to study these processes in detail.
The detection of X-ray emission from several synchrotron-emitting radio filaments is one of the most surprising Chandra results from the Galactic center. The filaments have been explained as magnetic instabilities formed in the wakes of in-falling molecular clouds (Shore & Larosa 1999) or shocks in outflows produced by massive stars (Yusef-Zadeh and K&oulm;nigl 2004). The first model assumes that ~1 mG fields are present in the Galactic Center, whereas the latter assumes < 0.1 mG fields. The larger fields also are expected if the gas fed into the Galactic center carries with it magnetic fields. So distinguishing between these models is crucial for understanding whether fields build up or are destroyed in the nuclei of galaxies.
References
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