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1996PASP..108..460S - Publ. Astron. Soc. Pac., 108, 460-460 (1996/May-0)

High-velocity clouds and superbubbles in nearby disk galaxies. (Dissertation summary)

SCHULMAN E.

Abstract (from CDS):

The galactic fountain model predicts that energetic stellar winds and supernovae in OB associations produce superbubbles containing hot gas that breaks out of the Galactic disk, cools radiatively as it rises upward, and recombines and returns to the disk ballistically. The hot (T ∼ 10^6 K) gas can be observed with X-ray telescopes, while the cool returning neutral hydrogen (H I) is detectable as 21 cm emission from high-velocity clouds (HVCs). In the Milky Way Galaxy, a combination of infalling material tidally torn from the Magellanic Clouds and a galactic fountain can explain the high-velocity clouds that cover about 10% of the sky down to a column density of 2 to 3 X 10^18 cm^-2.

Sensitive H I observations of nearby disk galaxies were performed with the Arecibo 305 m radio telescope to search for and measure the mass of HVCs in other galaxies. Ten of 14 galaxies have high-velocity wings that can be modeled as arising from a component of galactic gas with a velocity dispersion of 30 or 50 km s^-1. The HVC mass for the 10 galaxies ranges from 6 X 10^7 solar mass to 4 X 10^9 solar mass, which corresponds to 4 to 14% of the total H I in the galaxies. This is the first survey to search for HVCs in more than a few galaxies, and the results imply that Galactic HVCs are a disk-wide phenomenon with a characteristic distance of 10 to 20 kpc, containing a substantial fraction (∼10%) of the neutral hydrogen in the Galaxy and much of the random kinetic energy in neutral gas.

21 cm synthesis imaging of UGC 12732 and NGC 5668, performed with the Very Large Array, confirmed the Arecibo results that the former does not have high-velocity gas while the latter does. Two components of high-velocity gas are present in NGC∼5668; one may be from an accretion event, while the other is visible due to the increased H I velocity dispersion throughout the optical disk and may be galactic fountain gas. Neither of these components are visible in the observations of UGC 12732, and this galaxy can be successfully modeled as a rotating disk of neutral hydrogen with a velocity dispersion of 7 km s^-1 throughout. The star formation rate (SFR) in NGC 5668 is one to two orders of magnitude larger than the SFR in UGC 12732.

Optical and far-infrared (FIR; measured with the IRAS satellite) observations of the 14 sample galaxies revealed that galaxies with high-velocity H I wings have an average SFR an order of magnitude larger than that of the galaxies without such wings. The FIR to blue light and FIR to H-alpha ratios imply that the galaxies with high-velocity H I wings have more dust-enshrouded FIR sources than the galaxies without high-velocity wings. Both of these results are expected if a substantial fraction of the HVCs are produced in galactic fountains.

High-resolution X-ray observations of M33 were performed with the ROSAT satellite to search for superbubbles of hot gas within holes in the H I layer of this nearby spiral galaxy. These observations revealed a number of X-ray point sources (eight of which are coincident or nearly coincident with supernova remnants), diffuse X-ray emission around the nucleus of the galaxy, and two possible superbubbles. One has an X-ray luminosity of 2 X 10^37 ergs s^-1 within an H I hole of size 300 X 130 pc. The other has an X-ray luminosity of 8 X 10^37 ergs s^-1 and a gas temperature of 2 X 10^6 K. It is coincident with the giant H I region NGC 604 and is 13 ± 13 arcsec from a 150 X 130 pc H I hole.

The luminosity limit of the ROSAT observations for the detection of soft (kT= 0.2 to 0.4 keV) X-ray emission is about 2 S 10^37 ergs s^-1, similar to the X-ray luminosities of the brightest superbubbles that have been found in the Large Magellanic Cloud. These observations cannot, therefore, be used to perform the meaningful survey of the superbubbles in M33 that would be required to constrain the galactic fountain model. Future observations with the Advanced X-ray Astrophysics Facility (AXAF) should be able to perform such a survey, which would enable a good estimate to be made of the covering factor of holes through which hot gas from OB associations can escape into the halo.


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Journal keyword(s): Dissertation Summaries

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