An H\,I Superbubble in NGC 1313
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Session 19 -- Gas and Star Formation in Spiral Galaxies
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[19.10] An H\,I Superbubble in NGC 1313

S. D. Ryder (University of Alabama), L. Staveley-Smith (ATNF)

We report the discovery of a major H\,I bubble in the disk of the nearby late--type barred spiral NGC 1313. By using the Australia Telescope Compact Array to map the distribution of neutral hydrogen in NGC~1313, we have found a significant hole some $2'$ southwest of the nucleus, where the projected gas surface density is less than 10\% of the average value at that distance.

We attribute this hole to the existence of a bubble of H\,I some 2~kpc in diameter, and which is presently expanding at a rate of $\sim40$~km~s$^{-1}$. The bubble is unusual for its size in that its H\,I velocity profile exhibits both an approaching and a receding hemisphere, indicating that the bubble has yet to breach the galaxy's gas disk. From a preliminary analysis, we infer a dynamical age for the bubble of $\sim3\times10^{7}$~years, and a kinetic energy $\sim10^{54}$~ergs, putting this bubble in the realm of the ``supershells'' defined by Heiles (1979, ApJ, 229, 533). On the order of $10^{3}$ supernovae in the space of only a few million years would thus be required to account for this superbubble on the basis of violent star formation alone. Neither [S\,II], X-ray, nor radio continuum images reveal any evidence of very recent supernovae within this superbubble, but it is not uncommon for such large holes to apparently lack any association with recent massive star formation. The collision of a high velocity gas cloud onto the disk of NGC 1313 is unlikely to be the cause, owing to the remarkable symmetry of the superbubble. The superbubble does appear to be triggering regions of active star formation on its periphery, similar to the self-propagating Shapley Constellation~III complex in the LMC.

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