Neutral Hydrogen in the Edge‐on Spiral Galaxy NGC 3044—Global Properties and Discovery of HiSupershells

The first detailed VLA mapping of the neutral hydrogen distribution in theisolated, edge-on spiral galaxy NGC 3044 is presented. Physical parametersdetermined for this galaxy are typical for galaxies of its morphological class(SBc). We have modelled the HI spectra in order to derive its global densityand velocity distributions. An HI scale height of 420 h^(-1) pc is thus found.This can be compared to the 8 kpc radio continuum halo found previously. The present study reveals an asymmetry in the HI distribution as well asnumerous high-latitude HI structures at various galactocentric radii. Twelve high-latitude features were catalogued, of which four exhibit thesignature of an expanding shell. There is some correlation of these featureswith features observed in the radio continuum from independent data. The mostmassive shell (Feature 10) extends out to 6h^(-1) kpc above the galactic disk.The radii and masses of these shells range from 1.2h^(-1) - 2.0h^(-1) kpc and1.6 10^7 - 5.5 10^7h^(-2) solar masses, respectively. We have investigated thepossibility that the supershells could have been produced by external impactingclouds, but conclude that this scenario is unattractive, given the age of theshells, the isolation of the galaxy, and the lack of any observed featuressufficiently massive to form the shells in the vicinity of the galaxy.Therefore, an internal origin is suggested. Since the implied input energiesfrom supernovae are extremely high (e.g. from 1.4 10^(53)h^(-20 - 7.410^(55)h^(-2) ergs, corresponding to 400 - 74,000 supernovae), we suggest thatsome additional energy (e.g. from magnetic fields) may be needed to produce theobserved supershells.Comment: Latex 23 pages including 13 postscript figures, to be published in the Nov. 20 issue of Ap