Showing papers by "J. M. van der Hulst published in 1993"

Star formation thresholds in low surface brightness galaxies

Abstract: Low Surface Brightness (LSB) galaxies appear to have low star formation rates despite their often quite normal H I contents as judged from global H I properties such as M(H I)/L and M(H I)/M(T) ratios. H I imaging with the Very Large Array of the National Radio Astronomy Observatory (the NRAO is operated by Associated Universities Inc. under contract with the National Science Foundation) of eight LSB galaxies shows that the H I is extended compared with the optical size and has average surface densities which are about a factor 2 lower than in High Surface Brightness (HSB) galaxies of the same type. The resolution of the H I imaging allows a rough rotation curve analysis for evaluating the critical density for star formation as formulated by Kennicutt [ApJ, 344, 685 (1989)]. The observed H I surface densities systematically fall below this critical density for most of the galaxies in this sample, in agreement with the low current star formation rates. From the optical surface photometry we conclude that the galaxies studied are in general late-type galaxies dominated by an exponential disk with a typical scale length of a few kpc. The B-R and V-I colors of the LSB galaxies are a few tenths of a magnitude bluer than those of HSB galaxies indicating that the disks of these galaxies have a mean young age.

2 extreme h-i supershells in the disturbed edge-on galaxy ngc-4631

Abstract: Using Westerbork Synthesis Radio Telescope maps of 21 cm emission from NGC 463 1, we have discovered two very large supershells of atomic gas in the disk of this nearby, disturbed edge-on galaxy. The larger shell has a diameter of about 3 kpc and a swept up mass of roughly 1-2X10(8) M .. There is some evidence for expansion, with a velocity of 45 km s-1. Above a break at the top of the shell, there is a prominent ''worm'' of mass about 4X10(7) M .. The smaller shell has a diameter of 1.8 kpc and shows better evidence for expansion, at 35 km s-1, and has a mass of about 5-10X10(7) M .. The kinetic energy of the larger shell is roughly 2-5X10(54) erg, and the necessary input energy from supernovae and stellar winds to explain its current parameters is 2-5X10(55) erg, implying an association containing 10 000-35 000 supernova-producing OB stars! The smaller shell has energy parameters a few times lower. These would be extremely rich OB associations for a galactic disk if the supershells did indeed form in this way. A more attractive explanation as far as energetics are concerned is that small companions or massive gas clouds hit the disk of the galaxy and formed the shells. Unfortunately, there is no clear independent evidence for such collisions, and the near-complete ringlike appearance of the shells may be a problem for this explanation. The larger shell can be seen in the Halpha image of Rand et al. [ApJ, 396, 97 (1992)] as a bright, circular-shaped disturbance. Both shells are coincident with x-ray sources.

The VLA-WSRT survey of M33: statistical properties of a sample of optically selected supernova remnants

Abstract: A high-resolution, radio continuum survey of M33 has been carried out with the Very Large Array and the Westerbork Synthesis Radio Telescope at 4.84 and 1.42 GHz. The survey data have been inspected for radio emission from the 52 optically identified Supernova Remnants (SNR) in M33. We report the detection of 26 SNRs, significantly more than the previous total of 9. The results of this survey, as they apply to the radio properties of the optically identified SNRs of M33, are presented in the form of a radio catalog with some preliminary statistical analyses. In addition, an atlas is presented in the form of Hα, 6 cm and 20 cm contour and grey shade images of all 52 SNR fields. The major results of the statistical study are summarized below