H I in Four Star-forming Low-Luminosity E/S0 and S0 Galaxies*

摘要

We present H I data cubes of four low-luminosity early-type (E/S0 and S0) galaxies that are currently forming stars. These galaxies have absolute magnitudes in the range MB = -17.9 to -19.9 (H0 = 50 km s-1 Mpc-1). Their H I masses range between a few times 108 and a few times 109 M⊙, and the corresponding values for MHI/LB are between 0.07 and 0.42, so these systems are H I–rich for their morphological type. In all four galaxies, the H I is strongly centrally concentrated with high central H I surface densities, in contrast to what is typically observed in more luminous early-type galaxies. Star formation is occurring only in the central regions. In two galaxies (NGC 802 and ESO 118-G34), the kinematics of the H I suggests that the gas is in a strongly warped disk, which we take as evidence for recent accretion of H I. In the other two galaxies (NGC 2328 and ESO 027-G21), the H I must have been part of the systems for a considerable time. The H I properties of low-luminosity early-type galaxies appear to be systematically different from those of many more luminous early-type galaxies, and we suggest that these differences are due to a different evolution of the two classes. The star formation history of these galaxies remains unclear. Their UBV colors and Hα emission-line strengths are consistent with having formed stars at a slowly declining rate for most of the past 1010 yr. If so, their star formation history would be intermediate between late-type spiral disks and giant elliptical galaxies. However, the current data do not rule out a small burst of recent star formation overlaid on an older stellar population. Three of the galaxies have weak radio continuum emission, and the ratio of the far-infrared (FIR) to radio continuum emission is very similar to that of spirals of similar FIR or radio luminosity. We find that, except in the largest galaxy observed, the radio continuum emission can be accounted for solely by thermal (free-free) emission from H II regions, with no nonthermal (synchrotron) disk component. Thus, although these galaxies have gaseous disks, a disk magnetic field may be very weak or absent.