Counts and sizes of galaxies in the Hubble Deep Field South: Implications for the Next Generation Space Telescope

Science objectives for the Next Generation Space Telescope (NGST) include a large component of galaxy surveys, both imaging and spectroscopy. The Hubb le Deep Field data sets include the deepest observations ever made in the u ltraviolet, optical, and near-infrared, reaching depths comparable to that expected for NGST spectroscopy. We present the source counts, galaxy sizes, and isophotal filling factors of the Hubble Deep Field South (HDF-S) image s. The observed integrated galaxy counts reach over 500 galaxies per square arcminute at magnitudes AB < 30. We extend these counts to fainter levels and further into the infrared using galaxy-count models. It was determined from the HDF (North) and other deep Wide Field Planetary Camera 2 imaging t hat fainter galaxies are smaller. This trend continues to AB = 29 in the hi gh-resolution HDF-S Space Telescope Imaging Spectrograph (STIS) image, wher e galaxies have a typical half-light radius of 0 ".1. We have run extensive Monte Carlo simulations of the galaxy detection in the HDF-S, and we show that the small measured sizes are not due to selection effects until AB > 2 9. We compare observed sizes in the optical and near-infrared using the HDF -S Near Infrared Camera and Multi-Object Spectrometer image, showing that a fter taking into account the different point-spread functions and pixel siz es of the images, galaxies are smaller in the near-infrared than they are i n the optical. We analyze the isophotal filling factor of the HDF-S STIS im age and show that this image is mostly empty sky even at the limits of gala xy detection, a conclusion we expect to hold true for NGST spectroscopy. At the surface brightness limits expected for NGST imaging, however, about a quarter of the sky is occupied by the outer isophotes of AB < 30 galaxies, requiring deblending to detect the faintest objects. We discuss the implica tions of these data on several design concepts for the NGST near-infrared s pectrography. We compare the effects of resolution and the confusion limit of various designs, as well as the multiplexing advantages of either multio bject or full-field spectroscopy. We argue that the optimal choice for NGST spectroscopy of high-redshift galaxies is a multiobject spectrograph (MOS) with target selection by a microelectromechanical systems (MEMS) device. I f this technology does not become available in the next few years, then the second choice would be either a mechanical MOS using movable slits or fibe rs, or an integral field spectrograph.