The Feasibility Study

The pilot 21-cm survey for galaxies in the ZOA was conducted with the NRAO 300-ft telescope at Green Bank, WV. Five observing sessions during the period 1986-1988 covered 7200 search points. The beam width at 21 cm was about 10. About 60% of the lines of sight were in the ZOA; the rest were located in regions of low obscuration as a control. The search points were not concentrated in any particular area, but were spread over the sky accessible to the 300-ft. The velocity range covered, -400 to 6800 km s allowed the possibility of discovering new Local Group members, as well as spirals out to several thousand km s. The 300-ft was an extremely sensitive instrument, allowing detection of dwarfs over a large volume, and normal spirals to the limit set by the spectrometer bandwidth. A more complete description of the survey method is provided by Henning (1992).

The survey yielded 37 extragalactic objects, and two interesting high negative velocity sources which we believe are associated with our own Galaxy. Only the sample of 37 clearly extragalactic objects will be discussed here. Nineteen of these appear in optical catalogs. As expected, most of the optically-known objects lie at high galactic latitude, but one, NGC 2377, is quite close to the Galactic Plane, at b = 3. This illustrates the patchiness of the foreground obscuration. Of the full sample of 37 galaxies, 19 showed the typical two-horned or flat-topped HI profiles of unresolved spirals. But morphological classifications are shaky due to the random placement of the sources in the beam. Our rough classifications were accurate only about half of the time, judging from comparison with published morphological types of the optically-cataloged objects. Spirals far from the beam center tend to look one-horned, as do narrow-profile dwarf spirals. Therefore, all further study of the sample was based on follow-up observations with the Very Large Array (VLA) radio interferometer.

Interferometric observations were made for 25 galaxies, including all of the uncataloged objects and the weaker cataloged ones. The VLA was used in its most compact configuration, providing the highest sensitivity, but not allowing detailed mapping. From these observations, more accurate positions, HI masses, sizes, profile shapes, and linewidths were derived. The galaxies' HI masses range over two orders of magnitude, from 10 to 10 , small dwarfs to massive spirals. The profiles obtained look just like optically-unobscured HI galaxies (Henning 1992). Visual obscuration makes no difference whatsoever to HI.

Armed with accurate positions, we consulted the Infrared Astronomical Satellite (IRAS) Point Source Catalog to investigate any IR counterparts. Twelve of the 37 objects are associated with IR sources of appropriate color for galaxies. Ten of these are optically cataloged, and the other two appear on the Palomar Sky Survey. Why such a poor IR performance finding the hidden galaxies? There are two reasons. First, the IR sky is terribly confused at low latitudes, so our sources near the Galactic Plane were masked by Galactic IR emission. Even inspection of the more sensitive IRAS co-adds does not help pick out extragalactic sources at the very lowest latitudes. Second, IRAS is biased against dwarfs and dwarf spirals. The optically-cataloged dwarf and dwarf spirals in our sample were not listed in the Point Source Catalog, even though they were in regions of low obscuration.

© Copyright Astronomical Society of Australia 1997