In many cases the main driver for carrying out a large redshift project is the equipment that is available. This is true both for optical surveys as well as those carried out at 21-cm. Therefore, it seems appropriate to begin this review of HI redshift surveys by reminding the reader about the telescopes that have been available for carrying out such programs.
2.1. Summary of Major Facilities
The table below summarizes the major radio telescopes used for 21-cm redshift work. They are listed in order of decreasing collecting area. The beam size is for 21-cm, and the velocity range given corresponds to the simultaneous bandwidth obtained under normal HI redshift observing mode (approximate numbers only). The bandwidths come from papers presenting 21-cm observations; although an attempt was made to get the most up-to-date numbers possible, recent instrumentation upgrades may result in broader ranges in some cases. The numbers in the last column come from Table 9 of Huchtmeier & Richter (1989), and reflect the total number of published HI observations of galaxies through early 1989. This is NOT the number of unique galaxies observed per site, nor is it the number of new HI redshifts produced. However, it does appear to be indicative of the relative contribution of each telescope to the 21-cm redshift industry.
|Observatory||Diameter||Beam Size||Instantaneous||# Observed|
|[meters]||[arcmin]||Bandwidth [km s-1]||Galaxies|
|Nançay||300 × 30||3.6 × 22||5000||1200|
The two clear leaders in the 21-cm redshift survey field have been the Arecibo 305-m and NRAO Green Bank 91-m telescopes. Arecibo has the clear advantage of sheer size on its side, having more than eight times the collecting area of the next largest telescope, and 50 times more than the smallest telescope listed here. Its small beam size is a real plus when working in crowded regions or at higher redshift where the apparent diameters of galaxies approach one arcmin or less. Its obvious limitation is sky coverage. Because it has a fixed primary, it is only possible to observe sources that pass within 20 degrees of the zenith, meaning that it has a declination coverage of -2° 38°. Figure 2 of Huchtmeier & Richter (1989), which plots number of galaxies observed at 21-cm vs. declination, clearly shows the dominance of Arecibo as an HI telescope: the number of galaxies observed in the Arecibo declination range dwarfs the totals from all other portions of the sky.
Before its collapse in 1988, the Green Bank 91-m dish was also extremely productive as a 21-cm redshift machine. Despite being a transit telescope, and hence having very limited integration times (4 - 8 minutes per source, depending on declination), much of the early work on HI redshifts was carried out with this instrument. It had the advantage of relatively large aperture and complete declination coverage (north of = -18°). Its beam size was a good match for nearby spiral galaxies, and it had the advantage of being located in an area of low radio interference (the legislated radio quiet zone around Green Bank). This latter feature is a significant plus, especially in the current era where cellular phone transmissions and other types of radio noise are creating serious problems for nearly all radio observatories (see also Vanden Bout & Haynes, this volume).
The remaining telescopes in Table 1 have all played significant roles in collecting 21-cm data for galaxies. Parkes is the only radio telescope of significant size operating in the southern hemisphere, so despite the fact that it appears near the bottom of the current list it has been extremely important. The Bonn and Jodrell Bank telescopes are both fully steerable, which gives them a big advantage over those telescopes with limited sky coverage. This positive is counterbalanced by their smaller sizes and high radio interference environments, plus the fact that they are used at many other frequencies (particularly Bonn) which limits the time available for 21-cm work. Nançay, which employs the double reflector design (looking somewhat like the projection screen at a drive-in movie), has good sensitivity at southerly declinations (down to = -40°), which, coupled with its large collecting area, makes it a particularly useful instrument for some projects. Its main drawback is the large, odd-shaped beam, which makes source overlap and beam-dilution for smaller galaxies significant problems.
The astute reader will note that no mention has been made of interferometer systems such as the VLA or Westerbork in the comments above. Despite their great importance in HI mapping and dynamical studies, synthesis radio telescopes simply haven't been used for redshift survey work. The reasons should be obvious. For one thing, these instruments are so valuable (and unique) for a large range of scientific projects that using them for redshift surveys would be a poor use of their time. From a practical standpoint, although these arrays have good sensitivity, they have been designed to operate in spectral-line mode with rather narrow total bandwidths and relatively small numbers of channels. Therefore, while they are outstanding instruments for mapping HI distributions in galaxies whose velocities are known, they are not very efficient at sampling velocity space in order to find previously unknown redshifts. The plan for hardware upgrades for the VLA under development (Bastian & Bridle 1995) includes a new correlator that would provide a significant increase in the bandwidth and number of channels offered. With such an improvement, the VLA would offer significant opportunities for searching for galaxies found within the imaged field, for example in clusters and groups of galaxies or in a blind search mode.
2.2. Observational Methods
Before going on, it seems appropriate to mention briefly the standard observational techniques used in observing galaxies at 21-cm, and to specify the type of information obtained from a successful observation.
Data acquisition and reduction procedures are fairly standardized and automated at most facilities. The observations are typically done in one of two modes: (1) beam switching, where the telescope observes on-source for a specified length of time, and then is moved off the source and integrates for a comparable amount of time on "blank sky". In most cases the off-source position is chosen so that the telescope tracks over precisely the same path as it followed while on the source (i.e., the same altitude and azimuth); (2) frequency switching, where the central frequency of the observed band is alternated between two values during the course of the observation so that the expected signal moves from one side of the observed frequency range to the other. Frequency switching has the potential to be more efficient than beam switching since, with a judicious choice of frequency settings, one can observe the source 100% of the time. However, since frequency switching only works well over rather narrow bandwidths and the proper frequency settings aren't known ahead of time (i.e., one doesn't know the redshift), beam switching is used almost universally in redshift survey work. Frequency switching is useful when searching for objects with velocities close to those occupied by galactic hydrogen, when even the small differences in the local HI between the ON and OFF beams prevents instrumental removal via beam switching.
Once the data have been obtained, the processing and analysis is quite straightforward. A typical procedure includes differencing the ON and OFF source scans, dividing the difference by the OFF, fitting a low-order baseline to the resulting spectrum, and then measuring any signal present. Typically, the data processing and measurement procedure takes less than half the time required to acquire the data, and the observer comes away with fully processed values for the central velocity of the HI profile, the width of the profile (characteristic of the rotational velocity of the galaxy) and integrated flux (from which one derives an estimate of the total HI mass). For redshift survey projects the recessional velocity is the main value of interest, but the other two parameters are extremely useful as well.
2.3. Future Telescope Improvements
Future 21-cm redshift work will be aided by recent improvements in the telescopes and hardware available to the radio community. Improved receivers and larger bandwidth spectrometers can make the existing telescopes more sensitive and efficient. However, the biggest gains will occur through two major telescope projects.
The Arecibo 305-m telescope is undergoing a major upgrade which is expected to be completed by late 1996 or early 1997. The upgrade includes a number of enhancements to the facility. Most notable is the replacement of the old line feeds with a Gregorian subreflector system that will correct for the aberration of the spherical primary and focus the signal to a point. At this focal point a rotating turret containing several horn feeds will be located. The final system should have substantially higher sensitivity, and also be better shielded from local radio interference. It will also allow for much broader bandwidth observations, and a new spectrometer is planned that will provide greater than 100 MHz (> 21,000 km s-1) coverage in a single spectrum! Also included in the upgrade is a ground screen which rings the main reflector and greatly reduces spillover noise which was present for observations at declinations close to the telescope limit.
The radio community suffered a major setback when the NRAO 91-m collapsed in 1988. However, its replacement will soon be available and will be bigger and substantially better than its predecessor. The GBT - Green Bank Telescope - is a 100 × 110 meter off-axis paraboloid which is fully steerable and features an unblocked aperture. It will also have large bandwidth capability and be located at a low-noise site, so should prove to be a useful tool for 21-cm redshift work, particularly at declinations not covered by Arecibo. Its biggest drawback may be that it will also be in great demand for work at other frequencies as well, so HI redshift observers may have to stand in line. The GBT is slated to be in operation in late 1997.
The chapter in this volume by Vanden Bout & Haynes gives a complete description of the capabilities of these two major instruments, and hints at some of the exciting science that will be possible once they are operational.