The two telescopes which have been instrumental in obtaining high resolution radio maps of nearby galaxies are the Westerbork Synthesis Radio Telescope (WSRT) operated by the Netherlands Foundation for Research in Astronomy (NFRA) and the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) (1). No doubt, the compact array of the Australia Telescope (AT) will soon begin to supplement our knowledge by providing high resolution observations of galaxies which are located in the southern hemisphere.
In order to study the structure of the ISM it is necessary to work at linear resolutions of the order of 100-200 pc. Even for galaxies as nearby as M31, M33 or M81 this corresponds to angular resolutions of 10-20 arcsec, clearly only within the realm of synthesis telescopes. Table 1 lists some of the characteristic properties of the WSRT, VLA and AT and gives representative figures for the angular and velocity resolution and sensitivity which can be reached during a typical observing session. These figures are strictly for guidance only and are likely to change rapidly as more sensitive receivers and increasingly complicated correlators are brought on-line. General information about synthesis imaging can be found in the volume on Synthesis Imaging in Radio Astronomy (Perley et al. 1989).
|Number of elements||14||27||6|
|Antenna diameter (meter)||25||25||22|
|Primary beam (arcmin)||37.6||30||35|
|System temperature (K)||55||60||18|
|Sensitivity* (mJy beam-1)||2.0||0.7||1.1|
|Longest Baseline (km)||2.8||36.4||11.4||3.4||1.03||6.0|
|Shortest Baseline (km)||0.036||0.68||0.21||0.073||0.033||0.030|
|Synthesized beam (arcsec)||13 × 13 / sin||1.4||3.9||12.5||44||6.5 × 6.5 / sin|
|Largest structure (arcmin)||14||0.6||2||7||15||12|
*The sensitivity in mJy beam-1 corresponds to the one sigma rms noise reached after a 12h period. The frequency resolution of respectively 19.5, 12.2, and 15.6 kHz converts to a velocity resolution of 2.5-4.1 km s-1. The sensitivity in terms of surface brightness, expressed in Kelvin, assumes that the emission is distributed uniformly and fills the beam.
All instruments are extremely sophisticated and there usually is a wide choice in baseline coverage, defining the lowest and highest spatial frequency for which one is sensitive, and velocity resolution and coverage. In general the VLA is the preferred instrument for sources which lie within the declination band -30° < < 30° as the East-West oriented WSRT and AT rapidly loose resolution in the declination direction when approaching the equator.
1 The NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. Back.