10 Astronomical applications

We have described a synthesis telescope uniquely suited to observation of the Galactic ISM. While its resolution ($\sim1'$) is modest compared to some synthesis telescopes (e.g. the VLA), its sensitivity to extended structure is superior, and it offers significantly different performance from single-antenna telescopes. In this section we discuss a few examples of results obtained with the telescope, chosen to illustrate its unique capabilities. A more detailed discussion will be found in Taylor et al. (2000).

At distances of 2 to 10 kpc, the 1' angular resolution of the telescope translates into a physical resolution of 0.6 to 3 pc, which has proved to be a good match to significant ISM structures. The interstellar Hi shows extensive filamentary structure on these scales both within the Galactic plane (Normandeau et al. 1997) and at high latitudes (Joncas et al. 1992). While the origin of the fine structure in this emission is not completely understood, it seems very often to be related to interfaces between the atomic phase and some other constituent of the ISM, or to energy injection, as from stellar winds.

Experience with the telescope has shown that there are significant ISM phenomena which extend over many degrees, but require arcminute angular resolution for their detection. An example is provided by the discovery of a Galactic chimney by Normandeau et al. (1996). This structure is a conduit for radiation and material from the disk to the halo, and it is outlined by a sharp interface between the Hi exterior and the ionized interior. Its size is $\sim5$$^\circ$, equivalent to several hundred pc at the distance of the Perseus Arm ($\sim2$ kpc). It is nevertheless extremely difficult to detect in the survey data of Hartmann & Burton (1997): even though that survey has much higher nominal sensitivity than the Synthesis Telescope, its angular resolution is only 36'.

Similar considerations apply to the detection of thermal and non-thermal continuum objects. The continuum sensitivity of the telescope (listed in Table 7) translates into sensitivity to thermal emission measure (at both frequencies) of $\sim27$ cm^-6 pc and sensitivity to synchrotron emission of $\sim4 \ 10^{-23}$ W m^-2 Hz^-1 sr^-1, also at both frequencies. An example is provided by the detection of large supernova remnants (SNRs) of low surface brightness. For example, Landecker et al. (1990) discovered the SNR G65.1+0.6, whose size is $90' \times 51'$. The overall surface brightness of this object is about half of the nominal rms sensitivity quoted above, indicating the importance of filamentary structure in making such objects detectable.

Polarization imaging at 1420 MHz allows detailed analyses of discrete emitters of polarized radiation, particularly SNRs. For example, Leahy et al. (1997) have mapped the polarized emission from the full extent of the Cygnus Loop. However, only part of the SNR is strongly polarized, with some of the shell emission depolarized by strong Faraday rotation within a thermal electron component mixed in with the compressed fields and synchrotron emitting particles.

While SNRs are prominent objects in Stokes I images, most are barely significant in images of Stokes Q or U. The dominant feature in all polarization images made close to the Galactic Plane is widespread, low-level structure, seen most prominently in polarization angle. This structure is understood as the result of Faraday rotation in the magnetized ISM acting on polarized signals from the diffuse Galactic synchrotron radiation. It tells us less about the emitter than it does about the medium through which the polarized radiation has travelled.

The study of this phenomenon is providing a new window on the ISM, through its sensitivity to magnetic fields and low-density ionized material. The frequency of 1420 MHz seems ideal for such studies near the Galactic plane, where the properties of the Faraday screen lead to a change in polarization angle that is neither too small to produce measurable changes nor so large that it causes depolarization within the beam or the bandwidth of the telescope. The large elliptical feature seen in Fig. 7 (lower left panel), for example, which is very uniform in structure in contrast to its chaotic surroundings, is understood as just such a manifestation of ionized material in the inter-arm region (Gray et al. 1998). Other polarization results (Gray et al. 1999) have led to the detection of an extended envelope of a large Hii region, and the measurement of the magnetic field in that envelope. In both cases, the Hii regions themselves produce depolarization due to the turbulent, high-density ionized material within them.

Acknowledgements

The development and construction of the DRAO Synthesis Telescope has been the work of many people over many years. Notable for their skill and dedication have been Jean Bastien, Ron Casorso, Ed Danallanko, Jack Dawson, Ron McDougall, Harry Mielke, Diane Parchomchuk, Ev Sheehan and Rod Stuart. The work has also involved many students and others who have worked at DRAO for shorter periods. We are indebted to them all. The DRAO Synthesis Telescope is operated as a national facility by the National Research Council of Canada. Graduate students who have worked on the development of the telescope have been supported by grants to TLL, DR, and JFV from the Natural Science and Engineering Research Council.