The Radio Sky

The sky looks dramatically different at different wave bands and this is the primary reason multi-wavelength astronomy is interesting. In the optical band, the dominant emitters are stars, luminous clouds of gas, and galaxies all of which are thermal sources with temperatures in the range $10^3 - 10^4$ K. At these temperatures the emitted spectrum peaks in the optical band. Sources with temperatures outside this range and emitters of non thermal radiation are relatively weak emitters in the optical band but can be strong emitters in other bands. For example, cold ($\sim 100$ K) objects emit strongly in the infra red and very hot objects ( $> 10^5$ K) emit strongly in X-rays. Since the universe contains all of these objects one needs to make multiband studies in order to fully understand it.

For a thermal source with temperature greater than 100 K, the flux density in the radio band can be well approximated by the Rayleigh-Jeans Law^2.1, viz.

$$S = (2kT/\lambda^2) d\Omega$$ (2.2.2)

The predicted flux densities at radio wavelengths are miniscule and one might hence imagine that the radio sky should be dark and empty. However, radio observations reveal a variety of radio sources all of which have flux densities much greater than given by the Rayleigh-Jeans Law, i.e. the radio emission that they emit is not thermal in nature. Today it is known that the bulk of radio emission is produced via the synchrotron mechanism. Energetic electrons spiraling in magnetic fields emit synchrotron radiation. Unlike thermal emission where the flux density increases with frequency, for synchrotron emitters, the flux density increases with wavelength (see Figure 2.1). Synchrotron emitting sources are hence best studied at low radio frequencies.

Figure 2.1: Intensity as a function of frequency (``power spectra'') for synchrotron (dashed) and thermal (solid) radio sources.

The dominant sources seen in the radio sky are the Sun, supernova remnants, radio galaxies, pulsars etc. The Sun has a typical flux density of $10^5$ Jy while the next strongest sources are the radio galaxy Cygnus A and the supernova remnant Cassiopeia A, both of which have flux densities of $\sim 10^4$ Jy. Current technology permits the detection of sources as weak as a few $\mu$Jy. It turns out also that not all thermal sources are too weak to detect, the thermal emission from large and relatively nearby HII regions can also be detected easily in the radio band.

Radio emission from synchrotron and thermal emitters is ``broad band'', i.e. the emission varies smoothly (often by a power law) over the whole radio band. Since the spectrum is relatively smooth, one can determine it by measurements of flux density at a finite number of frequencies. This is a major advantage since radio telescopes tend to be narrow band devices with small frequency spreads ( $\Delta \nu/\nu \sim 0.1$). This is partly because it is not practical to build a single radio telescope that can cover the whole radio-band (see eg. Chapter 3) but mainly because radio astronomers share the radio band with a variety of other users ( eg. radar, cellular phones, pagers, TV etc.) all of who radiate at power levels high enough to completely swamp the typical radio telescope. By international agreement, the radio spectrum is allocated to different users. Radio astronomy has a limited number of protected bands where no one else is permitted to radiate and most radio telescopes work only at these protected frequencies.

Several atoms and molecules have spectral lines in the radio band. For example, the hyperfine transition of the Hydrogen atom corresponds to a line with a wavelength of $\sim 21$cm. Since atomic hydrogen (HI) is an extremely abundant species in the universe this line is one of the brightest naturally occurring radio lines. The HI 21cm line has been extensively used to study the kinematics of nearby galaxies. High quantum number recombination lines emitted by hydrogen and carbon also fall in the radio band and can be used to study the physical conditions in the ionized interstellar medium. Further the radio line emission from molecules like OH, SiO, H$_2$O etc. tend to be maser amplified in the interstellar medium and can often be detected to very large distances. Of course, these lines can be studied only if they fall within the protected radio bands. In fact, the presence of radio lines is one of the justifications for asking for protection in a specific part of the radio spectrum. While many of the important radio lines have been protected there are many outside the protected bands that cannot be studied, which is a source of concern. Further, with radio telescopes becoming more and more sensitive, it is possible to study lines like the 21cm line to greater and greater distances. Since in the expanding universe, distance translates to a redshift, this often means that these lines emitted by distant objects move out of the protected radio band and can become unobservable because of interference.

Footnotes

... Law^2.1

The Rayleigh-Jeans Law, as can be easily verified, is the limit of the Plank law when $h\nu << kT$. This inequality is easily satisfied in the radio regime for generally encountered astrophysical temperatures.