Low frequency observations are challenging. Radio Frequency Interference (RFI) causes issues (with some 30% of the data being rejected at 150 MHz) and one is required to use widefield imaging techniques to accurately image the data.
Apart from the technical challenges my interest in low frequency observations stems from my PhD project to try and detect radio emission from extrasolar planets (George & Stevens 2007). These results are summarised on the extrasolar planets pages, no detections were made but upper limits to any emission were made. For the detection of extrasolar planets we would ideally prefer to observe at lower frequencies. From these observations we completed a deep 150 MHz survey of a region around Epsilon Eridanus using the GMRT. The survey area was about 2 degrees and with a noise level of 3mJy these were some of the deepest 150 MHz observations at the time. We detected a total of 113 sources with most being point sources. We were able to detect five Ultra-Steep Spectrum sources which are candidate high-redshift radio galaxies (HzRGs).
The Ultra Steep Spectrum radio sources found were followed up with optical observations using the Liverpool Telescope; in all cases no sources are detected down to an R magnitude of ~23. By applying models and using the K-z relation we are able to suggest that these sources are possibly at high redshift. We discuss how 2m class telescopes can help with the identification of HzRGs from large-scale, low-frequency surveys. This is the realms of the 150 MHz GMRT all sky survey (TGSS).
In the above cases we are mostly interested in point sources but at 150 MHz extended sources are very interesting, showing older populations of electrons. I’ve used the GMRT to follow up 150 MHz observations at 610 and 1400 MHz for a few source interesting extended sources. GMRT J0137+4121 was a candidate double–double radio galaxy for which we have also used the VLA-A array at C band to resolve the core. These observations have allowed us to determine that this source is a normal radio galaxy with a core and a one sided jet. Prominent amongst the other extended sources is the giant radio galaxy, 4C39.04.