Deepest-ever low-frequency observations of Lockman Hole field
30 January 2017
Although the majority of the earliest radio surveys were carried out at very low frequencies, e.g. the Cambridge surveys and the Mills Cross surveys, in more recent years large-area radio surveys have primarily been performed at frequencies around 1 GHz such as the NRAO VLA Sky Survey (NVSS), the Sydney University Molonglo Sky Survey (SUMSS) and the Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey. With the advent of new radio interferometers using aperture arrays such as the LOw-Frequency ARray (LOFAR), we now have the ability to re-visit the low-frequency radio sky and survey large areas down to much fainter flux density levels, and at higher resolution, than these earlier surveys.
In a recent study, CAASTRO researcher Dr Elizabeth Mahoney (Univ. Sydney, formerly at ASTRON, NL) and colleagues present new 150-MHz LOFAR observations of the Lockman Hole field – a well-studied extragalactic field with a wealth of multi-band data, ideal for studying the radio source population. Reaching a noise level of 160 uJy/bm and covering an area of 34 square degrees, this is the deepest low-frequency radio data ever published for this field – more than an order of magnitude deeper than other surveys at this frequency
– providing new insight into the spectral index properties of the faint radio source population.
By cross-matching this new catalogue with other large surveys at different frequencies, the team determined that the vast majority of sources exhibit steep spectra, as expected for a low-frequency selected radio sample. They also found that the median spectral index became slightly flatter as the source flux density decreased down to around 10 mJy before flattening out and remaining constant below this flux level. There is tentative evidence for sources to become flatter between 60 MHz and 150 MHz, but further observations are needed to verify this. By studying such a large field, they were able to identify rarer types of objects such as potential high-redshift galaxies, sources with peaked or ultra-steep radio spectra and even a giant radio galaxy.
The depth reached in these observations closely matches that expected for the full all-sky LOFAR survey that is currently underway. As such, the results obtained here provide insight into what we can expect to detect in these future surveys. However, this study also highlights the need for complementary, sensitive data sets across a wide range in frequency to maximize the scientific return of low-frequency radio surveys.