Leah Morabito (Leiden) -- Low frequency radio astronomy with LOFAR: spectroscopy and long baselines

The Low Frequency Array (LOFAR) offers a unique combination of spatial and spectral resolution at low radio frequencies. Carbon radio recombination lines (CRRLs) at low frequencies (<500 MHz) trace the cold, diffuse phase of the interstellar medium, which is otherwise difficult to observe. I will present the first ever extragalactic detection of CRRLs, in M82. These CRRLs are seen in absorption, and cross-correlation of a template spectrum with the observed spectrum verifies that they originate at the systemic velocity of M82. By stacking 22 alpha-transitions from frequencies 48-64 MHz, we achieved an 8.5-sigma detection. The average line profile has properties which suggest that the CRRLs are associated with cold atomic gas in the direction of the nucleus of M82. Low-frequency CRRL absorption lines should be detectable at higher redshifts, and high redshift radio galaxies (HzRGs) are ideal candidates to search for CRRLs. The correlation between radio spectral steepness and redshift means that HzRGs will be bright at low frequencies, which is useful in signal-to-noise limited situations. Additionally, LOFAR can help shed light on the still unknown origin of the spectral steepness -- redshift correlation by providing spatially resolved spectral information at low frequencies, which is critical for constraining the particle acceleration processes in HzRGs. I will briefly present an ongoing survey of 11 of these objects, as well as preliminary results using the international stations to achieve ~arcsecond resolution around 60 MHz.