Sub-arcsecond resolution imaging of M 51 with the International LOFAR Telescope

TL;DR

This study uses the International LOFAR Telescope to produce a high-resolution image of galaxy M 51, revealing detailed supernova and nuclear activity, and highlighting the importance of international baselines for accurate flux measurements.

Contribution

First sub-arcsecond resolution LOFAR imaging of M 51, demonstrating the necessity of international baselines for precise flux density measurements of compact sources.

Findings

Detected SN 2011dh with flux density of 0.97 mJy, smaller than previous measurements.

Placed limits on radio evolution of SN 1994I and SN 2005cs.

Detected the galaxy's nucleus and other sources, including a High mass X-ray Binary.

Abstract

We present an International LOFAR Telescope sub-arcsecond resolution image of the nearby galaxy M 51 with a beam size of 0.436" x 0.366" and rms of 46 $\mu$Jy. We compare this image with an European VLBI Network study of M 51, and discuss the supernovae in this galaxy, which have not yet been probed at these low radio frequencies. We find a flux density of 0.97 mJy for SN 2011dh in the ILT image, which is about five times smaller than the flux density reported by the LOFAR Twometre Sky Survey at 6" resolution using the same dataset without the international stations. This difference makes evident the need for LOFAR international baselines to reliably obtain flux density measurements of compact objects in nearby galaxies. Our LOFAR flux density measurement of SN 2011dh directly translates into fitting the radio light curves for the supernova and constraining massloss rates of progenitor star. We do not detect two other supernovae in the same galaxy, SN 1994I and SN 2005cs, and our observations place limits on the evolution of both supernovae at radio wavelengths. We also discuss the radio emission from the centre of M 51, in which we detect the Active Galactic Nucleus and other parts of the nuclear emission in the galaxy, and a possible detection of Component N. We discuss a few other sources, including the detection of a High mass X-ray Binary not detected by LoTSS, but with a flux density in the ILT image that matches well with higher frequency catalogues.