2016MNRAS.455..511D

We report on the third epoch of very long baseline interferometry (VLBI) observations of the radio-bright supernova SN 2011dh, located in the nearby (7.8 Mpc) galaxy M51. The observations took place at t = 453 d after the explosion and at a frequency of 8.4 GHz. We obtained a fairly well-resolved image of the shell of SN 2011dh, making it one of only six recent supernovae for which resolved images of the ejecta are available. SN 2011dh has a relatively clear shell morphology, being almost circular in outline, although there may be some asymmetry in brightness around the ridge. By fitting a spherical shell model directly to the visibility measurements we determine the angular radius of SN 2011dh's radio emission to be 636±29 µas. At a distance of 7.8 Mpc, this angular radius corresponds to a linear radius of (7.4±0.3)x10¹⁶ cm and an average expansion velocity since the explosion of 19  000^+2800_-2400 km/s. We combine our VLBI measurements of SN 2011dh's radius with values determined from the radio spectral energy distribution under the assumption of a synchrotron-self-absorbed spectrum, and find all the radii are consistent with a power-law evolution, with R ∼ t0.97±0.01, implying almost free expansion over the period t = 4 d to 453 d.