Radio-continuum spectra of ram-pressure-stripped galaxies in the Coma Cluster

Abstract

The population of galaxies in the local Universe is bi-modal in terms of the specific star formation rate. This fact has led to a broad distinction between star-forming galaxies (typically cold-gas-rich and late-type) and quenched galaxies (typically cold-gas-poor and early-type). The ratio between quenched and star-forming galaxies is much higher in clusters than the field, and pinpointing which are the physical processes driving this excess quenching in clusters is an open question. We used the nearby Coma Cluster as a laboratory to probe the impact of ram pressure on star formation as well as to constrain the characteristic timescales and velocities for the stripping of the non-thermal interstellar medium. We used high-resolution ($6.5 kpc $), multi-frequency ($144\ MHz GHz $) radio continuum imaging of the Coma Cluster to resolve the low-frequency radio spectrum across the discs and tails of 25 ram-pressure-stripped galaxies. With resolved spectral index maps across these galaxy discs, we constrained the impact of ram pressure perturbations on galaxy star formation. We measured multi-frequency flux-density profiles along each of the ram-pressure-stripped tails in our sample. We then fitted the resulting radio continuum spectra with a simple synchrotron ageing model. We show that ram-pressure-stripped tails in Coma have steep spectral indices ($-2 -1$). The discs of galaxies undergoing ram pressure stripping have integrated spectral indices within the expected range for shock acceleration from supernovae ($-0.8 -0.5$), though there is a tail towards flatter values. In a resolved sense, there are gradients in the spectral index across the discs of ram-pressure-stripped galaxies in Coma. These gradients are aligned with the direction of the observed radio tails, with the flattest spectral indices being found on the `leading half'. From best-fit break frequencies, we estimate the projected plasma velocities along the tail to be of the order of hundreds of kilometres per second, with the precise magnitude depending on the assumed magnetic field strength.