Nascent Starbursts in Synchrotron-deficient Galaxies with Hot Dust

Abstract

Three nearby galaxies that have abnormally high infrared-to-radio continuum ratios, NGC 1377, IC 1953, and NGC 4491, are investigated with a view to understanding the physical origin of their peculiarity. We review the existing data and present new radio continuum measurements along with near-infrared integral-field spectroscopy and molecular gas observations. The three galaxies have low luminosities but starburst-like infrared colors; in NGC 1377, no synchrotron emission is detected at any wavelength; in IC 1953, the observed synchrotron component is attributable to the spiral disk alone and is lacking in the central regions; and the radio spectrum of NGC 4491 is unusually flat. We also compare and contrast them with NGC 4418, a heavily extinguished galaxy that shares some attributes with them. After examining various scenarios, we conclude that these galaxies are most likely observed within a few megayears of the onset of an intense star formation episode after being quiescent for at least ≈100 Myr. This starburst, while heating the dust, has not produced optical signatures or a normal amount of cosmic rays yet. We briefly discuss the statistics of such galaxies and what they imply for star formation surveys.