Dark supernova remnant

An almost perfect round hole of CO-line emission with a diameter of 3.7 pc was found in a molecular cloud (MC) centered on G35.75-0.25 (l = 35 ${{^{\circ}_{.}}}$ 75, b = -0 ${{^{\circ}_{.}}}$ 25) at radial velocity of 28 km s^-1. The hole is quiet in radio continuum emission, unlike the usual supernova remnants (SNR), and the molecular edge is only weakly visible in 8 and 24 μm dust emissions. The hole may be either a fully evolved molecular bubble around a young stellar object (YSO), or a relic of a radio-quiet SNR that has already stopped expansion after rapid evolution in the dense MC as a buried SNR. Because G35.75 exhibits quite different properties from YSO-driven bubbles of the same size, we prefer the latter interpretation. The existence of such a "dark" SNR would affect the estimation of the supernova rate, and therefore the star formation history, in the Galaxy.