G25.5 + 0.2: a very young galactic supernova remnant

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Abstract
Based on a Very Large Array (VLA) survey of catalogued radio sources in the galactic plane, radio emission has been detected from a compact source which satisfies the criteria for a very young galactic supernova remnant. G25.5 + 0.2 has a shell structure which appears to be at least partially filled, a total integrated flux density at 20 cm of 315 mJy, and a flat spectrum between 2 and 20 cm. Radio-recombination observations of the H 76 α line provide an upper limit to the line-to-continuum ratio of 0.01, indicating that the object is not a conventional compact H II region. Observations at 843 and 327 MHz indicate thermal absorption at low frequencies with a turnover in the spectrum near 1 GHz. The observations are consistent with either non-thermal emission in conjunction with absorbing thermal gas or with very high-velocity thermal gas responsible for both the observed radio emission and the low-frequency turnover. H I line observations establish a minimum distance of 7.2 kpc. Based on an average supernova expansion velocity of 10000 km s−1, the lower limit for the age is approximately 25 yr. A somewhat lower expansion velocity and a greater distance provide an approximate upper limit of 100 yr. G25.5 + 0.2 could be the youngest known supernova remnant in the Galaxy. Observations in the continuum (620.4 nm) and Hα made at Kitt Pfcak National Observatory show no coincidence between objects in the field of view and the position of the radio source.

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