Enigmatic ice leaves cold trail

Abstract

Solidified molecular material, called ices, can often be found in the cold, dense regions of space, such as molecular clouds or the environs around protostars. In 2007, the AKARI satellite serendipitously detected two icy objects during an infrared spectral Galactic plane survey. Takashi Shimonishi and colleagues have now performed follow-up observations with ALMA, discovering that the two regions do not bear the hallmarks of either molecular clouds or young stellar objects (YSOs), meaning that their nature is unknown, and potentially unique.

Both objects feature deep absorption features due to H₂O, CO₂ and CO ices in AKARI 2.5–13 µm spectra. The ALMA observations detected compact emission of both CO (J = 3–2) and SiO (J = 8–7) lines, and using kinematic information in the CO spectra, the sources were located at distances of 9.3 and 13.4 kpc, towards the Crux-Scutum arm of the Galaxy. Both appear to be compact (100–1000 au) and isolated from their surrounding molecular material, but are in areas of high visual extinction (A_V ~ 100 mag). Their spectral energy distributions peak around 5 µm, unusually blue for such dust-enshrouded objects. Both objects are incompatible with standard spectral energy distribution models of embedded YSOs. No dark molecular clouds are known in their locations, and adjacent stars are detected optically. The infrared characteristics are also incompatible with other potential sources of ices: protoplanetary disks, OH/IR stars, certain types of galaxy.