The Seahorse Nebula: New views of the filamentary infrared dark cloud G304.74+01.32 from SABOCA, Herschel, and WISE

Abstract: We mapped the filamentary infrared dark cloud (IRDC) G304.74+01.32 at 350 $\mu$m with the SABOCA bolometer. The new SABOCA data have a factor of 2.2 times higher resolution than our previous LABOCA 870 $\mu$m map of the cloud. We also employed the Herschel far-IR and submillimetre, and WISE IR data available for G304.74. The SABOCA data show that G304.74 is composed of a dense filamentary structure with a mean width of only $0.18\pm0.05$ pc. The percentage of LABOCA clumps that are found to be fragmented into SABOCA cores is $36\%\pm16\%$. The WISE data suggest that $65\%\pm18\%$ of the SABOCA cores host young stellar objects (YSOs). The mean dust temperature of the clumps, derived by comparing the Herschel/SPIRE flux densities, was found to be $15.0 \pm 0.8$ K. The mean mass, beam-averaged H$2$ column density, and H$_2$ number density of the LABOCA clumps are estimated to be $55\pm10$ M${\odot}$, $(2.0\pm0.2)\times10^{22}$ cm$^{-2}$, and $(3.1\pm0.2)\times10^4$ cm$^{-3}$. The corresponding values for the SABOCA cores are $29\pm3$ M$_{\odot}$, $(2.9\pm0.3)\times10^{22}$ cm$^{-2}$, and $(7.9\pm1.2)\times10^4$ cm$^{-3}$. The G304.74 filament is estimated to be thermally supercritical by a factor of $\gtrsim3.5$ on the scale probed by LABOCA, and by a factor of $ \gtrsim1.5$ for the SABOCA filament. Our data strongly suggest that G304.74 has undergone hierarchical fragmentation. The IRDC G304.74 has a seahorse-like morphology in the Herschel images, and the filament appears to be attached by elongated, perpendicular striations. Besides the presence of perpendicularly oriented, dusty striations and potential embedded intermediate-mass YSOs, G304.74 is a relatively nearby ($d\sim2.5$ kpc) IRDC, which makes it a useful target for future star formation studies. Owing to its observed morphology, we propose that G304.74 could be nicknamed the Seahorse Nebula.