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