Kavli Affiliate: Ke Wang
| First 5 Authors: Shenglan Sun, Ke Wang, Xunchuan Liu, Fengwei Xu,
| Summary:
Long and skinny molecular filaments running along Galactic spiral arms are
known as "bones", since they make up the skeleton of the Milky Way. However,
their origin is still an open question. Here, we compare spectral images of HI
taken by FAST with archival CO and Herschel dust emission to investigate the
conversion from HI to H$_2$ in two typical Galactic bones, CFG028.68-0.28 and
CFG047.06+0.26. Sensitive FAST HI images and an improved methodology enabled us
to extract HI narrow self-absorption (HINSA) features associated with CO line
emission on and off the filaments, revealing the ubiquity of HINSA towards
distant clouds for the first time. The derived cold HI abundances,
[HI]/[H$_2$], of the two bones range from $sim$(0.5 to 44.7)$times10^{-3}$,
which reveal different degrees of HI-H$_2$ conversion and are similar to that
of nearby, low-mass star forming clouds, Planck Galactic cold clumps and a
nearby active high-mass star forming region G176.51+00.20. The HI-H$_2$
conversion has been ongoing for 2.2 to 13.2 Myr in the bones, a timescale
comparable to that of massive star formation therein. Therefore, we are
witnessing young giant molecular clouds with rapid massive star formation. Our
study paves the way of using HINSA to study cloud formation in Galactic bones,
and more generally, in distant giant molecular clouds, in the FAST era.
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