Narrow-line HI and cold structures in the ISM
  Context. In the HI line profiles in the Leiden-Argentina-Bonn (LAB) all-sky
database, we have found a population of very cold HI clouds. So far, the role
of these clouds in the interstellar medium (ISM) has remained unclear. Aims. In
this paper, we attempt to confirm the existence of the narrow-line HI emission
(NHIE) clouds by using the data from the Parkes Galactic all-sky survey (GASS)
and try to find their place among other coldest constituents of the ISM.
Methods. We repeat the search of NHIE with the GASS data and derive or compile
some preliminary estimates for the distribution, temperatures, distances,
linear sizes, column and number densities, masses, and the composition of NHIE
clouds, and compare these data with corresponding estimates for HI
self-absorption (HISA) features, the Planck cold clumps (CC), and infrared dark
clouds (IRDC). Results. We demonstrate that from LAB and GASS we can separate
comparable NHIE complexes, and the properties of the obtained NHIE clouds are
very similar to those of HISA features, but both of these types of clouds are
somewhat warmer and more extended and have lower densities than the cores in
the Planck CC and IRDC. Conclusions. We conclude that NHIE may be the same type
of clouds as HISA, but in different observing conditions, in the same way as
the Planck CC and IRDC are most likely similar ISM structures in different
observing conditions and probably in slightly different evolutionary stages.
Both NHIE and HISA may be an intermediate phase between the diffuse cold
neutral medium and star-forming molecular clumps represented by the Planck CC
and IRDC.
