Gaussian decomposition of HI surveys. IV. Galactic intermediate- and high-velocity clouds

Abstract

Traditionally IVC and HVC were defined to be concentrations of HI gas, with line-of-sight velocities that are inconsistent with data on the differential rotation of the Galaxy. We demonstrate that IVCs and HVCs can be identified from certain density enhancements in (V_C, FWHM) distribution of Gaussians, representing the Galactic HI 21 cm radio lines. We study the Gaussians, which parameters fall into the regions of the phase-space density enhancements about $(V_C, FWHM) = (-131, 27), (164, 26) and (-49 km/s, 23 km/s). The sky distribution of the Gaussians, corresponding to the first two concentrations, very well represents the sky distribution of HVCs, as obtained on the basis of the traditional definition of these objects. The Gaussians of the last concentration correspond to IVCs. Based on this identification, the division line between IVCs and HVCs can be drawn at about |V_C| = 74 km/s, and IVCs can be identified down to velocities of about |V_C| = 24 km/s. Traces of both IVCs and HVCs can also be seen in the sky distribution of Gaussians with FWHM = 7.3 km/s. In HVCs, these cold cores have small angular dimensions and low observed brightness temperatures T_b. In IVCs, the cores are both larger and brighter. This definition of IVCs and HVCs is less dependant than the traditional one, on the differential rotation model of the Galaxy. The consideration of line-width information may enable IVCs and HVCs to be better distinguished from each other, and from the ordinary Galactic HI