Gaussian decomposition of H I surveys

IV. Galactic intermediate- and high-velocity clouds

Tartu Observatory, 61602 Tõravere, Tartumaa, Estonia e-mail: urmas@aai.ee

Received: 25 October 2007
Accepted: 22 January 2008

Abstract

Context. Traditionally intermediate- (IVC) and high-velocity hydrogen clouds (HVC) were defined to be concentrations of H I gas, with line-of-sight velocities that are inconsistent with data on the differential rotation of the Galaxy.

Aims. We attempt to demonstrate that IVCs and HVCs can be identified from density enhancements in parameter distributions of Galactic H I 21 cm radio lines.

Methods. To investigate the properties of the 21 cm radio lines, the profiles of “The Leiden/Argentine/Bonn (LAB) Survey of Galactic H I” are decomposed into Gaussian components using a fully automatic algorithm. We focus on some regions with an increased number of Gaussians in phase space, defined by the component central velocity (V_C) and the full width at the level of half maximum (FWHM). To separate the Gaussians responsible for the phase-space density enhancements, we model the width distributions of Gaussian components at equally-populated velocity intervals, using lognormal distributions.

Results. We study the Gaussians, which parameters fall into the regions of the phase-space density enhancements at and , where the indexes indicate the half widths at the level of half maximum (HWHM) of the enhancements. 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 = 74 km s^-1, and IVCs can be identified down to velocities of about = 24 km s^-1. Traces of both IVCs and HVCs can also be seen in the sky distribution of Gaussians with FWHM ≈ 7.3 km s^-1. 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.

Conclusions. When neglecting the general decrease in the amount of gas at higher , the IVCs and HVCs are observed as distinctive maxima in the distribution of the Gaussians, representing the structure of the 21 cm radio lines of the Galactic H I. This definition 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 H I.