Distribution of the high-velocity clouds in the Galactic halo

Universidad Nacional de La Plata, Facultad de Ciencias Astronómicas y Geofísicas, Paseo del Bosque, 1900 La Plata, Argentina e-mail: colano@lilen.fcaglp.unlp.edu.ar

Received: 27 March 2007
Accepted: 19 March 2008

Abstract

Context. There is a connection between the kinematics and sky distribution of the high velocity clouds (HVCs) and the spatial velocity and orbital plane of the Magellanic Clouds (MCs) that allow us to conclude that most HVCs were ejected from the Clouds, some hundreds of  Myrs ago, probably by an encounter between the Large (LMC) and Small Magellanic Cloud (SMC).

Aims. Based on the idea that HVCs have progenitors in the MCs, we attempt determine theoretical distances to the HVCs using observed data of the sky positions and radial velocities, and thus to map the spatial distribution of the HVCs.

Methods. i) In the context of a kinematical model we have found an analytical formula that gives the distance of an HVC; ii) we have developed two dynamical models in order to study the orbits of the HVCs under the gravitational forces exerted by the Galaxy and the MCs and forces of resistance due to the movement of the HVCs through the Galactic gaseous disk. We have adopted the gravitational potential of the Milky Way given by a dark matter halo (Model 1) plus a massive thick disk partially composed by dark matter (Model 2). By means of a fitting to certain Galactic, MC and HVC conditions we have obtained values for the free parameters of the Models. We have obtained the dynamical distances to the HVCs by constructing the theoretical relationship between the radial velocity and the distance in the line of sight of each HVC.

Results. The HVCs can be divided into three major groups: Population MS, Population W and Population A-C. Population MS comprises the Magellanic stream and the leading arm of the Magellanic system. The HVCs of this population are dispersed around the MCs within a volume of ≈ 200 kpc 200 kpc 60 kpc. Population W is situated at a mean distance of ≈15 kpc from the Sun, and has been braked by the gas of the Galactic disk. Population A-C includes Complexes A, C, H, L, M, and ACHVC. The dynamical models give two solutions for the distance of each HVC of Population A-C. The near-distance solutions place Population A-C at ≈6 kpc from the Sun, showing that this chain of HVCs started to interact with the outer parts of the Galactic gaseous disk about 100 Myr ago. While, the spatial distribution of Population A-C represented with the far distances has the shape of a gigantic ring with a radius of ≈125 kpc. Our results support the idea that the LMC-SMC encounter triggered a period of star formation bursts, in which part of the LMC bar was probably formed. The interactions of a few 10⁵ massive stars formed in this era with the then-existing low metallicity ISM of the MCs would have created the HVCs