AAS 198th Meeting, June 2001
Session 60. Galactic Halo

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[60.03] Modelling the Formation of HI Clouds in the Galactic Halo

M.A. de Avillez, M.-M. Mac Low (Amer. Mus. Natural Hist.)

Large scale modelling of the cycle of gas between the disk and the halo has been carried out with a 3D adaptive mesh refinement code. The model includes a gravitational field provided by the stars in the disk, an ideal-gas equation of state, and an approximation for the cooling curve, assuming collisional ionization equilibrium. Supernovae are set up at the beginning of their Sedov phases at a rate compatible with observations. Sixty percent of the SNe are set up within associations and the rest are set up at random sites.

After a transient startup period of roughly 200 Myr, dynamical balance between upward and downward flowing gas is reached. Gas at the disk-halo interface (z~1.5 kpc) flows into the halo in a turbulent convective flow at a rate of ~6 M\odot~yr-1, forming a large scale fountain. Ascending gas cools into clouds, with a large range of velocities, which we identify with observed H~{\sc i} clouds. These in turn rain down upon the disk. The descending clouds interact with the thick gas disk and eventually impact onto the thin disk leading to its deformation and disruption. The sizes of the clouds vary from a few pc to several tens of pc and their distribution in the halo varies with z. Intermediate velocity clouds (IVCs) are mainly distributed between z=0.8 and 4.2 kpc, whereas a large fraction

of high velocity clouds (HVCs) are found at greater heights. On average 55-60% of the clouds have negative velocities.

During a period of 50 Myr, approximately 18% of the total number of clouds have velocities between -90 and -160 km/s, whereas only ~3% of the clouds have vz<-160 km/s. The bulk of the HI clouds detected in the simulations have intermediate positive (40 to 90 km/s) and negative (-40 to -90 km/s) velocities. The former constitutes ~0% and the latter constitutes 25-27% of the total number of HI clouds detected.

Most of the clouds show a multiphase structure with a core of cold gas, having temperatures of some 103 K, embedded in a warmer phase.

M-MML and MA were partially supported by an NSF CAREER grant AST99-85392.


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