Warm gas accretion onto the Galaxy

Abstract

We present evidence that the accretion of warm gas onto the Galaxy today is at least as important as cold gas accretion. For more than a decade, the source of the bright H-alpha emission (up to 750 mR) along the Magellanic Stream has remained a mystery. We present a hydrodynamical model that explains the known properties of the H-alpha emission and provides new insights on the lifetime of the Stream clouds. The upstream clouds are gradually disrupted due to their interaction with the hot halo gas. The clouds that follow plough into gas ablated from the upstream clouds, leading to shock ionisation at the leading edges of the downstream clouds. Since the following clouds also experience ablation, and weaker H-alpha (100-200 mR) is quite extensive, a disruptive cascade must be operating along much of the Stream. In order to light up much of the Stream as observed, it must have a small angle of attack (~20 deg) to the halo, and this may already find support in new HI observations. Another prediction is that the Balmer ratio will be substantially enhanced due to the slow shock. We find that the clouds are evolving on timescales of 100-200 Myr, such that the Stream must be replenished by the Magellanic Clouds at a fairly constant rate (>0.1 Msun/yr). The ablated material falls onto the Galaxy as a warm drizzle; diffuse ionized gas at 10^4 K is an important constituent of galactic accretion. We consider the stability of HI clouds falling towards the Galactic disk and show that most of these must break down into smaller fragments that become partially ionized. The Galactic halo is expected to have huge numbers of smaller neutral and ionized fragments. When the ionized component is accounted for, the rate of gas accretion is ~0.4 Msun/yr, roughly twice the rate deduced from HI observations alone.