|Annu. Rev. Astron. Astrophys. 2012. 50:
Copyright © 2012 by Annual Reviews. All rights reserved
Cold halo clouds moving through hot diffuse galaxy halos are unlikely to survive for periods greater than a few hundred Myr unless a strong support mechanism is invoked (see below). These short lifetimes are a problem unless the clouds are continually regenerated on short timescales. In addition, some HVCs that are not clearly linked to satellite accretion have velocities that cannot be reached through gravitational acceleration within several hundred Myr (Benjamin & Danly 1997, Peek et al. 2007). The exact lifetime of a cloud depends on several factors such as cloud density, halo density, and velocity, but the total mass of the cloud seems to be one of the largest factors increasing their lifetimes (Heitsch & Putman 2009, Kwak et al. 2011). This is consistent with simulations that show the halo clouds are destroyed primarily via the Kelvin-Helmholtz (KH) instability. The characteristic growth time for the KH instability is tKH 1/2 Rcl / vrel, where is the density contrast between the cloud and the external medium, Rcl is a characteristic length of the cloud, and vrel is the relative speed between the two. The KH instability leads to gas being ablated from the cloud edges and forming a tail of lower column density material. Observationally this is evident in the head-tail clouds found throughout the Galactic halo (Putman et al. 2011b, Brüns et al. 2000, Ben Bekhti et al. 2006, Westmeier et al. 2005b), and potentially some of the detailed structure found along the edges of larger clouds (Peek et al. 2007, Winkel et al. 2011) as shown in Figure 4. O VI observations are also consistent with the interaction of halo clouds with the hot diffuse halo medium (Sembach et al. 2003, Kwak et al. 2011, Fox et al. 2006, Collins et al. 2007).
Possible methods of supporting the clouds to significantly extend their lifetimes include the presence of dark matter (Nichols & Bland-Hawthorn 2009, Braun & Burton 2000), magnetic fields (McClure-Griffiths et al. 2010), and/or shielding by an extended diffuse gaseous component (e.g., the extended medium surrounding the Magellanic Stream). The presence of dark matter extends a gas cloud's lifetime to timescales on the order of a Gyr (Quilis & Moore 2001); however despite the advantage of this preservation there is no strong evidence for dark matter in HVCs. Rather, even the small compact clouds are associated with large gaseous complexes (Figure 1; Saul et al. 2012, Putman et al. 2011b), and it would be difficult for gas to survive in such small dark matter halos through reionization (e.g., Ricotti et al. 2008). A magnetic field may help to preserve the clouds (Konz et al. 2002, Santillan et al. 2004, Kwak et al. (2011)], however most of the simulations done thus far are in two dimensions and some simulations indicate a strong magnetic field can aid in the disruption (Stone & Gardiner 2007).
Dynamical shielding by outer envelope layers may be the most likely mechanism of extending the lifetimes of HI clouds. As shown in Figures 7 and 8, the MW simulation finds cold neutral clouds are embedded within larger, warm, low density structures and Figure 1 shows that this is also observed. In this configuration, the HI clouds experience smaller relative velocities with respect to their immediate surroundings, which increases tKH. Head-tail structures or features indicative of shearing may be less common in the HI component of those clouds that are dynamically shielded by an outer, warmer layer. This may explain why clouds associated with the Leading Arm of the Magellanic System do not have detectable warm gas and also show abundant head-tail structures, while the tail of the Magellanic System has abundant warm diffuse gas and limited head-tail structures (e.g., Putman et al. 2011b).
For the Milky Way, since many of the HI halo clouds are at distances of ~ 10 kpc (or greater in the case of the clouds associated with the Magellanic System), and since the individual clouds within the complexes usually have masses < 105 M, the clouds are unlikely to make the trip through the Galactic halo unless significantly shielded by an outer, warmer component. For other galaxies, the detected large HI clouds are also likely to be collections of smaller clouds. The disrupted HI clouds will become part of the multi-phase halo medium, and any remaining over-densities will sink toward the Galactic disk. Some of the warm and warm-hot gas detected with absorption line systems may be the leftover over-densities from disrupted HI clouds. Whether these over-densities are able to re-cool depends on the magnitude of the over-density, and, if they cool, they may disrupt again before impacting the disk (Joung et al. 2012b, Vietri et al. 1997). The infall of these clouds can dissipate energy and help to heat the extended hot halo medium (Murray & Lin 2004). According to multi-wavelength observations, gaseous halos are complex multiphase places, which is consistent with an ongoing cycle of halo cloud destruction and cooling.