tokens: domain to a 4 kpc × 4 kpc × 24 kpc region of the Galaxy. The Galactic magnetic field model of Jansson Farrar ( 2012a ) has azimuthal variations in strength and direction of the field. We chose the galactocentric azimuthal angle to roughly agree with that of the point of impact 70 Myr ago shown in Nichols Bland-Hawthorn ( 2009 ), Figure 3 . The minihalos are necessarily constrained by the size of the domain and fill spheres of 2 kpc radius (the domain half-width). In Figure 1 , the minihalo barycenters are indicated by the “ X ” markers, and their simulation domain half-mass radii (distinct from r e ) are shown as black circles. Figure 1. Initial conditions for each simulation along slices through the y = 0 plane. log 10 of hydrogen number density is represented by the continuous color scale. log 10 of magnetic field strength is represented by contours. The dark matter barycenter is marked with an “ X ” and a circle at the half-mass radius. From left to right, the panels show Runs 1–6, as in Table 1 . 3. RESULTS All six clouds behave similarly as they fall through the halo. This can be seen in Figure 2 which shows the clouds at epoch t 1 (35 Myr), just before impact. As they pass through the halo, the clouds lose material due to ram pressure stripping. By epoch t 1 the clouds with minihalos have lost 16%–18% of their original baryonic mass, while those without have lost ∼36% (see Table 1 ). During this stage, the effect of the weak magnetic field is merely to become swept up and wrapped around the cloud. This is similar to the behavior in the 2D MHD simulations of Konz et al. ( 2002 ). Figure 2. Same as Figure 1 , but for epoch t 1 , equivalent to t = 35 Myr for all runs. Figure 3. Same as Figure 1 , but for epoch t 2 , equivalent to t = 52 Myr for Runs 1 and 2 and t = 45 Myr for Runs 3–6. Each of the clouds approaches the disk with a similar velocity (∼300 km s −1 ). However, the dark matter confined clouds are more compact and contain more baryonic mass and so carry larger kinetic energy densities than the dark-matter-free clouds. The collision of the cloud with the Galactic disk occurs at 36 Myr for those runs with a minihalo, and 37 Myr for the dark-matter-free runs. At the moment of collision, the minihalo separates from the cloud gas and continues, unhindered, through the Galactic disk. During the collision, the gas cloud ceases to exist as it mixes with and transfers momentum to the disk gas. This results in a hole in the disk and a long-lived, bubble-shaped structure filled with moderately dense (∼0.1 to ∼3 H cm −3 ), cool gas (∼1000 K) below the disk, as shown in Figure 3 at 52 Myr for Runs 1 and 2 and 45 Myr for Runs 3–6 (epoch t 2 ). These structures are composed of a mixture of gas from both the cloud and the disk, with the majority of their mass originating from the disk. Tenorio-Tagle et al. ( 1986 ) found similar structures in 2D hydrodynamic simulations of dark-matter-free clouds colliding with the Galactic thin disk. Over
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