H i Kinematics along the Minor Axis of M82

Abstract

M82 is one of the best studied starburst galaxies in the local universe, and is consequently a benchmark for studying star formation feedback at both low and high redshift. We present new VLA H I observations that reveal the cold gas kinematics along the minor axis in unprecedented detail. This includes the detection of H I up to 10 kpc along the minor axis toward the South and beyond 5 kpc to the North. A surprising aspect of these observations is that the line-of-sight H I velocity decreases substantially from about 120 km s −1 to 50 km s −1 from 1.5 to 10 kpc off the midplane. The velocity profile is not consistent with the H I gas cooling from the hot wind. We demonstrate that the velocity decrease is substantially greater than the deceleration expected from gravitational forces alone. If the H I consists of a continuous population of cold clouds, some additional drag force must be present, and the magnitude of the drag force places a joint constraint on the ratio of the ambient medium to the typical cloud size and density. We also show that the H I kinematics are inconsistent with a simple conical outflow centered on the nucleus, but instead require the more widespread launch of the H I over the ∼ 1 kpc extent of the starburst region. Regardless of the launch mechanism for the H I gas, the observed velocity decrease along the minor axis is sufficiently great that the H I may not escape the halo of M82. We estimate the H I outflow rate is much less than 1 M ⊙ yr −1 at 10 kpc off the midplane.

Cool outflows in galaxies and their implications

“…The total mass outflow rate in the colder phases seems to be considerably higher than that in the hot wind fluid, as modeled by Strickland and Heckman (2009). Martini et al (2018) point out that the H I kinematics are not compatible with an outflow launched only from the nuclear region, but instead require launching over a ∼ 1 kpc region that matches better the extent of the starburst. The kinematics are also not compatible with H I formation through cooling from the much faster X-ray emitting hot phase.…”

Cosmic rays and magnetic fields in the core and halo of the starburst M82: implications for galactic wind physics

“…However, note that we have ignored the magnetic tension force, which would change the total magnetic force depending on the global field topology. Indeed, the strong magnetic field could in principle trap material near M82 and inhibit an outflow, perhaps as seen in the HI wind dynamics reported by Martini et al (2018). Much more work is required to understand the dynamical importance of the magnetic field for wind driving.…”

“…We incorporate a spherical radial CR advection velocity profile, where the normalization of the wind density profile, M/V n , and the asymptotic CR advection speed, V 0 , are allowed to vary. The observed wind itself has been extensively studied and has been shown to have multiple components with different velocities, ranging from somewhat slower cold/cool molecular and neutral gas (Leroy et al 2015;Martini et al 2018)), to high velocity warm ionized gas of ∼600 km s −1 (Shopbell & Bland-Hawthorn 1998). Additionally, X-ray emission indicates the presence of a hot component that would have a large asymptotic velocity of ∼1000−2000 km s −1 (Strickland & Heckman 2009).…”

MusE GAs FLOw and Wind (MEGAFLOW) IV. A two sightline tomography of a galactic wind

“…Note that this potential velocity difference between the two sightlines could indicate deceleration of the gas with distance from the galaxy, as the quasar sightline is probing gas at a larger impact parameter than the back1 sightline does (16.8 kpc vs 8.8 kpc). Strong, nongravitational, deceleration in an outflow could be due to drag forces (in observations e.g., Martini et al 2018; in simulations e.g., Oppenheimer et al 2010). However, this interpretation would require the strong assumption that the two opposite cones have the same velocity profile.…”