Spectropolarimetry of the superwind filaments of the starburst galaxy M 82 II: Kinematics of the dust surrounding the nuclear starburst

Abstract

We performed deep spectropolarimetric observations of a prototypical starburst galaxy M82 with the Subaru Telescope in order to study the kinematics of the dust outflow. We obtained optical polarized emission-line spectra up to ∼4 kpc away from the nucleus of the galaxy along three position angles, 138 • , 150 • and 179 • within the conical outflowing wind (superwind). The Hα emission line in the superwind is strongly polarized and the polarization pattern shows dust scattering of central light sources, being consistent with the previous works. The intensity weighted polarization degree of the Hα line reaches ∼30% at maximum. There are at least two light sources at the central region of the galaxy; one of which is located at the nearinfrared nucleus and the other resides at one of the peaks of the 3 mm radio and molecular gas emission. The outer (> 1 kpc) dust is illuminated by the former, whereas the inner dust is scattering the light from the latter. We investigated as well the dust motion from the velocity field of the polarized Hα emission line. The dust is accelerated outward on the northwest side of the nucleus. A simple bi-conical dust outflow model shows that the outflow velocity of the dust reaches > ∼ 300 − 450 km s −1 at ∼ 4 kpc from the nucleus, suggesting that some portion of the dust escapes from the gravitational potential of M82 into the intergalactic space. At some regions on the southeast side, in particular along the position angle of 138 • , the dust has radial velocity slower than the systemic velocity of the galaxy, apparently suggesting inflowing motion toward the nucleus. These components are spatially consistent with a part of the molecular gas stream, which is kinematically independent of the outflow gas, thus the apparent inflow motion of the dust reflects the streaming motion associated with the molecular gas stream.

Cool outflows in galaxies and their implications

“…The warm dust in this screen will also produce polarized thermal emission, but in this case the polarization is parallel to the magnetic field. Spectropolarimetry in the UV-optical has also been used to measure the velocity of the dusty scatterers relative to that of the line-emitting gas (e.g., Yoshida et al 2011Yoshida et al , 2019.…”

“…Given the (tentative) evidence for elevated dust-to-gas ratios in the extraplanar material of some galaxies (Sect. 5.3) and decoupled dust/gas velocities in the ionized wind of M82 (Yoshida et al 2011(Yoshida et al , 2019, the possibility of the dust grains decoupling dynamically from the gas due to the stronger radiation forces acting on them should also be investigated, keeping tract of the sizes and charges of dust grains and the collisions, Coulomb interactions, and Lorentz forces with the molecular, neutral-atomic, and ionized gas making up the outflow (Draine and Salpeter 1979b, a;Krumholz and Thompson 2013;Thompson et al 2015).…”

“…The polarization traces an ordered component of the magnetic field that is mostly oriented along the axis of the outflow, perpendicular to the minor axis of M 82. Spectropolarimetric measurements in the optical emission lines suggest that the dust is moving substantially slower than the ionized gas in the wind of M 82 (Yoshida et al 2011(Yoshida et al , 2019.…”

Hydrogen recombination near-infrared line mapping of Centaurus A with IRSF/SIRIUS

“…The dust-scattered Hα emission from the host galaxy is actually observed in the galactic halo of M 82 (Yoshida et al 2019). Assuming that it is also the case in Cen A, we measured the Paβ flux of the host galaxy with an aperture radius of 2.…”

“…On the other hand, there is a possibility of the scattering of Hα photons from a host galaxy by dust grains in halo clouds. At about 4 kpc far from the galaxy disk of M 82, strongly polarized Hα emission is observed, and its polarization pattern suggests that the Hα emission originates from the dust scattering of Hα photons coming from the central near-InfraRed (IR) nucleus (Scarrott et al 1991;Yoshida et al 2019). Therefore, in order to evaluate the effect of the AGN feedback accurately, it is important to investigate whether or not the hydrogen recombination line is truly of the origin of star formation therein.…”