The Optical Structure of the Starburst Galaxy M82 – I: Dynamics of the Disk and Inner-wind

We present Gemini-North GMOS-IFU observations of the central starburst clumps and inner wind of M82, together with WIYN DensePak IFU observations of the inner 2× 0.9 kpc of the disk. These cover the emission lines of Hα, [N ii], [S ii], and [S iii] at a spectral resolution of 45–80 km s. The high signal-to-noise of the data is sufficient to accurately decompose the emission line profiles into multiple narrow components (FWHM ∼ 30–130 km s) superimposed on a broad (FWHM ∼ 150– 350 km s) feature. This paper is the first of a series examining the optical structure of M82’s disk and inner wind; here we focus on the ionized gaseous and stellar dynamics and present maps of the relevant emission line properties. Our observations show that ionized gas in the starburst core of M82 is dynamically complex with many overlapping expanding structures located at different radii. Localised line splitting of up to 100 km s in the narrow component is associated with expanding shells of compressed, cool, photoionized gas at the roots of the superwind outflow. We have been able to associate some of this inner-wind gas with a distinct outflow channel characterised by its dynamics and gas density patterns, and we discuss the consequences of this discovery in terms of the developing wind outflow. The broad optical emission line component is observed to become increasingly important moving outward along the outflow channel, and in general with increasing height above/below the plane. Following our recent work on the origins of this component, we associate it with turbulent gas in wind-clump interface layers and hence sites of mass loading, meaning that the turbulent mixing of cooler gas into the outflowing hot gas must become increasingly important with height, and provides powerful direct evidence for the existence of mass-loading over a large, spatially extended area reaching far into the inner wind. We discuss the consequences and implications of this. We confirm that the rotation axis of the ionized emission-line gas is offset from the stellar rotation axis and the photometric major axis by ∼12, not only within the nuclear regions but over the whole inner 2 kpc of the disk. This attests to the perturbations introduced from M82’s past interactions within the M81 group. Finally, finding a turn-over in the stellar and ionized gas rotation curves on both sides of the galaxy indicates that our sight line, in places, extends at least half way through disk, and conflicts with the high levels of obscuration usually associated with the nuclear regions of M82. Subject headings: galaxies: starburst – galaxies: individual: M82 – galaxies: kinematics and dynamics – galaxies: ISM – ISM: kinematics and dynamics