We present high-resolution optical HST Space Telescope Imaging Spectrograph ( STIS ) observations made with two slits crossing four of the optically brightest starburst clumps in the vicinity of the nucleus of M82 .
These provide H \alpha kinematics , extinction , electron density and emission measures .
From the radial velocity curves derived from both slits we confirm the presence of a stellar bar .
We also find that the super star cluster M82-A1 has a position and radial velocity consistent with it being at the end of one of the unique x _ { 2 } bar orbits formed by an inner Lindblad resonance .
We derive a new model for the orientation of the bar and disc with respect to the main starburst clumps , and propose that clump A has formed within the bar region as a result of gas interactions between the bar orbits , whereas region C lies at the edge of the bar and regions D and E are located further out from the nucleus but heavily obscured .
We derive extremely high interstellar densities of 500–900 cm ^ { -3 } , corresponding to ISM pressures of P / k \approx 0.5 – 1.0 \times 10 ^ { 7 } cm ^ { -3 } K , and discuss the implications of the measured gas properties surrounding the nuclear star clusters on the production and evolution of the galactic wind .
Despite varying pressures , the ionization parameter is uniform down to parsec-scales , and we discuss why this might be so .
Where the signal-to-noise ( S/N ) of our spectra are high enough , we identify multiple emission-line components .
Through detailed Gaussian line-fitting , we identify a ubiquitous broad ( 200–300 km s ^ { -1 } ) underlying component to the bright H \alpha line , and discuss the physical mechanism ( s ) that could be responsible for such widths .
We conclude that the evaporation and/or ablation of material from interstellar gas clouds caused by the impact of the high-energy photons and fast-flowing cluster winds produces a highly turbulent layer on the surface of the clouds from which the emission arises .