Galaxy-wide outflows driven by star formation are thought to be crucial drivers in galaxy evolution. Stellar feedback caused by intense central star formation activity can launch such outflows, leading to significant fractions of baryons that escape the main body of the galaxy. While evidence for galactic outflows is manifold, a detailed characterization is restricted to only a few local systems, where the relevant processes can be spatially resolved at high sensitivity.
In particular, the physical characterization of the outflowing gas mass is important as it influences a galaxy’s ability to form stars in the future. In this context the molecular gas phase is particularly relevant because it often carries the dominant mass fraction of all baryons. The fate of the molecular gas in the outflow is itself not clear.

M82 offers a unique laboratory to study galactic winds because of its close proximity (3.5 Mpc or 11.5 million lightyears) and its almost edge-on orientation. Previous observations of the molecular gas, however, could not resolve the structure within the outflow.
In 2019 to 2020, we obtained high-resolution observations over a large field-of-view in M82 with the Northern Extended Millimeter Array (NOEMA) and the IRAM 30m telescope. Compared to previous interferometric observations, we achieve a sensitivity that is 3 times deeper, a synthesized beam area that is 7 times smaller, and cover an area on the sky that is 3 times larger.

The molecular gas in and around M82 at the unprecedented resolution of 30pc. The image shows the peak intensity map of the CO(1-0) line. Krieger et al. (2021, submitted).

The dataset we obtained contains more than a dozen spectral lines featuring an impressive amount of detail. A first paper (Krieger et al. 2021, submitted) presents the observational details, data reduction, imaging and a first analysis of the molecular gas structure. With a watershed structure identification algorithm (Fellwalker), we decompose the molecular gas as traced by CO(1-0) into almost 2000 clouds. The velocity structure is highly complex and now allows us to resolve the gas on the near and far sides of the outflow cones and substructure in the streamers. Comparing cloud properties, such as size, velocity dispersion, mass or density, across the different environments (disk, outflows, streamers) reveals insights into the interaction of starburst and molecular clouds. To first order, the clouds are surprisingly similar on a statistical level despite living in very different environments. The dependence of these properties on the distance from the center of M82, however, reveals differences: clouds in the outflows appear to evaporate on their journey out of the galaxy while clouds in the streamers remain constant in their properties.

Almost 2000 molecular clouds in M82, the streamers (left and right features) and the outflows (towards top and bottom going out from the center). The colorcoding represents the velocity of each cloud. Outflows and streamers show a complex velocity structure with clouds of highly deviating velocities at the same projected position. This implies that we can now resolve the gas on the near and far sides of the outflow cones and resolve substructure in the streamers.

Banner image: M82 in optical and infrared light. The prominent red plumes are the ionized outflow (Hα).
Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF). Image in the public domain.