The Baryon cycle of M82

At only 4 Mpc, M82 is the nearest starburst galaxy. As such, it is a perfect laboratory for more distant starbursts. Here I use sub-millimeter CO observations to study the cold (T < 150 K) molecular gas within the starburst. This is a project that I started with Dr. Satoki Matsushita while on a NSF summer fellowship in the East Asian Pacific Summer Institute program (EAPSI). The paper was recently accepted by the AstroPhysical Journal, and a copy of it can be found here. 

Velocity map of the CO(3-2) emission. Notice how the S2 streamer region (upper left) is offset in velocity space by 37 km/s from the disk (i.e. S2 is red while the disk is pink).

Intensity and kinematics

I reduced and mapped CO(2-1) (the upper panel above) and CO(3-2) (the lower panel above) data taken on the SubMillimeter Array (SMA). We also included CO(1-0) emission from previous observations. This data is convolved to a common resolution of 3" (about 50 pc). We then map the intensity (above) and velocity (left) in M82. Above in the intensity maps, we mark some interesting diffuse gas features: an expanding super-bubble (Bub), and three "streamers" of molecular gas which jet out from the disk (S1 through S3). The velocity field (left) shows rotation of the gas within the disk with maximum rotation near 150 km/s. The S2 region stands out in velocity space as blueshifted from the  disk by 37 km/s, implying that the streamer is moving towards the observer relative to the disk of M82.

Temperature and Density

 Using the three CO lines we take a Bayesian approach to calculating the temperature and density of the CO gas using the non-LTE code RADEX. The temperature (upper panel) and density plots are shown on the right. The plots on the right show that the disk has a typical temperature of 100 K and a log mean H2 density of 3.6, consistent with previous observations. The super bubble region has a median H2 density (log(n) of 4), and a temperature of 67 K. Similarly, S2 has a high density (near log(n) = 4) and moderate temperature of 62 K.  

Temperature (upper) and density (lower) map of M82, with the CO(2-1) contours overlaid in black.

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A shocked shell of molecular gas

The density of the molecular outflow shows a thick dense shell of molecular gas with a low density channel punctured in it (figure to the left). We hypothesize that this is the base of a galactic outflow that is being accelerated by a hot, unobserved plasma. This hot plasma also interacts with the molecular gas, and is actually ripping a hole in the shell. This is seen by the low density channel. This molecular shell has a radius of about 100 pc and is centered on an evolved star cluster. These distances and densities are very similar to the distances I find for outflows probed by UV absorption lines. This may indicate that molecular outflows are the beginning of the large scale outflow which is progressively heated as it is accelerated. 

[Fe II] emission map of M82 with the CO(2-1) emission contours overlaid. [Fe II] emission tracers shocked gas. Notice a large knot of [Fe II] emission in the northwestern portion of the disk. This is where the S2 streamer intersects with the disk.

S2: a possible inflow?

[Fe II] 1.6 micron emission traces shocks, and  the Hubble Space Telescope image is shown on the left. There is substantial [Fe II] emission at the base of the S2 streamer, demonstrating that there is a large shock at the interface of S2 and the disk. Additionally, if S2 is in front of the disk we would expect 13 magnitudes of extinction, but we do not observe this in the optical images. We therefore postulate that S2 is a large cloud of cold gas that is falling into the disk of M82 with a velocity of  37 km/s, and mass inflow rate of 3-7 solar masses per year. This infall of cold gas replenishes some the gas lost via star formation and outflows, but it cannot replenish all of the gas. The galaxy is consuming molecular gas too rapidly, and the star formation rate must decrease in the next few Myr.