The Interstellar Medium in Starburst Galaxies with Inner Lindblad Resonances: M82 and M83

Facilities: JCMT (RxA, RxB, RxC)
Collaborator: Dr. C. D. Wilson
Status: Published in The Astrophysical Journal (see Petitpas & Wilson 1998, 2000)

Summary: We have studied the molecular gas in two nearby barred starburst galaxies that exhibit a double lobed morphology thought to indicate the presence of an Inner Lindblad Resonance (ILR). In M83, we found that while the lower J CO maps showed a double peaked appearance, the higher J maps suggested a single central peak. This suggests that molecular gas is being transported interior to the ILR, but is being heated such that it is emitting predominantly at higher J transitions of CO. In M82 we found temperature and density gradients that increase in opposing directions across the galaxy center. We believe this to be the result of vigorous star formation that both heats and depletes the molecular gas in the south-west side of M82.

M83 is a strongly barred galaxy containing a nuclear starburst. High resolution CO J=1-0 maps suggest that it has a twin-peaked distribution of molecular gas, indicating the presence of an Inner Lindblad Resonance (ILR). The question we wished to address is `How can there be a nuclear starburst if the molecular gas is getting trapped at the ILR?'. To answer this we mapped M83 in the higher J transitions of carbon monoxide as well as atomic carbon emission. We found M83 exhibits a `double-peaked' structure in the lower J transitions of CO, consistent with predictions of gas collection at an Inner Lindblad Resonance, but that this structure was not as predominant in the higher J transitions. In fact, in the nucleus the CO J=3-2 and CO J=1-0 maps show the double-peaked structure, while the CO J=4-3 maps are centrally concentrated (at approximately the recession velocity of M83). This suggests that the molecular gas is getting inside the ILR, but is being heated sufficiently that it is not showing up in the lower J transitions of CO. This also indicates that the lower J transitions of CO are not a good tracer of molecular gas in starburst galaxies, contrary to common belief. We also found that the atomic carbon emission seems to originate from the same hot regions of the galaxy as the high J transitions of CO, providing valuable insight into the conditions that may be conducive to the formation of CI (see Petitpas & Wilson 1998).

The nearby starburst galaxy M82 has long been considered the `prototypical' starburst galaxy. Its close proximity and strong CO emission make it one of the best studied extragalactic objects at sub-millimeter wavelengths. Previous 12CO and 13CO J=1-0 maps of M82 showed a double-peaked morphology that has long been thought to be a molecular torus of gas seen edge-on. In Petitpas & Wilson (2000) we present new data taken in the the higher J transitions of 12CO, 13CO, and C18O. Using these data in conjunction with radiative transfer models, we have determined that the double-peaked structure is likely the result of two individual clumps of molecular gas collecting at an ILR rather than an edge-on molecular torus. We also noticed some interesting variations in the emission line strengths at the different transitions. These variations were modeled using a radiative transfer code, and it was found that the unusual gradients in brightness could best be explained by a density gradient increasing from southwest to north-east, in conjunction with a temperature gradient that increases in the opposite direction. These gradients may be the result of an interaction with the nearby galaxy M81 which could be triggering increased star formation that both heats and depletes molecular gas in the south-west lobe of M82.



References

Petitpas, G. R., & Wilson, C. D., 1998, ApJ, 503, 219

Petitpas, G. R., & Wilson, C. D., 2000, ApJ, 538, L117