Early ALMA Science: The Structure of Molecular Gas in the Antennae Galaxies

• Paper title: ALMA CO and VLT/SINFONI H2 Observations of the Antennae overlap region: mass and energy dissipation
• Authors: Cinthya N. Herrera, Francois Boulanger, Nicole N. H. Nesvadba, Edith Falgarone
• First author’s affiliation: Institue d’Astrophysique Spatiale

The Atacama Large Millimeter Array (ALMA), a brand new billion-dollar submillimeter telescope, is coming online this year.  ALMA will be by far the most capable submillimeter telescope ever built and will be able to image molecular gas in unprecedented detail in nearby galaxies and detect and characterize the same gas at high redshift.  I don’t think I’m exaggerating too much when I say that ALMA will revolutionize our understanding of star formation, the cold ISM, and galaxy evolution.  We still have about a year to wait for results from the finished ALMA array, but it’s just about time to start hearing about early results from observations taken during the commissioning of ALMA.

The submillimeter and near IR observations analysed in this paper. At left, the ALMA CO observations are plotted as red contours on top of K-band image in blue. The VLT/SINFONI observations of shocked molecular hydrogen are shown in the right panel on top of the CO data in black contours. The locations of 106 solar mass super star clusters are marked with asterisks.

Today we’ll be discussing a paper that leverages some of these early ALMA observations as well as observations from the Very Large Telescope (VLT) in Chile to investigate the dynamical state of molecular gas in the ‘overlap’ region of the Antennae Galaxies – two nearby disk galaxies undergoing a major merger.  As the name of this region would imply, this is where the interstellar media of the two galaxies are slamming into each other at relative velocities of hundreds of kilometers per second.  Since gas is collisional, the momentum and energy of the collision are damped away by the emission of copious amounts of radiation in shocks.  The ALMA observations, tuned to detect emission from the CO molecule, are a good tracer of the bulk of the molecular gas in the observed region.  The VLT data, tuned to detect a vibrational emission line of molecular hydrogen, is a good tracer of shocked gas or gas that has been excited by an external ultraviolet radiation field from nearby star formation or AGN activity.  Since most of the gas in the Antennae is well-shielded from UV radiation by copious quantities of intervening dust, the H2 emission mostly traces shocked, turbulent gas.

Above, I’ve reproduced a figure from the paper that details the observations.  The ALMA observations, shown at left as red contours, were performed over three fields – two centered on the cores of each of the galaxies and one field centered on the overlap region.  The VLT observations, shown at right, focus on the super-giant molecular complexes (SGMCs) in the overlap region.

The position-velocity diagram of the emission associated with SGMC2. The emission shows two distinct peaks with separated by a few arc seconds. The emission peaks also seem to be offset in velocity space, slamming into each other with a relative velocity of about 150 kilometers per second.

The authors are able to extract several interesting details about the SGMCs.  First, using the ALMA data and making an assumption about the ratio of CO emission to molecular gas surface density (the so-called CO X-factor – yes, that really is the name), the authors infer that the SGMCs are extremely massive – several hundred million solar masses.  The largest agglomerations of molecular gas in the Milky Way, the giant molecular clouds, contain no more than a few million solar masses of gas.  The star forming regions in the Antennae are thus scaled up by a factor of a hundred or more compared to similar regions in the Milky Way.  Second, when looking at the position-velocity maps for the individual clouds – like the one at right – the authors observe two distinct velocity components that have been superposed in our field of view.  These observations seem to indicate that the individual SGMCs we see in the ALMA and VLT data are actually made up of two or more clouds that are colliding into each other with a relative velocity of about 150 kilometers per second.  Lastly, the authors detect a compact source of H2 emission in the SGMC2 molecular complex.  This object is unresolved in the ALMA observations, but is marginally resolved by the VLT, yielding a size of roughly 50 parsecs, comparable in size to galactic molecular clouds.  However, this compact source contains roughly 10 million solar masses of molecular gas, a factor of a few more than any Milky Way star forming region.  The compact source is also associated with a small ~104 solar mass star cluster and may be in the process of forming a much more massive 106 solar mass super star cluster.  This idea is bolstered by the ratio of emission from the compact source in the vibrational H2 line to emission in the Brackett-gamma line of neutral hydrogen – indicating that the emission is powered by shocks rather than embedded, invisible star formation.  This shock emission could be related to the decay of turbulent motions as the cloud contracts and forms new stars.  This compact source may very well be the incubator for a soon-to-be-born super star cluster.

When the full ALMA array is finished, astronomers will be able to map both galaxies at even higher resolution and with increased sensitivity, allowing a closer investigation of the dynamical state of molecular gas in a colliding system.  Stay tuned for more on this topic!

About Nathan Goldbaum

Nathan is a third year graduate student in Astrophysics at UCSC. His interests include the local ISM, molecular clouds, and the role of star formation in galactic evolution.

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