Radio Observations of Local Galaxies

The Local Volume HI Survey: Galaxy Kinematics (paper)

Emma M. Kirby, Barbel Koribalski, Helmut Jerjen, Angel Lopez Sanchez

This new high resolution study of neutral hydrogen (HI) in local galaxies, led by Baerbel Koribalska has a great name LVHIS (almost pronounced Elvis... which is reason enough to look at this work). It's also a fantastically thorough and exhaustive study into the kinematic properties of 12 nearby dwarf galaxies. The study of galaxy rotations using the HI line isn't anything new of course, but the dataset presented here represents the quality of data that we can routinely expect from the forthcoming Australian Square Kilometre Array Pathfinder (ASKAP) and hence is a valuable guide into the uses (and pitfalls) of high resolution kinematic data. 

So each galaxy was observed using the Australia Telescope Compact Array (ATCA), in three different configurations (the dishes are separated by different distances to improve resolution essentially) for 12 hours each. For those not in the know this is a seriously studied galaxy! As this is radio these datacubes can measure velocity along the line of sight (to better than 4km/s accuracy) for each resolved pixel of the galaxy (45 arcseconds, essentially the size of Jupiter in the nights sky from Earth). Now what can one do with such a dataset? 

By using complex software to model the rotational properties of the galaxy (in this case ROTCUR for those interested) the inclination of the galaxy to the observer can be measued... imagine an old vinyl record playing, the appearance of the lines on the surface change as you look directly above it (face-on) or with your head on the table looking at the disk (edge-on). The same information is available for us except the vinyl record is about 5 kpc (15,000 lightyears) across. Interestingly they point out circumstances were the inclination has been previously incorrectly calculated which completely ruined the estimates on the rotation velocity (and hence inferred gravitating mass) of the system - this demonstrates that even a seemingly simple problem as figuring out the rotation of a galaxy will need some work when you have 1000s of them from ASKAP!

The other fun result was that a system demonstrated disturbed kinematics, this was believed due to the presence of a neighbour - the gas is beginning to drift towards this neighbour (although tear is probably more apt!) Another system has a 'warp' in the disk, which is evidence of a recent accretion event... imagine a spinning pottery wheel (cue scence from Ghost), the effects of a satallite galaxy crashing into the disk which then buckles is similar to Patrick Swayze's hands ruining that poor clay pot. Perhaps that's a terrible analogue but it's Friday...

There then follows a brief discussion on the observed Tully-Fisher relation (essentially a tight correlation between the brightness of the galaxy and its rotational velocity- essentially bigger galaxies have more stars, and because they are bigger can hold onto material that rotates faster). These galaxies agree well with other works but more importantly because the entire kinematical structure is known we can be confident that the various definitions of 'rotation velocity' make sense - technical but important none the less.

In conclusion, this study is a textbook example on the kinematic study of HI in galaxies and more importantly demonstrates both the need for eyeball analysis and care required when automating this stufy in a future with ASKAP...