Paper Spotlight: “HALOGAS Observations of NGC 4559: Anomalous and Extra-planar HI and its Relation to Star Formation”

Carlos Vargas, a 5th year graduate student working with his adviser, Rene Walterbos, recently published a paper entitled “HALOGAS Observations of NGC 4559: Anomalous and Extra-planar HI and its Relation to Star Formation.”

For this study, Carlos and the other authors analyzed new observations made by HALOGAS, a survey dedicated to analyzing interactions between spiral galaxies and the neutral hydrogen gas that lies outside of their plane (that is, extra-planar gas). Using these observations, they studied the neutral hydrogen gas in NGC 4559, a nearby spiral galaxy. They chose this galaxy because its inclination made it a good test subject for code written to extract the locations of extra-planar gas.

The hydrogen gas observations were taken with a radio interferometer, which allows a 3-D data cube to be created, with x-y space on the 1st and 2nd axes and velocity on the 3rd. Using this 3rd velocity axis, the authors could map the motions of neutral hydrogen throughout the spatial extent of the galaxy. Using the aforementioned code, they found that between 10-20% of the total neutral hydrogen content lay outside the galaxy as extra-planar gas.

By modeling the trajectories of the gas, testing the emission line profiles of the gas, and looking for the stars that could have done the ejecting, the authors determined that this gas was most likely ejected from the plane of the galaxy by powerful supernovae and stellar winds. Eventually, this gas will respond to the gravitational pull of the galaxy and fall back into the plane, where it will provide fuel for future generations of stars. This circular feedback process is called the galactic fountain mechanism.

Along with this analysis, the authors discovered a previously undetected cloud of hydrogen associated with NGC 4559. They found two sources associated with this cloud in the Sloan Digital Sky Survey, and determined that this neutral gas cloud encompasses two young merging dwarf galaxies.

Take a look at Carlos’ paper by clicking this link.

Image: NGC 4559 in visible light
Image credit: Jeff Hapeman/Adam Block/NOAO/AURA/NSF

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