The ultraviolet GALEX satellite just revealed the largest known spiral galaxy! NGC 6872 (a barred spiral) is 522,000 light-years across from the tip of one outstretched arm to the tip of the other, which makes it about 5 times the size of our home galaxy, the Milky Way! We recently covered this story on our Facebook but we wanted to go a little more in-depth. So, here is an interview with one of the people involved with this discovery, grad student Rafael Eufrasio.
This composite of the giant barred spiral galaxy NGC 6872 combines visible light images from the European Southern Observatory’s Very Large Telescope with far-ultraviolet (1,528 angstroms) data from NASA’s GALEX and 3.6-micron infrared data acquired by NASA’s Spitzer Space Telescope. A previously unsuspected tidal dwarf galaxy candidate (circled) appears only in the ultraviolet, indicating the presence of many hot young stars. IC 4970, the small disk galaxy interacting with NGC 6872, is located above the spiral’s central region. Images of lower resolution from the Digital Sky Survey were used to fill in marginal areas not covered by the other data. Credit: NASA’s Goddard Space Flight Center/ESO/JPL-Caltech/DSS
Blueshift: What is your current role at Goddard? What research are you working on?
Rafel: I am a research assistant at Goddard, but I am also a PhD candidate at the Catholic University in DC. I am a 5th year PhD student and [have been] working at Goddard since I started it in 2008. If everything goes as planned, I am defending my thesis next year.
I am working on the Spectral Energy Distribution (SED) of nearby face-on spiral galaxies. For my research I use as much data as I can possibly gather for these galaxies, from the ultraviolet (UV) to the radio (i.e. UV, optical, IR, sub-millimeter, and radio), from space and ground-based telescopes. My goal is to describe the content of stars, gas, and dust of these galaxies based on the information from the SEDs, as well as their Star Formation History (SFH).
Blueshift: We read the news feature on the GALEX discovery of NGC 6872, which is currently the largest known spiral galaxy, more than five times the size of the Milky Way. Can you tell us more about the object and how this discovery came about? Did you stumble on this discovery or did you specifically use GALEX to study this object in more detail?
Rafael: I was introduced to this galaxy almost a year and a half ago by Dr. Duilia de Mello, a professor at the Catholic University who also works at Goddard. She was studying young stellar systems outside of galaxies, specially the ones with strong UV emission, that she likes to call “blue blobs”. She knew I was working on the SED of spiral galaxies and we started this project. Initially, [we] chose 16 large regions (10 kiloparsecs or kpc in diameter) all over the galaxy, [which] produced UV-to-IR [light] and we presented preliminary results at the American Astronomical Society (AAS) meeting in January of 2012.
However something seemed off when I checked the sizes of the regions, stellar masses, and star formation rates. These 10 kpc regions would encompass a large chunk of most spiral galaxies I am studying, but they looked small when superposed in NGC 6872.
Then we spent a year asking astronomers in different places and searching catalogs to finally claim that it is the largest-known spiral galaxy at the AAS meeting this January. With an extended disk of at least 160 kpc in diameter.
Blueshift: Why is this galaxy so much bigger than the Milky Way – and do you think this is an uncommon size for a galaxy, or have we just not discovered others like this yet? Once upon a time, we didn’t think exoplanets were common and now we know differently, for example.
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