Observing Red Galaxies With VIRUS-P
Hi Zoo fans,
My name is Peter Yoachim and I’m currently a postdoc working in the Astronomy Department at the University of Texas in Austin.
I got involved with the Galaxy Zoo after I saw Karen’s paper on Red Spirals. When I first read the paper I thought, “Wow, that’s really cool, spirals shouldn’t look red like that, wonder what happened to those galaxies.”, followed shortly by, “OMG, we have the perfect instrument to make follow-up observations of these objects!”
While I’ve been at UT, I’ve been making extensive use of VIRUS-P (Visible Integral-field Replicable Unit Spectrograph Prototype), a new instrument at McDonald Observatory in West Texas. Right now, VIRUS-P is mounted on the 2.7m Harlan J. Smith telescope. While modest in size by current standards, the 2.7m has been a scientific workhorse since 1968 although it is probably most famous for having several bullet holes in the primary mirror.
As I tell my 101 students, images of the sky are a great starting point, but if you want to do Astrophysics, you need to observe some spectra. With galaxies, the full spectra can tell us how different parts of the galaxy are moving (via the redshift and blueshift of light), what kind of stars are in the galaxy, and if there is any hot gas present. VIRUS-P is great for getting spectra, especially for targets like nearby galaxies.
In the bad old days (like when I was doing my thesis work 6 years ago), it was common to pass light from the telescope through a narrow slit, then bounce it off a grating to disperse the light onto the detector to observe the spectrum. The problem is that the narrow slit blocks most of the light from the galaxy. This is a tragedy! That light traveled for (literally) millions of years only to bounce off the slit mask at the last second.
Rather than use a long slit, VIRUS-P uses a fiber-optic bundle to pipe the light around. Here’s an example from a recent paper. NGC 6155 is just a nice normal galaxy, here’s an image of it from the Sloan survey:
When I observe the same galaxy with VIRUS-P, I see this:
Each circle represents a fiber. I’ve color-coded the fibers so that the brightest spectra are blue and the faintest are red. This isn’t too fancy, it even looks quite a bit worse than the Sloan image. But look what happens if I calculate the velocity from the redshift of the spectra in each fiber:
Now we can see the rotation of the disk. The top left of the galaxy is moving away form us, while the bottom right is moving towards us. The redshift of light only shows us the part of the motion that happens to be along our line of sight, but that’s still enough to get a good idea of how the stars and gas in the galaxy orbit the center. The next trick is to add up the spectra from multiple fibers to build up the signal to make it possible to measure accurate ages for the stars.
What we see here is the center of the galaxy is old (~7 billion years), while the disk is young and still forming stars (average age ~4-6 billion years). The youngest section that’s 4 billion years old corresponds to the bright blue spiral arms in the Sloan image. The cool part is the very outskirts of the disk are made of very old stars (8-10 billion years old), a result some of my coauthors actually predicted.
It should be clear now how VIRUS-P will be great for observing the red spirals. We can compare the motions of red spiral disks to regular spirals, and we can measure stellar ages to try and determine when star formation shut off in these galaxies.
The observing of the red spirals has been done by intrepid UT graduate student John Jardel. With the remnants of hurricane Alex blowing through, the observatory has received excessive rain this summer. All that rain makes it hard to observe, plus it lets the rattlesnakes and scorpions thrive. Here’s a scorpion I caught in the observatory lodge last week:
Despite the weather and wildlife, John was able to observe 5 galaxies. We’ve just finished our last observing run of the season, so we haven’t had a chance to analyze the data yet. But looking at the raw images, we already see something interesting:
The horizontal stripes are the signal from each individual fiber. The bright vertical lines are emission lines from the earth’s atmosphere. The two circles show 5 fibers where we can see bright spots. Those spots are emission from hot Hydrogen gas in the galaxy. If there’s gas, it’s possible these red galaxies could start forming stars again and turn back to regular blue spirals. Since the gas is hot and in emission, it could even be the case that there is star formation going on right now.