Using Our Largest Set of "Eyes"

While our data from the CANDELS program is nothing short of revolutionary, occasionally we need to step back and remember that Hubble is "only" a 2.4 meter telescope (meaning that the diameter of the primary mirror, which sets the telescopes light-gathering power, is 2.4 meters). Since Hubble is in space, above the atmosphere, its still able to do great work. However, some tasks are best left to larger, ground-based telescopes. One of these tasks is to followup some of the CANDELS sources with spectroscopic observations.

Spectroscopy spreads the light from a source out into its component wavelengths; much like placing a prism in a beam of sunlight. Through this technique, we can study a variety of properties about very distant galaxies, including their distance. In previous blog posts, we talked about how we can find very distant galaxies using their colors (we call this "photometric redshifts"). However, this technique provides only an approximate distance, and there's always a (small) chance that an object identified as being distant is really not quite so far (we call these "catastrophic outliers"). To ensure that our color-based distances are accurate, its always a good idea to measure exact distances for some subset of your distant galaxy sample.

Because these galaxies are so distant, we can just barely see them in our Hubble images. If we tried to use Hubble to do spectroscopy, splitting the light up into its colors, we would no longer be able to see them; essentially, it would be like taking all the light from the CANDELS image, and splitting it up into multiple images. Since we can just barely see it when all the light is added together, we would not be able to see it if we split the light with a spectrograph. So, to spectroscopically study the distant universe, I obtained time with the largest optical telescope on the ground, the Keck 10 meter telescope.

Most of the time on this telescope goes to its two major partners, CalTech, and the University of California.  I'm at the University of Texas at Austin, and we don't have our own access. However, NASA typically has 10-20 heavily contested nights to get away, and we wrote a proposal, and managed to get two nights with Keck in April.  We used these nights to observe distant galaxies with a brand new spectrograph, called MOSFIRE. This instrument is revolutionary, its it is highly sensitive, can observe many objects at once, and it observes in the near-infrared, all of which are crucial to observe our very faint, very distant galaxies.

Keck is one of my favorite observatories to go to, since although the telescope is located at 13,500 feet on top of Mauna Kea, we observe from the town of Waimea, at an elevation of 1000 feet, and only 15 minutes from the beach (and about 100 feet from Starbucks!). Joining me on the run was Mimi Song, a graduate student working with me at UT Austin, and Vithal Tilvi, a postdoctoral research from Texas A&M. Over our two nights, we obtained data on 43 very distant galaxies. I will write another post soon on the results from our run, so stay tuned!

Myself, Tilvi and Mimi in the Keck control room.