Megamasers: Distant Water and Precision Cosmology

Artist's conception of NGC 4258's maser disk. Image courtesy of NRAO/AUI and Artist: John Kagaya (Hoshi No Techou).

Measuring the distances to objects in space is surprisingly hard. When you look up at night, can you really tell anything about the distances to various stars? A "cosmic distance ladder" has been built, but larger assumptions must be made as you go to further and further distances. For a long time, accurate geometric distances could only be made to the nearest stars.

The Megamaser Cosmology Project, or MCP, has busted that model and is making accurate geometric distances to whole galaxies. Their latest result uses a clever technique to measure the distance to a galaxy, UGC 3789, 163 million light years away, (plus or minus 23 million light years)**.

A disk around a black hole at the center of UGC 3789, measured out in maser spots. Those coordinates are in milliarcseconds, or 0.0000003 of a degree! From Braatz et al., 2010 and Reid et al., 2009.

Distances on the scale of the solar system are based off of radar measurements and communication times to our interplanetary robot explorers. The distances to nearby stars are measured geometrically using parallax. Beyond that, you have to start to make some assumptions about the astrophysics of the object that you are studying. Therefore, the distances to galaxies hundreds of millions of light years away are not known to very high precision. Unless, that is, you can detect water megamasers.

Masers are just like lasers, emitting a coherent beam of light at a certain frequency, only they occur in the microwave regime of the electromagnetic spectrum. Thus, we can detect them with radio telescopes. Also, they can occur naturally under the right conditions.

Megamasers are extremely bright masers that can be seen in distant galaxies. In this case, the strong emission coming from near the supermassive black hole in the center of a galaxy excites water molecules in a disk around the black hole such that they act as a maser.

The top image is an artist's conception of such a disk from data taken of NGC 4258, the first such galaxy whose distance was measured in this way. The maser lines are typically discovered by surveys, using the very sensitive Green Bank Telescope. They can then be monitored for signs of acceleration in the masers that are along the line of the sight to the black hole. Then, the masers can be imaged with very high spatial resolution using a suite of telescopes around the Northern Hemisphere to make one large telescope.

This High Sensitivity Array combines the Very Long Baseline Array, the Very Large Array, the Green Bank Telescope, and the Effelsberg Radio Telescope.

A representation of the VLBA, just part of the High Sensitivity Array. Image courtesy of NRAO/AUI The maser spots follow Keplerian curves, that is, we can use simple Newtonian physics to determine their motions. Using the single-dish monitoring and the interferometry maps, a precise geometric distance can be measured. But why do we care how precise we are over such large distances?

The answer may surprise you: dark energy. With better precision in distance measurements, astronomers can get better precision on the Hubble constant, or the rate at which galaxies move away from each other due to the expansion of the universe. And then, astronomers can get a handle on one of the parameters that describes how dark energy works, w, the equation of state parameter. The value of that number will help physicists zero in on one of the many proposed types of dark energy. (Is it a cosmological constant? Vacuum energy? Something weirder? All you get now is a shrug.)

Although the first megamaser distance to NGC 4258 was impressive, that galaxy was too nearby to measure the Hubble constant. The MCP needs about 10 galaxies at the distance of UGC 3789 to really get a handle on the Hubble constant with the appropriate precision, and thus, a clue about dark energy. Several of the galaxies are being measured at the moment, while more are being searched for. It may be that powerful radio telescopes and simple geometry will give us our first glimpse at how dark energy works.

Okay, full disclosure time. I was on the project that detected the masers in UGC 3789, so I'm really excited to see this follow up! My advisor and I had split up the GBT time, and I was not the one on the telescope when this was discovered. However, I got the excited email shortly after and rushed to look at the data. It was definitely one of those WOW moments! No one had classified this galaxy as active before, though another colleague was gracious enough to get us an optical spectrum with which we could clearly show that the supermassive black hole at the center had matter falling into it, thus making it active. The universe can sometimes give you a clue when you least expect it.

** Wow. I can't do arithmetic. Many thanks to Charles Peterson for pointing out my errors!