You’ll hear a lot about neutral hydrogen on this blog, but ALFAFA also has science goals that don’t involve the 21 cm line. One such project is the detection of OH megamasers.
Lasers (Light Amplification of Stimulated Emission of Radiation) are familiar to most people. Masers are the same thing except that they emit in microwaves rather than optical wavelengths. In an OH maser, the line transition between two different electron levels in the molecule is strongly amplified. The megamasers get their name from the fact that they are a million (the mega prefix in scientific units) times stronger than the regular OH masers that were first observed in astronomical situations.
The lines that are commonly excited in OH megamasers occur at 1665 and 1667 MHz. These are at a higher frequency than the ALFALFA observations which occur in the range of 1335-1435 MHz. However, we can observe these lines when their are frequencies are red-shifted enough due to the recessional velocities of the galaxies they occur in. Due to the expansion of the universe, galaxies that are more distant from us appear to have higher recessional velocities; empirically, this phenomenon is known as Hubble’s Law. The redshift, z, of a galaxy is often used to describe how much the wavelength/frequency of light for an object is shifted. For the OH megamasers, the lines of interest move into ALFALFA’s frequency window for redshifts between 0.16 and 0.25. This corresponds to galaxies that are between 631 and 941 Mpc (2 to 3 billion light years) away.
OH megamasers are associated with merging of large galaxies. By finding and studying the distribution of these sources, we can learn about the merger rate during the history of the universe. As we observe objects that are further away, we are also seeing younger objects as the light has taken longer to reach us. Thus, we can try to understand how the merger rate varies with redshift, or age of the universe.
The redshift of an object is given by:
where is the wavelength and is the frequency of the light. The subscripts refer to whether it is the value of the wavelength/frequency emitted by the source or the value measured by an observer at some relative velocity.
The redshift is related to the recessional velocity of the source by:
The distance to an object can be calculated from its recessional velocity assuming pure Hubble flow:
where the currently accepted value of the Hubble constant () is 70 km/s/Mpc.