South Africa's MeerKAT radio telescope has detected the most distant "cosmic laser" yet: a hydroxyl megamaser from a merging galaxy about 8 billion light-years away. The result was published in the *Monthly Notices of the Royal Astronomical Society*.
What Is a Megamaser?
A maser is the microwave-band equivalent of a laser—molecules under high-density, high-turbulence conditions are pumped into amplifying radiation into a highly coherent, narrow-band, intense signal. Hydroxyl (OH) molecules are particularly good at this, with two spectral lines at 1665 MHz and 1667 MHz (the ~18 cm band). When a signal is about a million times brighter than an ordinary galactic maser, it is called a megamaser; a thousand times brighter still, a gigamaser. The detected power of this object approaches the gigamaser threshold. OH megamasers appear almost exclusively in violently merging galaxies, because mergers compress and stir the gas, pumping molecules far from equilibrium.
Two Remarkable Aspects
Distance. At 8 billion light-years, we are seeing the universe as it was roughly 5.8 billion years after the Big Bang—a chaotic era of frequent galaxy collisions. This is the most distant hydroxyl megamaser ever observed, pushing the observational window for such objects substantially further back.
Efficiency. The team was originally observing neutral hydrogen absorption. MeerKAT's wide bandwidth allowed them to capture the hydroxyl line in the same 5-hour observation—such objects normally require hundreds of hours of integration. Catching it in 5 hours owed half to MeerKAT's sensitivity and half to gravitational lensing: a foreground disk galaxy happened to lie along the line of sight, its curved spacetime amplifying this microwave signal emitted 8 billion years ago.
A Supercomputing Feat in Signal Processing
The signal is millions of times weaker than a mobile phone signal. The team used the IDIA (Inter-University Institute for Data Intensive Astronomy) supercomputers for days, running trillions of operations to wash out the noise and sharpen the signal.
Why It Matters
1. A new window on the early universe. Megamasers directly mark galaxy mergers and extreme star-forming environments. Mergers often accompany the inspiral of supermassive black hole pairs—the countdown stage before gravitational-wave events—so megamasers let us "preview" the host galaxies of future gravitational-wave sources.
2. A trailer for the SKA. MeerKAT will become part of the SKA's mid-frequency array. As Roger Deane put it, "it's a preview of what the SKA might achieve," with the US next-generation VLA (ngVLA) complementing at higher frequencies. If MeerKAT can find one in 5 hours, distant megamasers will likely go from rare discoveries to bulk catalogs in the SKA era.
3. Luck in radio astronomy is designed. Large bandwidth, high sensitivity, and a chance line-of-sight alignment—all three were necessary.
What to Watch Next
- How large is the population of gravitationally lensed, distant megamasers?
- Can the coexistence of neutral hydrogen absorption and hydroxyl emission along the same line of sight help reconstruct the full stratification of molecular gas, star-forming regions, and outer cold gas in merging galaxies?