The universe is getting more crowded by the day. New telescopes and technologies are allowing astronomers to discover new astronomical objects all the time, such as ten new galaxy clusters found recently by the Atacama Cosmology Telescope (ACT).
An international team of scientists led by Rutgers University astrophysicists have discovered these 10 new massive galaxy clusters from a large, uniform survey of the southern sky. They found the galaxies using a breakthrough technique that detects “shadows” of galaxy clusters on the cosmic microwave background radiation, a relic of the “big bang” that gave birth to the universe.
The Atacama Desert in Chile is an up-and-coming site for radio and microwave astronomy with the ACT and the Atacama Large Millimeter Array (ALMA), the latter of which is under construction. At an elevation of over 17,000 ft (over 5000 m), these telescopes get above much of the water vapor in the atmosphere that can absorb or distort the incoming cosmic radiation at these wavelengths.
These galaxies were not observed directly by the ACT but seen through the Sunyaev-Zel'dovic Effect, or SZE. Essentially, the hot gas molecules in the cluster scatter cosmic microwave background photons up to higher energies. Thus, a "hole" or "shadow" appears in an image made with microwave light.
Theorists Rashid Sunyaev and Yakov Zel’dovich predicted the SZE shadow phenomenon 40 years ago. The higher sensitivity and resolution of ACT now makes it practical for astronomers to essentially reverse the procedure – to search the cosmic background radiation for shadows that indicate the presence of unseen clusters.
Jack Hughes, professor of physics and astronomy at Rutgers, said: “The ‘shadows’ that ACT revealed are not shadows in the traditional sense, as they are not caused by the galaxy clusters blocking light from another source. Rather, the hot gases within the galaxy clusters cause a tiny fraction of the cosmic background radiation to shift to higher energies, which then makes them appear as shadows in one of ACT’s observing bands.”
Priyamvada Natarajan, professor of astronomy and physics at Yale University and a leading theoretical cosmologist not affiliated with the study, said: “The technical challenges involved in exploiting the S-Z technique are daunting, and it is fantastic to see this method working so well. It will build our inventory of the most massive and distant clusters in the universe, which will provide important constraints on the currently accepted cosmological model. I am personally excited to see the large number of strong lensing clusters that ACT is turning up.”
