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UW Students Compile Largest-Ever Catalog of Quasars

August 31, 2020

A group of University of Wyoming students put together the largest quasar catalog to date as part of the Sloan Digital Sky Survey (SDSS). Quasar broad emission line regions are the clouds of hot, ionized gas that surround the supermassive black holes feeding in the centers of galaxies.

“This is the release of the largest quasar catalog to date and possibly the largest for the next few years,” says Brad Lyke, a third-year UW graduate student from Oceanside, Calif., majoring in physics and astronomy. “The SDSS puts out a blog targeted for astronomy enthusiasts and undergraduates. I wrote a blog post on some of the ‘big picture’ surrounding our paper and where it fits into the cosmology research the SDSS is doing.”

Lyke was the lead and corresponding author of a paper, titled “The Sloan Digital Sky Survey Quasar Catalog: Sixteenth Data Release,” that was published Aug. 27 in The Astrophysical Journal Supplement, which is a peer-reviewed scientific journal of astrophysics and astronomy.

Alexandra Higley, a junior from Parker, Colo., double majoring in physics and astronomy/astrophysics; Jacob McLane, a third-year graduate student from Loveland, Colo., majoring in physics; and Danielle Schurhammer, a senior from Plainview, Minn., double majoring in physics  and astronomy/astrophysics, were the other UW students involved. All were co-authors of the paper.

Adam Myers, a UW associate professor of physics and astronomy, and Lyke’s Ph.D. adviser, oversaw the work.

Other contributors to the paper are from Ohio State University, University of Utah, University of Washington, New Mexico State University, Penn State University, University of Pittsburgh and Durham University in the U.K.

The SDSS has created the most detailed three-dimensional maps of the universe ever made, with deep multicolor images of one-third of the sky, and spectra for more than 3 million astronomical objects. The SDSS has been observing the skies from Apache Point Observatory in Sunspot, N.M., since 1998 and from Las Campanas Observatory in Vallenar, located in Atacama Region, Chile, since 2017.  

“I was the point person for the University of Wyoming team in collaboration meetings and for the quasar catalog project as a whole,” Lyke says.

The catalog work was coordinated through emails, online teleconferences and attending international conference meetings -- all under the umbrella of the SDSS-IV collaboration.

Lyke compiled the data from already-completed observations conducted by the SDSS. The SDSS targeted approximately 1.44 million objects from images that were highly likely to be quasars. However, the spectroscopic follow-up, the data Lyke worked with, showed not all of the objects were actually quasars. Many were actually stars and galaxies.

Out of the 1.44 million object images, just over 750,000 turned out to actually be quasars. Of that total, over 225,000 were newly discovered quasars.

“That was my team’s job, to separate the ‘incorrectly targeted’ non-quasars from the quasars,” Lyke explains. “Originally, I was only to take over the project compiling the catalog. However, another group responsible for visually inspecting a portion of the data -- about 20,000 spectra -- did not return for this last catalog.”

In the absence of any other volunteers, Lyke took on the part of visually inspecting the spectra. To complete this task, Lyke brought on Higley, McLane and Schurhammer to assist him.

“I trained them to look at and classify spectra. Then, we all worked on that part (visual inspections) together,” Lyke says. “We were all new to that part of the research and were taking over from a group of veterans of the project who had been doing it for years. I’m very proud of how my team did, especially under such circumstances.”

The team identified Damped Lyman Alpha quasars, which are distinguished by big dips in the light at certain wavelengths around the Lyman Alpha emission line(s). These emission lines are places where distant gas/dust clouds or other galaxies in front of the quasar have absorbed some of the light the team expected to see.

The group also identified Broad Absorption Line quasars, which are similar, but the absorbing clouds are usually in the same galaxy as the quasar. As a result, the absorption dips are in different places in the spectrum.

“In both cases, these ‘shadow puppets’ can give us information on the absorbing objects because we know what we should expect from the quasar itself, but what we see is different,” Lyke explains.

The catalog was released to the public July 20 and has two primary uses, Lyke says.

One, the cosmology research and analysis conducted with quasars, as part of the key project for SDSS-IV, required this catalog. That research needs the 3D position information of quasars across the sky, he says.

The team identified the first two coordinates for the quasars by their right ascension and declination coordinates. Redshifts, a measure of doppler shifting, were used to obtain the third coordinate. As an analogy, a redshift is similar to what happens to sound from a moving object -- such as a police siren driving past -- except with light.

And two, although the cosmology research doesn’t involve studying the physics of quasars themselves, quite a bit of other research does study the physics of quasars, Lyke says.

“This catalog will be used by those researchers as a list of known quasars with non-SDSS data included all in one file,” he says. “These SDSS quasar catalogs, and especially this last one, are widely used in the field of quasar research.”

The previous catalog, called DR14Q, was released in May 2018 and already has 148 citations linking back to it, Lyke says.

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