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Decade-long galaxy survey releases final catalog

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Lindsey Hadlock

The last data release and final official survey paper from the Arecibo Legacy Fast ALFA (ALFALFA) survey, led by Cornell astronomers, published July 2 in Astrophysical Journal Supplement.

The paper marks the conclusion of a vast project that required 4,400 hours of telescope time and resulted in more than 100 papers co-authored by faculty, staff and students at Cornell. The survey was the basis of 10 Cornell doctorates and an equal number by graduate students from other universities.

“The population of galaxies we’ve detected include the most isolated galaxies in the universe, the least clustered,” said Martha Haynes, co-principal investigator. “Studying these galaxies reveals how galaxies evolve in the absence of interactions with other galaxies or with the hot gas found in clusters of galaxies.”

Among ALFALFA’s discoveries are enigmatic objects such as the nearby faint dwarf Leo P, and Leoncino, a galaxy with very little metal content. ALFALFA also identified what the researchers call an “intriguing population” of massive galaxies with unusually high percentages of their mass composed of cool hydrogen gas.

ALFALFA fulfilled one of its primary goals by discovering a small population of galaxies with dark matter and gas and almost no detectable starlight. Researchers say they don’t yet understand why these galaxies haven’t turned their abundant gas into stars.

Since it began in 2005, ALFALFA has detected radio emissions from cool gas in more than 30,000 galaxies. It has produced the first comprehensive census of gas-bearing galaxies up to a distance of 800 million light years from our galaxy, the Milky Way, encompassing nearly one-sixth of the sky.

The survey was the first to use the Arecibo L-Band Feed Array (ALFA) radio camera on the 305-meter-diameter radio telescope at Arecibo Observatory in Puerto Rico, focusing on the frequency range that includes a spectral line emitted by neutral atomic hydrogen. Nearly every previous sky survey has been of optically, infrared- or X-ray-selected galaxies.

The importance of the hydrogen, explains co-principal investigator Riccardo Giovanelli, emeritus professor of astronomy, is that gas-rich systems of low mass that have not been able to convert their cosmic material into stars – so-called dark galaxies – are optically inert because they have few or no stars but can be detected by their hydrogen signature.

Data from the survey, including individual spectra, are available on the ALFALFA website and will be incorporated into the NASA Extragalactic Database.

While ALFALFA discoveries have been impressive, the project’s impact on teaching and learning through the Undergraduate ALFALFA Team has been equally profound, said Haynes. The team is a consortium of 23 mostly undergraduate colleges, for which Haynes serves as science lead.

“ALFALFA wasn’t a big-grant, big-team, big-science project but rather something that a small, distributed group has done to undertake a major scientific contribution,” said Haynes, the Goldwin Smith Professor of Astronomy in the College of Arts and Sciences. “It’s a legacy survey done without any professional staff; all the work was done by faculty, undergraduate and graduate students. We are very proud of the success of the project as part of the academic educational process.”

The Undergraduate ALFALFA Team’s latest project is the Arecibo Pisces-Perseus Supercluster Survey, which builds on ALFAFA data to measure the mass of the Pisces-Perseus Supercluster for the first time.

“ALFALFA is the source of many research projects and the seed of many discoveries to come,” said principal investigator Rebecca Koopmann of Union College. “It’s a gift to the community.”

Co-authors of “The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic Hi Source Catalog” include Elizabeth Adams, Ph.D. ’14; Gregory Hallenbeck, Ph.D. ’14; Kelley Hess ’05; Lyle Hoffman, Ph.D. ’83; Shan Huang, Ph.D. ‘13; Michael Jones, Ph.D. ’16; Brian Kent, Ph.D. ’08; David A. Kornreich, Ph.D. ’01; Lukas Leisman, Ph.D. ’17; and Emmanouil Papastergis, Ph.D. ’14.

The ALFALFA team at Cornell has been supported by the National Science Foundation and the Brinson Foundation.

Linda B. Glaser is a staff writer for the College of Arts and Sciences.

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