ARECIBO, PUERTO RICO – Fitted with its new compound eye on the
heavens, the National Science Foundation’s (NSF) Arecibo Observatory
telescope, the world’s largest and most sensitive single-dish radio
telescope, early tomorrow morning begins a years-long survey of
distant galaxies, perhaps discovering elusive “dark galaxies” –
galaxies that are devoid of stars.

Astronomers at Arecibo Observatory hope the new sky survey will
result in a comprehensive census of galaxies out to a distance of 800
million light years from our galaxy, the Milky Way, in nearly
one-sixth of the sky – or some 7,000 square degrees.

The search, conducted by an international team of students and
scholars, is the first of a series of large-scale Arecibo surveys
that will take advantage of a the telescope’s new instrument,
installed last year, called ALFA (for Arecibo L-Band Feed Array). The
device is essentially a seven-pixel camera with unprecedented
sensitivity for making radio pictures of the sky, allowing
astronomers to collect data about seven times faster than at present.
The project has been dubbed ALFALFA, for Arecibo Legacy Fast Alfa

“Fast” does not refer to the time necessary to carry out the survey,
which will require thousand hours of telescope time and a few years
to complete, but rather to the observing technique, which consists in
fast sweeps of broad swaths of sky.

The survey is supported by the National Astronomy and Ionosphere
Center (NAIC) at Cornell University, Ithaca, N.Y., which manages the
Arecibo Observatory for the NSF. In addition, support is being
provided through research grants from the NSF and the Brinson
Foundation to the project’s leader, Cornell professor of astronomy
Riccardo Giovanelli, and to Martha Haynes, a Goldwin Smith Professor
of Astronomy at Cornell.

Giovanelli explains that ALFA operates at radio frequencies near 1420
MegaHertz (MHz), a frequency range that includes a spectral line
emitted by neutral atomic hydrogen, the most abundant element in the
universe. ALFA detects this signature of hydrogen, which hopefully

signals the presence of an undiscovered galaxy. Nearly every previous
sky survey has been of optically, infrared- or X-ray-selected

ALFALFA will be six times more sensitive – meaning that it will go
much deeper in distance – than the only previous hydrogen wide-field
survey carried out in Australia in the late 1990s. “What has made
ALFALFA possible is the completion of the Gregorian upgrade to the
Arecibo telescope in 1997, which allowed feed arrays to be placed in
the telescope focal plane and expanded the instantaneous frequency
coverage of the telescope,” he says.

Besides providing a comprehensive census of the gaseous content of
the near universe, ALFALFA will explore galaxies in groups and
clusters and investigate the efficiency by which galaxies convert gas
into stars. What particularly intrigues astronomers is that ALFALFA
could determine whether gas-rich systems of low mass that have not
been able to convert their cosmic material into stars – the so-called
dark galaxies – actually exist. Because these galaxies, being
starless, are optically inert, it is hoped that they can be detected
by their hydrogen signature.

The galaxy survey is feasible now because ALFA lets the telescope see
seven spots – seven pixels – on the sky at once, greatly reducing the
time needed to make all-sky surveys. The Australian-built detector,
on the 305-meter (1,000-foot) diameter Arecibo radio telescope,
provides the imaging speed and sensitivity that astronomers will need
for their search.

Robert Brown, the NAIC’s director, said that a significant fraction
of the Arecibo telescope time in the next few years will be devoted
to extensive surveys with the ALFA array, such as ALFALFA. The new
survey consortium consists of 38 scientists from 10 countries,
including the United States, France, the United Kingdom, Italy,
Spain, Israel, Argentina, Chile, Russia and the Ukraine.
Several of the members are graduate students who will base their
Ph.D. theses on ALFALFA data. Among them are Cornell graduate
students Brian Kent, Sabrina Stierwalt and Amelie Saintonge.
Says Giovanelli: “My one and only paper published in an engineering
journal proposed the construction of a feed array at the upgraded
Arecibo telescope to carry out hydrogen line surveys of the sky. It
took 15 years of waiting, but I am finally going to do the

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ALFALFA schedules:

Arecibo Observatory: