The following text was adapted for an article entitled
"An EGG Hunt for Starless Galaxies" which appeared in the Winter 2004 Volume of
"Connecting with Cornell".
Cosmology from Cornell's own "Camuy Cave"
It may be 3 am, but the lights are on in a windowless room in the
Space Sciences Building adorned with toy bats and galaxy mobiles.
A computer screen refreshes itself every second with a new display,
monitoring incoming data from the
Arecibo telescope, 1,500 miles away.
A "talk" window allows an electronic conversation with the telescope operator in the
control room at the Observatory; a phone sits prominently on the desk
beside the screen ... just in case the internet goes on the blink.
A member of the Cornell
EGG (ExtraGalactic Group)
is spending another night observing remotely with the Arecibo telescope from the
room known locally as the "Camuy Cave", on a hunt for starless galaxies.
Starless galaxies? Aren't galaxies supposed to be conglomerates
of billions of stars? "Well, not exactly", an EGG astronomer will explain with a grin. It
turns out that the main constituent of galaxies is not matter of the kind
you and I are made of -- stars and planets, rocks, carbon, hydrogen... -- what scientists call
"baryonic matter", but rather matter of a mysterious form, referred
to as "dark". The label "dark" arises from the fact that dark matter is invisible;
it cannot be seen by any telescope. Although dark matter is nearly ten times more
abundant than baryonic matter, it can only
be detected through its gravitational interaction with the more familiar,
baryonic form. In a normal galaxy, baryonic matter -
stars and interstellar gas and dust - resides deep inside a huge,
massive envelope of dark matter, known as a "halo". By studying the
baryonic component, we hope to learn about the invisible, dark halo.
Cosmologists cannot construct
model universes in their laboratories, but they are able to simulate the
conditions believed to describe the universe at different stages in its
history through elaborate computer codes. Such "numerical simulations"
provide the details of the story of the formation and evolution of
galaxies and of even larger structures - the clusters and superclusters
of galaxies - since the Big Bang. They also predict that the lowest mass
galaxies make consist almost entirely of dark matter, containing so little
baryonic matter that they are unable to make any stars at all. Hence, these
low mass objects could be starless galaxies. Do such objects truly exist?
That's exactly what the Cornell EGGfolks are trying to find out.
You may ask: "If there are no stars, how can we know that a galaxy exists?"
The answer lies in the fact that even if the baryonic matter in the
starless galaxy failed to make any stars, there still might be small
amounts of hydrogen gas mixed in with the dark matter. Hydrogen is by far
the most abundant element in the Universe, and fortunately for
us, emits radio waves at wavelengths that can be detected by the giant Arecibo
telescope. So the Cornell EGG astronomers spend a lot of time
sitting before computers in the Camuy Cave, sifting through huge
amounts of radio noise, looking for the hydrogen signal of a starless galaxy.
Sort of the cyberspace equivalent of looking for a needle in the cosmic haystack.
The EGG search has been made possible by improvements in Arecibo's
capabilities in the last few years.
While the technology advances make the hunt feasible, it still remains
a daunting and complex task. The definition of the new search strategies,
the effects of ever-increasing radio frequency interference produced by
human activities and the discrimination of signals produced by
cosmic sources amid the radio noise are all challenges to the EGG team. Besides the
"EGGHeads", Professors Riccardo Giovanelli and Martha Haynes, the EGG
team includes Astronomy graduate students Kristine Spekkens, Chris
Springob, Karen Masters, Amelie Saintonge, Barbara Catinella and Brian
Kent, senior Lisa Wei and Visiting Professor Becky Koopmann, a sabbatic
visitor from Union College. Over the summer of 2003, NSF Research Experience for
Undergraduate summer students Ann Martin (SUNY--Buffalo) and Anna van Duzer
(U. South Dakota) also contributed by compiling a database of related
information from other telescopes.
The Arecibo radio telescope is the largest on Earth, capable of
collecting more photons of radiation at once than any other, thereby
enabling the discovery of the faintest radio sources. Yet, the
ongoing search for starless galaxies is only a pilot project,
in anticipation of grander things to come, after Arecibo overcomes
its "Cyclops complex". Powerful though it is, Arecibo today
is "single--eyed" and, until now, has been limited in its ability
to construct images of the sky. As for most single dish radio telescopes, an
image made with Arecibo today needs to be constructed by recording a
single pixel at a time.
A solution to this limitation was recognized many years ago, when
Giovanelli, then the head of the Radio Astronomy group at the Arecibo
Observatory, and others on the Arecibo staff investigated the feasibility
of constructing a multi-pixel detection system for Arecibo. The
implementation of such a device was made possible by the recent upgrade
of the telescope, which radically altered its optical design. The
so-called "Arecibo L-Band Feed Array" (ALFA) is now under construction
and will, within the next year, turn Arecibo into a 7-pixel "camera",
thereby making it possible to sweep the sky that much faster. Then,
the "ultimate" hunt for starless galaxies: an ALl-sky, Fast ALFA survey,
known to the EGGfolks as
"ALFALFA", will finally happen.
For Giovanelli and Haynes, the task of conceiving and implementing new
observing techniques at Arecibo is not new. As graduate students
they were among the first to use Arecibo for extragalactic studies
after the radio telescope was provided with a new antenna surface in
1974, enabling the first observations of hydrogen with the big dish.
Over the years, their long affiliations with both
Arecibo and Cornell have involved them in several new expansions of the
telescope's capabilities in observational cosmology. Their pioneering
work on mapping the large scale, filamentary nature of the distribution
of galaxies, largely conducted at Arecibo, brought them the
1989 Henry Draper Medal of the National Academy of Sciences.
Like Giovanelli and Haynes, who began their careers as young researchers
on the staff at Arecibo, the current EGG graduate students are gaining
experience and expertise far beyond what they might learn in a classroom:
the design of a major radio survey, the development of observing strategies,
the consideration of trade-offs in strategy, science goals and many
practicalities, the development of sophisticated software tools, the
handling of large volumes of data. As is typical of an effort of this
magnitude, results and answers don't come quickly, and all "grades" of
EGGfolks are contributing to the hunt for starless galaxies.
Long, observation-intensive projects like this one benefit enormously
from the ability to carry on observations remotely using graphical computer
tools developed by the Arecibo Observatory staff. On the other
hand, we EGGfolks do admit to being keen to travel to Puerto Rico for
observing runs, especially during the winter months(!).
Located a short distance from the Arecibo Observatory in the karst region of
Puerto Rico, the
real Camuy Cave is part of an extensive subterranean cave
system reported to be the largest in the western hemisphere. It's a lot
more scenic than its Cornell namesake!
|The right panel shows a series of radio spectra as obtained at
Arecibo, showing the radio flux detected over a range of different
frequencies every few seconds during one of our observing runs. Most of
the radio emission is simply static noise, but once in a while,
a signal "pops up" above the noise at some specific frequency.
A zoomed version of the signal is shown in the upper left.
Also shown is an almost-starless galaxy visible in the
accompanying optical image as the faint fuzzy patch along with its
brighter, normal spiral galaxy "parent", itself similar
to our own Milky Way. A truly starless galaxy would be detected
in the Arecibo map but would not be visible at all in the optical image.
The Arecibo hunt for starless galaxies involves searching for
similar signals among many hundreds of thousands of such radio spectra so that
identification of the signals must be performed automatically by
specially--designed signal detection algorithms rather than by eye.
Last modified: Fri Jan 7 13:34:36 EST 2005 by mph