Before you read this, if you have access to an FM radio, tune through the frequencies until you find one that has very little signal and is mostly static. Most of
that static is the mixed signals of human broadcasts, but a fraction of a percent of it is actually from the cosmos the sun, stars, galaxies and even the echo of the Big Bang.
Since cosmic radio signals are so weak, it takes enormous telescopes to detect them at sufficient strength to analyze. Thus, I and my fellow Adironack Public Observatory Director Jeff Miller spent last week at the largest single dish telescope in the world at Arecibo Observatory in Puerto Rico with a reflecting surface, "dish" 1,000 feet in diameter.
In the "light" of the electromagnetic spectrum from radio waves to gamma rays falling to Earth, the universe has been telling us about itself for all of history. In fact, other than a few moon rocks brought back by Apollo astronauts and Mars rocks found as meteorites, all that we know of the universe is from the light.
Once the positions of the stars were well mapped and the motions of the sun, moon and planets understood and predictable, gathering and learning how to see the light became the primary job of astronomers. As well as building bigger, better telescopes to create more detailed images of distant objects, learning to see the light required astronomers to explore and understand fine details in the spectra of individual objects and developing instruments to observe the universe in all the bands of the electromagnetic spectrum.
In 1888, Heinrich Hertz demonstrated that radio waves could be transmitted and received. In 1890, Thomas Edison proposed an experiment to detect radio waves from
cosmic sources. So the idea of radio astronomy is nearly as old as radio. As the technology progressed, Bell Labs set a young researcher to the task of discovering possible sources of static in transatlantic radio transmissions. Karl Jansky thus discovered radio emissions from the center of the Milky Way Galaxy in 1933. Hearing of this discovery, Grote Reber, a radio engineer and ham radio operator, built a radio telescope in his Wheaton, Ill. backyard in 1937. Other researchers followed.
Construction of the Arecibo Observatory Telescope began in 1960 and was completed in
Our work at Arecibo is part of the Arecibo Legacy Fast ALFA (ALFALFA) survey headed by astronomers from Cornell University that built and, until Oct. 1, 2011,
managed the observatory. ALFA is the Arecibo L-Band Feed Array, an instrument with seven radio "feeds" (individual receivers) that operates in the L-Band of the radio spectrum, between 1,200 and 1,800 MHz far off the upper end of the FM dial.
Installed in 2004, we use ALFA to take 14 100-MHz wide spectra each second (7 receivers x 2 polarizations).
The ALFALFA project is to observe all of the sky visible from Arecibo (a swath 36 degrees wide centered above Arecibo at 18.3 degrees north) outside the disk of our own Milky Way galaxy. When completed later this year, the survey will have required more than 4,000 hours of observing time and much more than that to process and analyze the terabytes of data. The wealth of data inspired the Cornell astronomers, Martha Haynes and Riccardo Giovanelli, to involve many undergraduate faculty and students in the project. Jeff and I, shown on our first observing run at Arecibo in 2006, have had the opportunity to take four students to Arecibo.
Six St. Lawrence University students have done their senior research projects (in physics and computer science) with ALFALFA data.
What we observe with ALFA are galaxy-sized clouds of cool hydrogen, almost all of which are associated with galaxies known from optical surveys of the sky. We hope to find some "dark galaxies" where the gas is abundant but has been unable to create stars due to insufficient density or disruptive currents.
The reason we observe spectra instead of creating images is that spectra allow us to determine the distances to the clouds we detect. As discovered by Edwin Hubble in the
1920's, the light from all galaxies is shifted to the red (to lower frequencies) and the shift increases with distance. This is thought to be due to the Doppler shift of the light due to motion of the source. Think of the sound of a car race. As the cars pass by the stands, their sounds descend in pitch. This is because the sound waves of the approaching cars are squeezed by their motion, shifting them to a higher pitch while the
waves of the receding cars are stretched to a lower pitch. The redshifts of the galaxies are interpreted to mean that all galaxies are moving away and more distant galaxies are moving away faster.
Hence, measuring the redshift of a galaxy provides a measurement of its distance. Since cool (atomic) hydrogen radiates at 1420 MHz, the frequency at which a cloud is observed in our spectrum immediately reveals its redshift.
From our data we can then build three-dimensional cubes with galaxy positions on the sky and distance. The lowest frequency we detect, 1335 MHz corresponds to a distance
of 850 million light years. However distant that seems, it's a mere 6.5 percent of the most distant galaxy's 13.2 billion light years. Apart from any discovery of dark galaxies, this
3-D model of the nearby universe will be a vital contribution to our knowledge of our cosmic neighborhood.
If you have questions about radio astronomy or any other astronomical topic, please visit the Adirondack Public Observatory website at apobservatory.org or email Aileen