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Courtesy of Farhad Zadeh/Northwestern University

An image of a molecular cloud (top) is seen to be compatible with its “envelope,” as Professor Farhad Zadeh explained, in the radio image (bottom). Zadeh’s radio wave technique allows for  detection of dark, dusty regions of clouds and stars not detectable in other wavelengths.


Radio wave technique illuminates the dark regions of space

by Fouad Egbaria
Jan 24, 2013


A new radio wave technique makes what was otherwise invisible, visible.

As modern science steers humanity through the dark corridors of the cosmos, radio waves can serve as a guide for mapping some of our galaxy's darker spots, according to a technique developed by Farhad Zadeh, a professor of physics and astronomy at Northwestern University.

Zadeh has developed a technique utilizing radio waves to locate and identify dusty stars and clouds in our Milky Way galaxy. The use of radio waves allows for the identification of dark galactic features that would not be caught by other spectra, such as X-rays, infrared and optical light.

“The concept is very simple. You have a big bath of radiation from stars or galaxies and you just put a cold gas, cold cloud in it, just punch it through. So what happens is that when you look at it, you have less radiation coming when there is cold gas and you see more outside of it,” Zadeh said.

Zadeh earned his doctorate in radio astronomy from Columbia University in 1986. He has been studying the distribution of radio emission in the nucleus region of our galaxy for about 20 years. He presented his findings on Jan. 8 at a meeting of the American Astronomical Society in Long Beach, Calif.

“[When] you do radio interferometry (a branch of radio astronomy) … it creates an artifact, and these artifacts have to be accounted for,” Zadeh said. These artifacts show up on the image as dark, he said. “It is generally very obvious whenever you see a dark feature that it is an artifact of the image, an artifact of ‘cleaning the image’ itself as radio astronomers call it.” An artifact is a product of image construction that does not actually exist in reality.

Doyal Harper, a professor in the Department of Astronomy and Astrophysics at the University of Chicago, explained the advantage of interferometry for radio astronomers. “[Interferometry] allows you to get much more detailed pictures … that can show the structure of often complex regions where this star formation is occurring.”

After further inspecting these images, Zadeh realized that these dark features were not just artifacts. They were something else.

“In the last couple of years I’ve realized that actually there’s something there, it’s unusual,” he said. After inspecting the images using different radio maps, Zadeh noticed that these dark features coincided almost exactly with very cold molecular clouds, like “a hand in a glove.”

Zadeh described the discovery process as a slow one, but he found himself excited about having discovered something new. Eventually, he came to understand its significance. “You see something and say ‘Oh well hmm, do we see more of these clouds?’ And you just look more … you say ‘oh wow look at this, and look at this, look at this.’ I found six, seven different types of clouds, all showing the same structure.”

What at first may have been a coincidence required further data collection in order to establish the hand in a glove correlation. Zadeh acquired more and more data before reaching two conclusions: the dark structures he observed were in fact real (that is, not mere artifacts) and that there were dense clouds of gas sitting in these dark, cold spots not visible in other spectra of electromagnetic radiation. Similarly, Zadeh more recently found an analogous relationship between such dark areas and dusty stars. As such, these dark clouds and stars are referred to as “radio dark” clouds and stars.

“Good scientists are always suspicious at first,” Zadeh said, laughing. “And then when they get excited it’s always good also.”

Harper works mostly with the far infrared radition, which is between visible light and radio (in terms of wavelength) on the electromagnetic spectrum. However, one of his primary research interests is dense regions where young stars are forming, “exactly the kind of region that [can be] studied using this technique,” Harper said.

We know where a lot of star-forming regions in our own galaxy are, but it’s difficult to study them in great detail because of their great distance from Earth, Harper noted.

This technique, however, also opens doors for the study of space beyond our galaxy. “[This] also opens up the opportunity of isolating particular molecular clouds in distant galaxies, things that are much farther away than the star-forming regions in the Milky Way,” Harper said.

Zadeh is also currently studying whether these dark features are in fact pointing toward a black hole –a dense body in space from which nothing can escape– and if the holes are pulling some of this galactic dust to it, in a sense “feeding” it. 

Zadeh will be giving a public talk at 7 p.m. Feb. 8 at the Adler Planetarium on the black hole at the center of our galaxy, also known as Sagittarius A*. He stressed the importance of communication between the astronomical community and the general public, adding that he enjoyed opportunities to speak with so called “amateur astronomers.”

“I love these public talks because you talk for an hour and people are really interested in the science but they’re not professionals,” Zadeh said. “They are people who are just interested in astronomy.”