By Rebecca Canning and Norbert Werner
The atmosphere that enshrouds the Earth and provides us with wonderful things—like air to breathe, and beautiful sunrises and sunsets, and rainbows—unfortunately also absorbs light at many wavelengths and limits us from having a transparently clear view of the universe. To be certain, the visible wavelength light entering our atmosphere from the surrounding universe brings us astounding information about distant stars and galaxies. However, most information about the cosmos is hidden from our human eyes, unable to penetrate the Earth’s atmosphere. In fact nearly 90% of the ordinary matter in the Universe cannot be observed with visible-light telescopes—it is either too cold, too hot, or too enshrouded bydust.
Some of the most interesting events in the universe, such as the birth of stars from cold gas and dust clouds, are too obscured or too cool to shine in visible light, but emit infrared radiation instead. This infrared light, with wavelengths of between about 1 millionth to 300 millionths of a meter (or 1-300 microns) is difficult to observe from the Earth. The longer wavelengths in this range are referred to as the “far-infrared” and as most of this type of light is absorbed by water vapor and carbon dioxide, we need to make observations from high altitudes where the atmosphere is thin. But even the highest mountains on Earth (~29,000 ft)—where the atmosphere is too thin for humans to live—are not high enough to avoid all of the absorption.
To see the far-infrared light from the cosmos, we must go even higher. Space-based telescopes offer fantastic sensitivity to the infrared — however, these missions are very expensive and less versatile than ground-based instruments where one can easily change or upgrade a detector. Another innovative approach is to fly a telescope in the highest parts of the Earth’s atmosphere, high above the absorbing water vapor. This is the purpose of SOFIA: the Stratospheric Observatory For Infrared Astronomy.
Above: This figure shows which wavelengths of light are blocked by the Earth’s atmosphere at ground level, and which are allowed through. (Image credit:http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irwindow...)
SOFIA is an airborne observatory, a Boeing 747SP with a 2.5 meter diameter telescope on board. Recently, we were fortunate enough to observe on this very special airplane operated jointly by NASA and the German Aerospace Center (DLR) from the Armstrong Flight Research Center in Palmdale, California.
Above: Becky in front of SOFIA plane.
We were granted five hours of observing time on SOFIA to look at six nearby giant elliptical galaxies. This type of galaxy has been historically described as "red-and-dead" owing to the light from it coming primarily from old reddish stars and the lack of any bright, blue, young star formation within it. While cold gas is abundant in spiral galaxies with lively star formation, the lack of it in giant ellipticals seemed to explain the absence of new stars. However, our previous observations with the now defunct Herschel Space Observatory (which took data in the far infrared and submillimeter wavelengths) showed that some of the ellipticals do host large reservoirs of cold and molecular gas—the vital raw material from which stars are born. These observations left us with more questions than answers:
Why do some giant ellipticals harbor reservoirs of cold gas while others don't?
Why are these galaxies apparently unable to efficiently convert this gas to stars?
Are the powerful jets emanating from the centers of these galaxies the result of the infall of cold gas towards the central supermassive black holes?
Do the jets eventually destroy this gas?
Addressing these questions was the aim of our SOFIA observations.
Above: Norbert in front of SOFIA plane.
We arrived in Palmdale for safety training the day before our flight. SOFIA must fly much higher than commercial airplanes (42,000-46,000 ft compared with 28,000-35,000 ft) in order to escape the majority of the infrared-absorbing atmosphere, and therefore the safety requirements are somewhat different than we are normally used to. At these altitudes the air is so thin that if the cabin accidentally depressurizes, a human will remain conscious for only a few seconds compared with a minute or so on a commercial flight.
SOFIA requires precision flying and therefore it is piloted by experienced NASA test pilots. The low air density at high altitudes results in less lift being generated for a given airspeed and angle of attack (angle between the wing and the horizontal flight path), which means the higher an airplane flies the greater its minimum speed must be. This is especially true when turning as the wing loading will increase, which necessitates yet more lift. One also has to be careful of the maximum allowed speed; as air flows over the wing it is accelerated, and as the wing’s acceleration progresses towards the speed of sound, shocks form over it, reducing lift. But the speed of sound decreases with altitude—colder air at high altitude has a lower sound speed. Thus, the higher a plane flies, the closer its high and low speed limits approach one another. The pilots flying SOFIA have a margin of error of only about 5 knots—too much faster or slower and the plane could stall. Precision is also required as the main pointing of the telescope is controlled by the pilots with only fine adjustments made by the telescope and instrument teams.
We observed with an instrument called the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS). FIFI-LS is an integral field spectrometer which means that it not only produces an image, but for each image pixel we also get a spectrum of the dispersed infrared light that hit that pixel. The camera on the telescope needs to stare at an object for a long time to see faint emission and during the time the camera is collecting the light it must be held steadily on the target to produce a sharp image. Steadying a camera is a non-trivial task on Earth, let alone on an aircraft bouncing in turbulent air more than 13 km high in the sky. The pointing is stabilized using gyroscopes which use thin layers of air and oil as lubrication, and sensitive measurements of the torques on the telescope. Amazingly, the telescope on SOFIA can maintain a pointing accuracy of 0.5 arc seconds (the equivalent of the width of a dime seen from 2.5 miles away)—even in substantially choppy atmosphere!
Above: Some of the control screens for the telescope, which can itself be seen at the back of the picture peering out of the airplane (Photo credit: R.Canning, N.Werner).
SOFIA is an incredible feat of technology enabling a unique access to the infrared sky. It is also ever-improving; as time passes new instruments will be added, further increasing its capabilities. For us, hopefully, these observations were just the beginning—there is so much more that this instrument can teach us about how the largest galaxies in the Universe evolve and why they remain red-and-dead.
We are currently in the process of analyzing all the great data we acquired, and plan to follow up with the actual results of our observations in a future post!