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UCSD astronomers to target Hubble to Quasars, active galaxies and
supernova remnants
April 4, 1990
Contact: Yvonne Baskin, (619) 534-0362
UCSD ASTRONOMERS WILL TARGET SPACE TELESCOPE TO QUASARS, ACTIVE GALAXIES,
SUPERNOVA REMNANTS
The University of California, San Diego team will devote much of its observing time to quasars, the most
distant and energetic objects in the universe. The goal is to try to understand the nature and evolution of these
enigmatic objects that are as compact as our solar system yet as bright as 100 trillion stars.
Other targets include potential black holes lurking in the centers of active galaxies.
The key to interpreting the light picked up from these celestial objects by the Faint Object Spectrograph (FOS)
is a phenomenon called redshift.
Elements such as hydrogen and iron emit light at specific wavelengths or spectral lines when tested in the
laboratory. Yet the light from objects such as galaxies and quasars is skewed from these laboratory values and
shifted toward longer wavelengths--the red end of the spectrum--by the time it reaches the Earth.
Since 1929, orthodox theory has attributed this "redshift" to the Doppler effect, the same effect that causes the
sound of a passing train to drop in pitch as it moves farther away. Thus, the redshift in an object's spectral lines is
used to calculate how fast it is receding from the Earth due to the expansion of the universe and how far away it is
in space and time.
Here is a list of the primary targets for the UCSD team's guaranteed observing time from the FOS:
I. The ultraviolet spectra of low redshift quasars
"Some quasars have such extremely high redshifts that lines in the ultraviolet end of the spectrum are actually
pulled into the visible range," says Margaret Burbidge, University professor of physics at UCSD and leader of the
UCSD team. "We've been able to study these with ground-based telescopes."
The Earth's atmosphere, however, absorbs ultraviolet radiation and makes it impossible to study the UV
spectrum of low redshift quasars from the ground. These objects are a key target for the FOS.
Because of their low redshift, these quasars are presumed to be younger in time and nearer to the Earth than
better-studied high redshift quasars. Burbidge's team wants to compare data on older and younger objects for
clues to the evolution of quasars.
2. The ultraviolet spectra of very high redshift quasars
"A redshift greater than 3.1 is just large enough that the spectral lines of both ionized and neutral helium are
pulled from the far ultraviolet into the visible range of the Faint Object Spectrograph," Burbidge says. "Helium is
a very important element for cosmologists. It was supposed to have been created in the Big Bang, and various
theories about the Big Bang call for differing amounts of helium.
"So we want to know, how much helium is left in intergalactic space? In quasars? What was the primordial
amount?"
Looking at a quasar with a redshift of four, for instance, corresponds to looking back 10 billion years in time to
the period shortly after the Big Bang.
3. The spectra of intermediate redshift quasars
Light emitted by hydrogen is the most prominent line in the spectra of quasars and is called the Lyman alpha
emission line. A redshift of two or greater will pull this emission line from the ultraviolet into the visible range. A
large number of weaker absorption lines produced by Lyman alpha from these quasars remain obscured in the
UV range, however, at wavelengths just short of the transition to the visible range. These have been named the
"Lyman alpha forest," and one goal of the UCSD FOS team is to measure just how far back into the blue end of
the spectrum this forest of lines goes.
The forest, Burbidge notes, may represent light not from the quasar itself but from diffuse gas clouds that lie
along our line of sight to the quasar and absorb some of its spectrum.
"These gas clouds may be in a primordial region, perhaps evolving into a cluster of galaxies around the
quasar," Burbidge says. "Or perhaps it's an area of star formation and the quasar itself is the early stage of an
evolving galaxy."
4. BL Lacertae (BL Lac) objects and gravitational lenses
Einstein's General Theory of Relativity predicted that even rays of light can be bent by the gravitational pull of
a massive object. Such an object acts like a lens--a gravitational lens--distorting passing light rays in such a way
that a distant observer may see multiple objects where only one exists.
Some astronomers have proposed that strange quasar-like phenomena called BL Lac objects may actually
be the result of gravitational lensing by quasars. The UCSD team will use FOS along with the space telescope's
Wide-Field Planetary Camera to study these objects and other suspected lensing phenomena.
5. Nuclei of active galaxies and radio galaxies
The nuclei of some galaxies such as M51, the Whirlpool galaxy, actively emit radio waves or high-speed gas
jets. Could black holes be creating this radiation? The UCSD team will use the FOS to resolve the spectral lines
as close to the nuclei of such galaxies as possible. By using a blocking disk to filter out the emissions from the
nuclei, they will also try to find out what's happening in the regions around the centers of these galaxies.
6. Globular clusters in the Milky Way
The globular clusters are dense clusters of stars that form a halo around our own galaxy, the Milky Way.
"These areas contain some of our oldest stars, the ones that are least rich in elements heavier than hydrogen,"
says Burbidge. "With FOS, we'll try to resolve the centers of these clusters and determine the source of their
strong X-ray emissions. One theory is that these emissions come from hot gases turning into an accretion disk
around a collapsing neutron star."
7. Other targets: supernova remnants to radiation jets
"The Crab Nebula is our nearest and best-studied supernova remnant, but no one has gotten the ultraviolet
spectrum of it," says Burbidge. "That's one of our goals."
Another goal is the UV spectrum of the synchrotron radiation jet coming out of a radio galaxy called M87.
And a final challenge is to get the optical and UV spectrum of a radiation jet coming from the quasar 3C273,
one of the lowest redshift and brightest quasars known. "In 1963 this was the first quasar ever identified,"
Burbidge says. "But all attempts to separate out the spectrum of the jet from the quasar itself have failed. The
FOS should resolve it."
(April 4, 1990)