Number 210, January 13, 1995 by Phillip F. Schewe and Ben Stein|
"A GIANT LEAP BACKWARDS IN OUR UNDERSTANDING OF QUASARS" is
the way John Bahcall of the Institute for Advanced Study assesses his new
observations of quasars with the Hubble Space Telescope (HST). Speaking
at this week's meeting of the American Astronomical Society (AAS) in Tucson,
Bahcall presented a study of 14 bright and relatively nearby quasars. His
goal was to use HST's sharp vision to image the galaxies which, according
to current theories, should act as hosts for the quasars insofar as they
would provide fuel for the presumed supermassive black hole at the heart
of the quasar. Host galaxies are hard to see because of the glare from
the quasar itself. In Bahcall's sample of 14, several host galaxies were
found, but for 8 quasars no hosts were found. Such "naked quasars"
represent a challenge to quasar models. Another surprise was the observation
of a quasar attracting and distorting a companion (but not host) galaxy.
THE INTERIOR LIFE OF SUPERNOVAS has been made visible through sophisticated
supercomputer simulations. The new models show what happens when the massive
outer portion of a dying star falls in on the blizzard of neutrinos streaming
out of the just-collapsed core. Older models, tending to look only at one-dimensional,
pencil-beam sections of the evolving supernova explosion, were unsatisfactory
since they too often predicted that supernovas would stall out, not for
want of energy but because the tremendous energy stored in the nascent
explosion (10**53 ergs) could not effectively be transmitted to the cooler
stellar matter above. At the AAS meeting, Adam Burrows and Willy Benz of
the University of Arizona and Marc Herant of Los Alamos showed off their
multi-dimensional models allowing colossal convective plumes of rising
hot material and sinking cool material to interpenetrate, facilitating
a more efficient movement of explosion energy. (Herant compared all of
this to heating a thick sauce in a pan. Left unstirred, the sauce will
burn on the bottom and be cool on top. Stirring allows heat to get to the
top.) Not only does the complex mixing permit the outward-going shock wave
to proceed on schedule---eventually ripping apart the outer envelope of
the star---but it helps to explain other features of supernovas poorly
handled in older models. These features include the dispersal of heavy
elements in the subsequent debris blown into space, the overall asymmetric
shape of supernovas, and the observed sideways recoil velocities (amounting
to hundreds of km/sec) imparted to the pulsar remnant of some supernovas.
THE BEST EVIDENCE YET FOR A BLACK HOLE comes from observations of maser
emissions (coherent microwave radiation analogous to laser light) from
a dusty torus very near the center of galaxy NGC 4258. From the speed of
the dust (900 km/sec) rotating around the galactic center, astronomers
have calculated that the mass of the central gravitating body has a mass
of almost 40 million solar masses, all within a radius of 0.13 parsec.
This works out to a density of 100 million solar masses per light year,
a density some 10,000 times greater than that of any known cluster of stars.
James Moran of the Harvard-Smithsonian Center for Astrophysics, reporting
the new results at the AAS meeting on behalf of his Japanese-American collaboration,
believes that the profile of high-speed gas provides "compelling evidence"
that the central object is a black hole. The high-resolution pictures of
the heart of NGC 4258 were made with the Very Long Baseline Array, a network
of radio telescopes located from Hawaii to the Virgin Islands.