Arxiu del divendres, 31/05/2019

Faster than light

divendres, 31/05/2019

January 6, 1999

John Biretta
Space Telecsope Science Institute, Baltimore

          HUBBLE DETECTS FASTER-THAN-LIGHT MOTION IN GALAXY M87

Astronomers reported today discovering clouds which appear to
move many times faster than the speed of light, shooting out 
from the region of a black hole at the center of the M87 galaxy.  
These results help explain the nature of distant quasars and 
``BL Lac'' objects.

``We see almost a dozen clouds which appear to be moving out from the 
galaxy's center at between four and six times the speed of light.  
These are all located in a narrow jet of gas streaming out from the 
region of the black hole at the galaxy's center," said Dr. John Biretta 
of the Space Telescope Science Institute.  ``We believe this apparent
speed translates into an actual velocity just slightly below that of light
itself."

Biretta and his colleagues Duccio Macchetto, William Sparks, and 
Eric Perlman announced the results at a meeting of the American 
Astronomical Society in Austin, Texas.
 
The speeds reported are two to three times faster than the 
fastest motions previously recorded in M87, the only nearby 
galaxy to show evidence for superluminal motion. 
``This discovery goes a long way towards confirming that
radio galaxies, quasars and exotic BL Lac objects are basically 
the same beast, powered by super massive black holes, and differ 
only in orientation with respect to the observer," Biretta said.
``Here we have, for the first time, a fairly normal radio galaxy with 
both excellent evidence for a super-massive black hole, as well as 
superluminal jet speeds similar to those seen in distant quasars 
and BL Lac objects."

The result also represents a first for the Hubble Space Telescope.
``This is the first time superluminal motion has been seen
with any optical telescope, and this discovery was made possible by
the extremely fine resolution obtained by Hubble,'' said Macchetto.

The measurements compared observations made between 1994 and 1998 
with Hubble's Faint Object Camera. In those four years,
despite the astonishing speed involved, the clouds traveled only 
about one-tenth of an arcsecond across the sky -- or about the 
width of a human hair seen from 100 yards away.
 
``Current so-called `unified' models hypothesize that radio galaxies, 
quasars, and BL Lac objects are all the same type of object, but merely 
seen from different directions.  In the case of quasars and BL Lacs, we 
must be observing from a special direction where the jet is nearly
aimed at us.  For this orientation, effects related to Einstein's
theory of relativity cause the jet to appear very bright, move 
rapidly, and vary in brightness -- all properties associated with 
quasars and BL Lacs.  And in the more common radio galaxies the jet is 
hypothesized to be seen side-ways, so it appears relatively 
undistinguished.  ``Our discovery of fast superluminal speeds in M87, 
a fairly normal radio galaxy, essentially confirms the unified model," 
said Biretta.

The term `superluminal motion' is something of a misnomer.  While it 
accurately describes the speeds measured, scientists still believe the 
actual speed falls just below the speed of light.

``It's an illusion created by the finite speed of light and rapid motion," 
Biretta said.  ``Our present understanding is that this `superluminal motion' 
occurs when these clouds move towards Earth at speeds very close to that 
of light, in this case, more than 98 percent of the speed of light.  At 
these speeds the clouds nearly keep pace with the light they emit as they 
move towards Earth, so when the light finally reaches us, the motion appears 
much more rapid than the speed of light.  Since the moving clouds travel 
slightly slower than the speed of light, they do not actually violate 
Einstein's theory of relativity which sets light as the speed limit."

M87 is a faint [9th magnitude] galaxy about 50 million light
years distant in the constellation Virgo.  Its jet was first discovered 
in 1918 by an optical astronomer, Heber Curtis, who noticed a very faint
wisp of light extending outward from the center of the galaxy.
Decades later, radio telescopes were turned to this object, and
Curtis's slightly peculiar galaxy turned out to be one of the brightest 
sources of radio waves in the sky. Such galaxies with strong radio emission 
are termed ``radio galaxies'' and are now known to be fairly common.

The evidence for a super massive black hole at the center of M87 comes from 
spectroscopic observations made with HST by groups led by Drs. Holland Ford
(Johns Hopkins Univ.) and Duccio Macchetto.   Macchetto's group found there 
must be a mass of 3 billion solar masses within 10 light years of the center, 
which virtually rules out any explanation other than a black hole.

The details of how black holes form jets with speeds approaching
that of light are still unclear, but current models favor mechanisms where 
rotation of the accretion disk surrounding the black hole acts to twist-up the 
magnetic field lines much like a rubber band.  This energy is released by 
flinging material, probably thin clouds of electrons and positrons, down the 
field lines along the rotation axis, hence forming a narrow jet moving very 
nearly at the speed of light.

This work is made possible by grants from the National Aeronautics and 
Space Administration and European Space Agency.

                                   -===-