The Chandra X-Ray Observatory image of the elliptical galaxy NGC 4261 revealed
dozens of black holes and neutron stars strung out across tens of
thousands of light years like beads on a necklace. The spectacular
structure, which is not apparent from the optical image of the galaxy,
is thought to be the remains of a collision between galaxies a few
billion years ago, when a smaller galaxy was captured and pulled apart
by the gravitational tidal forces of NGC 4261. As the doomed galaxy fell
into the larger galaxy, large streams of gas were pulled out into long tidal tails. Shock waves in these tidal tails triggered the formation of many massive stars.
Over the course of a few million years, these stars evolved into neutron
stars or black holes. A few of these collapsed stars had companion
stars, and became bright X-ray sources as gas from the companions was
captured by their intense gravitational fields.
The currently favored view is that elliptical galaxies are produced
by collisions between spiral galaxies. Computer simulations of galaxy collisions support
this idea, and optical evidence of tails, shells, ripples, arcs and
other structures have been interpreted as evidence for this theory.
However as the image above shows, the optical evidence rather quickly
fades into the starry background of the galaxy, whereas the X-ray
signature lingers for hundreds of millions of years. Chandra's image of
NGC 4261 shows that X-ray observations may be the best way to identify
the ancient remains of mergers between galaxies.
The Hubble Space Telescope picture
below of the center of NGC 4261 tells a dramatic tale. The gas and dust
in this disk are swirling into what is almost certainly a super,
perhaps super-super massive black hole.
The disk is probably what remains of the smaller galaxy that fell in
hundreds of millions of years ago. Collisions like this may be a common
way of creating such active galactic nuclei as quasars. Strangely, the center of this fiery whirlpool is offset from the exact center of the galaxy - for a reason that for now remains an astronomical mystery.
Source: The Daily Galaxy via Chandra X-Ray Observatory and NASA
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