Appearances can be deceiving. In this NASA Hubble Space Telescope image, an odd celestial duo, the spiral galaxy NGC 4319 [center]
and a quasar called Markarian 205 [upper right], appear to be neighbors. In reality, the two objects don't even live in the same city. They are separated by time and space.
NGC 4319 is 80 million light-years from Earth. Markarian 205 (Mrk 205) is more than 14 times farther away, residing 1 billion
light-years from Earth. The apparent close alignment of Mrk 205 and NGC 4319 is simply a matter of chance. Astronomers used two methods to determine the distances to these objects. First, they measured how their
light has been stretched in space due to the universe's expansion. Then they measured how much the ultraviolet light from Mrk 205 dimmed as it passed through the interstellar gas of NGC 4319.
The Hubble image shows the inner region of NGC 4319. In addition to the galaxy's inner spiral arms, an outer arm is faintly
visible at lower left. The unusually dark and misshapen dust lanes in the galaxy's inner region are evidence of a disturbance, probably caused by an earlier interaction with another galaxy, NGC 4291, which is
not in the photograph.
At a distance of 1 billion light-years, Mrk 205 is a relatively nearby quasar. Many quasars reside much farther away. Quasars, once
known only as mysterious point-like objects, are now known to be distant galaxies that have extremely bright cores. These powerhouses of light are probably fueled by massive black holes. With powerful telescopes
like Hubble, it is often possible to see the quasar's surrounding halo of faint starlight, as is clearly visible around Mrk 205.
Mrk 205 has a companion, a compact galaxy just below it. The objects appear to be interacting. The compact galaxy may be responsible
for the structure in Mrk 205's halo.
The Hubble image shows that interacting galaxies and disturbances within galaxies are a rich source of information about galaxy
structure and evolution.
The pair, NGC 4319 and Mrk 205, is of particular interest to astronomers because of the role they played in
a three-decades-old scientific controversy. Astronomer Halton Arp identified a number of galaxy-quasar pairings and argued that the quasars were at the same distance as the galaxies, not the much larger distances
indicated by their redshifts. NGC 4319 and Mrk 205 were the most famous example because in some ground-based images they appeared to be linked by a "luminous bridge". An overwhelming abundance of evidence
long ago convinced virtually all astronomers that quasars are indeed at the vast distances indicated by their redshifts, but this controversy remains as an interesting example of how science judges and incorporates
new ideas and novel phenomena. You can learn more about this controversy by visiting the More Images link on this month's Hubble Heritage image page.
Credits: NASA and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: R. Knacke (Penn State Erie)