| What
are these objects?
The spiral
galaxy, NGC 4319, in this Hubble
Heritage image is classified as
a "Barred Spiral" (SBb)
galaxy. The HST field shows just
the inner regions. A longer exposure
shows the outer spiral arms (faintly
visible in the lower left corner
of the release image). The image
below from the Digitized
Sky Survey
shows the bar and spiral arms
clearly. Barred spiral galaxies
are relatively common. This one
shows interesting structure and
dust lanes in the inner region.
|
|
| Markarian
205 (Mrk 205) is object number
205 in a catalog compiled by
theArmenian
astronomer, Benik
Markarian. These are galaxies
withstrong ultraviolet emission;
many of them active galaxies.
It's usually classified as a Seyfert
galaxy, one with a bright nucleus,
or as a low-luminosity quasar.
Just northeast
of Mrk 205 is a compact galaxy.
It is at the same distance from
us as Mrk 205, but doesn't show
an envelope of stars and gas.
Some distance away and outside
the HST image is the elliptical
galaxy NGC 4291. Its redshift
is close to that of NGC 4319.
The two galaxies may have undergone
a near collision recently. Perhaps
this could account for the structure
in NGC 4319.
|
WFPC2 image of
Mrk 205 (F814W - red) |
What
is the faint filament between Mrk 205
and NGC 4319?
This
is hard to see in the Hubble image. Faint
features like this tend to stand out better
in a negative image and they can be enhanced
by artificially "stretching"
faint features with an image processing
program. Two such stretched and enhanced
images are shown.
Enhanced
stretches of the HST image show
a debatable "luminous bridge"
between NGC 4319 and Mrk205:
(left) animated gif 2.3MB, (center)
inverse image and (right) contour
plot.
Animation
Courtesy of NASA and Z. Levay
(STScI)
Still Images Courtesy of R.
Knacke (Penn State Erie)
|
WFPC2 image of
NGC 4319 and Mrk205 identifying
spiral arms and dust lanes. |
The
figures do show some nebulosity lying
between NGC 4319 and Mrk 205, as Halton
Arp and other astronomers noticed many
years ago. However, the question is whether
this nebulosity implies that there is
a real, physical connection between the
two galaxies, or whether it is just a
little bit of irregularity in the structure
of NGC 4319 or Mrk 205, that happens to
lie between the images. Notice that there
are similar nebulous features on the edges
of both objects in other places as well,
not just between them. I don't think that
these images demonstrate that there is
a real connection between the objects,
but you can make up your own mind.
What's
the issue about the redshifts?
NGC
4319 has a redshift (the fractional amount
that observed wavelengths of spectral
lines in a galaxy are shifted relative
to the wavelengths at rest, (lobs
- l rest)
/ lrest
) of 0.00468, while Mrk 205 has a redshift
of 0.071. If redshifts imply distance,
as almost all astronomers believe, then
Mrk 205 is almost 15 times farther away
than NGC 4319.
Mrk
205 is projected in the sky within the
spiral arms of NGC 4319. In 1971 Halton
Arp, who compiled an important catalog
of peculiar galaxies called the Arp
Catalog, wondered if this is not just
a chance superposition, but rather evidence
that the quasar-like galaxy really lies
within NGC 4319. He found support for
this view in the filamentary structure
between the two objects.
If
this were so, then redshifts would not
be distance indicators in all cases. Needless
to say it was a radical suggestion that,
if true, would have upset some of the
fundamental tenets of cosmology. It stirred
up a lot of controversy about the meaning
of redshifts and whether they were "cosmological,"
that is, due to the universal expansion,
in all cases. Arp found numerous other
examples of quasars near galaxies, although
few as dramatic as this one.
In
the view of most astronomers, the juxtapositions
are just due to chance. The filamentary
connection became less convincing as better
images became available. John Bahcall
and collaborators made a noteworthy contribution
when they showed that NGC 4319 absorbs
some of the light from Mrk 205, just as
expected if NGC 4319 is projected in front
of Mrk 205 (Astrophysical
Journal 1992). In time, many
quasars were found to lie in galaxies
with exactly the same redshift, providing
powerful evidence that quasars are an
event that occurs in the nucleus of galaxies.
Today
the redshift controversy has almost faded
from view. Only a few astronomers still
think there is reasonable evidence for
noncosmological redshifts; a recent summary
making their case was published by Geoffrey
Burbidge (Publications
of the Astronomical Society of the Pacific
2001). The vast majority of
astronomers think that the evidence is
overwhelming that redshifts show distances
to objects in the expanding universe.
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