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NGC 4319 and Markarian 205
by Roger Knacke (Penn State Erie)

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.

 


Ground-based image from the
Digitized Sky Survey

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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