Redshift challenge in Seyfert's Sextet

Seyfert's Sextet
image NOAO

The National Optical Astronomy Observatory (NOAO) site tells about the image: "This picture was taken at the Kitt Peak National Observatory's 2.1-meter telescope in July of 1998, as part of the KPNO Research Experiences for Undergraduates program, sponsored by the National Science Foundation. North is up, east is to the left."

The accompanying image shows labeling and redshifts expressed in speeds km/s. While most redshifts in the sextet are in the range 4017 km/s - 4482 km/s there is one odd galaxy in the center with the speed of 19813 km/s. This is another story, altogether.

Image NOAO

Always ready for new adventures in Science, aren't we!

"Five of the members show very similar redshifts, from 4000 to 4500 km/s, while the fifth is measured at nearly 20000 km/s. Conventional wisdom argues that this is a chance projection of a distant background galaxy: some would have otherwise and require a complete rethink of modern cosmology and the introduction of completely new physics."

NOAO

Well, such complete rethink of modern cosmology etc. has happened - an exception, anomaly in observed data that challenges generally accepted theories and at the end smashing them. So who will win in this case, classic redshift physics or human mind generating a new theory explaining the strange redshift reading for the sixth object in Seyfert's Sextet?

Well, let's take a closer look at the sextet taken by a camera located above Earth's distorting atmosphere.

Seyfert's Sextet NASA APOD

"The sextet actually contains only four interacting galaxies, though. Near the center of this Hubble Space Telescope picture,

• the small face-on spiral galaxy lies in the distant background and appears only by chance aligned with the main group. 
• Also, the prominent condensation on the upper left is likely not a separate galaxy at all, but a tidal tail of stars flung out by the galaxies' gravitational interactions." (APOD)

What you see is what you get - the sextet turns out to be a quartet!

As for classic redshift physics, common wisdom has survived this challenge and demonstrated once again the value of optical spectrometry to Astronomy!