NGC7217

This image shows the unbarred spiral galaxy NGC7217.  This small (4 arc second) gas poor galaxy lies about 50 million light years away in the constellation Pegasus.  This galaxy has an interesting structure described by Sil’chenko et al that consists of several rings of stars concentric to its nucleus.  The outermost ring is very metal poor and about 2 billion years old.  It contains a starforming area that is about 1 billion years old.  The inner ring is about 5 billion years old.  The core of the galaxy is very old (13-15 billion years old).

It has been suggested these features were caused by a catastrophic event, likely a merger with another galaxy and, in fact, computer simulations show that NGC 7217 could have been a large lenticular galaxy that merged with one or two smaller gas-rich ones of late Hubble type becoming the spiral galaxy we see today.  

Some of these features can be seen in the image below that was generated from some WFPC2 data that I downloaded from the Hubble Legacy Archive and processed locally.  The details in the inner areas are clearly seen while the details in the outer areas, while there, are much dimmer.

While the Hubble image above shows some of the details, other details are more visible in the WISE data.  This animation (warning, 8Mb file -- might take time to download) shows an animation of a fade from the visible data image taken at SSRO to an image that was generated from WISE IR data.  The bright dust rings in the center of this galaxy and in one of the outer rings are easy to see as is an extended area that surrounds the galaxy.  Note that this animation also shows a fade from the visible data from SSRO to the Hubble data and then to the WISE data.