Nuclear Rings (star-forming) are not lenses
by Budgieye moderator
Nuclear rings are not lenses, they rings of hot star-forming gas.
Here is a nuclear ring, which mimics a nucleus-hugging lens.
NGC 1097 (Hubble)
But it is not a lens.
It obviously exists for the sole purpose of making the lens hunter's life harder.
The sequence of events for forming a nuclear ring seems to be something like this:
INFALL OF GAS
In galaxies with bars or that have an oval shape, gas is disturbed by the bar. The gas loses energy, and spirals in towards the nucleus.
FORMATION OF GAS RING
The gas is molecular hydrogen H2.
The hydrogen gas stops at a resonance point, becomes hot by being shocked, and starts to emit light.
At this point, the ring of gas will be smooth and hot, and most likely to be mistaken for a lens.
If the hydrogen gas funnels down the two spiral arms, then the ring will have two bright spots, which may be mistaken for an arc and counter-arc.
It would take a spectral chart to distinguish between a nuclear ring and a lens.
The hydrogen molecules combine to form large stars, and then star clusters. The ring starts looking clumpy, with "hot spots"
Large stars have a short life, then explode and contribute to the brightness of the ring.
At this point, the ring will become ragged and look like a circular spiral arm of a galaxy.
wikipedia/common NGC4314 by HST1998
http://www.galaxyzooforum.org/index.php?topic=278908.0 Star formation rings
by Budgieye moderator
http://apod.nasa.gov/apod/ap030726.html Spiral Galaxy NGC 7742
posted by elisabeth
quote from http://en.wikipedia.org/wiki/NGC_7742
The galaxy is unusual in that it contains a ring but no bar. Typically, bars are needed to produce a ring structure. The bars' gravitational forces move gas to the ends of the bars, where it forms into the rings seen in many barred spiral galaxies. In this galaxy, however, no bar is present, so this mechanism cannot be used to explain the formation of the ring. O. K. Sil'chenko and A. V. Moiseev proposed that the ring was formed partly as the result of a merger event in which a smaller gas-rich dwarf galaxy collided with NGC 7742. As evidence for this, they point to the unusually bright central region, the presence of highly-inclined central gas disk, and the presence of gas that is counterrotating (or rotating in the opposite direction) with respect to the stars.
Informative message for the hesitating and unsure classifyers.. Thx you Budgieye.