It's a little baffling to me how the dust and star forming regions completely ignore
the bar and appear as a continuous spiral overlapping the bar. It is something I have never noticed before but seems to be typical for bars so this is a normal example rather than an exception.
I don't think the bar of NGC 2903 is typical. I think it is weird.
You will forgive me for once again referring to James D Wray's The Color Atlas of Galaxies
. In that atlas, Wray very often compared galaxies with one another. So, for example, Wray wrote about NGC 2841:
The broad outer disk is similar to that of NGC 2775, although long dust lanes are more in evidence here.
He wrote about NGC 2959:
Compare with NGC 2787 and NGC 936.
So Wray often asked his readers to understand a galaxy better by looking at a picture of another galaxy that is similar to it. (That, by the way, is why I often like to point out other galaxies that are similar to this or that galaxy portrait of this or that APOD - I try to take after James D Wray.)
But Wray could see no counterpart to the bar of NGC 2903. I really think that the bar of NGC 2903 is highly unusual. However, I think there just might be a counterpart, and that might be M108, NGC 3556. According to Wikipedia
, M108 is a barred galaxy, and photographs show it
to have a disturbed disk with out-of-plane dust features.
Tammy Plotner wrote
Located about 45 million light years away from Earth and running away from us at 772 kilometers per second, this disturbed looking galaxy is rich in dark dust, star forming regions and a supershell.
“Since this galaxy is isolated, the supershells are unlikely to have been created through impacting external clouds, yet the required input energy is also greater than that available from the observed internal star formation rate. Thus it would appear that some form of energy enhancement (such as magnetic fields) must also be important in creating these features. The supershells are so dominant that they distort the outer major axis.
Perhaps, if we had a more face-on view of M108, its bar might look a bit broken by the out-of-plane supershell.
Personally I don't think that NGC 2903 looks like it might be the product of a recent merger. Its outer features are far too "settled"
and regular for that. Also M108 may or may not - probably not - be the product of a merger. But galaxies may have upheavals for other, more mysterious reasons. It's anybody's guess why isolated NGC 1313
is having the star formation chaos tantrum that it, in fact, does have.
So I don't think the weird bar of NGC 2903 is the product of a merger, although something strange has clearly occurred inside this galaxy.