In your post, AVAO, we can see the two dust lanes feeding gas into the nuclear ring. One of the dust lanes "hits the ring" at about 7 o'clock(ish), and the other one is hitting the ring at about 12 o'clock. One dust lane is "coming in from the lower left", and the other is "coming in from the upper right".
In this post, you can very clearly see the two dust lanes feeding gas and dust into the nuclear ring of NGC 1512.
What happens in the nuclear ring is that gas is being fed and compressed from two directions, fuelling star birth. Why doesn't the gas fall straight into the black hole that is certainly there at the center of any large galaxy? All I can say is that in any stable feature, such as a nuclear ring, the material that makes it up must be in orbit around the massive central object. That is what is happening in the Solar system, after all: the Earth isn't falling into the Sun, and neither is any of the other planets, because we are all orbiting the Sun. In the case of the nuclear rings of galaxies, new gas being fed into the ring is captured by the ring and prevented from falling (straight) into the black hole. I'm not saying that some of the gas might not wind up there eventually!
NGC 4314. Image Credit: G. Fritz Benedict, Andrew Howell,
Inger Jorgensen, David Chapell, Jeffery Kenney , and Beverly J. Smith , and NASA
Finally, take a look at two barred spiral galaxies that have run out of gas. One of them, NGC 4314, still has a little bit of gas near its center, and it still has two short dust lanes leading gas into its nuclear ring, fuelling fresh star birth there and creating one single splash of color in the entire galaxy. But NGC 936 has run out of gas altogether, leaving the galaxy "red and dead".
You can still recognize the major features of these two galaxies. There is a bright round central object where a huge number of stars are in orbit around a black hole. You can clearly see the bar of both galaxies, and you can make out the outer ring outside the bar. In NGC 4314, you can see two long elegant but "dead" (non-starforming) spiral arms. In NGC 936, every trace of spiral arms is gone, and there is only a featureless disk.
Why are the main features of the two galaxies (apart form the nuclear ring of NGC 4314) not the same color, if they are both made up of old stars? It is because they have been photographed through different filters and the images have been processed and mapped differently. Both galaxies are completely dominated by old stars, but there is still a little gasp of gas and blue fire left in NGC 4314.
Ann
M95: Spiral Galaxy with an Inner Ring
