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Question 1: What is known about how or whether the bars affect star formation in the ring?


Exaggerated central ultraviolet ring
and bar in NGC 6782

The inner bar is made up of older stars which go around the small central galaxy bulge in preferred orbits (e.g. a collection of highly elliptical or rather "pointy'' orbits). These orbits have resonances that gives the appearance of an elongated bar-like structure. The bar rotates essentially like a solid body, even though it is only composed of gas and stars (and dust) --- hence that at every point its velocity around the galaxy center increases linearly with the distance to the center. This bar structure ends around another resonance, called the Inner Lindblad Resonance, where surrounding gas piles up causing a density wave that forms new young and hot stars that we primarily see in the UV, explaining the very blue color of this ring-like structure.

 


Exaggerated outer star-forming ring, bar and outer spiral arms.



Ground-based image from the
Digitized Sky Survey

Question 2: What is it about the bar that would have the effect you describe above?

The gas beyond or outside the location of the bar goes around differentially, i.e. particles further away from the center than the bar go around at a speeds slower than the one the bar would have at the same distance (the circular velocity along the bar increases linearly with distance from the center). It is in a sense like placing a straight solid ruler (the bar) down on a large soft table-cloth (the gas disk), and rotating the ruler in one direction at a uniform angular speed. At both ends of the ruler you will see the table-cloth pile up and even form some kind of "spiral structure", with the outer parts of the cloth drag behind the rotating bar ("differential rotation"). The gas will compress in this region, which has the shape of a ring, and condensed shocked gas is the place where new young and hot stars will form. (The analogy is not perfect of course, since the forces on the table-cloth are not the same as in the galaxy, and the bar is not a solid body -- it just acts like one for a while at least). Note indeed that two dim spiral arms are emanating from the two points where the inner bar ends in the inner ring, although they appear mostly as dim dust streaks that spiral from the inner to the outer rings (if young hot stars were present here, they'd be largely obscured by thus dust).

Question 3: Why are the bars there in the first place? Why doesn't the material in the bars spread out into a spiral pattern like the rest of the galaxy?

The bars are thought to be composed of these older stars that go around in these preferred high ecccentric orbits that conspire to form something that has the appearance of a bar-like structure. An extreme example of this in the absence of a spiral disk is the so called triaxial elliptical galaxy, where similar orbits are found. The resulting stellar population finds equilibrium in some cigar-shaped ("triaxial") morphology, although none of the three elliptical axes necessarily have the same length. (In a normal cigar, at least two axes -- the two smaller ones -- have the same length).
Unlike in an isolated triaxial elliptical galaxy, these bars in spiral galaxies are believed by some astronomers to not last forever, but possibly dissolve in a more loosely flattish distribution of stars.


Illustration Credit: L. Frattare (STScI)

This we may be witnessing in the outer dim bar of NGC6782 (the one beyond the blue inner ring). This outer dim bar goes along the major axis of the system and reaches to the outer dim blue ring of spiral structure. We observe this region to be very flat in its red light distribution. Our guess, but really only a guess is that the outer bar is dissolving into this quite unremarkable older disk population (brownish green in the image), and leaving an outer and dimmer ring behind.

These questions were presented to astronomer Rogier Windhorst (ASU) by press affiliate Robert Roy Britt from SPACE.com.

Read Robert Roy Britt's SPACE.com article on NGC 6782.

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