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
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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.
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Exaggerated outer star-forming ring, bar
and outer spiral arms.
Ground-based image from the
Digitized
Sky Survey
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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).
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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.
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| 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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