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Grand design spiral galaxies are named for the majestic twin-armed spiral patterns that swirl outward from the galactic centers. In optical images, these bright arms are often seen to be dotted at regular intervals with luminous star-forming regions. In radio-frequency images, arms are seen to be abundant in cold, dense, molecular gas -- the raw material from which these new stars are created. |
 M51 (NGC 5194), the
Whirpool galaxy (courtesty of S. Vogel)
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Spiral arms and spurs
in M51: red patches are star-forming regions; spurs are dark "dust
lanes" |
When observed at very high resolution, the morphology of spiral galaxies becomes more complex. From each primary spiral arm, a series of smaller structures emerges, sweeping outward and backward into the interarm region. These substructures are seen as dark dust lanes overlying the bright background in optical images, as long filaments in infrared images that trace emission from the dust, and as extensions from the densest large-scale molecular gas concentrations in millimeter-wavelength radio images. These substructures are referred to as spiral-arm "spurs" or "feathers". In some cases, star-forming clusters are also dotted along the spurs. |
Theoretical models have identified the dynamical processes
that lead
to the formation of these spurs, and make it possible to see spur
formation in action. These studies have also revealed more details of
the series of steps that transforms diffuse gas into dense gas, and
subsequently into stars. It is the collapse and fragmentation of dense
gas clouds that ultimately creates the luminous clusters of young
stars that we see.
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The first step in the
process is the flow of diffuse gas from
interarm regions
into spiral arms. Because gas is flowing supersonically
relative to the arm pattern (which itself spins about the galactic
center), it shocks when it encounters the arm. This shock drives up
the density of the gas, which enhances the abundance of molecules
(formed when atoms that have stuck to grain surfaces encounter each
other). At the high densities present within arms, gravity is
stronger, and this gravity begins to cause the gas to concentrate
locally -- leaving rarefied regions in between the growing
condensations.
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 Schematic of gas flowing across a spiral arm. (Kim & Ostriker 2002) |
If the shock is strong, gas condensations grow very
rapidly. Some molecular gas condensations, containing up to ten million
times the
mass of the Sun, collapse all the way to make gravitationally bound
clouds in the arm. Embedded star clusters form within these giant
molecular clouds.
| Other overdensities do not grow as rapidly, but the gas is still concentrated when it emerges downstream from (outside of) the spiral arm. Since shear is stronger in interarm regions than arm regions, the concentrations get stretched into spurs with a ``backswept'' morphology. At the enhanced densities within spurs, condensations can continue to grow, and this sometimes leads to formation of star clusters in the interarm regions. |
Snapshots of a spiral arm
segment at successive stages of spur formation and fragmentation
(Kim & Ostriker 2002)
 |
| Only a small fraction (typically 5%) of the gas in giant molecular clouds is transformed into stars; the remaining is dispersed by the energy input from young stars. With the conversion of dense gas to the diffuse phase in interarm regions, the cycle returns to its starting point -- poised for the next encounter with a spiral arm. | Simulation including
feedback from star formation, which disperses gas that has become
concentrated by gravity (Shetty
& Ostriker 2008)  |
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This material is based upon work supported by the National Science Foundation (NSF). Any opinions, findings, conclusions, or recommendations expressed here are those of the author(s) and do not necessarily reflect the views of the NSF.