The Sagittarius Spiral Arm of the Galaxy: Now You See It, Now You Don't

Details The Sagittarius Spiral Arm of the Galaxy: Now You See It, Now You Don't The Sagittarius Spiral Arm of the Galaxy: Now You See It, Now You Don't

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20924205b&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #242.05; Bulletin of the American Astronomical Society,

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We present an analysis of star count data using the GLIMPSE Point Source Catalog of Spitzer/IRAC sources focusing on the directions of the Sagitarrius arm tangency (galactic longitude l=49-53) and the (Scutum)-Crux arm tangency (l=306-309). Nearly all models of Galactic spiral structure place the location of these tangencies at the same distance from the Sun (d 6 kpc). We clearly detect the Crux spiral arm tangency over galactic longitude range l=306-309 as a 10-30% excess in star counts over the magnitude range m=6.5-12.5. We demonstrate that (1) the asymmetric shape of the excess as a function of longitude is consistent with the expectation of a stellar spiral arm seen in tangency, (2) the direction is consistent with the claims of a spiral arm tangency seen in gas tracers, and (3) the physical width of the Crux arm (w=300+/40 pc) is consistent with other spiral galaxies. We use data from WHAM (Wisconsin H-alpha Mapper) to demonstrate that the Sagittarius arm tangency at l=50 is clearly seen in H-alpha emission, but find no evidence of an excess in infrared star counts. This confirms the analysis of COBE K-band light by Drimmel (2000) and Drimmel & Spergel (2001), and provides further evidence that the mass of the Galaxy (traced by infrared light) is organized in a two-armed, not four-armed, spiral structure. We also discuss the relationship of the Scutum-Crux and Perseus arms to the stellar bar of the Galaxy.

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