Observations of stars at relatively low latitude are key to revealing the structure, kinematics and composition of the Galactic disks. However, the interstellar medium – clouds of gas and dust that permeate the disks – modifies the starlight that reaches telescopes on Earth. The ISM can block or change the observed color of light from stars, and the amount of extinction and reddening depends on how much dust lies between a particular star and the observer.

Modern dust maps are two-dimensional on the sky; a better understanding of the three-dimensional distribution of dust in the Galaxy is essential in order to accurately correct for the effects of the ISM. SEGUE will be instrumental in helping to create such a three-dimensional map of the ISM.

SEGUE will combine its optical catalogs with the 2MASS point source catalog in order to better determine the amount of reddening for the brighter stars observed by SEGUE and help resolve ambiguities in stellar identifications.

Determination of Extinction and Reddening

SDSS utilizes the SFD dust map, derived from maps of the infrared emission from dust across the sky, to predict the extinction in each of the five SDSS ugriz colors. The resulting extinction maps are used to correct the observed magnitudes of quasars and galaxies before spectroscopic target selection.

SEGUE scans provide an important check of the SFD map, which is crucial for

• correcting observations of extragalactic objects for extinction
• measuring an important CMB "foreground"
• understanding and improving the assumed model of the dust mass, composition and distribution.

Spatial Mapping of the Local ISM

SEGUE will use M dwarf stars to probe the three-dimensional structure of the local dust at distances ranging from 100 to 1000 pc (where the bulk of the extinction is expected to occur). M dwarfs are numerous, have a well-defined color-absolute magnitude relation, and their g - r colors lie in a narrow locus about g - r = 1.4. The spatial variation of M dwarf g -r colors observed by SEGUE will be transformed in a map of cloud locations using techniques such as maximum entropy reconstruction.

Red Emission from Dust

A diffuse red glow can be seen around the edges of dust clouds imaged by SDSS (see e.g. figure 13 in the SEGUE 2004 White Paper). This glow may be related to diffuse extended red emission (ERE), a broad emission band centered near 7000 Å and attributed to photoluminescence from carbon nonparticles. SEGUE imaging over large areas containing a wide range of dust densities and starlight illumination may help to establish the origin of this diffuse glow and offer insights into the size range and composition variations of dust paricles in the ISM.

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For more details, please see the SEGUE 2004 White Paper.