SDSS imaging data are produced in five photometric bands, namely u, g, r, i, and z (Fukugita et al. 1996, Gunn et al. 1998, Gunn et al. 2006, Hogg et al. 2001). Thanks to the efforts of many people, the data are automatically processed through pipelines to measure photometric and astrometric properties (Lupton, Gunn, & Szalay 1999, Stoughton et al. 2002, Smith et al. 2002, Pier et al. 2003, Ivezic et al. 2004).
To carry out a systematic survey, the stars with 16 r 22 are first binned into 10' × 10' regions in right ascension and declination. Then, a running window of size 1° × 1° is used to compute the background. All bins that are more than 3 away from the background are selected. Known satellite galaxies and globular clusters are removed using the list of van den Bergh (2000a). Visual inspection is used to discard a few obvious contaminants, such as resolved stellar associations in background galaxies. All the candidates are ranked according to the signal-to-noise. The two strongest candidates that remain are the Canes Venatici dSph (Zucker et al. 2006) and the object studied in this Letter, which is named Boo after the constellation of Boötes in which it lies.
The upper left panel of Figure 1 shows a grayscale SDSS image of the sky centered on Boo. There is no obvious object. However, on plotting the density of all objects classified by the SDSS pipeline as stars, a curiously-shaped overdensity is readily visible (upper middle and right panels). Plotting these stars in a color-magnitude diagram (CMD) reveals a clear red giant branch and horizontal branch (lower panels). This evidence of a localized overdensity of stars with a distinct color-magnitude diagram suggests that this is a new satellite - possibly a dwarf galaxy.
Figure 1. The Boötes Satellite: Upper left: Combined SDSS g,r,i images of a 1° × 1° field centered on the overdensity. and are the relative offsets in right ascension and declination, measured in arcdegrees. The dotted circle indicates a radius of 0.15°. Upper middle: The spatial distribution of all objects classified as stars in the same area. Upper right: Binned spatial density of all stellar objects. The inner dotted circle marks a radius of 0.15° and encloses the same area as the two outer circles, which have a radius of 0.4° and 0.43° respectively. Bins are 0.033° × 0.033°, smoothed with a Gaussian with a FWHM of 0.1°. Lower left: CMD of all stellar objects within the inner 0.15° radius circle. There is a clear red giant and horizontal branch, even without removal of field contamination. Lower middle: Control CMD, showing all stellar objects in the annulus between 0.4° and 0.45° of the center. Lower right: A color-magnitude density plot (Hess diagram), showing the inner CMD minus the control CMD, normalized to the number of stars in each CMD. A mask is shown around the satellite's sequence.