Identification of A‐colored Stars and Structure in the Halo of the Milky Way from Sloan Digital Sky Survey Commissioning Data

A sample of 4208 objects with magnitude 15 < g* < 22 and colors of mainsequence A stars has been selected from 370 square degrees of Sloan Digital SkySurvey (SDSS) commissioning observations. The data is from two long, narrowstripes, each with an opening angle of greater than 60 deg, at Galacticlatitudes 36 < abs(b) < 63 on the celestial equator. An examination of thesample's distribution shows that these stars trace considerable substructure inthe halo. Large overdensities of A-colored stars in the North at (l,b,R) =(350, 50, 46 kpc) and in the South at (157, -58, 33 kpc) and extending overtens of degrees are present in the halo of the Milky Way. Using photometry toseparate the stars by surface gravity, both structures are shown to contain asequence of low surface gravity stars consistent with identification as a bluehorizontal branch (BHB). Both structures also contain a population of highsurface gravity stars two magnitudes fainter than the BHB stars, consistentwith their identification as blue stragglers (BSs). From the numbers ofdetected BHB stars, lower limits to the implied mass of the structures are6x10^6 M_sun and 2x10^6 M_sun. The fact that two such large clumps have beendetected in a survey of only 1% of the sky indicates that such structures arenot uncommon in the halo. Simple spheroidal parameters are fit to a completesample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit toa halo distribution with flattening (c/a = 0.65+/-0.2) and a density falloffexponent of alpha = -3.2+/-0.3.Comment: AASTeX v5_0, 26 pages, 1 table, 20 figures, ApJ accepte