TOUCHING THE VOID: A STRIKING DROP IN STELLAR HALO DENSITY BEYOND 50 kpc

• Published in: The Astrophysical journal Vol. 787; no. 1; pp. 1 - 16
• Main Authors: Deason, A J, Belokurov, V, Koposov, S E, Rockosi, C M
• Format: Journal Article
• Language: English
• Published: United States 20.05.2014
• Subjects: Accretion; Alpha rays; Astronomical models; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; COMPUTERIZED SIMULATION; DENSITY; DISTRIBUTION; Halos; MASS; MILKY WAY; PHOTOMETRY; Slopes; STARS
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/787/1/30

We use A-type stars selected from Sloan Digital Sky Survey data release 9 photometry to measure the outer slope of the Milky Way stellar halo density profile beyond 50 kpc. A likelihood-based analysis is employed that models the ugr photometry distribution of blue horizontal branch and blue straggler stars. In the magnitude range 18.5 < g < 20.5, these stellar populations span a heliocentric distance range of: 10 [lap] less than D sub(BS)/kpc [lap] less than 75, 40 [lap] less than D sub(BHB)/kpc [lap] less than 100. Contributions from contaminants, such as QSOs, and the effect of photometric uncertainties, are also included in our modeling procedure.We find evidence for a very steep outer halo profile, with power-law index [alpha] ~ 6 beyond Galactocentric radii r = 50 kpc, and even steeper slopes favored ([alpha] ~ 6-10) at larger radii. This result holds true when stars belonging to known overdensities, such as the Sagittarius stream, are included or excluded. We show that, by comparison to numerical simulations, stellar halos with shallower slopes at large distances tend to have more recent accretion activity. Thus, it is likely that the Milky Way has undergone a relatively quiet accretion history over the past several gigayears. Our measurement of the outer stellar halo profile may have important implications for dynamical mass models of the Milky Way, where the tracer density profile is strongly degenerate with total mass estimates.