Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 782, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/2041-8205/782/2/L24
Keywords
cosmology: observations; galaxies: evolution; galaxies: halos; Galaxy: halo; Galaxy: structure
Categories
Funding
- NSF [AST-1312376]
- NSERC
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1312376] Funding Source: National Science Foundation
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We use a new telescope concept, the Dragonfly Telephoto Array, to study the low surface brightness outskirts of the spiral galaxy M101. The radial surface brightness profile is measured down to mu(g) similar to 32 mag arcsec(-2), a depth that approaches the sensitivity of star count studies in the Local Group. We convert surface brightness to surface mass density using the radial g-r color profile. The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R = 70 kpc, corresponding to 18 disk scale lengths. Fitting a bulge + disk + halo model we find that the best-fitting halo mass M-halo = 1.7(-1.7)(+3.4) x 10(8) M-circle dot. The total stellar mass of M101 is M-tot,M-* = 5.3(-1.3)(+1.7) x 10(10) M-circle dot, and we infer that the halo mass fraction f(halo) = M-halo/M-tot,M-* = 0.003(-0.003)(+0.006). This mass fraction is lower than that of the Milky Way (f(halo) similar to 0.02) and M31 (f(halo) similar to 0.04). All three galaxies fall below the f(halo)-M-tot,M-* relation predicted by recent cosmological simulations that trace the light of disrupted satellites, with M101's halo mass a factor of similar to 10 below the median expectation. However, the predicted scatter in this relation is large, and more galaxies are needed to better quantify this possible tension with galaxy formation models. Dragonfly is well suited for this project: as integrated-light surface brightness is independent of distance, large numbers of galaxies can be studied in a uniform way.
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