Reconciling observed and simulated stellar halo masses
We use cosmological hydrodynamical simulations of Milky Way–mass galaxies from the FIRE project to evaluate various strategies for estimating the mass of a galaxy's stellar halo from deep, integrated-light images. We find good agreement with integrated-light observations if we mimic observational methods to measure the mass of the stellar halo by selecting regions of an image via projected radius relative to the disk scale length or by their surface density in stellar mass. However, these observational methods systematically underestimate the accreted stellar component, defined in our (and most) simulations as the mass of stars formed outside of the host galaxy, by up to a factor of 10, since the accreted component is centrally concentrated and therefore substantially obscured by the galactic disk. Furthermore, these observational methods introduce spurious dependencies of the estimated accreted stellar component on the stellar mass and size of galaxies that can obscure the trends in accreted stellar mass predicted by cosmological simulations, since we find that in our simulations, the size and shape of the central galaxy are not strongly correlated with the assembly history of the accreted stellar halo. This effect persists whether galaxies are viewed edge-on or face-on. We show that metallicity or color information may provide a way to more cleanly delineate in observations the regions dominated by accreted stars. Absent additional data, we caution that estimates of the mass of the accreted stellar component from single-band images alone should be taken as lower limits.
© 2018 The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2017 December 13; revised 2018 September 26; accepted 2018 October 22; published 2018 December 6. The authors thank Laura Sales, Lydia Elias, and Kyle Stewart for helpful suggestions during the GalFRESCA 2017 meeting, organized by Shea Garrison-Kimmel and Coral Wheeler, and Alison Merritt for helpful background on her measurement techniques. R.E.S. was supported by an NSF Astronomy & Astrophysics Postdoctoral Fellowship under grant AST-1400989. Support for S.G.K. was provided by NASA through Einstein Postdoctoral Fellowship grant number PF5-160136 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. A.W. was supported by a Caltech-Carnegie Fellowship, in part through the Moore Center for Theoretical Cosmology and Physics at Caltech, and by NASA through grants HST-GO-14734 and HST-AR-15057 from STScI. T.K.C. was supported by NSF grant AST-1412153. Support for P.F.H. was provided by an Alfred P. Sloan Research Fellowship, NSF Collaborative Research Grant #1715847, and CAREER grant #1455342. D.K. acknowledges support from NSF grants AST-1412153 and AST-1715101 and the Cottrell Scholar Award from the Research Corporation for Science Advancement. E.E. was supported by a Ford Foundation Predoctoral Fellowship. C.-A.F.-G. was supported by the NSF through grants AST-1412836, AST-1517491 and AST-1715216 and CAREER award AST-1652522 and by NASA through grant NNX15AB22G. Numerical calculations were run on the Caltech computer cluster "Wheeler," allocations from XSEDE TG-AST130039 and PRAC NSF.1713353 supported by the NSF, NASA HEC SMD-16-7592, and the High Performance Computing at Los Alamos National Labs.
Submitted - 1712.05808.pdf
Published - Sanderson_2018_ApJ_869_12.pdf