The Accretion Origin of the Milky Way’s Stellar Halo

By Eric F. Bell, D. Zucker, V. Belokurov, Sanjib Sharma, K. Johnston, James S. Bullock, David W. Hogg, K. Jahnke, J. D. de Jong, Timothy C. Beers and 8 more. 2007

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We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to explore the overall structure and substructure of the stellar halo of the Milky Way using ~4 million color-selected main-sequence turnoff stars with 0.2 < g − r < 0.4 and 18.5 ⩽ r < 22.5. We fit oblate and triaxial broken power law models to the data, and found a “best-fit” oblateness of the stellar halo 0.5 < c/a < 0.8, and halo stellar masses between galactocentric radii of 1 and 40 kpc of 3.7 ± 1.2 × 108 M☉. The density profile of the stellar halo is approximately ρ ∝ r−α, where –2 > α > − 4. Yet, we found that all smooth and symmetric models were very poor fits to the distribution of stellar halo stars because the data exhibit a great deal of spatial substructure. We quantified deviations from a smooth oblate/triaxial model using the rms of the data around the model profile on scales ≳100 pc, after accounting for the (known) contribution of Poisson uncertainties. Within the DR5 area of the SDSS, the fractional rms deviation σ/total of the actual stellar distribution from any smooth, parameterized halo model is ≳40%: hence, the stellar halo is highly structured. We compared the observations with simulations of galactic stellar halos formed entirely from the accretion of satellites in a cosmological context by analyzing the simulations in the same way as the SDSS data. While the masses, overall profiles, and degree of substructure in the simulated stellar halos show considerable scatter, the properties and degree of substructure in the Milky Way’s halo match well the properties of a “typical” stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point toward a picture in which an important fraction of the stellar halo of the Milky Way has been accreted from satellite galaxies.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

ON THE SHOULDERS OF GIANTS: PROPERTIES OF THE STELLAR HALO AND THE MILKY WAY MASS DISTRIBUTION (2014, background): “A smaller break radius also complies with the study of SDSS main sequence turn-off stars in Bell et al. (2008) who conclude that the slope of the density profile atr .”
THE GHOSTS SURVEY. I. HUBBLE SPACE TELESCOPE ADVANCED CAMERA FOR SURVEYS DATA (2011, background): “For instance, whether the halo is built entirely from disrupted satellites with no smooth underlying component is still unclear (Bell et al. 2008).”
Dynamics for galactic archaeology (2013, background): “By counting faint stars in the outer reaches of both our Galaxy and the Andromeda nebula, M31, it has been shown that the outer parts of galaxies are a mass of stellar streams and full of faint satellite galaxies (McConnachie et al., 2009; Belokurov et al., 2006; Bell et al., 2008).”
Tracing Galaxy Formation with Stellar Halos. II. Relating Substructure in Phase and Abundance Space to Accretion Histories (2008, background): “…% of its stars is substructure with of a dozen streams brighter than 35th mag arcsec−2 (see §4.1 and §4.2) as seems to be the case for both Milky Way (Bell et al. 2008) and Andromeda (Ibata et al. 2007); (ii) the brightest of these streams more metal rich than the smooth component of the halo…”
The Milky Way’s stellar disk (2013, methodology): “While mainsequence turn-off stars have been widely used to make 3D maps of the Milky Way (mostly its halo; e.g., Belokurov et al. 2006; Jurić et al. 2008; Bell et al. 2008), they are among the stellar types abundantly found in typical surveys the ones for which precise (< 20”
THE SPATIAL STRUCTURE OF MONO-ABUNDANCE SUB-POPULATIONS OF THE MILKY WAY DISK (2011, methodology): “We can estimate the expected number of halo stars in our sample using the Bell et al. (2008) density fits to the smooth stellar halo.”
THE LUMINOSITY PROFILE AND STRUCTURAL PARAMETERS OF THE ANDROMEDA GALAXY (2011, background): “If M31’s halo is anything like the Milky Way’s, our assumption of a spherical halo is well justified (Majewski et al. 2003; De Propris et al. 2010; however see Bell et al. 2008).”
Substructure revealed by RR Lyraes in SDSS Stripe 82 (2009, background): “Together with earlier SDSS discoveries (Belokurov et al. 2007; Jurić et al. 2008), the patchyand clumpy nature of the RR Lyrae distribution adds support to the picture of an outer halo composed of overdensities and voids, perha s entirely devoid of any smooth component (e.g. Bell et al. 2008).”
Young Massive Star Clusters (2010, background): “…globular cluster data is available online http://www.astro.caltech.edu/∼george/glob/data.html (Djorgovski & Meylan 1993). stellar halo (Hut & Djorgovski 1992, Bell et al. 2008, see also §4), although we note that the spatial distributions of halo stars and GCs differ (Forte, Vega & Faifer 2009).”
THE LARGE-SCALE STRUCTURE OF THE HALO OF THE ANDROMEDA GALAXY. I. GLOBAL STELLAR DENSITY, MORPHOLOGY AND METALLICITY PROPERTIES (2013, background): “It is very interesting that the incidence of the halo substructures appears to be similar to that found in cosmologically-motivated stellar halo formation simulations (Bell et al. 2008).”
Galactic Searches for Dark Matter (2012, methodology): “One model is described by a pure power law in density, ρs ∝ r−α, which is appropriate for a stellar halo population (Bell et al., 2008) or probably for the dSph population (Evans and Wilkinson, 2000).”
Galactic stellar haloes in the CDM model (2009, methodology): “Current observational estimates (e.g. Bell et al. 2008) imply that the stellar halo of the Milky Way is intermediate in mass between our haloes Aq-C and Aq-D; if its accretion history is, in fact, qualitatively similar to these many-progenitor haloes, Fig.”
The bifurcated age–metallicity relation of Milky Way globular clusters and its implications for the accretion history of the galaxy (2013, background): “…comparing the ob- served number of halo clusters from the split AMR and the total stellar mass of the MW stellar halo (∼ 1±0.4×109M ; Morrison 1993; Bell et al. 2008; Deason, Belokurov & Evans 2011) to simulations of the merging history of the MW to check whether the GC population and stellar…”
THE DUAL ORIGIN OF STELLAR HALOS (2009, methodology): “Bell et al. (2008) have found, using F and G turnoff stars in the Sloan Digital Sky Survey (SDSS), that the distribution of halo stars is highly structured, and the outer halo is consistent with being formed almost entirely from accreted subhalos.”