THE RADIAL PROFILE AND FLATTENING OF THE MILKY WAY’S STELLAR HALO TO 80 kpc FROM THE SEGUE K-GIANT SURVEY

Abstract

We characterize the radial density, metallicity and flattening profile of the Milky Way's stellar halo, based on the large sample of spectroscopically confirmed giant stars from SDSS/SEGUE-2 (Xue et al. 2014), spanning galactocentric radii 10 kpc ≤ r GC ≤ 80 kpc. After excising stars that were algorithmically attributed to apparent halo substructure (including the Sagittarius stream) the sample has 1757 K giants, with a typical metallicity precision of 0.2 dex and a mean distance accuracy of 16%. Compared to blue horizontal branch stars or RR Lyrae variables, giants are more readily understood tracers of the overall halo star population, with less bias in age or metallicity. The well-characterized selection function of the sample enables forward modeling of those data, based on ellipsoidal stellar density models, ν * (R, z), with Einasto profiles and (broken) power laws for their radial dependence, combined with a model for the metallicity gradient and the flattening profile. Among models with constant flattening, these data are reasonably well fit by an Einasto profile of n = 3.1±0.5 with an effective radius r eff = 15 ± 2 kpc and a flattening of q = 0.7 ± 0.02; or comparably well by an equally flattened broken power law, with radial slopes of α in = 2.1 ± 0.3 and α out = 3.8 ± 0.1, with a break radius of r break = 18 ± 1 kpc; this is largely consistent with earlier work. We find a modest but significant metallicity gradient within the 'outer' stellar halo, [Fe/H] decreasing outward. If we allow for a variable flattening q = f (r GC ), we find the distribution of halo giants to be considerably more flattened at small radii, q(10 kpc) = 0.55 ± 0.02, compared to q(> 30kpc) = 0.8 ± 0.03. Remarkably, the data are then very well fit by a single power law with index of 4.2 ± 0.1 on the variable r q ≡ R 2 + (z/q(r)) 2 .

The Profile of the Galactic Halo from Pan-STARRS1 3π RR Lyrae

“…Similar approaches were carried out by, e.g., Sesar et al (2013b), Xue et al (2015), Cohen et al (2015), and Iorio et al (2017), but all with either a smaller sample size than in our analysis or probing a smaller distance range.…”

“…Others presume a broken power law (BPL; e.g., Xue et al 2015), where inner and outer power-law indices are defined. The change in the power-law index then occurs by a step function at the break radius.…”

“…The change in the power-law index then occurs by a step function at the break radius. As our sample starts at a galactocentric radius of 20 kpc, and the break radius is found to be around or below 20 kpc (e.g., Xue et al 2015), we cannot compare to the results by Xue et al (2015). However, in order to compare to the findings by Deason et al (2014), who find a BPL with three ranges of subsequently steepening slope, where one of the breaks is occurring within the distance range present in our sample, we fit a BPL: The Einasto profile (Einasto 1965;Einasto & Haud 1989) is the 3D analog to the Sérsic profile (Sérsic 1963) for surface brightnesses and has been used to describe the halo density distribution Deason et al 2011;Sesar et al 2011;Xue et al 2015;Iorio et al 2017) and dark matter halos Navarro et al 2010).…”

“…Thus, the mean luminosity of an RRab variable, and hence its distance, can be determined with knowledge of the light curve only. RRab stars were used by many previous studies, including those of Hawkins (1984), Saha (1984), Wetterer et al (1996), Ivezić et al (2000), Vivas & Zinn (2006), Jurić et al (2008), Catelan (2009), Watkins et al (2009), de Jong et al (2010, Sesar et al (2010Sesar et al ( , 2011, Deason et al (2011), Akhter et al (2012), Drake et al (2014), Torrealba et al (2015), Cohen et al (2015), Xue et al (2015), Soszyński et al (2016), Bland-Hawthorn & Gerhard (2016), Iorio et al (2017), and Cohen et al (2017).…”

“…One approach was to use brighter tracer stars, usually K giants and usually selected from the Sloan Digital Sky Survey (SDSS), but with larger distance uncertainties and only modest sample sizes (see, e.g., Xue et al 2015, who probe the Galactic halo out to 80 kpc using 1757 stars from the SEGUE K-giant Survey). There have also been efforts to reach the outer halo using blue horizontal branch (BHB) stars, which to first order have a fixed luminosity similar to that of RRab (see, e.g., Deason et al 2014), but these run into problems of confusion with much more numerous blue stragglers at the same apparent magnitude and with quasars.…”