The progenitors of the Milky Way stellar halo: big bricks favoured over little bricks

Abstract

We present a census of blue horizontal branch (BHB) and blue straggler (BS) stars belonging to dwarf galaxies and globular clusters, and compare these counts to that of the Milky Way stellar halo. We find, in agreement with earlier studies, that the ratio of BS-to-BHB stars in these satellite populations is dependent on stellar mass. Dwarf galaxies show an increasing BS-to-BHB ratio with luminosity. In contrast, globular clusters display the reverse trend, with N BS /N BHB ( 1) decreasing with luminosity. The faintest (L < 10 5 L ⊙ ) dwarfs have similar numbers of BS and BHB stars (N BS /N BHB ∼ 1), whereas more massive dwarfs tend to be dominated by BS stars (N BS /N BHB ∼ 2−40). We find that the BS-to-BHB ratio in the stellar halo is relatively high (N BS /N BHB ∼ 5 − 6), and thus inconsistent with the low ratios found in both ultra-faint dwarfs and globular clusters. Our results favour more massive dwarfs as the dominant "building blocks" of the stellar halo, in good agreement with current predictions from ΛCDM models.

On the use of the number count of blue horizontal branch stars to infer the dominant building blocks of the Milky Way halo

“…Both our models and observations suggest that small number of BHB stars are the main driver of the high NBS/NBHB ratios. Interestingly, the NBS/NBHB ratio of NGC 362 and Terzan 7 is quite comparable with dwarf spheroidals ranging from 2 to 40 and the outer halo field ranging from 5 to 6 (Deason et al 2015). Furthermore, no presence of BHB stars in Palomar 12, Ruprecht 106, Palomar 13, and Pyxis would increase the NBS/NBHB ratio of each GC to the infinity.…”

“…The selection boxes for BHB (blue box) and BS stars (red dashed box) are from Deason et al (2015). The adopted N BS /N HB ratio is 0.3 and the total number of HB stars is 200.…”

“…Furthermore, no presence of BHB stars in Palomar 12, Ruprecht 106, Palomar 13, and Pyxis would increase the NBS/NBHB ratio of each GC to the infinity. These GCs without BHB stars were not included in the analysis of Deason et al (2015) because of their selection criteria (NBS > 1 and NBHB > 1). Therefore, their analysis was further biased because of these selection criteria, which had led them to select against these GCs with only FG.…”

“…Recently, Deason et al (2015) suggested that the Milky Way outer halo formed mostly from big building blocks, such as dwarf spheroidal galaxies, based on the similar number ratio of blue straggler (BS) stars to blue horizontal-branch (BHB) stars. Here we demonstrate, however, that this result is seriously biased by not taking into detailed consideration on the formation mechanism of BHB stars from helium enhanced second-generation population.…”

The central spheroids of Milky Way mass-sized galaxies

“…Contemporary studies show that the MW halo also comprises diverse stellar populations (Carollo et al 2007(Carollo et al , 2010, with a significant number of structures and overdensities related to satellite accretion (Helmi & White 1999;Gilbert et al 2013;Ibata et al 2013;Deason et al 2015;Re Fiorentin et al 2015;Grillmair & Carlin 2016;Merritt et al 2016). Carollo et al (2007Carollo et al ( , 2010 and Beers et al (2012) report that stars in the diffuse inner and outer haloes exhibit different spatial distributions, kinematics, dynamics, and chemical composition, suggesting different formation mechanisms.…”