Co-formation of the disc and the stellar halo★

Abstract

Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ∼10 kpc from the Sun) stellar halo. We demonstrate that the halo's velocity ellipsoid evolves strongly with metallicity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared to the tangential one) is mildly radial with 0.2 < β < 0.4. However, for stars with [Fe/H]> −1.7 we measure extreme values of β ∼ 0.9. Across the metallicity range considered, i.e. −3 <[Fe/H]−1, the stellar halo's spin is minimal, at the level of 20 <v θ (kms −1 ) < 30. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo were deposited in a major accretion event by a satellite with M vir > 10 10 M ⊙ around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor's orbit, amplified by the action of the growing disc.

Where Is the Flux Going? The Long-term Photometric Variability of Boyajian’s Star

“…Two additional stars in this sample have photometric amplitudes above but very close to the threshold. Interestingly, Boyajian's Star itself is not clearly variable in the Gaia data, with an amplitude slightly below the Belokurov et al (2017) threshold. Gaia DR1 includes observations from 2014 July to 2015 September (Gaia Collaboration et al 2016a), and it is possible that the star was relatively quiescent over this period.…”

“…A powerful new resource for detecting photometric variability across the entire sky is Gaia (e.g., Eyer et al 2017;Belokurov et al 2017;Deason et al 2017). While variability information is not included in the first Gaia data release (Gaia Collaboration et al 2016a), Belokurov et al (2017) showed that it is possible to identify variable stars by comparing the reported flux uncertainties in the Gaia DR1 catalog to the expected photon noise.…”

“…While variability information is not included in the first Gaia data release (Gaia Collaboration et al 2016a), Belokurov et al (2017) showed that it is possible to identify variable stars by comparing the reported flux uncertainties in the Gaia DR1 catalog to the expected photon noise. We calculate the Amplitude metric defined by Belokurov et al (2017) for the fourteen candidate variables in the same ASAS-SN field as Boyajian's Star and find that eleven easily satisfy the conservative variability threshold of Amp > 0.22G − 4.87 set by Belokurov et al This result suggests that at least ∼ 70% of the stars with > 4σ trends in ASAS-SN are actually variable. Two additional stars in this sample have photometric amplitudes above but very close to the threshold.…”

Apocenter Pile-up: Origin of the Stellar Halo Density Break

“…Evidence for the latter scenario has rapidly been growing (e.g., Deason et al 2015;Fiorentino et al 2015;Belokurov et al 2018a;Lancaster et al 2018). Belokurov et al (2018b) recently showed that ∼2/3 of the material in the inner stellar halo exhibits extreme radial anisotropy, making it appear "sausage-like" in velocity space-this, they argue, is a consequence of the accretion of a massive dwarf galaxy on a highly eccentric orbit. Fast on the heels of the second Gaia data release (DR2) Myeong et al (2018) found that N=8 of the Milky Way globular clusters are likely related to this markedly radial accretion event.…”

SMASH: Survey of the MAgellanic Stellar History

“…More recently, Mackey et al (2016) used Dark Energy Survey (DES; Dark Energy Survey Collaboration et al 2016) data to detect an "arclike" structure in the periphery of the LMC (∼15°from the center), which is likely a tidally disturbed portion of the LMC disk due to a recent interaction with the SMC. In addition, Belokurov et al (2017) used DES data to map streams of Blue Horizontal Branch (BHB) stars around the MCs with some extending to R∼40°.…”