P. Zivick | Searcharxiv

arXiv2018

Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud

We use GAIA DR2 proper motions of the RIOTS4 field OB stars in the Small Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data reveal that the SMC Wing has a systemic peculiar motion relative to the SMC Bar of (v_RA, v_Dec) = (62 +/-7, -18+/-5) km/s and relative radial velocity +4.5 +/- 5.0 km/s. This unambiguously demonstrates that these two regions are kinematically distinct: the Wing is moving away from the Bar, and towards the Large Magellanic Cloud with a 3-D velocity of 64 +/- 10 km/s. This is consistent with models for a recent, direct collision between the Clouds. We present transverse velocity distributions for our field OB stars, confirming that unbound runaways comprise on the order of half our sample, possibly more. Using eclipsing binaries and double-lined spectroscopic binaries as tracers of dynamically ejected runaways, and high-mass X-ray binaries (HMXBs) as tracers of runaways accelerated by supernova kicks, we find significant contributions from both populations. The data suggest that HMXBs have lower velocity dispersion relative to dynamically ejected binaries, consistent with the former corresponding to less energetic supernova kicks that failed to unbind the components. Evidence suggests that our fast runaways are dominated by dynamical, rather than supernova, ejections.

astro-ph.GA↗

arXiv2018

The Orbit and Origin of the Ultra-faint Dwarf Galaxy Segue 1

We present the first proper motion measurement for an ultra-faint dwarf spheroidal galaxy, Segue 1, using SDSS and LBC data as the first and second epochs separated by a baseline of $\sim 10$ years. We obtain a motion of $μ_α\,\cos(δ) = -0.37\pm0.57$ mas yr$^{-1}$ and $μ_δ =-3.39\pm0.58$ mas yr$^{-1}$. Combining this with the known line-of-sight velocity, this corresponds to a Galactocentric V$_\mathrm{rad}=84\pm9$ and V$_\mathrm{tan}=164^{+66}_{-55}$ km s$^{-1}$. Applying Milky Way halo masses between 0.8 to 1.6$\times 10^{12}$ M$_\odot$ results in an apocenter at 33.9$^{+21.7}_{-7.4}$ kpc and pericenter at 15.4$^{+10.1}_{-9.0}$ kpc from the Galactic center, indicating Segue~1 is rather tightly bound to the Milky Way. Since neither the orbital pole of Segue 1 nor its distance to the Milky Way is similar to the more massive classical dwarfs, it is very unlikely that Segue 1 was once a satellite of a massive known galaxy. Using cosmological zoom-in simulations of Milky Way-mass galaxies, we identify subhalos on similar orbits as Segue~1, which imply the following orbital properties: a median first infall 8.1$^{+3.6}_{-4.3}$ Gyrs ago, a median of 4 pericentric passages since then and a pericenter of 22.8$^{+4.7}_{-4.8}$ kpc. This is slightly larger than the pericenter derived directly from Segue 1 and Milky Way parameters, because galaxies with a small pericenter are more likely to be destroyed. Of the surviving subhalo analogs only 27\% were previously a satellite of a more massive dwarf galaxy (that is now destroyed), thus Segue 1 is more likely to have been accreted on its own.

astro-ph.GA↗