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Register a new userCharacterizing the Infall Times and Quenching Timescales of Milky Way Satellites with $Gaia$ Proper Motions
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Observations of low-mass satellite galaxies in the nearby Universe point towards a strong dichotomy in their star-forming properties relative to systems with similar mass in the field. Specifically, satellite galaxies are preferentially gas poor and no longer forming stars, while their field counterparts are largely gas rich and actively forming stars. Much of the recent work to understand this dichotomy has been statistical in nature, determining not just that environmental processes are most likely responsible for quenching these low-mass systems but also that they must operate very quickly after infall onto the host system, with quenching timescales $lesssim 2~ {rm Gyr}$ at ${M}_{star} lesssim 10^{8}~{rm M}_{odot}$. This work utilizes the newly-available $Gaia$ DR2 proper motion measurements along with the Phat ELVIS suite of high-resolution, cosmological, zoom-in simulations to study low-mass satellite quenching around the Milky Way on an object-by-object basis. We derive constraints on the infall times for $37$ of the known low-mass satellite galaxies of the Milky Way, finding that $gtrsim~70%$ of the `classical satellites of the Milky Way are consistent with the very short quenching timescales inferred from the total population in previous works. The remaining classical Milky Way satellites have quenching timescales noticeably longer, with $tau_{rm quench} sim 6 - 8~{rm Gyr}$, highlighting how detailed orbital modeling is likely necessary to understand the specifics of environmental quenching for individual satellite galaxies. Additionally, we find that the $6$ ultra-faint dwarf galaxies with publicly available $HST$-based star-formation histories are all consistent with having their star formation shut down prior to infall onto the Milky Way -- which, combined with their very early quenching times, strongly favors quenching driven by reionization.
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The second data release from the Gaia mission (DR2) provides a comprehensive and unprecedented picture of the motions of astronomical sources in the plane of the sky, extending from the solar neighborhood to the outer reaches of the Milky Way. I present proper motion measurements based on Gaia DR2 for 17 ultra-faint dwarf galaxies within 100 kpc of the Milky Way. I compile the spectroscopically-confirmed member stars in each dwarf bright enough for Gaia astrometry from the literature, producing member samples ranging from 2 stars in Triangulum II to 68 stars in Bootes I. From the spectroscopic member catalogs I estimate the proper motion of each system. I find good agreement with the proper motions derived by the Gaia collaboration for Bootes I and Leo I. The tangential velocities for 14 of the 17 dwarfs are determined to better than 50 km/s, more than doubling the sample of such measurements for Milky Way satellite galaxies. The orbital pericenters are well-constrained, with a median value of 38 kpc. Only one satellite, Tucana III, is on an orbit passing within 15 kpc of the Galactic center, suggesting that the remaining ultra-faint dwarfs are unlikely to have experienced severe tidal stripping. As a group, the ultra-faint dwarfs are on high-velocity, eccentric, retrograde trajectories, with nearly all of them having space motions exceeding 370 km/s. In a low-mass (M_vir = 0.8 x 10^12 M_sun) Milky Way potential, eight out of the 17 galaxies lack well-defined apocenters and appear likely to be on their first infall, indicating that the Milky Way mass may be larger than previously estimated or that many of the ultra-faint dwarfs are associated with the Magellanic Clouds. The median eccentricity of the ultra-faint dwarf orbits is 0.79, similar to the values seen in numerical simulations, but distinct from the rounder orbits of the more luminous dwarf spheroidals.
Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf spheroidal galaxies (dSphs) of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve, with total mass ranging from $2.8times10^{11}$ $M_{odot}$ to $15times10^{11}$ $M_{odot}$. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dSphs are highly concentrated close to their pericenter, rather than to their apocenter as expected from Keplers law. This may challenge the fact that most dSphs are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dSphs lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dSphs.
We have derived absolute proper motions of the entire Galactic bulge region from VIRAC and Gaia. We present these as both integrated on-sky maps and, after isolating standard candle red clump (RC) stars, as a function of distance using RC magnitude as a proxy. These data provide a new global, 3-dimensional view of the Milky Way barred bulge kinematics. We find a gradient in the mean longitudinal proper motion, $<mu_l^star>$, between the different sides of the bar, which is sensitive to the bar pattern speed. The split RC has distinct proper motions and is colder than other stars at similar distance. The proper motion correlation map has a quadrupole pattern in all magnitude slices showing no evidence for a separate, more axisymmetric inner bulge component. The line-of-sight integrated kinematic maps show a high central velocity dispersion surrounded by a more asymmetric dispersion profile. $sigma_{mu_l} / sigma_{mu_b}$ is smallest, $sim1.1$, near the minor axis and reaches $sim1.4$ near the disc plane. The integrated $<mu_b>$ pattern signals a superposition of bar rotation and internal streaming motion, with the near part shrinking in latitude and the far part expanding. To understand and interpret these remarkable data, we compare to a made-to-measure barred dynamical model, folding in the VIRAC selection function to construct mock maps. We find that our model of the barred bulge, with a pattern speed of 37.5 $mathrm{km , s^{-1} , kpc^{-1}}$, is able to reproduce all observed features impressively well. Dynamical models like this will be key to unlocking the full potential of these data.
With the release of Gaia DR2, it is now possible to measure the proper motions (PMs) of the lowest mass, ultra-faint satellite galaxies in the Milky Ways (MW) halo for the first time. Many of these faint satellites are posited to have been accreted as satellites of the Magellanic Clouds (MCs). Using their 6-dimensional phase space information, we calculate the orbital histories of 13 ultra-faint satellites and five classical dwarf spheroidals in a combined MW+LMC+SMC potential to determine which galaxies are dynamically associated with the MCs. These 18 galaxies are separated into four classes: i.) long-term Magellanic satellites that have been bound to the MCs for at least the last two consecutive orbits around the MCs (Carina 2, Carina 3, Horologium 1, Hydrus 1); ii.) Magellanic satellites that were recently captured by the MCs $<$ 1 Gyr ago (Reticulum 2, Phoenix 2); iii.) MW satellites that have interacted with the MCs (Sculptor 1, Tucana 3, Segue 1); and iv.) MW satellites (Aquarius 2, Canes Venatici 2, Crater 2, Draco 1, Draco 2, Hydra 2, Carina, Fornax, Ursa Minor). Results are reported for a range of MW and LMC masses. Contrary to previous work, we find no dynamical association between Carina, Fornax, and the MCs. Finally, we determine that the addition of the SMCs gravitational potential affects the longevity of satellites as members of the Magellanic system (long-term versus recently captured), but it does not change the total number of Magellanic satellites.
We present the star formation histories (SFHs) of 20 faint M31 satellites ($-12 lesssim M_V lesssim -6$) that were measured by modeling sub-horizontal branch (HB) depth color-magnitude diagrams constructed from Hubble Space Telescope (HST) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration $3-6$ Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent ($<3$ Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SFHs: one with exponentially declining SFHs ($tau sim 2$ Gyr) and one with rising SFHs with abrupt quenching. We speculate how these two groups could be related to scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury survey of M31 satellites will provide well-constrained ancient SFHs to go along with the quenching times we measure here. The discovery and characterization of M31 satellites with $M_V gtrsim -6$ would help quantify the relative contributions of reionization and environment to quenching of the lowest-mass satellites.
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