ABSTRACT
We use the continuity equation to derive a method for measuring the pattern speed of the Milky Way’s bar/bulge from proper motion data. The method has minimal assumptions but requires ...complete coverage of the non-axisymmetric component in two of the three Galactic coordinates. We apply our method to the proper motion data from a combination of Gaia DR2 and VISTA Variables in the Via Lactea (VVV) to measure the pattern speed of the bar as $\Omega _\mathrm{p}=(41\pm 3)\, \mathrm{km\, s^{-1}\, kpc^{-1}}$ (where the error is statistical). This puts the corotation radius at $(5.7\pm 0.4)\, \mathrm{kpc}$, under the assumptions of the standard peculiar motion of the Sun and the absence of non-axisymmetric streaming in the Solar neighbourhood. The obtained result uses only data on the near side of the bar which produces consistent measurements of the distance and velocity of the centre of the Galaxy. Addition of the data on the far side of the bar pulls the pattern speed down to $\Omega _\mathrm{p}=(31\pm 1)\, \mathrm{km\, s^{-1}\, kpc^{-1}}$ but requires a lower transverse velocity for the Galactic centre than observed. This suggests systematics of $5-10\, \mathrm{km\, s^{-1}kpc^{-1}}$ dominate the uncertainty. We demonstrate using a dynamically formed bar/bulge simulation that even with the limited field of view of the VVV survey our method robustly recovers the pattern speed.
A number of dwarf spheroidal (dSph) galaxies are known to contain a more extended, metal-poor population with a flattish velocity dispersion profile, and a more concentrated, metal-rich population ...with a velocity dispersion declining with radius. The two populations can be modelled with Michie-King distribution functions (DFs) in the isothermal and in the sharply truncated limits, respectively. We argue that the truncation of the metal-rich population can be traced back to the spatial distribution of the star-forming gas. Suppose δ is the exponent of the first non-constant term in the Taylor expansion of the total potential at the centre (δ= 1 for Navarro-Frenk-White or NFW haloes, δ= 2 for cored haloes). Then, we show that the ratio of the half-light radii of the populations R
δ/2
h, 2/R
h, 1
δ/2 must be smaller than the ratio of the line-of-sight velocity dispersions σlos, 2(R
h, 2)/σlos, 1(R
h, 1).
Specializing to the case of the Sculptor dSph, we develop a technique to fit simultaneously both populations with Michie-King DFs. This enables us to determine the mass profile of the Sculptor dSph with unprecedented accuracy in the radial range 0.2 < r < 1.2 kpc. We show that cored halo models are preferred over cusped halo models, with a likelihood ratio test rejecting NFW models at any significance level higher than 0.05 per cent. Even more worryingly, the best-fitting NFW models require concentrations with c ≲ 20, which is not in the cosmologically preferred range for dwarf galaxies. We conclude that the kinematics of multiple populations in dSphs provides a substantial new challenge for theories of galaxy formation, with the weight of available evidence strongly against dark matter cusps at the centre.
ABSTRACT We have used the publicly released Dark Energy Survey (DES) data to hunt for new satellites of the Milky Way (MW) in the southern hemisphere. Our search yielded a large number of promising ...candidates. In this paper, we announce the discovery of nine new unambiguous ultra-faint objects, whose authenticity can be established with the DES data alone. Based on the morphological properties, three of the new satellites are dwarf galaxies, one of which is located at the very outskirts of the MW, at a distance of 380 kpc. The remaining six objects have sizes and luminosities comparable to the Segue 1 satellite and cannot be classified straightforwardly without follow-up spectroscopic observations. The satellites we have discovered cluster around the LMC and the SMC. We show that such spatial distribution is unlikely under the assumption of isotropy, and, therefore, conclude that at least some of the new satellites must have been associated with the Magellanic Clouds in the past.
ABSTRACT
We examine the capacity to identify binary systems from astrometric errors and deviations alone. Until the release of the fourth Gaia data release, we lack the full astrometric time-series ...that the satellite records, but as we show can still infer the presence of binaries from the best-fitting models, and their error, already available. We generate a broad catalogue of simulated binary systems within 100 pc, and examine synthetic observations matching the Gaia survey’s scanning law and astrometric data processing routine. We show how the unit weight error (UWE) and proper motion anomaly (PMA) vary as a function of period, and the properties of the binary. Both UWE and PMA peak for systems with a binary period close to the time baseline of the survey. Thus UWE can be expected to increase or remain roughly constant as we observe the same system over a longer baseline, and we suggest UWEeDR3 > 1.25 and ΔUWE/UWEeDR3 > −0.25 as criteria to select astrometric binaries. For stellar binaries, we find detectable significant astrometric deviations for 80–90 per cent of our simulated systems in a period range from months to decades. We confirm that for systems with periods less than the survey’s baseline the observed UWE scales ∝ ϖ (parallax), a (semimajor axis), and $\Delta =\frac{|q-l|}{(1+q)(1+l)}$, where q and l are the mass and light ratio, respectively, with a modest dependence on viewing angle. For longer periods the signal is suppressed by a factor of roughly ∝P−2 (period). PMA is largest in orbits with slightly longer periods but obeys the same approximate scaling relationships.
Abstract
We model the fastest moving ($v_{\rm tot} \gt 300 \, {\rm km}\, \, {\rm s}^{-1}$) local (D ≲ 3 kpc) halo stars using cosmological simulations and six-dimensional Gaia data. Our approach is ...to use our knowledge of the assembly history and phase-space distribution of halo stars to constrain the form of the high-velocity tail of the stellar halo. Using simple analytical models and cosmological simulations, we find that the shape of the high-velocity tail is strongly dependent on the velocity anisotropy and number density profile of the halo stars – highly eccentric orbits and/or shallow density profiles have more extended high-velocity tails. The halo stars in the solar vicinity are known to have a strongly radial velocity anisotropy, and it has recently been shown the origin of these highly eccentric orbits is the early accretion of a massive ($M_{\rm star}\sim 10^9 \, \mathrm{M}_\odot$) dwarf satellite. We use this knowledge to construct a prior on the shape of the high-velocity tail. Moreover, we use the simulations to define an appropriate outer boundary of 2r200, beyond which stars can escape. After applying our methodology to the Gaia data, we find a local (r0 = 8.3 kpc) escape speed of $v_{\rm esc}(r_0) = 528^{+24}_{-25} \, {\rm km}\, \, {\rm s}^{-1}$. We use our measurement of the escape velocity to estimate the total Milky Way mass, and dark halo concentration: $M_{200, \rm tot} = 1.00^{+0.31}_{-0.24} \times 10^{12}\, \mathrm{M}_\odot$, $c_{200}=10.9^{+4.4}_{-3.3}$. Our estimated mass agrees with recent results in the literature that seem to be converging on a Milky Way mass of $M_{200, \rm tot} \sim 10^{12}\, \mathrm{M}_\odot$.
Abstract
We consider the orbital evolution of satellites in galaxy mergers, focusing on the evolution of eccentricity. Using a large suite of
N
-body simulations, we study the phenomenon of satellite ...orbital radialization—a profound increase in the eccentricity of its orbit as it decays under dynamical friction. While radialization is detected in a variety of different setups, it is most efficient in cases of high satellite mass, not very steep host density profiles, and high initial eccentricity. To understand the origin of this phenomenon, we run additional simulations with various physical factors selectively turned off: satellite mass loss, reflex motion and distortion of the host, etc. We find that all these factors are important for radialization because it does not occur for point-mass satellites or when the host potential is replaced with an unperturbed initial profile. The analysis of forces and torques acting on both galaxies confirms the major role of self-gravity of both host and satellite in the reduction of orbital angular momentum. The classical Chandrasekhar dynamical friction formula, which accounts only for the forces between the host and the satellite, but not for internal distortions of both galaxies, does not match the evolution of eccentricity observed in
N
-body simulations.
Binary deviations from single object astrometry Penoyre, Zephyr; Belokurov, Vasily; Wyn Evans, N ...
Monthly Notices of the Royal Astronomical Society,
06/2020, Letnik:
495, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Most binaries are undetected. Astrometric reductions of a system using the assumption that the object moves like a single point mass can be biased by unresolved binary stars. The discrepancy ...between the centre of mass of the system (which moves like a point mass) and the centre of light (which is what we observe) introduces additional motion. We explore the extent to which binary systems affect single object model fit to astrometric data. This tells us how observations are diluted by binaries and which systems cause the largest discrepancies – but it also allows us to make inferences about the binarity of populations based on observed astrometric error. By examining a sample of mock observations, we show that binaries with periods close to 1 yr can mimic parallax and thus bias distance measurements, while long-period binaries can introduce significant apparent proper motion. While these changes can soak up some of the error introduced by the binary, the total deviation from the best-fitting model can be translated into a lower limit on the on-sky separation of the pair. Throughout, we link these predictions to data from the Gaia satellite, while leaving the conclusions generalizable to other surveys.
ABSTRACT The total gas mass of a protoplanetary disk is a fundamental, but poorly determined, quantity. A new technique has been demonstrated to assess directly the bulk molecular gas reservoir of ...molecular hydrogen using the HD J = 1-0 line at 112 m. In this work we present a Herschel Space Observatory10 survey of six additional T Tauri disks in the HD line. Line emission is detected at >3 significance in two cases: DM Tau and GM Aur. For the other four disks, we establish upper limits to the line flux. Using detailed disk structure and ray-tracing models, we calculate the temperature structure and dust mass from modeling the observed spectral energy distributions, and we include the effect of UV gas heating to determine the amount of gas required to fit the HD line. The ranges of gas masses are 1.0-4.7 × 10−2 for DM Tau and 2.5-20.4 × 10−2 for GM Aur. These values are larger than those found using CO for GM Aur, while the CO-derived gas mass for DM Tau is consistent with the lower end of our mass range. This suggests a CO chemical depletion from the gas phase of up to a factor of five for DM Tau and up to two orders of magnitude for GM Aur. We discuss how future analysis can narrow the mass ranges further.
ABSTRACT
We examine the capacity to identify binary systems from astrometric deviations alone. We apply our analysis to the Gaia eDR3 and DR2 data, specifically the Gaia Catalogue of Nearby Stars. We ...show we must renormalize (R)UWE over the local volume to avoid biasing local observations, giving a local unit weight error (LUWE). We use the simple criterion of LUWE>2, along with a handful of quality cuts to remove likely contaminants, to identify unresolved binary candidates. We identify 22 699 binary candidates within 100 pc of the Sun (just under 10 per cent of sources in this volume). We find an astrometric binary candidate fraction of around 20 per cent for giant stars, 10 per cent on the main sequence and lower than 1 per cent for white dwarfs. We also look for Variability Induced Movers, by computing the correlation between photometric variability and astrometric noise – and show that VIMs may dominate the binary population of sub-Solar mass MS stars. We discuss the possibility and limitations of identifying non-luminous massive companions from astrometry alone, but find that our method is insensitive to these. Finally, we compare the astrometric deviations of MS binaries to the simulated sample from paper I, which show excellent agreement, and compute the astrometric candidate binary fraction as a function of absolute magnitude.
We estimate the mass of the Milky Way (MW) within 21.1 kpc using the kinematics of halo globular clusters (GCs) determined by Gaia. The second Gaia data release (DR2) contained a catalog of absolute ...proper motions (PMs) for a set of Galactic GCs and satellite galaxies. We select from the catalog only halo GCs, identifying a total of 34 GCs spanning , and use their 3D kinematics to estimate the anisotropy over this range to be , in good agreement with, though slightly lower than, a recent estimate for a sample of halo GCs using Hubble Space Telescope (HST) PM measurements further out in the halo. We then use the Gaia kinematics to estimate the mass of the MW inside the outermost GC to be , which corresponds to a circular velocity at of The implied virial mass is . The error bars encompass the uncertainties on the anisotropy and on the density profile of the MW dark halo, and the scatter inherent in the mass estimator we use. We get improved estimates when we combine the Gaia and HST samples to provide kinematics for 46 GCs out to 39.5 kpc: , , and . We show that these results are robust to potential substructure in the halo GC distribution. While a wide range of MW virial masses have been advocated in the literature, from below 1012 to above 2 × 1012 , these new data imply that an intermediate mass is most likely.
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