The distribution of shapes of galaxies’ dark halos provides a basic test for models of galaxy formation. To-date, few dark halo shapes have been measured, and the results of different methods appear contradictory. Here, we add to the sample of measured shapes by calculating the flattening of the Milky Way’s dark halo based on the manner in which the gas layer in the Galaxy flares with radius. We also test the validity of this technique – which has already been applied to several other galaxies – by comparing the inferred halo flattening to that obtained from a stellar-kinematic analysis, which can only be applied to the Milky Way. Both methods return consistent values for the shape of the Milky Way’s halo, with a shortest-to-longest axis ratio for the dark matter distribution of q = 0.75±0.25. However, this consistency is only achieved if we adopt a value of R0 = 7 ± 1kpc for the Sun’s distance to the Galactic center. Although this value is smaller than the IAU-sanctioned R0 = 8.5kpc, it is quite consistent with current observations. Whatever value of R0 is adopted, neither method returns halo parameters consistent with a disklike mass distribution, for which q � 0.2. This finding rules out cold molecular gas and decaying massive neutrinos as dark matter candidates.
In order to test the reliability of determinations of the shapes of galaxies' dark matter halos, we have made such measurements for the Milky Way by two independent methods, which make use of the stellar kinematics in the solar neighbourhood and the observed flaring of the Galactic HI layer to estimate the flattening of the Galactic dark halo. These techniques are found to produce a consistent estimate for the halo shape, with a shortest-to-longest axis ratio of q ~ 0.8, but only if one adopts somewhat non-standard values for the distance to the Galactic centre, R_0, and the local Galactic rotation speed, Theta_0. For consistency, one requires values of R_0 < 7.6 kpc and Theta_0 < 190 km/s. Although differing significantly from the current IAU-sanctioned values, these upper limits are consistent with all existing observational constraints. If future measurements confirm these lower values for the Galactic constants, then the validity of the gas layer flaring method will be confirmed. Further, dark matter candidates such as cold molecular gas and massive decaying neutrinos, which predict very flat dark halos with q < 0.2, will be ruled out. Conversely, if the Galactic constants were found to be close to the more conventional values, then there would have to be some systematic error in the methods for measuring dark halo shapes, so the existing modeling techniques would have to be viewed with some scepticism.
The Origins Billions Star Survey is a mission concept addressing the astrophysics of extrasolar planets, Galactic structure, the Galactic halo and tidal streams, the Local Group and local supercluster of galaxies, dark matter, star formation, open clusters, the solar system, and the celestial reference frame by determining the position, parallax, and proper motion, as well as photometry, for billions of stars down to 23rd visual magnitude. It is capable of surveying the entire celestial sphere or dwelling on a star field by varying the cadence of observations. The mission's ability to measure objects fainter than 17th magnitude allows a large number of extragalactic compact objects to be observed, making the astrometric measurements absolute. The project mission accuracy is comparable to Gaia for a survey mission. Improved accuracy can be achieved by dwelling on a particular star field or by using the Gaia positions at 14th magnitude to improve the positions of objects at the 18th–23rd visual magnitudes.
The Oort constants describe the local spatial variations of the stellar streaming field. The classic way for their determination employs their effect on stellar proper motions. We discuss various problems arising in this procedure. A large, hitherto apparently overlooked, source of potential systematic error arises from longitudinal variations of the mean stellar parallax, caused by intrinsic density inhomogeneities and interstellar extinction. Together with the reflex of the solar motion, these variations by mode mixing create contributions to the longitudinal proper motions (l) that are indistinguishable from the Oort constants at ≲20% of their amplitude. Fortunately, we can correct for this mode mixing using the latitudinal proper motions μb(l). We use about 106 stars from the ACT/Tycho-2 catalogs brighter than V ≈ 11 with median proper-motion error of ≈3 mas yr-1, taking every precaution to avoid or correct for the various sources of systematic error, significant deviations from expectations based on a smooth axisymmetric equilibrium, in particular nonzero C for old red giant stars. We also find variations of the Oort constants with the mean color, which correlate nicely with the asymmetric drift of the subsample considered. In addition, these correlations are different in nature than those expected for an axisymmetric galaxy. The most reliable tracers for the "true" Oort constants are red giants, which are old enough to be in equilibrium and distant enough to be unaffected by possible local anomalies. For these stars we find, after correction for mode mixing and the axisymmetric asymmetric-drift effects, A ≈ 16, B ≈ -17, A - B ≈ 33, and C ≈ -10 km s-1 kpc-1 with internal errors of about 1-2 and external error of perhaps the same order. These values are consistent with our knowledge of the Milky Way (flat rotation curve and Ω ≡ A - B ≈ 28 ± 2), based on observations made with the ESA Hipparcos astrometry satellite.
For decades optical time-domain searches have been tuned to find ordinary supernovae, which rise and fall in brightness over a period of weeks. Recently, supernova searches have improved their cadences and a handful of fast-evolving luminous transients (FELTs) have been identified. FELTs have peak luminosities comparable to Type Ia supernovae, but rise to maximum in $<10$ days and fade from view in $
Three planned astrometry survey satellites, FAME, DIVA, and GAIA, all aim at observing magnitude-limited samples. We argue that substantial additional scientific opportunities are within the reach of these mission if they devote a modest fraction of their catalogs to selected targets that are fainter than their magnitude limits. We show that the addition of ~10^6 faint (R>15) targets to the 40 10^6 object FAME catalog can improve the precision of the reference frame by a factor 2.5, to 7 micro-as/yr, increase Galactocentric distance at which halo rotation can be precisely (2 km/s) measured by a factor 4, to 25 kpc, and increase the number of late M dwarfs, L dwarfs, and white dwarfs with good parallaxes by an order of magnitude. In most cases, the candidate quasars, horizontal branch stars, and dim dwarfs that should be observed to achieve these aims are not yet known. We present various methods to identify candidates from these classes, and assess the efficiencies of these methods. The analysis presented here can be applied to DIVA with modest modifications. Application to GAIA should be deferred until the characteristics of potential targets are better constrained.
We propose a MIDEX-class space mission with the goal to find and characterize roughly 10,000 transiting planets. When transits occur, a much more detailed characterization of the planet is possible and so a large data base of transiting planets will provide planets with a large range in periods and radii for follow-up studies. Our survey will be all-sky and focused on stars brighter than V=14.8. Down to V=12, LEAVITT will be able to detect Neptune-sized objects. Because of it's high cadence, LEAVITT is about 100 times more sensitive at detecting transits than GAIA, while it will find more than 20 times as many transits as KEPLER. LEAVITT has multi-band photometric capability implemented via a low-resolution dispersive element which can obtain 0.2% (2 mmag) photometry down to V=14.8. LEAVITT's high multi-band photometric accuracy reduces the number of false-positives significantly.
The distribution of shapes of galaxies' dark halos provides a basic test for models of galaxy formation. To-date, few dark halo shapes have been measured, and the results of different methods appear contradictory. Here, we add to the sample of measured shapes by calculating the flattening of the Milky Way's dark halo based on the manner in which the gas layer in the Galaxy flares with radius. We also test the validity of this technique -- which has already been applied to several other galaxies -- by comparing the inferred halo flattening to that obtained from a stellar-kinematic analysis, which can only be applied to the Milky Way. Both methods return consistent values for the shape of the Milky Way's halo, with a shortest-to-longest axis ratio for the dark matter distribution of q = 0.75 +/- 0.25. However, this consistency is only achieved if we adopt a value of R_0 = 7 +/- 1 kpc for the Sun's distance to the Galactic center. Although this value is smaller than the IAU-sanctioned R_0 = 8.5 kpc, it is quite consistent with current observations. Whatever value of R_0 is adopted, neither method returns halo parameters consistent with a disk-like mass distribution, for which q ~ 0.2. This finding rules out cold molecular gas and decaying massive neutrinos as dark matter candidates.
