Accurate Extra-Galactic Distances and Dark Energy: Anchoring the Distance Scale with Rotational Parallaxes

We investigate how the uncertainty on the Hubble constant (H_0) affects the uncertainty in the Equation of State (EOS) of Dark Energy and the total density of the Universe (Omega_tot). We use the approximate relations between the cosmological parameters [Spergel etal (2007)] and use error-propagation to estimate the effects of improving the CMB parameters and H_0 on the EOS of Dark Energy (DE). First we assume that the additional data does not improve significantly, but decrease the error on H_0 by a factor <~10. Second, we allow improved additional data but current H_0 errors (i.e., the DE Task Force case). In the 1st scenario, improvements of the CMB parameters hardly change the accuracy of the EOS and Omega_tot, unless H_0 can be measured with an accuracy of a few %. We find that a combination of moderate improvements for both H_0 and other data significantly constrains the evolution of dark energy, but at a reduced cost. We review several methods (and their strengths and weaknesses) that might yield extra-galactic distances with errors of about 1%. We review: the Velocity Field method, two Maser methods, two Light Echo techniques, the Binary Star method, and the Rotational Parallax (RP) technique. Because these methods substantially rely on geometry rather than astrophysics or cosmology, their results are quite robust. We focus on the advantages of the RP technique which can provide single-step, bias-free distances to nearby spirals. These distances can be used to improve the zero-point for other methods which in turn allow for a much improved H_0 errors. Achieving an accuracy of ~2% in the distances to M31, M33 and the LMC by the RP method requires proper motions from future astrometric missions (SIM, GAIA and OBSS, or the SKA).Comment: 20 pages, 2 figures. Minor changes. Now includes a review of methods capable of achieving unbiased 1% extra-galactic distances. MNRAS: Accepted for publicatio

Refining the Oort and Galactic constants

The local stellar kinematics of the Milky Way offer a useful tool for studying the rotation curve of the Galaxy. These kinematics -- usually parameterized by the Oort constants A and B -- depend on the local gradient of the rotation curve as well as its absolute value (Theta_0), and the Sun's distance to the Galactic center (R_0). The density of interstellar gas in the Milky Way is shown to vary non-monotonically with radius, and so contributes significantly to the local gradient of the rotation curve. We have therefore calculated mass models for the Milky Way that include this component, and have derived the corresponding radial variation in the Oort constants. Between 0.9R_0 and 1.2R_0, the Oort functions, A(R) and B(R), differ significantly from the general Theta/R dependence. Various previously-inexplicable observations are shown to be consistent with these predictions. These models can explain the 40% difference between the values for 2 A R_0 derived from radial velocity data originating in the inner and outer Galaxy. They also go some way toward explaining the different shapes of the velocity ellipsoids of giant and dwarf stars in the solar neighbourhood. However, a consistent picture only emerges if one adopts small values of R_0 = 7.1 +/- 0.4 kpc and Theta_0 = 184 +/- 8 km/s. With these Galactic constants, the Milky Way's rotation curve declines slowly in the outer Galaxy; V_rot(20 kpc) = 166 kms. Our low value for R_0 agrees well with the only direct determination (7.2 +/- 0.7 kpc, Reid 1993). Using these Galactic constants, we find that the proper motion of Sgr A^* is consistent with the observational constraints. The radial velocities and proper motions of our best fit model are entirely consistent with the radial velocities of Cepheids and the Hipparcos measurements of their proper motions.Comment: 11 pages, LaTeX, including 5 figures, using mn and epsf style files. Accepted for publication in MNRA

Two measures of the shape of the Milky Way's dark halo

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.Comment: Accepted for publication in MNRAS. 10 pages, 6 figures, uses mn.sty and epsf.st

Luminous and Dark Matter in the Milky Way

(Abridged) Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants (R_0 and T_0), the stellar columndensity (S_*) and the shape of the dark matter (DM) halo. Here we present analytical relations that can be used to investigate the effects of the uncertain gaseous velocity dispersion on the HI flaring constraints. The contribution of cosmic rays and magnetic fields to the pressure gradients is small. A significantly flattened dark matter halo is only possible if R_0 <~ 6.8 kpc. If R_0 is larger than ~7 kpc, or T_0 >~ 170 km/s, we can rule out two DM candidates that require a highly flattened DM halo: 1) decaying massive neutrinos; and 2) a disk of cold molecular hydrogen. It is only possible to construct self-consistent models of the Galaxy based on the IAU-recommended values for the Galactic constants in the unlikely case that the the stellar columndensity is smaller than ~18 M_sun/pc^2. If we assume that the halo is oblate and S_* = 35 +/- 5 M_sun/pc^2, R_0 <~ 8 kpc and T_0 <~ 200 km/s. Combining the best kinematical and star-count estimates of S_*, we conclude that: 25 <~ S_* <~ 45 M_sun/pc^2. Kuijken & Gilmore's (1991) determination of the columndensity of matter with |z|<=1.1 kpc is robust and valid over a wide range of Galactic constants. Our mass models show that the DM density in the Galactic centre is uncertain by a factor 1000. In the Solar neighbourhood we find: rho_DM ~0.42 GeV/c^2/cm^3 or (11 +/- 5) mM_sun/pc^3 -- roughly 15% of rho_tot.Comment: Accepted for publication in MNRA

Case Notes

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 have been identified(1-5). These have peak luminosities comparable to type Ia supernovae, but rise to maximum in less than ten days and fade from view in less than one month. Here we present the most extreme example of this class of object thus far: KSN 2015K, with a rise time of only 2.2 days and a time above half-maximum of only 6.8 days. We show that, unlike type Ia supernovae, the light curve of KSN 2015K was not powered by the decay of radioactive elements. We further argue that it is unlikely that it was powered by continuing energy deposition from a central remnant (a magnetar or black hole). Using numerical radiation hydrodynamical models, we show that the light curve of KSN 2015K is well fitted by a model where the supernova runs into external material presumably expelled in a pre-supernova mass-loss episode. The rapid rise of KSN 2015K therefore probes the venting of photons when a hypersonic shock wave breaks out of a dense extended medium.NASA NNH15ZDA001N NNX17AI64G Australian Research Council Centre of Excellence for All-sky Astrophysics CE11000102

Old open clusters in the outer Galactic disk

The outer parts of the Milky Way disk are believed to be one of the main arenas where the accretion of external material in the form of dwarf galaxies and subsequent formation of streams is taking place. The Monoceros stream and the Canis Major and Argo over-densities are notorious examples. VLT high resolution spectra have been acquired for five distant open clusters. We derive accurate radial velocities to distinguish field interlopers and cluster members. For the latter we perform a detailed abundance analysis and derive the iron abundance [Fe/H] and the abundance ratios of several $\alpha$ elements. Our analysis confirms previous indications that the radial abundance gradient in the outer Galactic disk does not follow the expectations extrapolated from the solar vicinity, but exhibits a shallower slope. By combining the metallicity of the five program clusters with eight more clusters for which high resolution spectroscopy is available, we find that the mean metallicity in the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H] $\approx -0.35$, with only marginal indications for a radial variation. In addition, all the program clusters exhibit solar scaled or slightly enhanced $\alpha$ elements, similar to open clusters in the solar vicinity and thin disk stars. We investigate whether this outer disk cluster sample might belong to an extra-galactic population, like the Monoceros ring. However, close scrutiny of their properties - location, kinematics and chemistry - does not convincingly favor this hypothesis. On the contrary, they appear more likely genuine Galactic disk clusters. We finally stress the importance to obtain proper motion measurements for these clusters to constrain their orbits.Comment: 19 pages, 9 eps figure, in press in A&A, abstract rephrased to fit i