We describe an algorithm that can fit the properties of the dwarf galaxy progenitor of a tidal stream, given the properties of that stream. We show that under ideal conditions (the Milky Way potential, the orbit of the dwarf galaxy progenitor, and the functional form of the dwarf galaxy progenitor are known exactly), the density and angular width of stars along the stream can be used to constrain the mass and radial profile of both the stellar and dark matter components of the progenitor dwarf galaxy that was ripped apart to create the stream. Our provisional fit for the parameters of the dwarf galaxy progenitor of the Orphan Stream indicates that it is less massive and has fewer stars than previous works have indicated.

The stellar disc of the Milky Way exhibits clear departures from planarity, the most conspicuous manifestation being the Galactic Warp but also includes an apparent corrugation pattern in number counts around 15kpc from the Galactic centre, a wave like pattern in the vertical velocities of stars as a function of guiding radius, asymmetries about the midplane in both number counts and bulk motions, and phase spirals in the z–vz projection of the local stellar distribution function. We discuss the physics of these phenomena and, in particular, suggest a possible avenue for inferring the vertical force in the Solar Neighbourhood from phase spirals. We apply Dynamic Mode Decomposition, a technique widely used in the realm of fluid mechanics, to simulations of disc galaxy simulations. This method appears to be particularly well-suited to the study of nonlinear processes such as the coupling of warps and spirals, first discussed by Masset and Tagger.

Recent observations from SEGUE, RAVE, and LAMOST have revealed tantalizing evidence that the local stellar disk of the Milky Way is in a state of disequilibrium. In particular, the disk appears to exhibit bending and breathing waves normal to its midplane within 2 kiloparsecs of our position within the disk. There also appear to be bending waves or corrugations at larger Galactocentric radii. These waves may be linked to other time-dependent disk phenomena such as the bar, spiral structure, and warp, or they may be the result of a passing dark matter subhalo or dwarf galaxy. Here, we discuss the observational evidence for these waves, the theory of bending and breathing waves in (simulated) stellar disks, and implications of disequilibrium for attempts to determine the local vertical force and dark matter density (the Oort problem). We also discuss the types of analyses that one might do with the Gaia database.

We perform the first observational test of dark matter in the form of cold (3 K) fractal clouds, as described by Pfenniger et al. (1994) and Pfenniger & Combes (1994). This is accomplished by probing for HI absorption in the halo of NGC 3079 against the background quasar, Q 0957+561, which is separated from the centre of NGC 3079 by 64 kpc, in projection. No absorption is detected to a limit of 3ΔTb/(–Tc) = 0·01. We have considered models for HI + H2 clouds characterised by the cloud radius and fractal dimension. Using the upper limit on absorption, we have ruled out a limited but interesting region of this parameter space. The observations do not rule out the possibility that all the dark matter could be hidden in the form of cold fractal clouds. By contrast, if the gas is diffuse with unity filling factors, then HI cannot constitute more than ∼ 10−5, by mass, of the galaxy's dark matter.

Courteau et al. (2007a) reported on the dependence of the ratio of a galaxy's maximum circular velocity, Vc, to its central velocity dispersion, σ0, on morphology, or equivalently total light concentration. This Vc − σ0–concentration relation, which involves details about the local and global galaxy physics, poses a fundamental challenge for galaxy structure models. Furthermore, not only must these models reproduce the Vc − σ0 relation and its various dependences, they must simultaneously match other fundamental scaling relations such as the velocity-size-luminosity and color-luminosity relations. We focus here on the interpretation of parameters that enter the Vc − σ0 relation to enable proper data-model comparisons and follow-up studies by galaxy modelers and observers.