The new Milky Way satellites: alignment with the VPOS and predictions for proper motions and velocity dispersions

The evidence that stellar systems surrounding the Milky Way (MW) are distributed in a Vast Polar Structure (VPOS) may be observationally biased by satellites detected in surveys of the northern sky. The recent discoveries of more than a dozen new systems in the southern hemisphere thus constitute a critical test of the VPOS phenomenon. We report that the new objects are located close to the original VPOS, with half of the sample having offsets less than 20 kpc. The positions of the new satellite galaxy candidates are so well aligned that the orientation of the revised best-fitting VPOS structure is preserved to within 9 degrees and the VPOS flattening is almost unchanged (31 kpc height). Interestingly, the shortest distance of the VPOS plane from the MW center is now only 2.5 kpc, indicating that the new discoveries balance out the VPOS at the Galactic center. The vast majority of the MW satellites are thus consistent with sharing a similar orbital plane as the Magellanic Clouds, confirming a hypothesis proposed by Kunkel & Demers and Lynden-Bell almost 40 years ago. We predict the absolute proper motions of the new objects assuming they orbit within the VPOS. Independent of the VPOS results we also predict the velocity dispersions of the new systems under three distinct assumptions: that they (i) are dark-matter-free star clusters obeying Newtonian dynamics, (ii) are dwarf satellites lying on empirical scaling relations of galaxies in dark matter halos, and (iii) obey MOND.Comment: 17 pages, 6 figures, 5 tables. Updated to match version accepted for publication in MNRA

The halo by halo missing baryon problem

The global missing baryon problem - that the sum of observed baryons falls short of the number expected form BBN - is well known. In addition to this, there is also a local missing baryon problem that applies to individual dark matter halos. This halo by halo missing baryon problem is such that the observed mass fraction of baryons in individual galaxies falls short of the cosmic baryon fraction. This deficit is a strong function of circular velocity. I give an empirical estimate of this function, and note the presence of a critical scale of ~900 km/s therein. I also briefly review Omega_b from BBN, highlighting the persistent tension between lithium and the CMB, and discuss some pros and cons of individual galaxies and clusters of galaxies as potential reservoirs of dark baryons.Comment: 10 pages, review for IAU Symposium 244, Dark Galaxies & Lost Baryon

Distinguishing Between CDM and MOND: Predictions for the Microwave Background

Two hypothesized solutions of the mass discrepancy problem are cold dark matter (CDM) and modified Newtonian dynamics (MOND). The virtues and vices of these very different hypotheses are largely disjoint, making the process of distinguishing between them very dependent on how we weigh disparate lines of evidence. One clear difference is the nature of the principal mass constituent of the universe (CDM or baryons). This difference in the baryon fraction ($f_b \approx 0.1$ vs. 1) should leave a distinctive signature in the spectrum of fluctuations in the cosmic microwave background. Here I discuss some of the signatures which should be detectable in the near future. The most promising appears to be the ratio of the amplitudes of the first two peaks relative to the intervening trough.Comment: 8 pages, 1 figure, AASTeX. Accepted for publication in ApJ Letter

The dark and baryonic matter content of low surface brightness disk galaxies

We present mass models of a sample of 19 low surface brightness (LSB) galaxies and compare the properties of their constituent mass components with those of a sample of high surface brightness (HSB) galaxies.We find that LSB galaxies are dark matter dominated. Their halo parameters are only slightly affected by assumptions on stellar mass-to-light ratios. Comparing LSB and HSB galaxies we find that mass models derived using the maximum disk hypothesis result in the disks of LSB galaxies having systematically higher stellar mass-to-light ratios than HSB galaxies of similar rotation velocity. This is inconsistent with all other available evidence on the evolution of LSB galaxies. We argue therefore that the maximum disk hypothesis does not provide a representative description of the LSB galaxies and their evolution. Mass models with stellar mass-to-light ratios determined by the colors and stellar velocity dispersions of galactic disks imply that LSB galaxies have dark matter halos that are more extended and less dense than those of HSB galaxies. Surface brightness is thus related to the halo properties. LSB galaxies are slowly evolving, low density and dark matter dominated galaxies.Comment: 23 pages Latex, 12 postscript figures, uses mn.sty. Accepted for publication in MNRA