4,006 research outputs found
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 ( 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
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