28,803 research outputs found
Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies
Maximum disk mass models fitted to galaxy rotation curves are used to show
that dark matter (DM) halos in late-type and dwarf spheroidal (dSph) galaxies
satisfy well defined scaling laws. Halos in less luminous galaxies have smaller
core radii, higher central densities, and smaller central velocity dispersions.
Implications: (1) A single, continuous physical sequence of increasing mass
extends from the tiniest dSphs to the most luminous spirals. (2) The high DM
densities in dSph galaxies are normal for such dwarf galaxies. Since virialized
density depends on collapse redshift z, the smallest dwarfs formed about delta
z = 7 earlier than the biggest spirals. (3) The high DM densities of dSphs
implies that they are real galaxies formed from primordial density
fluctuations. They are not tidal fragments. (4) Because dwarf galaxies become
more numerous and more nearly dominated by DM as luminosity decreases, there
may be a large population of objects that are completely dark. Such objects are
a canonical prediction of cold DM theory. (5) The slopes of the DM parameter
correlations provide a measure on galactic mass scales of the slope n of the
power spectrum of primordial density fluctuations. Our results not yet
corrected for baryonic compression of DM give n = -1.9 +- 0.2. This is
consistent with cold DM theory.Comment: 19 pages, 5 Postscript figures; requires IAUS215.sty; to appear in
"IAU Symposium 220, Dark Matter in Galaxies", ed. Ryder, Pisano, Walker, and
Freeman, San Francisco: ASP, in pres
Evidence of Early Enrichment of the Galactic Disk by Large-Scale Winds
Large-scale homogeneous surveys of Galactic stars may indicate that the
elemental abundance gradient evolves with cosmic time, a phenomenon that was
not foreseen in existing models of Galactic chemical evolution (GCE). If the
phenomenon is confirmed in future studies, we show that this effect, at least
in part, is due to large-scale winds that once enriched the disk. These set up
the steep abundance gradient in the inner disk (R <14 kpc). At the close of the
wind phase, chemical enrichment through accretion of metal-poor material from
the halo onto the disk gradually reduced the metallicity of the inner region,
whereas a slow increase in the metallicity proceeded beyond the solar circle.
Our "wind+infall" model accounts for flattening of the abundance gradient in
the inner disk, in good agreement with observations. Accordingly, we propose
that enrichment by large-scale winds is a crucial factor for chemical evolution
in the disk. We anticipate that rapid flattening of the abundance gradient is
the hallmarks of disk galaxies with significant central bulges.Comment: 9 pages including 5 figures, accepted for publication in PAS
Structure and kinematics of edge-on galaxy discs -- V. The dynamics of the stellar discs
In earlier papers in this series we determined the intrinsic stellar disc
kinematics of fifteen intermediate to late type edge-on spiral galaxies using a
dynamical modeling technique. From the photometry we find that intrinsically
more flattened discs tend to have a lower face-on central surface brightness
and a larger dynamica mass-to-light ratio. This observation suggests that at a
constant maximum rotational velocity lower surface brightness discs have
smaller vertical stellar velocity dispersions.Although the individual
uncertainties are large, we find from the dynamical modeling that at least
twelve discs are submaximal. The average disc contributes 534 percent to
the observed rotation at 2.2 disc scalelengths, with a 1 scatter of 15
percent. This percentage becomes somewhat lower when effects of finite disc
flattening and gravity by the dark halo and the gas are taken into account.
Since boxy and peanut-shaped bulges are probably associated with bars, the
result suggests that at 2.2 the submaximal nature of discs is
independent of barredness. The possibility remains that very high surface
brightness discs are maximal.We confirm that the radial stellar disc velocity
dispersion is related to the galaxy maximum rotational velocity. The scatter in
this relation appears to correlate with the disc
flattening, face-on central surface brightness and dynamical mass-to-light
ratio. Low surface brightness discs tend to be more flattened and have smaller
stellar velocity dispersions. The findings are consistent with the observed
correlation between disc flattening and dynamical mass-to-light ratio.Comment: Accepted for publication by Mon. Not. R.A.
Stable quantum memories with limited measurement
We demonstrate the existence of a finite temperature threshold for a 1D
stabilizer code under an error correcting protocol that requires only a
fraction of the syndrome measurements. Below the threshold temperature, encoded
states have exponentially long lifetimes, as demonstrated by numerical and
analytical arguments. We sketch how this algorithm generalizes to higher
dimensional stabilizer codes with string-like excitations, like the toric code.Comment: 11 Pages, 7 Figure
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