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 53$\pm$4 percent to the observed rotation at 2.2 disc scalelengths, with a 1$\sigma$ 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$h_{\rm R}$ 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 $\sigma-v_{\rm max}$ 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