Simulated Dark-Matter Halos as a Test of Nonextensive Statistical Mechanics

In the framework of nonextensive statistical mechanics, the equilibrium structures of astrophysical self-gravitating systems are stellar polytropes, parameterized by the polytropic index n. By careful comparison to the structures of simulated dark-matter halos we find that the density profiles, as well as other fundamental properties, of stellar polytropes are inconsistent with simulations for any value of n. This result suggests the need to reconsider the applicability of nonextensive statistical mechanics (in its simplest form) to equilibrium self-gravitating systems.Comment: Accepted for publication in Physical Review

A search for disk-galaxy lenses in the Sloan Digital Sky Survey

We present the first automated spectroscopic search for disk-galaxy lenses, using the Sloan Digital Sky Survey database. We follow up eight gravitational lens candidates, selected among a sample of ~40000 candidate massive disk galaxies, using a combination of ground-based imaging and long-slit spectroscopy. We confirm two gravitational lens systems: one probable disk galaxy, and one probable S0 galaxy. The remaining systems are four promising disk-galaxy lens candidates, as well as two probable gravitational lenses whose lens galaxy might be an S0 galaxy. The redshifts of the lenses are z_lens ~ 0.1. The redshift range of the background sources is z_source ~ 0.3 - 0.7. The systems presented here are (confirmed or candidate) galaxy-galaxy lensing systems, that is, systems where the multiple images are faint and extended, allowing an accurate determination of the lens galaxy mass and light distributions without contamination from the background galaxy. Moreover, the low redshift of the (confirmed or candidates) lens galaxies is favorable for measuring rotation points to complement the lensing study. We estimate the rest-frame total mass-to-light ratio within the Einstein radius for the two confirmed lenses: we find M_tot/L_I = 5.4 +- 1.5 within 3.9 +- 0.9 kpc for SDSS J081230.30+543650.9, and M_tot/L_I = 1.5 +- 0.9 within 1.4 +- 0.8 kpc for SDSS J145543.55+530441.2 (all in solar units). Hubble Space Telescope or Adaptive Optics imaging is needed to further study the systems.Comment: ApJ, accepte