\newcommand{\hmsun}{h^{-1}{\rm M}_\odot} \newcommand{\hmpc}{h^{-1}{\rm Mpc}} \newcommand{\hkpc}{h^{-1}{\rm kpc}} \newcommand{\msun}{{\rm M}_\odot} \newcommand{\mpc}{{\rm Mpc}} \newcommand{\kpc}{{\rm kpc}} \newcommand{\kms}{{\rm ~km~s^{-1}}} \documentclass{article} \usepackage{amsfonts,amsmath,amssymb} \usepackage{graphicx} \usepackage{color} \usepackage{url} \bibliographystyle{plain}