\newcommand{\Msun}{\mathrm{M}_\odot}
\newcommand{\Mjup}{\mathrm{M_{Jup}}}
\newcommand{\AU}{\mathrm{AU}}
\newcommand{\yr}{\mathrm{yr}}

\def\t{\thinspace}
\def\msol{{\rm\thinspace M${}_\odot$}}
\def\fig{{\rm\thinspace Figure}}
\def\kms{{km s${}^{-1}$}}
\def\etal{{\it et al.\thinspace}}
\def\eg{{\it e.g.\ }}
\def\etc{{\it etc.\ }}
\def\ie{{\it i.e.\ }}
\def\cf{{\it c.f.\ }}
\def\msolyr{{\rm\thinspace M${}_\odot\; {\rm yr}^{-1}$}}
\def\gcm3{{g cm${}^{-3}$}}
%       Simple units
\def\arcsec{\hbox{$^{\prime\prime}$}}
\def\cm{{\rm\thinspace cm}}
\def\deg{^o}
\def\erg{{\rm\thinspace erg}}
\def\eV{{\rm\thinspace eV}}
\def\g{{\rm\thinspace g}}
\def\ga{{\rm\thinspace gauss}}
\def\K{{\rm\thinspace K}}
\def\keV{{\rm\thinspace keV}}
         \def\km{{\rm\thinspace km}}
\def\kpc{{\rm\thinspace kpc}}
\def\Lsun{\hbox{$\rm\thinspace L_{\odot}$}}
\def\m{{\rm\thinspace m}}
\def\mag{{\rm\thinspace mag}}
\def\Mpc{{\rm\thinspace Mpc}}
\def\Msun{\hbox{$\rm\thinspace M_{\odot}$}}
\def\msol{\hbox{$\rm\thinspace M_{\odot}$}}
\def\msolyr{\hbox{$\rm\thinspace M_{\odot} \; {\rm yr}^{-1}$}}
\def\pc{{\rm\thinspace pc}}
\def\s{{\rm\thinspace s}}
\def\st{{\rm\thinspace ster}}
\def\yr{{\rm\thinspace yr}}
\def\yrs{{\rm\thinspace yrs}}
\def\h50{\hbox{$\rm\thinspace h_{50}$}}
\def\h50m1{\hbox{$\rm\thinspace h_{50}^{-1}$}}
%       Compound units
\def\cmps{\hbox{$\cm\s^{-1}\,$}}
\def\col{\hbox{$\cm^{-2}\,$}}
\def\ergpcmps{\hbox{$\erg\cm^{-3}\s^{-1}\,$}}
\def\ergps{\hbox{$\erg\s^{-1}\,$}}
\def\flux{\hbox{$\keV\cm^{-2}\s^{-1}\st^{-1}\keV^{-1}\,$}}
\def\gpcm{\hbox{$\g\cm^{-3}\,$}}
\def\gpcmps{\hbox{$\g\cm^{-3}\s^{-1}\,$}}
\def\gps{\hbox{$\g\s^{-1}\,$}}
\def\kmps{\hbox{$\km\s^{-1}\,$}}
\def\kmpspMpc{\hbox{$\km\s^{-1}\Mpc^{-1}\,$}}
\def\Lsunppc{\hbox{$\Lsun\pc^{-3}\,$}}
\def\Msunpc{\hbox{$\Msun\pc^{-3}\,$}}
\def\Msunpkpc{\hbox{$\Msun\kpc^{-1}\,$}}
\def\Msunppc{\hbox{$\Msun\pc^{-3}\,$}}
\def\Msunppcpyr{\hbox{$\Msun\pc^{-3}\yr^{-1}\,$}}
\def\Msunpyr{\hbox{$\Msun\yr^{-1}\,$}}
\def\Msunpyrpkpc{\hbox{$\Msun\yr^{-1}\kpc^{-1}\,$}}
\def\pcm{\hbox{$\cm^{-3}\,$}}
\def\pcmK{\hbox{$\cm^{-3}\K$}}
\def\pyr{\hbox{$\yr^{-1}\,$}}
 
\def\Msun{\mathop{\rm M_{\odot}}\nolimits}
\def\Lsun{\mathop{\rm M_{\odot}}\nolimits}
\def\keV{\mathop{\rm keV}\nolimits}
\def\Mpc{\mathop{\rm Mpc}\nolimits}
\def\kpc{\mathop{\rm kpc}\nolimits}
\def\erg{\mathop{\rm erg}\nolimits}
\def\s{\mathop{\rm s}\nolimits}
\def\K{\mathop{\rm K}\nolimits}
%\def\Ga{\mathop{\rm Ga}\nolimits}
\def\Ga{\mathop{\times 10^9 \yrs}\nolimits}
\def\yrs{\mathop{\rm yrs}\nolimits}
\def\cm{\mathop{\rm cm}\nolimits}
\def\munit{\mathop{\rm munit}\nolimits}
\def\kunit{\mathop{\rm kunit}\nolimits}
\def\k{\mathop{\rm k}\nolimits}
\def\km{\mathop{\rm km}\nolimits}
\def\G{\mathop{\rm G}\nolimits}
\def\g{\mathop{\rm g}\nolimits}
\def\kps{\mathop{\rm km/s}\nolimits}
\def\msol{\hbox{$\rm\thinspace M_{\odot}$}}
\def\etal{{\it et al.\thinspace}}
\def\eg{{\it e.g.\ }}
\def\etc{{\it etc.\ }}
\def\ie{{\it i.e.\ }}
\def\cf{{\it c.f.\ }}
\def\fig{figure}
\def\Fig{Fig.\thinspace}
\def\figs{figures}
\def\Figs{Figs.\thinspace}
\def\p3m{P${}^3$M}
\def\ap3m{AP${}^3$M}
\def\-{{\em{---}}}
\def\Hydra{{\small HYDRA} }
\newcommand{\expd}[1]{
 \times 10^{#1}
}
 
\newcommand{\mean}[1]{
 \langle #1 \rangle
}
 
\def\bmath{\boldmath}
\def\shortcite{\cite}
\def\nabr{{\nabla_{\bf r}}}
\def\nabx{{\nabla_{\bf x}}}
\def\nabi{{\nabla_i}}
\def\vx{{{\bf v_x}}}
\def\vr{{{\bf v_r}}}
\def\Wr{{W({\bf r}_{ij},h_i)}}
\def\Wx{{W({\bf x}_{ij},h_i^c)}}
\def\dxij{{\,\Delta {\bf x}_{ij}}}
\def\dvxij{{\,\Delta {\bf v_x}_{ij}}}
\def\drij{{\,\Delta {\bf r}_{ij}}}
\def\dvij{{\,\Delta {\bf v}_{ij}}}
\def\br{{{\bf r}}}
\def\bv{{{\bf v}}}
\def\bx{{{\bf x}}}
\def\bvx{{{\bf v_x}}}
\def\tot{{{2 \over 3}}}
\def\prho{{{P_i \over \rho_i^2}}}
\def\prhoc{{{P_i^c \over \rho_i^{c^2}}}}
\def\do{{\tt DO }}
\def\accel{{\bf accel }}
\def\force{{\bf force }}
\def\shgravsph{{\bf shgravsph }}
\def\updaterv{{\bf updaterv }}
\def\refforce{{\bf refforce }}
\def\rfinit{{\bf rfinit }}
\def\uload{{\bf uload }}
\def\load{{\bf load}}
\newcommand{\bmh}[1]{\boldmath{#1}}





\newcommand\Hmol{{\rm\,H_2}} 
\newcommand\Hatom{{\rm\,HI}} 
\newcommand\SSSMC{{\rm\,S^{3}MC}} 
\newcommand\XCO{{X_{\rm\,CO}}}