r    = 20.*1.e3*pc                 ; radius of galaxy
t    = 2.e7                        ; temperature of gas
mu   = 0.5
mass = (2.*boltz*t)/(g*m_h*mu)*r/msun
print,' mass = ',mass,' solar masses = ',mass*msun/1.d3,' kg'
;
sigma0 = sqrt((boltz * t)/(m_h*mu))
print,' sigma = ',sigma0/km

; escape velocity
r_0 = 10.*kpc
r_g = 20.*kpc
ve  = sqrt(4.*sigma0^2*(1.+alog(r_g/r_0)))/km
print,' escape velocity = ',ve,' km/s',' at ',r_0/kpc,' kpc'