def compute_U(Sigma_sfr, Sigma_g):
    """
    Ionizaton parameter as a function of star formation and gas surface densities. 
    reference:
      Eq. 38 Ferrara et al. 2019
    inputs:
      Sigma_sfr in Msun/yr/kpc^2
      Sigma_g   in Msun/kpc^2
    """
    out= (1.7e+14)*(Sigma_sfr/(Sigma_g*Sigma_g))
    return out