def R_sfg_halflight(mstar, alpha=0.115, beta=0.898, gamma=0.199, M0=3.016e10):
    """
    Half-light radius of star-forming galaxies, assuming exponential profile. 
    Taken from Eq. 15 of Bonaldi et al. (arXiv:1805.05222).
    
    Parameters
    ----------
    mstar : array_like
        Stellar mass, in Msun.
    
    alpha : float, optional
        Scaling parameter. Default: 0.115.
    
    beta : float, optional
        Second scaling parameter. Default: 0.898.
    
    gamma : float, optional
        Overall amplitude parameter. Default: 0.199.
    
    M0 : float, optional
        Reference mass in correction factor, in Msun. Default: 3.016e10.
    
    Returns
    -------
    R : array_like, float
        Half-light radius of galaxy, in kpc.
    """
    return gamma * mstar**alpha * (1. + mstar/M0)**(beta - alpha)