Feedback and the fudamental line of low-luminosity LSB/dwarf galaxies

Abstract

We study in simple terms the role of feedback in establishing the scaling relations of low-surface-brightness and dwarf galaxies with stellar masses in the range 6x10^5 <M*< 3x10^10 Msun. These galaxies, as measured from SDSS and in the Local Group, show tight correlations of internal velocity, metallicity and surface brightness (or radius) with M*. They define a fundamental line distinguishing them from the brighter galaxies of high surface brightness and metallicity. The idealized model assumes spherical collapse of CDM haloes to virial equilibrium and angular-momentum conservation. The relations for bright galaxies are reproduced by assuming that M* is a constant fraction of the halo mass M. The upper bound to the low-luminosity LSBs coincides with the virial velocity of haloes in which supernova feedback could significantly suppress star formation, V<100km/s (Dekel & Silk 1986). We argue that the energy fed to the gas obeys E_sn \prop M* despite the radiative losses, and equate it with the binding energy of the gas to obtain M*/M \prop V^2. This idealized model provides surprisingly good fits to the scaling relations of low-luminosity LSBs and dwarfs, which indicates that supernova feedback had a primary role in determining the fundamental line. The apparent lower bound for galaxies at V~10km/s may be due to the cooling barrier at T~10^4 K. Some fraction of the dark haloes may show no stars due to complete gas removal either by supernova winds from neighboring galaxies or by radiative feedback after cosmological reionization at z_ion. Radiative feedback may also explain the distinction between dwarf spheroidals (dE) and irregulars (dI), where the dEs of V<30km/s form stars before z_ion and are then cleaned out of gas, while the dIs with V>30km/s retain gas-rich discs with feedback-regulated star formation.