Effect of the Remnant Mass in Estimating the Stellar Mass of Galaxies

Abstract

Abstract The definition of the galactic stellar mass estimated from the spectral energy distribution is ambiguous in the literature; whether the stellar mass includes the mass of the stellar remnants, i.e., white dwarfs, neutron stars, and black holes, is not well described. The remnant mass fraction in the total (living + remnant) stellar mass of a simple stellar population monotonically increases with the age of the population, and the initial mass function and metallicity affect the increasing rate. Since galaxies are composed of a number of stellar populations, the remnant mass fraction may depend on the total stellar mass of galaxies in a complex way. As a result, the shape of the stellar mass function of galaxies may change, depending on the definition of the stellar mass. In order to explore this issue, we ran a cosmological hydrodynamical simulation, and subsequently found that the remnant mass fraction indeed correlates with the total stellar mass of galaxies. However, this correlation is weak, and the remnant fraction can be regarded as a constant that depends only on the redshift. Therefore, the shape of the stellar mass function is almost unchanged, but it simply shifts horizontally if the remnant mass is included or not. The shift is larger at lower redshift, and it reaches 0.2-dex at z = 0 for a Chabrier IMF. Since this causes a systematic difference, we should take care of the definition of the ‘stellar’ mass, when comparing one's result with others.