The growth history of local M 33-mass bulgeless spiral galaxies

NGC 7793, NGC 300, M 33, and NGC 2403 are four nearby undisturbed and bulgeless low-mass spiral galaxies whose morphology and stellar mass are similar. They are ideal laboratories for studying disc formation scenarios and the histories of stellar mass growth. We constructed a simple chemical evolution model by assuming that discs grow gradually with continuous metal-free gas infall and metal-enriched gas outflow. By means of the classical chi(2) method, applied to the model predictions, the best combination of free parameters capable of reproducing the corresponding present-day observations was determined, that is, the radial dependence of the infall timescale tau = 0.1r/R-d + 3.4 Gyr (R-d is the disc scale length) and the gas outflow efficiency b(out) = 0.2. The model results agree excellently with the general predictions of the inside-out growth scenario for the evolution of spiral galaxies. About 80% of the stellar mass of NGC 7793 was assembled within the last 8 Gyr, and 40% of the mass was assembled within the last 4 Gyr. By comparing the best-fitting model results of the three other galaxies, we obtain similar results: 72% (NGC 300), 66% (NGC 2403), and 79% (M 33) of the stellar mass were assembled within the last similar to 8 Gyr (i.e. z = 1). These four disc galaxies simultaneously increased their sizes and stellar masses in time, and they grew in size at similar to 0.30 times the rate at which they grew in mass. The scale lengths of these four discs now are 20%-25% larger than at z = 1. Our best-fitting model predicted the stellar mass-metallicity relation and the metallicity gradients, constrained by the observed metallicities from HII-region emission line analysis, agree well with the observations measured from individual massive red and blue supergiant stars and population synthesis of Sloan Digital Sky Survey galaxies.