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 χ 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 τ = 0.1 r / 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 ∼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 ∼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.