The growth history of local M33-mass bulgeless spiral galaxies

TL;DR

This study models the growth and chemical evolution of four nearby bulgeless spiral galaxies, showing they grow inside-out with significant stellar mass assembled in the last 8 Gyr, aligning well with observational metallicity data.

Contribution

It introduces a simple chemical evolution model with specific infall and outflow parameters that accurately reproduces observed properties of bulgeless spiral galaxies.

Findings

80% of NGC7793's stellar mass formed in the last 8 Gyr

Galaxies grow in size at about 0.30 times their mass growth rate

Model predictions match observed metallicity gradients and relations

Abstract

NGC7793, NGC300, M33 and NGC2403 are four nearby undisturbed and bulgeless low-mass spiral galaxies with similar morphology and stellar mass. They are ideal laboratories to study disc formation scenarios and stellar mass growth histories. We construct 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 $\chi^{2}$ methodology, applied to the model predictions, the best combination of free parameters capable of reproducing the corresponding present-day observations is determined, i.e. the radial dependence of the infall timescale $\tau=0.1r/{R_{\rm d}}+3.4\,{\rm Gyr}$ ($R_{\rm d}$ is the disc scale-length) and the gas outflow efficiency $b_{\rm out}=0.2$. The model results are in excellent agreement 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 is assembled within the last 8\,Gyr and 40\% within the last 4\,Gyr. By comparing the best-fitting model results of the three other galaxies we obtain similar results, 72\% (NGC300), 66\% (NGC2403) and 79\% (M33) stellar mass were assembled within the past $\sim\rm 8\,Gyr$ (i.e. $z\,=\,1$). These four disc galaxies simultaneously increase their sizes and stellar masses as time goes by and they grow in size at $\sim\,0.30$ times the rate at which they grow in mass. The scale-lengths of these four discs are now 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-regions emission line analysis, agree well with the observations measured from individual massive red and blue supergiant stars and population synthesis of SDSS galaxies.