The Origin of the Light Distribution in Spiral Galaxies

TL;DR

This study uses a detailed cosmological simulation to investigate the origin of double-exponential light profiles in spiral galaxy discs, revealing that star formation decline and stellar migration shape these features.

Contribution

It provides new insights into the formation of light profile breaks, emphasizing the roles of gas disc warping and stellar radial migration, with implications for understanding galaxy evolution.

Findings

Break in light profile linked to gas disc warping and star formation decline.

Majority of stars beyond the break migrated from inner disc.

No significant difference in break features with galaxy inclination.

Abstract

We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy simulation, to study the source of the double-exponential light profiles seen in many stellar discs, and the effects of stellar radial migration upon the spatio-temporal evolution of both the disc age and metallicity distributions. We find a "break" in the pure exponential stellar surface brightness profile, and trace its origin to a sharp decrease in the star formation per unit surface area, itself produced by a decrease in the gas volume density due to a warping of the gas disc. Star formation in the disc continues well beyond the break. We find that the break is more pronounced in bluer wavebands. By contrast, we find little or no break in the mass density profile. This is, in part, due to the net radial migration of stars towards the external parts of the disc. Beyond the break radius, we find that ~60% of the resident stars migrated from the inner disc, while ~25% formed in situ. Our simulated galaxy also has a minimum in the age profile at the break radius but, in disagreement with some previous studies, migration is not the main mechanism producing this shape. In our simulation, the disc metallicity gradient flattens with time, consistent with an "inside-out" formation scenario. We do not find any difference in the intensity or the position of the break with inclination, suggesting that perhaps the differences found in empirical studies are driven by dust extinction.