Disk Formation and the Size-sSFR Relation of Dwarf Galaxies

TL;DR

This study uses high-resolution simulations to explore how isolated dwarf galaxies form disks and grow in size, highlighting the role of mergers with high angular momentum satellites in their evolution.

Contribution

It reveals that stellar disk growth through high angular momentum mergers is key to size evolution in dwarf galaxies, a process not well understood before.

Findings

Over half of the simulated dwarfs grow in size through disk formation.

High angular momentum mergers contribute about 30% of cold gas at z=0.

Disk growth correlates with secular star formation in angular momentum-supported gas.

Abstract

Dwarf galaxies are dark matter-dominated systems that are sensitive to feedback and display a diversity of baryonic morphologies. This makes them excellent probes for understanding dark matter and galaxy evolution. This work investigates the physical processes that influence the sizes of isolated dwarf galaxies using high-resolution cosmological zoom-in simulations of $39$ dwarf galaxies drawn from the Marvelous Massive Dwarfs simulation suite ($7.5 < \log(M_{\star}/M_{\odot}) < 9.1$). Our simulations show that dwarf galaxies initially form as compact galaxies ($R_e < 2$ kpc). However, several of these galaxies ($54\%$) experience periods of gradual size growth at relatively stable sSFR, allowing them to become extended galaxies. We find that the growth of rotation-supported stellar disks is the primary means by which isolated dwarfs become extended in size. These stellar disks are formed by mergers with high orbital angular momentum satellites on high angular momentum (spiraling-in) orbits, which spin up the gas surrounding the central galaxy and contribute $\approx 30 \%$ of the cold gas mass at $z=0$. For these systems, star formation in the angular momentum supported gas and the gradual build up of stars in the disk result in secular size growth.