Declining rotation curves at $z=2$ in $\Lambda$CDM galaxy formation simulations

TL;DR

This study shows that declining rotation curves and low dark matter fractions in $z=2$ disk galaxies naturally occur in $ ext{Lambda}$CDM simulations, aligning with recent observations and highlighting the role of baryonic physics during peak star formation.

Contribution

It demonstrates that such galaxy features at $z=2$ are consistent with $ ext{Lambda}$CDM and baryonic physics, providing a theoretical explanation for observed phenomena.

Findings

Nearly half of $z=2$ disk galaxies show declining rotation curves.

Galaxies with these features evolve into diverse morphological types by $z=0$.

Some dispersion-dominated galaxies at $z=2$ have similar rotation curves to disks.

Abstract

Selecting disk galaxies from the cosmological, hydrodynamical simulation Magneticum Pathfinder we show that almost half of our poster child disk galaxies at $z=2$ show significantly declining rotation curves and low dark matter fractions, very similar to recently reported observations. These galaxies do not show any anomalous behavior, reside in standard dark matter halos and typically grow significantly in mass until $z = 0$, where they span all morphological classes, including disk galaxies matching present day rotation curves and observed dark matter fractions. Our findings demonstrate that declining rotation curves and low dark matter fractions in rotation dominated galaxies at $z=2$ appear naturally within the $\Lambda$CDM paradigm and reflect the complex baryonic physics, which plays a role at the peak epoch of star-formation. In addition, we find some dispersion dominated galaxies at $z=2$ which host a significant gas disk and exhibit similar shaped rotation curves as the disk galaxy population, rendering it difficult to differentiate between these two populations with currently available observation techniques.