Galactic Dynamics Drives Cyclic Star Formation in Dipole Dwarf Galaxies

Abstract

Dwarf galaxies are dark matter-dominated systems that are sensitive to feedback and display bursty star formation histories. This makes them ideal probes of the processes that regulate star formation. This work investigates the timescales of star formation rate (SFR) variability in isolated dwarf galaxies using high-resolution cosmological zoom-in simulations of $50$ dwarf galaxies drawn from the Marvelous Massive Dwarfs and Extended Marvelous Dwarfs simulation suites ($7.3 <$ \lmstar $< 9.1$). We measure star formation histories in $10$ Myr bins and characterize their variability with power spectral densities (PSDs). At the population level, the PSDs of both disk and non-disk galaxies follow a broken power-law, with a break timescale near $100$ Myr. Individually, our galaxies show both smooth and bursty star formation. A subset of disk galaxies show quasi-periodic SFR oscillations about their mean, which we refer to as cyclic star formation. The dominant timescale of these oscillations scales linearly with half-light radius and is approximated by the orbital period at that radius. Cyclic galaxies have Magellanic Spiral (Sm) morphologies and host lopsided dipole gas disks. In these systems, the bulk of the gas in the lopsided disk moves between its maximum and minimum distance from the galactic center, compressing the star-forming gas once per radial orbit and driving the SFR cycle. Cyclic star formation adds a narrow Lorentzian component to the PSD, peaked at radial orbital frequency of the disk ($\sim 200-500$ Myr), which is comparable to burst timescales inferred from observations.