Real and counterfeit cores: how feedback expands halos and disrupts tracers of inner gravitational potential in dwarf galaxies

Abstract

The tension between the diverging density profiles in Lambda Cold Dark Matter ($\Lambda$CDM) simulations and the constant-density inner regions of observed galaxies is a long-standing challenge known as the `core-cusp' problem. We demonstrate that the \texttt{SMUGGLE} galaxy formation model implemented in the \textsc{Arepo} moving mesh code forms constant-density cores in idealized dwarf galaxies of $M_{\star }\approx 8\times 10^7$ M${}_{\odot }$ with initially cuspy dark matter halos of $M_{200}\approx 10^{10}$ M${}_{\odot }$. Identical initial conditions run with the Springel and Hernquist (2003; SH03) feedback model preserve cuspiness. Literature on the subject has pointed to the low density threshold for star formation, ${\rho }_{\text{th}}$, in SH03-like models as an obstacle to baryon-induced core formation. Using a \texttt{SMUGGLE} run with equal ${\rho }_{\text{th}}$ to SH03, we demonstrate that core formation can proceed at low density thresholds, indicating that ${\rho }_{\text{th}}$ is insufficient on its own to determine whether a galaxy develops a core. We suggest that the ability to resolve a multiphase interstellar medium at sufficiently high densities is a more reliable indicator of core formation than any individual model parameter. In \texttt{SMUGGLE}, core formation is accompanied by large degrees of non-circular motion, with gas rotational velocity profiles that consistently fall below the circular velocity $v_{\text{circ}}=\sqrt{GM/R}$ out to $\sim 2$ kpc. This may artificially mimic larger core sizes when derived from observable quantities compared to the size measured from the dark matter distribution ($\sim 0.5$ kpc), highlighting the need for careful modeling in the inner regions of dwarfs to infer the true distribution of dark matter.

18 pages, 15 figures, submitted to MNRAS

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