Unveiling Kinematic Structure in the Starburst Heart of NGC 253

We investigate the kinematics of ionized gas within the nuclear starburst of NGC 253 with observations of the Brackett $\alpha$ recombination line at 4.05 $\mu$m. The goal is to distinguish motions driven by star-formation feedback from gravitational motions induced by the central mass structure. Using NIRSPEC on Keck II, we obtained 30 spectra through a $0''.5$ slit stepped across the central $\sim$5$''\times 25''$ (85 $\times$ 425 pc) region to produce a spectral cube. The Br$\alpha$ emission resolves into four nuclear sources: S1 at the infrared core (IRC), N1 at the radio core near nonthermal source TH2, and the fainter sources N2 and N3 in the northeast. The line profile is characterized by a primary component with $\Delta v_{\mathrm{primary}}$$\sim$90-130 km s$^{-1}$ (FWHM) on top of a broad {blue} wing with $\Delta v_{\mathrm{broad}}$$\sim$300-350 km s$^{-1}$, and an additional redshifted narrow component in the west. The velocity field generated from our cube reveals several distinct patterns. A mean NE-SW velocity gradient of +10 km s$^{-1}$ arcsec$^{-1}$ along the major axis traces the solid-body rotation curve of the nuclear disk. At the radio core, isovelocity contours become S-shaped, indicating the presence of secondary nuclear bar of total extent $\sim$5$''$ (90 pc). The symmetry of the bar places the galactic center near the radio peak TH2 of the galaxy rather than the IRC, and makes this the most likely location of a SMBH. A third kinematic substructure is formed by blueshifted gas on the southeast side of the IRC. This feature provides evidence for a $\sim$100-250 km s$^{-1}$ starburst-driven outflow potentially responsible for powering the kpc-scale galactic wind of NGC 253.