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.
