Unveiling Kinematic Structure in the Starburst Heart of NGC 253
Abstract
We investigate the kinematics of ionized gas within the nuclear starburst of NGC 253 with observations of the Brackett α recombination line at 4.05 μm. 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 5''× 25'' (85 × 425 pc) region to produce a spectral cube. The Brα 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 vprimary90-130 km s-1 (FWHM) on top of a broad blue wing with vbroad300-350 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 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 100-250 km s-1 starburst-driven outflow potentially responsible for powering the kpc-scale galactic wind of NGC 253.