Abstract:
The most rapidly evolving regions of galaxies often display complex optical spectra with
emission lines excited by massive stars, shocks and accretion on to supermassive black holes.
Standard calibrations (such as for the star formation rate) cannot be applied to such mixed
spectra. In this paper, we isolate the contributions of star formation, shock excitation and
active galactic nucleus (AGN) activity to the emission line luminosities of individual spatially
resolved regions across the central 3 × 3 kpc2 region of the active barred spiral galaxy NGC
613. The star formation rate and AGN luminosity calculated from the decomposed emission
line maps are in close agreement with independent estimates from data at other wavelengths.
The star formation component traces the B-band stellar continuum emission, and the AGN
component forms an ionization cone which is aligned with the nuclear radio jet. The optical line
emission associated with shock excitation is cospatial with strong H2 and [Fe II] emission and
with regions of high ionized gas velocity dispersion (σ 100 km s−1). The shock component
also traces the outer boundary of the AGN ionization cone and may therefore be produced by
outflowing material interacting with the surrounding interstellar medium. Our decomposition
method makes it possible to determine the properties of star formation, shock excitation and
AGN activity from optical spectra, without contamination from other ionization mechanisms.
