Spatially Resolved Ultraviolet Spectroscopy of the Liner Galaxy NGC 3998

Abstract

We report results of recent IUE observations of the low-ionization nuclear emission-line region (LINER) galaxy NGC 3998. Spatial structure in the long-wavelength emission is apparent; the spatial profiles in the LWP images show a two-component structure, with an unresolved central component superposed on extended underlying emission. We have used new software, which exploits knowledge of the LWP point spread function and its variation with wavelength, to model the spatial profiles and to separate the LWP emission into component spectra. The profiles can be modeled as the sums of unresolved (point source) components and extended Gaussians having intrinsic sigmas of ~4.5". The centers of the unresolved and extended components coincide to within 0.2". The unresolved component spectrum is considerably flatter than the spectrum of the extended component, which is dominated by fairly late-type stars (late F to early G). The unresolved component also shows strong C II] λ2326 and broad Mg II λ280O emission. The width of the Mg II emission is ~8000 km s^-1^, and its profile agrees quite well with the broad wings of the Hα emission. The prominence of Mg II relative to C Il] indicates that the broad-line emitting gas must be dense (n_e_ ~> 10^9^ cm^-3^. Finally, the spectrum of the unresolved component is also dominated by starlight, but allows a photoionizing power-law continuum of sufficient flux to account for the emission-line fluxes if the emission-line spectrum is similar to that seen in M81. These results strengthen the resemblance between NGC 3998 and M81, and further support the idea that NGC 3998 may contain a "dwarf" Seyfert nucleus.