Physical Conditions in the Narrow-Line Region of Markarian 3. I. Observational Results

We use Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) long-slit low-resolution spectroscopy from 1150 to 10300 Å to study the physical conditions in the narrow-line region (NLR) of the Seyfert 2 galaxy Markarian 3. We find from the He II λ1640/λ4686 line ratio and the Balmer decrement that the extinction within Mrk 3 along the line of sight to the NLR is best characterized by an LMC-type extinction curve. We observe an extinction gradient increasing from west to east along the STIS slit (at position angle 71° measured east from north) in both line and continuum emission. We infer from this gradient that the host galaxy disk is tilted toward the observer in the east: the line of sight to the eastern emission-line cone intersects more dust in the plane of the galaxy than that to the western cone. From emission-line diagnostics we find that the NLR gas is photoionized by the hidden active galactic nucleus continuum and that its density decreases with increasing distance from the center. We model the observed continuum as a combination of reddened host galaxy light from an old stellar population, reddened H+ and He+2 recombination continua, and less reddened scattered light from the central engine with spectral index α = 1 (Lν ∝ ν-α). The host galaxy-to-scattered light ratio is estimated to be 3 : 1 at 8125 Å in a 0.″1 × 1.″8 aperture. Using a two-component power-law model for the ionizing continuum (α = 2 for 13.6 eV < E < 0.2 keV and α = 1 for 0.2 keV < E < 50 keV), we find that the covering factor (normalized for our observation aperture) of the NLR gas is greater than 0.7%. We estimate that the amount of intrinsic nonionizing UV continuum scattered into our line of sight is 0.04%. This is consistent with our estimate of the scattering fraction for broad C IV λλ1548, 1551 emission.