HST/ACS EMISSION LINE IMAGING OF LOW-REDSHIFT 3CR RADIO GALAXIES. I. THE DATA

We present 19 nearby (z < 0.3) 3CR radio galaxies imaged at low and high excitation as part of a Cycle 15 Hubble Space Telescope (HST) snapshot survey with the Advanced Camera for Surveys (ACS). These images consist of exposures of the H alpha (6563 angstrom, plus [N II] contamination) and [O III]lambda 5007 emission lines using narrowband linear ramp filters adjusted according to the redshift of the target. To facilitate continuum subtraction, a single-pointing 60 s line-free exposure was taken with a mediumband filter appropriate for the target's redshift. We discuss the steps taken to reduce these images independently of the automated recalibration pipeline so as to use more recent ACS flat-field data as well as to better reject cosmic rays. We describe the method used to produce continuum-free (pure line-emission) images, and present these images along with qualitative descriptions of the narrow-line region morphologies we observe. We present H alpha+[N II] and [O III] line fluxes from aperture photometry, finding the values to fall expectedly on the redshift-luminosity trend from a past HST/WFPC2 emission line study of a larger, generally higher redshift subset of the 3CR. We also find expected trends between emission line luminosity and total radio power, as well as a positive correlation between the size of the emission line region and redshift. We discuss the associated interpretation of these results, and conclude with a summary of future work enabled by this data set.