CONTINUUM SUBTRACTING LYMAN-ALPHA IMAGES: LOW-REDSHIFT STUDIES USING THE SOLAR BLIND CHANNEL OF HST/ACS

We are undertaking an imaging study of local star-forming galaxies in the Lyman-alpha (Ly alpha) emission line using the Solar Blind Channel (SBC) of the Advanced Camera for Surveys onboard the Hubble Space Telescope. Observations have been obtained in Ly alpha and H-alpha (H alpha) and six line-free continuum filters between similar to 1500 angstrom and the I band. In a previous article, we demonstrated that the production of Ly alpha line-only images (i.e., continuum subtraction) in the SBC-only data set is nontrivial and that supporting data is a requirement. We here develop various methods of continuum subtraction and assess their relative performance using a variety of spectral energy distributions (SEDs) as input. We conclude that simple assumptions about the behavior of the ultraviolet continuum consistently lead to results that are wildly erroneous, and determine that a spectral fitting approach is essential. Moreover, fitting of a single component stellar or stellar+nebular spectrum is not always sufficient for realistic template SEDs and, in order to successfully recover the input observables, care must be taken to control the contribution of nebular gas and any underlying stellar population. Independent measurements of the metallicity must first be obtained, while details of the initial mass function play only a small role. We identify the need to bin together pixels in our data to obtain signal-to-noise ratios (S/Ns) of around 10 in each band before processing. At S/N = 10, we are able to recover Ly alpha fluxes accurate to within around 30% for Ly alpha lines with intrinsic equivalent width (W-Ly alpha) of 10 angstrom. This accuracy improves to less than or similar to 10% for W-Ly alpha = 100 angstrom. We describe the method of image processing applied to the observations presented in Ostlin et al. and the associated data release. We also present simulations for an observing strategy for an alternative low-redshift Ly alpha imaging campaign using ACS/SBC using adjacent combinations of long-pass filters to target slightly higher redshift.