Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST-ACS - AstrophysicsReport as inadecuate

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Abstract: ABRIDGED We are undertaking an imaging study of local star-forming galaxiesin the Lyman-alpha Lya emission line using the Solar Blind Channel SBC ofthe ACS onboard the HST. Observations have been obtained in Lya and H-alphaHa and 6 continuum filters between ~1500AA and the I-band. Previously Hayeset al 2005 we demonstrated that the production of Lya line-only images in theSBC-only data-set is non-trivial and that supporting data is a requirement. Wehere develop various methods of continuum subtraction and assess their relativeperformance using a variety of spectral energy distributions SED as input. Weconclude that simple assumptions about the behavior of the ultravioletcontinuum consistently lead to results that are wildly erroneous, and determinethat an SED fitting approach is essential. Moreover, a single component stellaror stellar+nebular spectrum is not always sufficient for realistic templateSEDs and, in order to successfully recover the input observables, care must betaken to control contributions of nebular gas and underlying stellarpopulations. Independent measurements of the metallicity must first beobtained, but details of the initial mass function play only a small role. Weidentify the need to bin together pixels in our data to obtain signal-to-noiseS-N of ~10 in each band before processing. At S-N=10 we can recover Lyafluxes accurate to ~30% for Lya lines with intrinsic equivalent width WLyaof 10AA. This accuracy improves to ~10% for WLya=100AA. We describe the imageprocessing applied to the observations presented in \-Ostlin et al. 2009 andthe associated data-release. We also present simulations for an observingstrategy for an alternative low-redshift SBC Lya imaging campaign usingadjacent combinations of long-pass filters at slightly higher redshift.

Author: Matthew Hayes 1, Goran Ostlin 2, J. Miguel Mas-Hesse 3, Daniel Kunth 4 1 Geneva Observatory, 2 Stockholm Observatory, 3 CSIC-INTA



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