THE Lyα REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259

Abstract

The Lyman-alpha reference sample (LARS) is a program with the Hubble Space\nTelescope (HST) that provides a sample of local universe laboratory galaxies in\nwhich to study the astrophysics of the visibility and strength of the\nLyman-alpha (Lya) line of hydrogen. This article presents an overview of the\nsurvey, its selection function and HST imaging observations. The sample was\nselected from the GALEX+SDSS catalogue at z=0.028-0.19, in order to allow Lya\nto be captured with combinations of long pass filters in the Solar Blind\nChannel (SBC) of HST/ACS. In addition, LARS utilises Halpha and Hbeta narrow,\nand U, B, i broad-band imaging with ACS and WFC3. In order to study galaxies in\nwhich large numbers of Lya photons are produced we demanded an Halpha\nequivalent width > 100{\\AA}. The sample of 14 galaxies covers far UV (FUV)\nluminosities that overlaps with those of high-z Lya emitters and Lyman Break\nGalaxies (LBGs). We present the reduction steps used to obtain the Lya images,\nincluding our LARS eXtraction software (LaXs) which utilises pixel-by-pixel\nspectral synthesis fitting to subtract the continuum at Lya. We demonstrate\nthat the use of SBC long pass filters dramatically increase the signal to noise\ncompared to the nominal Lya filter in SBC. To exemplify, we present results for\nMrk 259. This irregular galaxy shows extended but strongly asymmetric Lya\nemission. Spectroscopy from HST/COS show a moderate outflow in the neutral ISM\nand an asymmetric Lya profile. Radiative transfer modeling is able to reproduce\nthe Lya line profile, confirming the presence of an outflow. The integrated\nphotometry provides a Lya luminosity of 1.3E42 erg/s an equivalent width W(Lya)\n=45{\\AA} and a FUV absolute magnitude -19.2. Mrk 259 would hence be detectable\nin high-z Lya and LBG surveys. The total Lya escape fraction is 12%, similar to\nthat observed at z>4. (shortened)\n