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