Spectroscopic Measurements of the Far-Ultraviolet Dust Attenuation Curve
  at z~3
  We present the first measurements of the shape of the far-ultraviolet
(far-UV; lambda=950-1500 A) dust attenuation curve at high redshift (z~3). Our
analysis employs rest-frame UV spectra of 933 galaxies at z~3, 121 of which
have very deep spectroscopic observations (>7 hrs) at lambda=850-1300 A, with
the Low Resolution Imaging Spectrograph on the Keck Telescope. By using an
iterative approach in which we calculate the ratios of composite spectra in
different bins of continuum color excess, E(B-V), we derive a dust curve that
implies a lower attenuation in the far-UV for a given E(B-V) than those
obtained with standard attenuation curves. We demonstrate that the UV composite
spectra of z~3 galaxies can be modeled well by assuming our new attenuation
curve, a high covering fraction of HI, and absorption from the Lyman-Werner
bands of H2 with a small (<20%) covering fraction. The low covering fraction of
H2 relative to that of the HI and dust suggests that most of the dust in the
ISM of typical galaxies at z~3 is unrelated to the catalysis of H2, and is
associated with other phases of the ISM (i.e., the ionized and neutral gas).
The far-UV dust curve implies a factor of ~2 lower dust attenuation of Lyman
continuum (ionizing) photons relative to those inferred from the most commonly
assumed attenuation curves for L* galaxies at z~3. Our results may be utilized
to assess the degree to which ionizing photons are attenuated in HII regions
or, more generally, in the ionized or low column density (N(HI)<10^17.2 cm^-2)
neutral ISM of high-redshift galaxies.
