The Metagalactic Ionizing Background: A Crisis in UV Photon Production
  or Incorrect Galaxy Escape Fractions?
  Recent suggestions of a "photon underproduction crisis" (Kollmeier \etal\
2014) have generated concern over the intensity and spectrum of ionizing
photons in the metagalactic ultraviolet background (UVB). The balance of
hydrogen photoionization and recombination determines the opacity of the
low-redshift intergalactic medium (IGM). We calibrate the hydrogen
photoionization rate ($\Gamma_{\rm H}$) by comparing {\it Hubble Space
Telescope} spectroscopic surveys of the low-redshift column density
distribution of \HI\ absorbers and the observed ($z < 0.4$) mean \Lya\ flux
decrement, $D_A = (0.014)(1+z)^{2.2}$, to new cosmological simulations. The
distribution, $f(N_{\rm HI}, z) \equiv d^2 {\cal N} / d(\log N_{\rm HI}) dz$,
is consistent with an increased UVB that includes contributions from both
quasars and galaxies. Our recommended fit, $\Gamma_{\rm H}(z) = (4.6 \times
10^{-14}$ s$^{-1})(1+z)^{4.4}$ for $0 < z < 0.47$, corresponds to
unidirectional LyC photon flux $\Phi_0 \approx 5700$~cm$^{-2}$~s$^{-1}$ at $z =
0$. This flux agrees with observed IGM metal ionization ratios (\CIII/\CIV\ and
\SiIII/\SiIV) and suggests a 25-30\% contribution of \Lya\ absorbers to the
cosmic baryon inventory. The primary uncertainties in the low-redshift UVB are
the contribution from massive stars in galaxies and the LyC escape fraction
($f_{\rm esc}$), a highly directional quantity that is difficult to constrain
statistically. We suggest that both quasars and low-mass starburst galaxies are
important contributors to the ionizing UVB at $z < 2$. Their additional
ionizing flux would resolve any crisis in photon underproduction.
