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<jats:title>Abstract</jats:title> <jats:p>Ly<jats:italic>α</jats:italic> emission is widely used to detect and confirm high-redshift galaxies and characterize the evolution of the intergalactic medium (IGM). However, many galaxies do not display Ly<jats:italic>α</jats:italic> emission in typical spectroscopic observations, and intrinsic Ly<jats:italic>α</jats:italic> emitters represent a potentially biased set of high-redshift galaxies. In this work, we analyze a set of 703 galaxies at 2 ≲ <jats:italic>z</jats:italic> ≲ 3 with both Ly<jats:italic>α</jats:italic> spectroscopy and measurements of other rest-frame ultraviolet and optical properties in order to develop an empirical model for Ly<jats:italic>α</jats:italic> emission from galaxies and understand how the probability of Ly<jats:italic>α</jats:italic> emission depends on other observables. We consider several empirical proxies for the efficiency of Ly<jats:italic>α</jats:italic> photon production, as well as the subsequent escape of these photons through their local interstellar medium. We find that the equivalent width of metal-line absorption and the O3 ratio of rest-frame optical nebular lines are advantageous empirical proxies for Ly<jats:italic>α</jats:italic> escape and production, respectively. We develop a new quantity, <jats:inline-formula> <jats:tex-math> <?CDATA ${X}_{\mathrm{LIS}}^{{\rm{O}}3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab4993ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, that combines these two properties into a single predictor of net Ly<jats:italic>α</jats:italic> emission, which we find describes ∼90% of the observed variance in Ly<jats:italic>α</jats:italic> equivalent width when accounting for our observational uncertainties. We also construct conditional probability distributions demonstrating that galaxy selection based on measurements of galaxy properties yield samples of galaxies with widely varying probabilities of net Ly<jats:italic>α</jats:italic> emission. The application of the empirical models and probability distributions described here may be used to infer the selection biases of current galaxy surveys and evaluate the significance of high-redshift Ly<jats:italic>α</jats:italic> (non)detections in studies of reionization and the IGM.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We address the spatial scale, ionization structure, mass, and metal content of gas at the Milky Way disk–halo interface detected as absorption in the foreground of seven closely spaced, high-latitude halo blue horizontal branch stars with heights <jats:italic>z</jats:italic> = 3–14 kpc. We detect transitions that trace multiple ionization states (e.g., Ca <jats:sc>ii</jats:sc>, Fe <jats:sc>ii</jats:sc>, Si <jats:sc>iv</jats:sc>, C <jats:sc>iv</jats:sc>) with column densities that remain constant with height from the disk, indicating that the gas most likely lies within <jats:italic>z</jats:italic> &lt; 3.4 kpc. The intermediate ionization state gas traced by C <jats:sc>iv</jats:sc> and Si <jats:sc>iv</jats:sc> is strongly correlated over the full range of transverse separations probed by our sight lines, indicating large, coherent structures greater than 1 kpc in size. The low ionization state material traced by Ca <jats:sc>ii</jats:sc> and Fe <jats:sc>ii</jats:sc> does not exhibit a correlation with either <jats:italic>N</jats:italic> <jats:sub>H <jats:sc>i</jats:sc> </jats:sub> or transverse separation, implying cloudlets or clumpiness on scales less than 10 pc. We find that the observed ratio log(<jats:italic>N</jats:italic> <jats:sub>Si <jats:sc>iv</jats:sc> </jats:sub>/<jats:italic>N</jats:italic> <jats:sub>C <jats:sc>iv</jats:sc> </jats:sub>), with a median value of −0.69 ± 0.04, is sensitive to the total carbon content of the ionized gas under the assumption of either photoionization or collisional ionization. The only self-consistent solution for photoionized gas requires that Si be depleted onto dust by 0.35 dex relative to the solar Si/C ratio, similar to the level of Si depletion in DLAs and in the Milky Way interstellar medium. The allowed range of values for the areal mass infall rate of warm, ionized gas at the disk−halo interface is 0.0003 &lt; <jats:italic>dM</jats:italic> <jats:sub>gas</jats:sub>/<jats:italic>dtdA</jats:italic> [<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> kpc<jats:sup>−2</jats:sup> yr<jats:sup>−1</jats:sup>] &lt;0.006. Our data support a physical scenario in which the Milky Way is fed by complex, multiphase processes at its disk−halo interface that involve kiloparsec-scale ionized envelopes or streams containing parsec-scale, cool clumps.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze a set of 89 type Ia supernovae (SNe Ia) that have both optical and near-infrared (NIR) photometry to derive distances and construct low-redshift (<jats:italic>z</jats:italic> ≤ 0.04) Hubble diagrams. We construct mean light curve (LC) templates using a hierarchical Bayesian model. We explore both Gaussian process (GP) and template methods for fitting the LCs and estimating distances, while including peculiar-velocity and photometric uncertainties. For the 56 SNe Ia with both optical and NIR observations near maximum light, the GP method yields a NIR-only Hubble-diagram with a root mean square (rms) of <jats:inline-formula> <jats:tex-math> <?CDATA $0.117\,\pm \,0.014$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab2a16ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> mag when referenced to the NIR maxima. For each NIR band, a comparable GP method rms is obtained when referencing to NIR-max or <jats:italic>B</jats:italic>-max. Using NIR LC templates referenced to <jats:italic>B</jats:italic>-max yields a larger rms value of <jats:inline-formula> <jats:tex-math> <?CDATA $0.138\,\pm \,0.014$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab2a16ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> mag. Fitting the corresponding optical data using standard LC fitters that use LC shape and color corrections yields larger rms values of 0.179 ± 0.018 mag with SALT2 and <jats:inline-formula> <jats:tex-math> <?CDATA $0.174\,\pm \,0.021$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab2a16ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> mag with SNooPy. Applying our GP method to subsets of SNe Ia NIR LCs at NIR maximum light, even without corrections for LC shape, color, or host-galaxy dust reddening, provides smaller rms in the inferred distances, at the ∼2.3–4.1<jats:italic>σ</jats:italic> level, than standard optical methods that correct for those effects. Our ongoing RAISIN program on the <jats:italic>Hubble Space Telescope</jats:italic> will exploit this promising infrared approach to limit systematic errors when measuring the expansion history of the universe in order to constrain dark energy.</jats:p>