We report on a sample of 48 nearby, star-forming galaxies observed with the Cosmic Origin Spectrograph on the Hubble Space Telescope. We measure the kinematics of warm gas in galactic outflows using a combination of four Si II absorption lines. We use multi-wavelength ancillary data to estimate stellar masses (M * ), star formation rates (SFR), circular velocities (v circ ), and morphologies. The galaxies cover four orders of magnitude in M * and SFR, and sample a wide range of morphologies from starbursting mergers to normal star-forming galaxies. We derive 3.0-3.5σ relations between outflow velocity and SFR, M * , and v circ . The outflow velocities scale as SFR with the range depending on whether we use a maximum or a central velocity to quantify the outflow velocity. After accounting for their increased SFR, mergers drive 32% faster outflows than nonmerging galaxies, with all of the highest velocity outflows arising from mergers. Low-mass galaxies (log (M * / M e ) < 10.5) lose some low-ionization gas through galactic outflows, while more massive galaxies retain all of their low-ionization gas, unless they undergo a merger.

We infer the properties of massive star populations using the far-ultraviolet stellar continua of 61 star-forming galaxies: 42 at low redshift observed with the Hubble Space Telescope and 19 at z∼2 from the MEGaSaURA sample. We fit each stellar continuum with a linear combination of up to 50 single-age and single-metallicity STARBURST99 models. From these fits, we derive light-weighted ages and metallicities, which agree with stellar wind and photospheric spectral features, and infer the spectral shapes and strengths of the ionizing continua. Inferred light-weighted stellar metallicities span 0.05-1.5 Z e and are similar to the measured nebular metallicities. We quantify the ionizing continua using the ratio of the ionizing flux at 900 Å to the non-ionizing flux at 1500 Å and demonstrate the evolution of this ratio with stellar age and metallicity using theoretical single-burst models. These single-burst models only match the inferred ionizing continua of half of the sample, while the other half are described by a mixture of stellar ages. Mixed-age populations produce stronger and harder ionizing spectra than continuous star formation histories, but, contrary to previous studies that assume constant star formation, have similar stellar and nebular metallicities. Stellar population age and metallicity affect the far-UV continua in different and distinguishable ways; assuming a constant star formation history diminishes the diagnostic power. Finally, we provide simple prescriptions to determine the ionizing photon production efficiency (ξ ion ) from the stellar population properties. The ξ ion inferred from the observed star-forming galaxies has a range of log(ξ ion )=24.4-25.7 Hz erg −1 that depends on the stellar population age, metallicity, star formation history, and contributions from binary star evolution. These stellar population properties must be observationally determined to accurately determine the number of ionizing photons generated by massive stars.

The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to indirectly measure the escape fraction of high-redshift galaxies, it is unlikely that we will know what reionized the universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within 1.4σ) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Lyα escape fraction and the optical [O III]/[O II] flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions (fesc ≤ 1%), but 11 have escape fractions > 1%. The methods presented here will measure the escape fractions of high-redshift galaxies, enabling future telescopes to determine whether star-forming galaxies reionized the early universe.

Context. The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. Aims. To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the H i covering fractions and column densities using ultraviolet (UV) H i and metal absorption lines of 18 star-forming galaxies that have Lyman series observations. Nine of these galaxies are confirmed LyC emitters Methods. We fit the stellar continuum, dust attenuation, metal, and H i properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We used synthetic interstellar absorption lines to explore the systematics of our measurements. Then we applied our method to the observed UV spectra of low-redshift and z ∼ 2 galaxies. Results. The observed H i lines are found to be saturated in all galaxies. An indirect approach using O i column densities and the observed O/H abundances yields H i column densities of log(N H I ) ∼ 18.6 − 20 cm −2 . These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have H i covering fractions less than unity. Ionizing photons escape through optically thin channels in a clumpy interstellar medium. Our simulations confirm that the H i covering fractions are accurately recovered. The Si ii and H i covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z ∼ 3. Thus, with an empirical correction, the Si ii absorption lines can also be used to determine the H i coverage. Finally, we show that a consistent fitting of dust attenuation, continuum, and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation. Conclusions. These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper.

Galactic outflows are believed to play an important role in regulating star formation in galaxies, but estimates of the outflowing mass and momentum have historically been based on uncertain assumptions. Here, we measure the mass, momentum, and energy outflow rates of seven nearby star-forming galaxies using ultraviolet absorption lines and observationally motivated estimates for the density, metallicity, and radius of the outflow. Low-mass galaxies generate outflows faster than their escape velocities with mass outflow rates up to twenty times larger than their star formation rates. These outflows from low-mass galaxies also have momenta larger than provided from supernovae alone, indicating that multiple momentum sources drive these outflows. Only 1-20% of the supernovae energy is converted into kinetic energy, and this fraction decreases with increasing stellar mass such that low-mass galaxies drive more efficient outflows. We find scaling relations between the outflows and the stellar mass of their host galaxies (M * ) at the 2-3σ significance level. The mass-loading factor, or the mass outflow rate divided by the star formation rate, scales as M −0.4 * and with the circular velocity as v −1.6 circ . The scaling of the mass-loading factor is similar to recent simulations, but the observations are a factor of five smaller, possibly indicating that there is a substantial amount of unprobed gas in a different ionization phase. The outflow momenta are consistent with a model where star formation drives the outflow while gravity counteracts this acceleration.