We present a systematic study of the shape of the dust attenuation curve in star‐forming galaxies from the far‐ultraviolet (far‐UV) to the near‐infrared (NIR; ∼0.15–2 μ m), as a function of specific star formation rate (ψS) and axial ratio (b/a), for galaxies with and without a significant bulge. Our sample comprises 23 000 (15 000) galaxies with a median redshift of 0.07, with photometric entries in the Sloan Digital Sky Survey (SDSS), UKIRT Infrared Deep Sky Survey‐Large Area Survey and Galaxy Evolution Explorer‐All‐Sky Imaging Survey catalogues and emission‐line measurements from the SDSS spectroscopic survey. We develop a new pair‐matching technique to isolate the dust attenuation curves from the stellar continuum emission. The main results are: (i) the slope of the attenuation curve in the optical varies weakly with ψS, strongly with b/a, and is significantly steeper than the Milky Way extinction law in bulge‐dominated galaxies; (ii) the NIR slope is constant and matches the slope of the Milky Way extinction law; (iii) the UV has a slope change consistent with a dust bump at 2175 Å which is evident in all samples and varies strongly in strength with b/a in the bulge‐dominated sample; (iv) there is a strong increase in emission‐line‐to‐continuum dust attenuation (τV, line/τV, cont) with both decreasing ψS and increasing b/a; and (v) radial gradients in dust attenuation increase strongly with increasing ψS, and the presence of a bulge does not alter the strength of the gradients. These results are consistent with the picture in which young stars are surrounded by dense ‘birth clouds’ with low covering factor which disperse on time‐scales of ∼107 yr and the diffuse interstellar dust is distributed in a centrally concentrated disc with a smaller scaleheight than the older stars that contribute the majority of the red and NIR light. Within this model, the path‐length of diffuse dust, but not of birth‐cloud dust, increases with increasing inclination and the apparent optical attenuation curve is steepened by the differential effect of larger dust opacity towards younger stars than towards older stars. Additionally, our findings suggest that: (i) galaxies with higher star formation rates per unit stellar mass have a higher fraction of diffuse dust, which is more centrally concentrated; (ii) the observed strength of the 2175‐Å dust feature is affected predominantly by global geometry; and (iii) only highly inclined discs are optically thick. We provide new empirically derived attenuation curves for correcting the light from star‐forming galaxies for dust attenuation.

We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z 7 − 9 selected to have intense rest-frame optical line emission by Roberts-Borsani et al. (2016). Previous spectroscopic follow-up has revealed Lyα emission in two of the four galaxies. Our new MOSFIRE observations confirm that Lyα is present in the entire sample. We detect Lyα emission in the galaxy COS-zs7-1, confirming its redshift as z Lyα = 7.154, and we detect Lyα in EGS-zs8-2 at z Lyα = 7.477, verifying a tentative detection presented in an earlier study. The ubiquity of Lyα emission in this unique photometric sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this surprising result in more detail, we have initiated a campaign to target UV metal line emission in the four Lyα emitters as a probe of both the ionizing radiation field and the velocity offset of Lyα at early times. Here we present the detection of very large equivalent width [CIII], CIII] λλ1907,1909Å emission in EGS-zs8-1 (W CIII],0 = 22 ± 2Å), a galaxy from this sample previously shown to have Lyα emission at z = 7.73. Photoionization models indicate that an intense radiation field (log 10 ξ * ion [erg −1 Hz] 25.6) and moderately low metallicity (0.11 Z ) are required to reproduce the CIII] line emission and intense optical line emission implied by the broadband SED. We argue that this extreme radiation field is likely to affect the local environment, increasing the transmission of Lyα through the galaxy. Moreover, the centroid of CIII] emission indicates that Lyα is redshifted from the systemic value by 340 km sec −1 . This velocity offset is larger than that seen in less luminous systems and provides an additional explanation for the transmission of Lyα emission through the intergalactic medium. Since the transmission is further enhanced by the likelihood that such systems are also situated in the densest regions with accelerated evolution and the largest ionized bubbles, the visibility of Lyα at z > 7 is expected to be strongly luminosity-dependent, with the most effective transmission occurring in systems with intense star formation.

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z /2. Such tests are particularly important for interpreting the surprising high-ionization UV line emission detected at z > 6 in recent years. We present HST /COS ultraviolet spectra of ten nearby metal-poor star-forming galaxies selected to show He ii emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼ 10 2 Gyr −1 ). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C iii] equivalent widths comparable to those seen in systems at z ∼ 6 − 7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H 8.0 (Z/Z 1/5) presenting minimal stellar wind features and prominent nebular emission in He ii and C iv. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He + -ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C iv and He ii powered by these stars may provide useful metallicity constraints in the reionization era.

We present a new-generation tool to model and interpret spectral energy distributions (SEDs) of galaxies, which incorporates in a consistent way the production of radiation and its transfer through the interstellar and intergalactic media. This flexible tool, named beagle (for BayEsian Analysis of GaLaxy sEds), allows one to build mock galaxy catalogues as well as to interpret any combination of photometric and spectroscopic galaxy observations in terms of physical parameters. The current version of the tool includes versatile modeling of the emission from stars and photoionized gas, attenuation by dust and accounting for different instrumental effects, such as spectroscopic flux calibration and line spread function.We show a first application of the beagle tool to the interpretation of broadband SEDs of a published sample of ∼10 4 galaxies at redshifts 0.1 z 8. We find that the constraints derived on photometric redshifts using this multi-purpose tool are comparable to those obtained using public, dedicated photometric-redshift codes and quantify this result in a rigorous statistical way. We also show how the post-processing of beagle output data with the Python extension pyp-beagle allows the characterization of systematic deviations between models and observations, in particular through posterior predictive checks. The modular design of the beagle tool allows easy extensions to incorporate, for example, the absorption by neutral galactic and circumgalactic gas, and the emission from an active galactic nucleus, dust and shock-ionized gas. Information about public releases of the beagle tool will be maintained on http://www.jacopochevallard.org/beagle.