ALBERT

Description: We present a method to estimate the total gas column density, dust-to-gas and dust-to-metal ratios of distant galaxies from rest-frame optical spectra. The technique exploits the sensitivity of certain optical lines to changes in depletion of metals on to dust grains and uses photoionization models to constrain these physical ratios along with the metallicity and dust column density. We compare our gas column density estimates with $\mathrm{H\,\small {I}}$ and CO gas mass estimates in nearby galaxies to show that we recover their total gas mass surface density to within a factor of 2 up to a total surface gas mass density of ~75 M pc –2 . Our technique is independent of the conversion factor of CO to H 2 and we show that a metallicity-dependent X CO is required to achieve good agreement between our measurements and that provided by CO and $\mathrm{H\,\small {I}}$ . However, we also show that our method cannot be reliably aperture corrected to total integrated gas mass. We calculate dust-to-gas ratios for all star-forming galaxies in the Sloan Digital Sky Survey Data Release 7 and show that the resulting dependence on metallicity agrees well with the trend inferred from modelling of the dust emission of nearby galaxies using far-IR data. We also present estimates of the variation of the dust-to-metal ratio with metallicity and show that this is poorly constrained at metallicities below 50 per cent solar. We conclude with a study of the inventory of gas in the central regions, defined both in terms of a fixed physical radius and as a fixed fraction of the half-light radius, of ~70 000 star-forming galaxies from the Sloan Digital Sky Survey. We show that their central gas content and gas depletion time are not accurately predicted by a single parameter, but in agreement with recent studies we find that a combination of the stellar mass and some measure of central concentration provides a good predictor of gas content in galaxies. We also identify a population of galaxies with low surface densities of stars and very long gas depletion times.

Description: We present a new approach to investigate the content and spatial distribution of dust in structurally unresolved star-forming galaxies from the observed dependence of integrated spectral properties on galaxy inclination. Motivated by the observation that different stellar populations reside in different spatial components of nearby star-forming galaxies, we develop an innovative combination of generic models of radiative transfer in dusty media with a prescription for the spectral evolution of galaxies, via the association of different geometric components of galaxies with stars in different age ranges. We start by showing that a wide range of radiative transfer models all predict a quasi-universal relation between slope of the attenuation curve at any wavelength, from the ultraviolet to the near-infrared, and V -band attenuation optical depth in the diffuse interstellar medium (ISM), at all galaxy inclinations. This relation predicts steeper (shallower) dust attenuation curves than both the Calzetti and Milky Way curves at small (large) attenuation optical depths, which implies that geometry and orientation effects have a stronger influence on the shape of the attenuation curve than changes in the optical properties of dust grains. We use our new, combined radiative transfer and spectral evolution model to interpret the observed dependence of the Hα/Hβ ratio and ugrizYJH attenuation curve on inclination in a sample of about 23 000 nearby star-forming galaxies, which we correct for systematic biases by developing a general method based on importance sampling. From the exploration of the model parameter space by means of a Bayesian Markov chain Monte Carlo technique, we measure the central face-on B -band optical depth of this sample to be B   1.8 ± 0.2 (corresponding to an angle-averaged ${\langle \hat{\tau }^\mathrm{ISM}_V\rangle _\theta }\approx 0.3$ ). We also quantify the enhanced optical depth towards newly formed stars in their birth clouds, finding this to be significantly larger in galaxies with bulges than in disc-dominated galaxies, while B is roughly similar in both cases. This can arise if, for example, galaxies with significant bulges have higher central star formation efficiencies than their disc-dominated counterparts at a fixed specific star formation rate, and dustier stellar birth clouds because of the higher metallicity. We find that over 80 per cent of the attenuation in galaxies in our sample is characteristic of that affecting thin-disc stars in radiative transfer models. The median unattenuated V -band luminosity ratio of thick-disc to thin-disc stars is 0.1–0.2, in good agreement with the results from spatially resolved studies of nearby edge-on disc galaxies. Finally, we show that neglecting the effect of geometry and orientation on attenuation can severely bias the interpretation of galaxy spectral energy distributions, as the impact on broad-band colours can reach up to 0.3–0.4 mag at optical wavelengths and 0.1 mag at near-infrared ones. This paper also contains an original application of Gaussian random processes to extend the wavelength range of dust attenuation curves.

Description: Deep spectroscopic observations of z 6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Ly α emission. While this may offer valuable insight into the end of the reionization process, it presents a challenge to the detailed spectroscopic study of bright photometrically-selected distant sources now being found via deep Hubble Space Telescope imaging, and particularly those highly magnified sources viewed through foreground lensing clusters. In this paper, we demonstrate the validity of a new way forward via the detection of an alternative diagnostic line, C iii ] 1909 Å, seen in spectroscopic exposures of a star-forming galaxy at z Lyα = 6.029. We also report tentative detection of C iii ] 1909 Å in a galaxy at z Lyα = 7.213. The former 3.3 detection is based on a 3.5 h XShooter spectrum of a bright ( J 125 = 25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter 2.8 detection is based on a 4.2 h MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J 140 = 25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Ly α), ensuring that any C iii ] emission would be located in a favourable portion of the near-infrared sky spectrum. Since the availability of secure Ly α redshifts significantly narrows the wavelength range where C iii ] is sought, this increases confidence in these, otherwise, low-signal-to-noise ratio detections. We compare our C iii ] and Ly α equivalent widths in the context of those found at z ~= 2 from earlier work and discuss the motivation for using lines other than Ly α to study galaxies in the reionization era.

Description: In the context of observations of the rest-frame ultraviolet and optical emission from distant galaxies, we explore the emission-line properties of photoionization models of active and inactive galaxies. Our aim is to identify new line-ratio diagnostics to discriminate between gas photoionization by active galactic nuclei (AGN) and star formation. We use a standard photoionization code to compute the emission from AGN narrow-line regions and compare this with calculations of the nebular emission from star-forming galaxies achieved using the same code. We confirm the appropriateness of widely used optical spectral diagnostics of nuclear activity versus star formation and explore new diagnostics at ultraviolet wavelengths. We find that combinations of a collisionally excited metal line or line multiplet, such as C iv 1548, 1551, O iii ] 1661, 1666, N iii ] 1750, [Si iii ] 1883+Si iii ] 1892 and [C iii ] 1907+C iii ] 1909, with the He ii 1640 recombination line are individually good discriminants of the nature of the ionizing source. Diagrams involving at least three of these lines allow an even more stringent distinction between active and inactive galaxies, as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation. Several line ratios involving Ne-based emission lines, such as [Ne iv ] 2424, [Ne iii ] 3343 and [Ne v ] 3426, are also good diagnostics of nuclear activity. Our results provide a comprehensive framework to identify the sources of photoionization and physical conditions of the ionized gas from the ultraviolet and optical nebular emission from galaxies. This will be particularly useful to interpret observations of high-redshift galaxies with future facilities, such as the James Webb Space Telescope and extremely large ground-based telescopes.

Description: We present a new model of the nebular emission from star-forming galaxies in a wide range of chemical compositions, appropriate to interpret observations of galaxies at all cosmic epochs. The model relies on the combination of state-of-the-art stellar population synthesis and photoionization codes to describe the ensemble of H ii regions and the diffuse gas ionized by young stars in a galaxy. A main feature of this model is the self-consistent yet versatile treatment of element abundances and depletion on to dust grains, which allows one to relate the observed nebular emission from a galaxy to both gas-phase and dust-phase metal enrichment. We show that this model can account for the rest-frame ultraviolet and optical emission-line properties of galaxies at different redshifts and find that ultraviolet emission lines are more sensitive than optical ones to parameters such as C/O abundance ratio, hydrogen gas density, dust-to-metal mass ratio and upper cut-off of the stellar initial mass function. We also find that, for gas-phase metallicities around solar to slightly subsolar, widely used formulae to constrain oxygen ionic fractions and the C/O ratio from ultraviolet and optical emission-line luminosities are reasonable faithful. However, the recipes break down at non-solar metallicities, making them inappropriate to study chemically young galaxies. In such cases, a fully self-consistent model of the kind presented in this paper is required to interpret the observed nebular emission.

Description: By adopting a semiparametric approach, the ‘traditional’ regional knowledge production function is developed in three complementary directions. First, the model is augmented with region-specific time trends to account for endogeneity due to selection on unobservables . Second, the nonparametric part of the model relaxes the standard assumptions of linearity and additivity regarding the effect of R&D and human capital. Finally, the assumption of homogeneity in the effects of R&D and human capital is also relaxed by explicitly accounting for the differences between developed and lagging regions. The analysis of the genesis of innovation in the regions of the European Union unveils nonlinearities, threshold effects, complex interactions and shadow effects that cannot be uncovered by standard parametric formulations.

Description: We present Keck/MOSFIRE observations of UV metal emission lines in four bright ( H  = 23.9–25.4) gravitationally lensed z ~= 6–8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a highly magnified star-forming galaxy with a Lyα redshift of z  = 7.045, reveals a confident detection of the nebular C iv 1548 emission line (unresolved with full width at half-maximum〈125 km s –1 ). UV metal emission lines are not detected in the three other galaxies. At z ~= 2–3, nebular C iv emission is observed in just 1 per cent of UV-selected galaxies. The presence of strong C iv emission in one of the small sample of galaxies targeted in this paper may indicate that hard ionizing spectra are more common at z ~= 7. The total estimated rest-frame equivalent width of the C iv doublet and C iv /Lyα flux ratio are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular C iv line can also be reproduced by a young stellar population, with very hot metal-poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the C iv , we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z ~= 2, producing more H ionizing photons per unit 1500 Å luminosity (log( ion /erg – 1  Hz) = 25.68) and a larger flux density at 30–50 eV. If such extreme radiation fields are typical in UV-selected systems at z   7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyα emitters at z   7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high-ionization emission lines in galaxies with and without Lyα detections will help clarify whether hard ionizing spectra are common in the reionization era.

Description: We present star formation histories (SFHs) for a sample of 104 massive (stellar mass M  〉 10 10 M ) quiescent galaxies (MQGs) at z = 1.0–1.5 from the analysis of spectrophotometric data from the Survey for High- z Absorption Red and Dead Sources (SHARDS) and HST /WFC3 G102 and G141 surveys of the GOODS-North field, jointly with broad-band observations from ultraviolet (UV) to far-infrared (far-IR). The sample is constructed on the basis of rest-frame UVJ colours and specific star formation rates (sSFRs = SFR/Mass). The spectral energy distributions (SEDs) of each galaxy are compared to models assuming a delayed exponentially declining SFH. A Monte Carlo algorithm characterizes the degeneracies, which we are able to break taking advantage of the SHARDS data resolution, by measuring indices such as MgUV and D4000. The population of MQGs shows a duality in their properties. The sample is dominated (85 per cent) by galaxies with young mass-weighted ages, $\overline{t_{\rm M}}$  〈 2 Gyr, short star formation time-scales, 〈〉 ~ 60–200 Myr, and masses log( M /M ) ~ 10.5. There is an older population (15 per cent) with $\overline{t_{\rm M}}$ = 2–4 Gyr, longer star formation time-scales, 〈〉 ~ 400 Myr, and larger masses, log( M /M ) ~ 10.7. The SFHs of our MQGs are consistent with the slope and the location of the main sequence of star-forming galaxies at z  〉 1.0, when our galaxies were 0.5–1.0 Gyr old. According to these SFHs, all the MQGs experienced a luminous infrared galaxy phase that lasts for ~500 Myr, and half of them an ultraluminous infrared galaxy phase for ~100 Myr. We find that the MQG population is almost assembled at z  ~ 1, and continues evolving passively with few additions to the population.