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 λλ1548, 1551, O iii λλ1661, 1666, N iii λ1750, Si iii λ1883+Si iii λ1892 and C iii λ1907+C iii λ1909, with the He ii λ1640 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 λ2424, Ne iii λ3343 and Ne v λ3426, 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.
We present a new model for computing the spectral evolution of stellar populations at ages between 1 × 105 and 2 × 1010 yr at a resolution of 3 Å across the whole wavelength range from 3200 to 9500 Å ...for a wide range of metallicities. These predictions are based on a newly available library of observed stellar spectra. We also compute the spectral evolution across a larger wavelength range, from 91 Å to 160 μm, at lower resolution. The model incorporates recent progress in stellar evolution theory and an observationally motivated prescription for thermally pulsing stars on the asymptotic giant branch. The latter is supported by observations of surface brightness fluctuations in nearby stellar populations. We show that this model reproduces well the observed optical and near-infrared colour–magnitude diagrams of Galactic star clusters of various ages and metallicities. Stochastic fluctuations in the numbers of stars in different evolutionary phases can account for the full range of observed integrated colours of star clusters in the Magellanic Clouds. The model reproduces in detail typical galaxy spectra from the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We exemplify how this type of spectral fit can constrain physical parameters such as the star formation history, metallicity and dust content of galaxies. Our model is the first to enable accurate studies of absorption-line strengths in galaxies containing stars over the full range of ages. Using the highest-quality spectra of the SDSS EDR, we show that this model can reproduce simultaneously the observed strengths of those Lick indices that do not depend strongly on element abundance ratios. The interpretation of such indices with our model should be particularly useful for constraining the star formation histories and metallicities of galaxies.
ABSTRACT
We explore the production and escape of ionizing photons in young galaxies by investigating the ultraviolet and optical emission-line properties of models of ionization-bounded and ...density-bounded H ii regions, active-galactic-nucleus (AGN) narrow-line regions, and radiative shocks computed all using the same physically consistent description of element abundances and depletion on to dust grains down to very low metallicities. We compare these models with a reference sample of metal-poor star-forming galaxies and Lyman-continuum (LyC) leakers at various redshifts, which allows the simultaneous exploration of more spectral diagnostics than typically available at once for individual subsamples. We confirm that current single- and binary-star population synthesis models do not produce hard-enough radiation to account for the high-ionization emission of the most metal-poor galaxies. Introducing either an AGN or radiative-shock component brings models into agreement with observations. A published model including X-ray binaries is an attractive alternative to reproduce the observed rise in He iiλ4686/H β ratio with decreasing oxygen abundance in metal-poor star-forming galaxies, but not the high observed He iiλ4686/Hβ ratios of galaxies with large EW(Hβ). A source of harder ionizing radiation appears to be required in these extreme objects, such as an AGN or radiative-shock component, perhaps linked to an initial-mass-function bias towards massive stars at low metallicity. This would also account for the surprisingly high O i/O iii ratios of confirmed LyC leakers relative to ionization-bounded models. We find no simple by-eye diagnostic of the nature of ionizing sources and the escape of LyC photon, which require proper simultaneous fits of several lines to be discriminated against.
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 τB⊥ 1.8 ± 0.2 (corresponding to an angle-averaged
). 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 τ
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.
ABSTRACT The Large Early Galaxy Census (LEGA-C) is a Public Spectroscopic Survey of ∼3200 K-band selected galaxies at redshifts z = 0.6 − 1.0 with stellar masses M * > 10 10 M , conducted with VIMOS ...on ESO's Very Large Telescope. The survey is embedded in the COSMOS field (R.A. = 10h00; decl . = + 2 deg ). The 20 hr long integrations produce high-signal-to-noise ratio continuum spectra that reveal ages, metallicities and velocity dispersions of the stellar populations. LEGA-C's unique combination of sample size and depth will enable us for the first time to map the stellar content at large lookback time, across galaxies of different types and star formation activity. Observations started in 2014 December and are planned to be completed by mid 2018, with early data releases of the spectra and value-added products. In this paper we present the science case, the observing strategy, an overview of the data reduction process and data products, and a first look at the relationship between galaxy structure and spectral properties, as it existed 7 Gyr ago.
ABSTRACT
We present a method to self-consistently propagate stellar-mass $\hbox{$\hbox{${\rm M}$}_{\star }$}=\log (\hbox{${\rm M}$}/\hbox{${\rm M}_{\odot }$})$ and star-formation-rate $\hbox{${\Psi ...}$}=\log (\hbox{${\psi }$}/\hbox{${\rm M}_{\odot }$}\, {\rm yr}^{-1}$) uncertainties on to intercept (α), slope (β), and intrinsic-scatter (σ) estimates for a simple model of the main sequence of star-forming galaxies, where $\hbox{${\Psi }$}= \alpha + \beta \hbox{$\hbox{${\rm M}$}_{\star }$}+ \mathcal {N}(0,\sigma)$. To test this method and compare it with other published methods, we construct mock photometric samples of galaxies at z ∼ 5 based on idealized models combined with broad- and medium-band filters at wavelengths 0.8–5 μm. Adopting simple Ψ estimates based on dust-corrected ultraviolet luminosity can underestimate σ. We find that broad-band fluxes alone cannot constrain the contribution from emission lines, implying that strong priors on the emission-line contribution are required if no medium-band constraints are available. Therefore, at high redshifts, where emission lines contribute a higher fraction of the broad-band flux, photometric fitting is sensitive to Ψ variations on short (∼10 Myr) time-scales. Priors on age imposed with a constant (or rising) star formation history (SFH) do not allow one to investigate a possible dependence of σ on $\hbox{${\rm M}$}_{\star }$ at high redshifts. Delayed exponential SFHs have less constrained priors, but do not account for Ψ variations on short time-scales, a problem if σ increases due to stochasticity of star formation. A simple SFH with current star formation decoupled from the previous history is appropriate. We show that, for simple exposure-time calculations assuming point sources, with low levels of dust, we should be able to obtain unbiased estimates of the main sequence down to $\mathrm{ log}(\hbox{${\rm M}$}/\hbox{${\rm M}_{\odot }$})\sim 8$ at z ∼ 5 with the James Webb Space Telescope while allowing for stochasticity of star formation.
We present a new determination of the ultraviolet (UV) galaxy luminosity function (LF) at redshift z 7 and 8, and a first estimate at z 9. An accurate determination of the form and evolution of the ...galaxy LF during this era is of key importance for improving our knowledge of the earliest phases of galaxy evolution and the process of cosmic reionization. Our analysis exploits to the full the new, deepest Wide Field Camera 3/infrared imaging from our Hubble Space Telescope (HST) Ultra-Deep Field 2012 (UDF12) campaign, with dynamic range provided by including a new and consistent analysis of all appropriate, shallower/wider area HST survey data. Our new measurement of the evolving LF at z 7 to 8 is based on a final catalogue of 600 galaxies, and involves a step-wise maximum-likelihood determination based on the photometric redshift probability distribution for each object; this approach makes full use of the 11-band imaging now available in the Hubble Ultra-Deep Field (HUDF), including the new UDF12 F140W data, and the latest Spitzer IRAC imaging. The final result is a determination of the z 7 LF extending down to UV absolute magnitudes M
1500 = −16.75 (AB mag) and the z 8 LF down to M
1500 = −17.00. Fitting a Schechter function, we find M1500
* = −19.90+0.23
−0.28, log φ* = −2.96+0.18
−0.23 and a faint-end slope α = −1.90+0.14
−0.15 at z 7, and M1500* = −20.12+0.37
−0.48, log φ* = −3.35+0.28
−0.47 and α = −2.02+0.22
+0.23 at z 8. These results strengthen previous suggestions that the evolution at z > 7 appears more akin to 'density evolution' than the apparent 'luminosity evolution' seen at z 5 − 7. We also provide the first meaningful information on the LF at z 9, explore alternative extrapolations to higher redshifts, and consider the implications for the early evolution of UV luminosity density. Finally, we provide catalogues (including derived z
phot, M
1500 and photometry) for the most robust z ∼ 6.5-11.9 galaxies used in this analysis. We briefly discuss our results in the context of earlier work and the results derived from an independent analysis of the UDF12 data based on colour-colour selection.
ABSTRACT
The Hubble Frontier Fields represent the opportunity to probe the high-redshift evolution of the main sequence of star-forming galaxies to lower masses than possible in blank fields thanks ...to foreground lensing of massive galaxy clusters. We use the beagle SED-fitting code to derive stellar masses, $\rm{{M_{\star }}}=\log ({\it M}/{\rm{M_{\odot }}})$, SFRs, $\rm{{\Psi }}=\log (\rm{{\psi }}/{\rm{M_{\odot }}}\, {\rm{yr}}^{-1})$, and redshifts from galaxies within the astrodeep catalogue. We fit a fully Bayesian hierarchical model of the main sequence over 1.25 < z < 6 of the form $\rm{{\Psi }}= \rm{\alpha _\mathrm{9.7}}(z) + \rm{\beta }({\rm{M_{\star }}}-9.7) + \mathcal {N}(0,\rm{\sigma }^2)$ while explicitly modelling the outlier distribution. The redshift-dependent intercept at $\rm{{M_{\star }}}=9.7$ is parametrized as $\rm{\alpha _\mathrm{9.7}}(z) = \log {\it N}(1+{\it z})^{\rm{\gamma }} + 0.7$. Our results agree with an increase in normalization of the main sequence to high redshifts that follows the redshift-dependent rate of accretion of gas on to dark matter haloes with $\rm{\gamma }=2.40^{+0.18}_{-0.18}$. We measure a slope and intrinsic scatter of $\rm{\beta }=0.79^{+0.03}_{-0.04}$ and $\rm{\sigma }=0.26^{+0.02}_{-0.02}$. We find that the sampling of the SED provided by the combination of filters (Hubble + ground-based Ks-band + Spitzer 3.6 and 4.5 μm) is insufficient to constrain M⋆ and Ψ over the full dynamic range of the observed main sequence, even at the lowest redshifts studied. While this filter set represents the best current sampling of high-redshift galaxy SEDs out to z > 3, measurements of the main sequence to low masses and high redshifts still strongly depend on priors employed in SED fitting (as well as other fitting assumptions). Future data sets with JWST should improve this.
AIMS: To help cells to better resist the stressful conditions associated with the freeze‐drying process during starter production, we investigated the effect of various osmotic conditions on growth, ...survival and acidification activity of Lactobacillus buchneri R1102, after freeze‐drying and during storage for 3 months at 25°C. METHODS AND RESULTS: High survival rates during freeze‐drying, but not during storage, were obtained when 0·1 mol l⁻¹KCl was added at the beginning of fermentation, without any change in membrane properties and betaine accumulation. This condition made it possible to maintain a high acidification rate throughout the process. In contrast, the addition of 0·6 mol l⁻¹KCl concentrations at the beginning of fermentation led to a high survival rate during storage that was related to high intracellular betaine levels, low membrane fluidity and high cycC19:0 concentrations. However, these modifications induced the degradation of acidification activity during storage. When a moderate stress was applied by combining 0·1 mol l⁻¹KCl at the beginning and 0·6 mol l⁻¹KCl at the end of fermentation, betaine accumulated in the cells without any membrane alteration, allowing them to maintain high acidification activity and survival rate during storage. CONCLUSION: Specific osmotic conditions during fermentation induced intracellular betaine accumulation and modifications of membrane character‐istics, thus affecting stress resistance of Lact. buchneri R1102. A slight osmotic stress made it possible to maintain a high acidification activity, whereas a high osmotic stress at the end of fermentation led to the preservation of cell survival during freeze‐dried storage. SIGNIFICANCE AND IMPACT OF THE STUDY: This study revealed that the survival and preservation of acidification activity of freeze‐dried Lact. buchneri R1102 during starter production can be improved by using appropriate osmotic conditions.
We present a comprehensive study of the physical properties of ∼ 105 galaxies with measurable star formation in the Sloan Digital Sky Survey (SDSS). By comparing physical information extracted from ...the emission lines with continuum properties, we build up a picture of the nature of star-forming galaxies at z < 0.2. We develop a method for aperture correction using resolved imaging and show that our method takes out essentially all aperture bias in the star formation rate (SFR) estimates, allowing an accurate estimate of the total SFRs in galaxies. We determine the SFR density to be 1.915+0.02−0.01 (random)+0.14−0.42 (systematic) h7010−2 M⊙ yr−1 Mpc−3 at z= 0.1 (for a Kroupa initial mass function) and we study the distribution of star formation as a function of various physical parameters. The majority of the star formation in the low-redshift Universe takes place in moderately massive galaxies (1010–1011 M⊙), typically in high surface brightness disc galaxies. Roughly 15 per cent of all star formation takes place in galaxies that show some sign of an active nucleus. About 20 per cent occurs in starburst galaxies. By focusing on the SFR per unit mass we show that the present to past average SFR, the Scalo b-parameter, is almost constant over almost three orders of magnitude in mass, declining only at M* > 1010 M⊙. The volume averaged b parameter is 0.408+0.005−0.002 (random)+0.029−0.090 (systematic)h−170. We use this value to constrain the star formation history of the Universe. For the concordance cosmology the present-day Universe is forming stars at at least 1/3 of its past average rate. For an exponentially declining cosmic star formation history this corresponds to a time-scale of 7+0.7−1.5 Gyr. In agreement with other work we find a correlation between b and morphological type, as well as a tight correlation between the 4000-Å break (D4000) and b. We discuss how D4000 can be used to estimate b parameters for high-redshift galaxies.
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