Evolution of gas-phase metallicity across cosmic time

Chemical enrichment of the interstellar medium (ISM) in galaxies by generations of star-formation is a crucial ingredient to tracing galaxy evolution. Local galaxies have been the rigorously studied owing to their proximity, whereas distant and relatively faint high-redshift (1 < z < 4) galaxies remain poorly understood. In order to trace chemical evolution in high-z galaxies, it is necessary to develop and test new UV nebular diagnostics because at high-z we primarily observe the rest-frame UV spectra. We compare new rest-frame UV and existing optical nebular emission line diagnostics, by applying both sets, for the first time, to the brightest known lensed galaxy at cosmic noon (z~2) RCS0327-E. We infer the metallicity (12+log(O/H)), ionisation parameter (log (q)), electron temperature (T_e), electron density (n_e) and ISM pressure (log(P/k)) via UV and optical emission line diagnostics. Moreover, we extend an existing Bayesian inference code (IZI) to 3D (IZIP), enabling it to infer 12+log(O/H), log (q) and log (P/k) simultaneously. We find that 12+log(O/H) is harder to infer based on UV emission lines alone while inferred values log (q) and T_e are broadly similar for UV and optical diagnostics. UV diagnostics yield a higher (~1.5 dex) pressure and density than optical, because the UV lines probe the inner, denser parts of nebula. We employ the new UV diagnostics to obtain metallicities for the full MEGaSaURA sample -- consisting of rest-frame UV spectra of 22 bright, star-forming, gravitationally lensed galaxies in redshift 1.6 < z < 3.6. We focus on the N3O3 ([N III] 1750/[O III] 1660,6) diagnostic because this ratio has the highest number of detections (N = 11) in our sample. Our sample expands the literature of lensed galaxy metallicities in the above redshift range by ~30%. We compare the N3O3 metallicities with those inferred from IZIP, as well as stellar and optical gas-phase metallicities from the literature. We find it difficult to draw strong conclusions regarding the redshift evolution of metallicity in the MEGaSaURA sample because of large scatter in metallicities -- both intrinsic to the sample and across different diagnostics. Probing distant galaxies not only requires us to study the rest-frame UV regime, but also leads to worsening spatial resolution of the observations. This impacts studies of the spatial distribution of metals, particularly the metallicity gradient { which is crucial to understand gas flow history of galaxies. We quantify the impact of spatial resolution and signal-to-noise ratio (SNR) on inferred metallicity gradients in integral field unit (IFU) datacubes by modeling a suite of synthetic IFU data cubes from a simulation of an isolated, Milky Way-type disk galaxy. We find that coarse resolution leads to artificially shallow inferred metallicity gradients, and that recovery of the true gradient with an accuracy of ~10% requires resolving the galaxy scale length by at least 4-5 resolution elements. Based on these numerical experiments, we propose a method to correct observed metallicity gradients for spatial resolution effects. Thereafter, we correct the observed metallicity gradients of the MaNGA, CALIFA and SAMI samples and study the mass-metallicity gradient relation. We find that while the correction has little effect on the overall average mass-metallicity gradient relation, individual galaxies can undergo significant correction depending on the resolution. Our results are qualitatively consistent across the three different IFU samples -- more massive galaxies exhibit steeper gradients, up to log (M_*/M_sun) < 10.5, above which the the gradients are shallower, likely due to an increased prevalence of major mergers leading to gas mixing. Our models and method of correcting for resolution effects, along with our cross-survey comparisons, will facilitate planning, analysing and comparing current and future large surveys

The effect of non-Gaussianity on error predictions for the Epoch of Reionization (EoR) 21-cm power spectrum

The Epoch of Reionization (EoR) 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We have used semi-numerical simulations to study how this affects the error predictions for the EoR 21-cm power spectrum. We expect $SNR=\sqrt{N_k}$ for a Gaussian random field where $N_k$ is the number of Fourier modes in each $k$ bin. We find that non-Gaussianity is important at high $SNR$ where it imposes an upper limit $[SNR]_l$. For a fixed volume $V$, it is not possible to achieve $SNR > [SNR]_l$ even if $N_k$ is increased. The value of $[SNR]_l$ falls as reionization proceeds, dropping from $\sim 500$ at $\bar{x}_{HI} = 0.8-0.9$ to $\sim 10$ at $\bar{x}_{HI} = 0.15$ for a $[150.08\, {\rm Mpc}]^3$ simulation. We show that it is possible to interpret $[SNR]_l$ in terms of the trispectrum, and we expect $[SNR]_l \propto \sqrt{V}$ if the volume is increased. For $SNR \ll [SNR]_l$ we find $SNR = \sqrt{N_k}/A$ with $A \sim 0.95 - 1.75$, roughly consistent with the Gaussian prediction. We present a fitting formula for the $SNR$ as a function of $N_k$, with two parameters $A$ and $[SNR]_l$ that have to be determined using simulations. Our results are relevant for predicting the sensitivity of different instruments to measure the EoR 21-cm power spectrum, which till date have been largely based on the Gaussian assumption.Comment: Accepted for publication in MNRAS Letters. The definitive version is available at http://mnrasl.oxfordjournals.org/content/449/1/L4

Quantifying the effects of spatial resolution and noise on galaxy metallicity gradients

Metallicity gradients are important diagnostics of galaxy evolution, because they record the history of events such as mergers, gas inflow and star-formation. However, the accuracy with which gradients can be measured is limited by spatial resolution and noise, and hence measurements need to be corrected for such effects. We use high resolution (~20 pc) simulation of a face-on Milky Way mass galaxy, coupled with photoionisation models, to produce a suite of synthetic high resolution integral field spectroscopy (IFS) datacubes. We then degrade the datacubes, with a range of realistic models for spatial resolution (2 to 16 beams per galaxy scale length) and noise, to investigate and quantify how well the input metallicity gradient can be recovered as a function of resolution and signal-to-noise ratio (SNR) with the intention to compare with modern IFS surveys like MaNGA and SAMI. Given appropriate propagation of uncertainties and pruning of low SNR pixels, we show that a resolution of 3-4 telescope beams per galaxy scale length is sufficient to recover the gradient to ~10-20% uncertainty. The uncertainty escalates to ~60% for lower resolution. Inclusion of the low SNR pixels causes the uncertainty in the inferred gradient to deteriorate. Our results can potentially inform future IFS surveys regarding the resolution and SNR required to achieve a desired accuracy in metallicity gradient measurements.Comment: 21 pages, 11 figures, 20 pages Supplementary Online Material provided with 10 additional figures, accepted for publication in MNRA

Spatially Resolved Patchy Lyman-α\alpha Emission Within the Central Kiloparsec of a Strongly Lensed Quasar Host Galaxy at z = 2.8

We report the detection of extended Lyman-$\alpha$ emission from the host galaxy of SDSS~J2222+2745, a strongly lensed quasar at $z = 2.8$. Spectroscopic follow-up clearly reveals extended Lyman-$\alpha$ in emission between two images of the central active galactic nucleus (AGN). We reconstruct the lensed quasar host galaxy in the source plane by applying a strong lens model to HST imaging, and resolve spatial scales as small as $\sim$200 parsecs. In the source plane we recover the host galaxy morphology to within a few hundred parsecs of the central AGN, and map the extended Lyman-$\alpha$ emission to its physical origin on one side of the host galaxy at radii $\sim$0.5-2 kpc from the central AGN. There are clear morphological differences between the Lyman-$\alpha$ and rest-frame ultraviolet stellar continuum emission from the quasar host galaxy. Furthermore, the relative velocity profiles of quasar Lyman-$\alpha$, host galaxy Lyman-$\alpha$, and metal lines in outflowing gas reveal differences in the absorbing material affecting the AGN and host galaxy. These data indicate the presence of patchy local intervening gas in front of the central quasar and its host galaxy. This interpretation is consistent with the central luminous quasar being obscured across a substantial fraction of its surrounding solid angle, resulting in strong anisotropy in the exposure of the host galaxy to ionizing radiation from the AGN. This work demonstrates the power of strong lensing-assisted studies to probe spatial scales that are currently inaccessible by other means.Comment: Accepted to ApJ Letters; 7 pages, 5 figure

A Comparison of UV and Optical Metallicities in Star-forming Galaxies

Our ability to study the properties of the interstellar medium in the earliest galaxies will rely on emission-line diagnostics at rest-frame ultraviolet (UV) wavelengths. In this work, we identify metallicity-sensitive diagnostics using UV emission lines. We compare UV-derived metallicities with standard, well-established optical metallicities using a sample of galaxies with rest-frame UV and optical spectroscopy. We find that the He2-O3C3 diagnostic (He II λ1640 Å/C III] λ1906,1909 Å versus [O III] λ1666 Å/C III] λ1906,9 Å) is a reliable metallicity tracer, particularly at low metallicity (12 log O H 8 + 10( ), where stellar contributions are minimal. We find that the Si3-O3C3 diagnostic ([Si III] λ1883 Å/C III] λ1906 Å versus [O III] λ1666 Å/C III] λ1906,9 Å) is a reliable metallicity tracer, though with large scatter (0.2-0.3 dex), which we suggest is driven by variations in gas-phase abundances. We find that the C4-O3C3 diagnostic (C IV λ 1548,50 Å/[O III] λ 1666 Å versus [O III] λ 1666 Å/C III] λ 1906,9 Å) correlates poorly with optically derived metallicities. We discuss possible explanations for these discrepant metallicity determinations, including the hardness of the ionizing spectrum, contribution from stellar wind emission, and non-solar-scaled gas-phase abundances. Finally, we provide two new UV oxygen abundance diagnostics, calculated from polynomial fits to the model grid surface in the He2-O3C3 and Si3-O3C3 diagrams