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Register a new userRest-frame UV and optical emission line diagnostics of ionised gas properties: a test case in a star-forming knot of a lensed galaxy at z~1.7
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We examine the diagnostic power of rest-frame ultraviolet (UV) nebular emission lines, and compare them to more commonly used rest-frame optical emission lines, using the test case of a single star-forming knot of the bright lensed galaxy RCSGA 032727-132609 at redshift z~1.7. This galaxy has complete coverage of all the major rest-frame UV and optical emission lines from Magellan/MagE and Keck/NIRSPEC. Using the full suite of diagnostic lines, we infer the physical properties: nebular electron temperature (T_e), electron density (n_e), oxygen abundance (log(O/H)), ionisation parameter (log(q)) and interstellar medium (ISM) pressure (log(P/k)). We examine the effectiveness of the different UV, optical and joint UV-optical spectra in constraining the physical conditions. Using UV lines alone we can reliably estimate log(q), but the same is difficult for log(O/H). UV lines yield a higher (~1.5 dex) log(P/k) than the optical lines, as the former probes a further inner nebular region than the latter. For this comparison, we extend the existing Bayesian inference code IZI, adding to it the capability to infer ISM pressure simultaneously with metallicity and ionisation parameter. This work anticipates future rest-frame UV spectral datasets from the James Webb Space Telescope (JWST) at high redshift and from the Extremely Large Telescope (ELT) at moderate redshift.
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For the extremely bright lensed galaxy SDSS J1723+3411 at z=1.3293 , we analyze spatially integrated MMT, Keck, and Hubble Space Telescope spectra that fully cover the rest-frame wavelength range of 1400 to 7200 Angstroms. We also analyze near-IR spectra from Gemini that cover H alpha for a portion of the lensed arc. We report fluxes for 42 detected emission lines, and upper limits for an additional 22. This galaxy has extreme emission line ratios and high equivalent widths that are characteristic of extreme emission-line galaxies. We compute strong emission line diagnostics from both the rest-frame optical and rest-frame ultraviolet (UV), to constrain physical conditions and test the spectral diagnostics themselves. We tightly determine the nebular physical conditions using the most reliable diagnostics, and then compare to results from other diagnostics. We find disappointing performance from the UV--only diagnostics: they either are unable to measure the metallicity or dramatically under-estimate it; they over-estimate the pressure; and the UV diagnostic of ionization parameter has a strong metallicity dependence in this regime. Based on these results, we suggest that upcoming James Webb Space Telescope spectroscopic surveys of galaxies in the reionization epoch should invest the additional integration time to capture the optical [O II] and [O III] emission lines, and not rely solely on the rest-frame UV emission lines. We make available the spectra; they represent one of the highest-quality emission line spectral atlases of star-forming galaxy available beyond the local universe, and will aid planning observations with JWST.
We report on the galaxy MACSJ0032-arc at z=3.6314 discovered during the Herschel Lensing snapshot Survey of massive galaxy clusters, and strongly lensed by the cluster MACSJ0032.1+1808. The successful detections of its rest-frame UV, optical, FIR, millimeter, and radio continua, and of its CO emission enable us to characterize, for the first time at such a high redshift, the stellar, dust, and molecular gas properties of a compact star-forming galaxy with a size smaller than 2.5 kpc, a fairly low stellar mass of 4.8x10^9 Msun, and a moderate IR luminosity of 4.8x10^11 Lsun. We find that the bulk of the molecular gas mass and star formation seems to be spatially decoupled from the rest-frame UV emission. About 90% of the total star formation rate is undetected at rest-frame UV wavelengths because of severe obscuration by dust, but is seen through the thermal FIR dust emission and the radio synchrotron radiation. The observed CO(4-3) and CO(6-5) lines demonstrate that high-J transitions, at least up to J=6, remain excited in this galaxy, whose CO spectral line energy distribution resembles that of high-redshift submm galaxies, even though the IR luminosity of MACSJ0032-arc is ten times lower. This high CO excitation is possibly due to the compactness of the galaxy. We find evidence that this high CO excitation has to be considered in the balance when estimating the CO-to-H2 conversion factor. The inferred depletion time of the molecular gas in MACSJ0032-arc supports the decrease in the gas depletion timescale of galaxies with redshift, although to a lesser degree than predicted by galaxy evolution models. Instead, the measured molecular gas fraction as high as 60-79% in MACSJ0032-arc favors the continued increase in the gas fraction of galaxies with redshift as expected, despite the plateau observed between z~1.5 and z~2.5.
We present the rest-frame optical spectrum of a strongly lensed galaxy at redshift z =1.7 behind the cluster Abell 1689. We detect the temperature sensitive auroral line [O III] 4363, which allows the first direct metallicity measurement for galaxies at z > 1. Our high signal-to-noise spectrum indicates that the target is an extremely low metallicity star-forming galaxy.We estimate an intrinsic absolute B band magnitude of M_{B}=-18.3 pm 0.1$, with a stellar mass of 4.4$pm1.2times10^{8}$ M$_{odot}$. This galaxy extends the luminosity-metallicity relation of star-forming galaxies at z > 2 by more than an order of magnitude. Given the double-nuclei like morphology and velocity profile of ha, we tentatively suggest that it could be a merger or a proto-rotating disk galaxy.
We use extensive spectroscopy from the MOSFIRE Deep Evolution Field (MOSDEF) survey to investigate the relationships between rest-frame optical emission line equivalent widths ($W$) and a number of galaxy and ISM characteristics for a sample of $1134$ star-forming galaxies at redshifts $1.4lesssim zlesssim 3.8$. We examine how the equivalent widths of [OII]$lambdalambda 3727, 3730$, H$beta$, [OIII]$lambdalambda 4960, 5008$, [OIII]$+$H$beta$, H$alpha$, and H$alpha$+[NII]$lambdalambda 6550, 6585$, depend on stellar mass, UV slope, age, star-formation rate (SFR) and specific SFR (sSFR), ionization parameter and excitation conditions (O32 and [OIII]/H$beta$), gas-phase metallicity, and ionizing photon production efficiency ($xi_{rm ion}$). The trend of increasing $W$ with decreasing stellar mass is strongest for [OIII] (and [OIII]+H$beta$). More generally, the equivalent widths of all the lines increase with redshift at a fixed stellar mass or fixed gas-phase metallicity, suggesting that high equivalent width galaxies are common at high redshift. This redshift evolution in equivalent widths can be explained by the increase in SFR and decrease in metallicity with redshift at a fixed stellar mass. Consequently, the dependence of $W$ on sSFR is largely invariant with redshift, particularly when examined for galaxies of a given metallicity. Our results show that high equivalent width galaxies, specifically those with high $W({rm [OIII]})$, have low stellar masses, blue UV slopes, young ages, high sSFRs, ISM line ratios indicative of high ionization parameters, high $xi_{rm ion}$, and low metallicities. As these characteristics are often attributed to galaxies with high ionizing escape fractions, galaxies with high $W$ are likely candidates for the population that dominates cosmic reionization.
We have used FMOS on Subaru to obtain near-infrared spectroscopy of 123 far-infrared selected galaxies in COSMOS and obtain the key rest-frame optical emission lines. This is the largest sample of infrared galaxies with near-infrared spectroscopy at these redshifts. The far-infrared selection results in a sample of galaxies that are massive systems that span a range of metallicities in comparison with previous optically selected surveys, and thus has a higher AGN fraction and better samples the AGN branch. We establish the presence of AGN and starbursts in this sample of (U)LIRGs selected as Herschel-PACS and Spitzer-MIPS detections in two redshift bins (z~0.7 and z~1.5) and test the redshift dependence of diagnostics used to separate AGN from star-formation dominated galaxies. In addition, we construct a low redshift (z~0.1) comparison sample of infrared selected galaxies and find that the evolution from z~1.5 to today is consistent with an evolving AGN selection line and a range of ISM conditions and metallicities from the models of Kewley et al. (2013b). We find that a large fraction of (U)LIRGs are BPT-selected AGN using their new, redshift-dependent classification line. We compare the position of known X-ray detected AGN (67 in total) with the BPT selection and find that the new classification line accurately selects most of these objects (> 70%). Furthermore, we identify 35 new (likely obscured) AGN not selected as such by their X-ray emission. Our results have direct implications for AGN selection at higher redshift with either current (MOSFIRE, KMOS) or future (PFS, MOONS) spectroscopic efforts with near-infrared spectral coverage.
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