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Register a new userObtaining Global Galaxy Metallicities Using Emission Line EWs
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Added by
Henry A. Kobulnicky
Publication date
2003
fields
Physics
and research's language is
English
Authors
Chip Kobulnicky
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We develop a prescription for estimating the interstellar medium oxygen abundances of distant star-forming galaxies using the ratio EWR_{23} formed from the equivalent widths of the [O II] 3727, [O III] 4959,5007 and Hbeta nebular emission lines. This EWR_{23} approach essentially identical to the widely-used R_{23} method of Pagel etal (1979). Using data from three spectroscopic surveys of nearby galaxies, we conclude that the emission line equivalent width ratios are a good substitute for emission line flux ratios in galaxies with active star formation. The RMS dispersion between EWR_{23} and the reddening-corrected R_{23} values is sigma(log R_{23})leq0.08 dex. This dispersion is comparable to the emission-line measurement uncertainties for distant galaxies in many spectroscopic galaxy surveys, and is somewhat smaller than the uncertainties of sigma(O/H)~0.15 dex inherent in strong-line metallicity calibrations. Because equivalent width ratios are partially insensitive to interstellar reddening effects, emission line equivalent width ratios should be superior to flux ratios when reddening corrections are not available. The EWR_{23} method presented here is likely to be most useful for statistically estimating the mean metallicities for large samples of galaxies to trace their chemical properties as a function of redshift or environment. The approach developed here is used in a companion paper (Kobulnicky etal 2003) to measure the metallicities of star-forming galaxies at z=0.2 - 0.8 in the Deep Extragalactic Evolutionary Probe spectroscopic survey of the Groth Strip.
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We develop a prescription for estimating the interstellar medium oxygen abundances of distant star-forming galaxies using the ratio EWR_{23} formed from the equivalent widths of the [O II] 3727, [O III] 4959,5007 and Hbeta nebular emission lines. This EWR_{23} approach essentially identical to the widely-used R_{23} method of Pagel et. al (1979). Using data from three spectroscopic surveys of nearby galaxies, we conclude that the emission line equivalent width ratios are a good substitute for emission line flux ratios in galaxies with active star formation. The RMS dispersion between EWR_{23} and the reddening-corrected R_{23} values is sigma(log(R_{23})) < 0.08 dex. This dispersion is comparable to the emission-line measurement uncertainties for distant galaxies in many spectroscopic galaxy surveys, and is smaller than the uncertainties of sigma(O/H) ~ 0.15 dex inherent in strong-line metallicity calibrations. Because equivalent width ratios are, to first order, insentitive to interstellar reddening, emission line equivalent width ratios may actually be superior to flux ratios when reddening corrections are not available. The EWR_{23} method presented here is likely to be most useful for statistically estimating the mean metallicities for large samples of galaxies to trace their chemical properties as a function of redshift or environment. The approach developed here is used in a companion paper (Kobulnicky et. al 2003) to measure the metallicities of star-forming galaxies at z=0.2-0.8 in the Deep Extragalactic Evolutionary Probe spectroscopic survey of the Groth Strip.
We derive direct measurement gas-phase metallicities of $7.4 < 12 + log(O/H) < 8.4$ for 14 low-mass Emission Line Galaxies (ELGs) at $0.3 < z < 0.8$ identified in the Faint Infrared Grism Survey (FIGS). We use deep slitless G102 grism spectroscopy of the Hubble Ultra Deep Field (HUDF), dispersing light from all objects in the field at wavelengths between 0.85 and 1.15 microns. We run an automatic search routine on these spectra to robustly identify 71 emission line sources, using archival data from VLT/MUSE to measure additional lines and confirm redshifts. We identify 14 objects with $0.3 < z < 0.8$ with measurable O[III]$lambda$4363 AA emission lines in matching VLT/MUSE spectra. For these galaxies, we derive direct electron-temperature gas-phase metallicities with a range of $7.4 < 12 + log(O/H) < 8.4$. With matching stellar masses in the range of $10^{7.9} M_{odot} < M_{star} < 10^{10.4} M_{odot}$, we construct a mass-metallicity (MZ) relation and find that the relation is offset to lower metallicities compared to metallicities derived from alternative methods (e.g.,$R_{23}$, O3N2, N2O2) and continuum selected samples. Using star formation rates (SFR) derived from the $Halpha$ emission line, we calculate our galaxies position on the Fundamental Metallicity Relation (FMR), where we also find an offset toward lower metallicities. This demonstrates that this emission-line-selected sample probes objects of low stellar masses but even lower metallicities than many comparable surveys. We detect a trend suggesting galaxies with higher Specific Star Formation (SSFR) are more likely to have lower metallicity. This could be due to cold accretion of metal-poor gas that drives star formation, or could be because outflows of metal-rich stellar winds and SNe ejecta are more common in galaxies with higher SSFR.
We measured the metallicity Z in the broad emission line regions (BELRs) of 43 SDSS quasars with the strongest N IV] and N III] emission lines. These N-Loud QSOs have unusually low black hole masses. We used the intensity ratio of N lines to collisionally-excited emission lines of other heavy elements to find metallicities in their BELR regions. We found that 7 of the 8 line-intensity ratios that we employed give roughly consistent metallicities as measured, but that for each individual QSO their differences from the mean of all metallicity measurements depends on the ionization potential of the ions that form the emission lines. After correcting for this effect, the different line-intensity ratios give metallicities that generally agree to within the 0.24 dex uncertainty in the measurements of the line-intensity ratios. The metallicities are very high, with mean log Z for the whole sample of 5.5 Z_sun and a maximum of 18 Z_sun. Our results argue against the possibility that the strong N lines represent an overabundance only of N but not of all heavy elements. They are compatible with either (1) the BELR gas has been chemically enriched by the general stellar population in the central bulge of the host galaxy but the Locally Optimally-emitting Cloud model used in the analysis needs some fine tuning, or (2) that instead this gas has been enriched by intense star formation on the very local scale of the active nucleus that has resulted in an abundance gradient within the BELR.
The Sloan Digital Sky Survey IV extended Baryonic Oscillation Spectroscopic Survey (SDSS-IV/eBOSS) will observe 195,000 emission-line galaxies (ELGs) to measure the Baryonic Acoustic Oscillation standard ruler (BAO) at redshift 0.9. To test different ELG selection algorithms, 9,000 spectra were observed with the SDSS spectrograph as a pilot survey based on data from several imaging surveys. First, using visual inspection and redshift quality flags, we show that the automated spectroscopic redshifts assigned by the pipeline meet the quality requirements for a reliable BAO measurement. We also show the correlations between sky emission, signal-to-noise ratio in the emission lines, and redshift error. Then we provide a detailed description of each target selection algorithm we tested and compare them with the requirements of the eBOSS experiment. As a result, we provide reliable redshift distributions for the different target selection schemes we tested. Finally, we determine an target selection algorithms that is best suited to be applied on DECam photometry because they fulfill the eBOSS survey efficiency requirements.
We present multiwaveband photometric and optical spectropolarimetric observations of the R=15.9 narrow emission line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infra-red and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray errorbox and so conclude that R117_A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117_A.
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