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Low gas-phase metallicities of ultraluminous infrared galaxies are a result of dust obscuration

Abstract

Optical spectroscopic measurements show that gas in dusty, starbursting galaxies known as ultraluminous infrared galaxies (ULIRGs) in the local Universe has a significantly lower metal content than that of gas in star-forming galaxies with similar masses. This low metal content has resulted in the claim that ULIRGs are primarily fuelled by metal-poor gas falling into those galaxy merger systems from large distances. Here we report a new set of gas-phase metal abundance measurements taken in local ULIRGs using emission lines at far-infrared wavelengths tracing oxygen and nitrogen. These new data show that ULIRGs lie on the fundamental metallicity relation determined by the stellar mass, metal abundance and star formation rate as the key observational parameters. Instead of metal-poor gas accretion, the new data suggest that the underabundance of metals derived from optical emission lines is probably due to heavy dust obscuration associated with the starburst. As dust-obscured, infrared-bright galaxies dominate the star formation rate density of the Universe during the peak epoch of star formation, we caution the use of rest-frame optical measurements alone to study the metal abundances of galaxies at redshifts of 2–3.

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Fig. 1: MZR of ULIRGs whose metallicities were measured using optical and FIR spectral emission lines.
Fig. 2: Gas-phase metallicities of ULIRGs versus star-forming galaxies in the local Universe.

Data availability

Data supporting this study are publicly available or will be available by June 2022 through the NASA/IPAC Infrared Science Archive at https://irsa.ipac.caltech.edu/applications/sofia/ under Plan ID 08-0095. SOFIA/FIFI-LS observations are publicly available for all the sources except for IRAS 12112+0305 and Mrk273 which will be made available by June 2022.

Code availability

The SOFIA data reduction pipelines and SOSPEX are publicly available at https://github.com/SOFIA-USRA/sofia_redux and https://github.com/darioflute/sospex, respectively. All other code used in this work is available upon reasonable request.

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Acknowledgements

Results in this paper are based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract number NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract number 50-OK-0901 to the University of Stuttgart. Financial support for part of this work was also provided by NASA through award number 80NSS20K0437. J.W. acknowledges support from an STFC Ernest Rutherford Fellowship (ST/P004784/2).

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Contributions

N.C. and A.C. authored the draft version of this paper. N.C. measured line fluxes and FIR metallicities of the sample and conducted the analysis of this paper. A.C., J.M., H.N. and J.W. were PI/co-I in the successful SOFIA proposal and performed the observations. D.F. reduced the SOFIA/FIFI-LS data for the ULIRG sample used in this work. All other coauthors contributed extensively to interpreting the results of this paper and provided comments on this manuscript as part of an internal review process.

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Correspondence to Asantha Cooray.

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Extended data

Extended Data Fig. 1 Metal abundances of galaxies using the oxygen and nitrogen spectral emission lines.

Left: N/O as a function of O/H. The blue line is a third-order polynomial fit to the observed N/O-O/H relation47, which is employed in FIR calibration. The calibration for optical metallicity is based on a relation46 which is shown with an orange line. The discrepancy between the two models directly affects the metallicity measurements and one needs to correct this systematic offset resulting from different assumptions. Right: The blue line shows the FIR metallicity calibration employing observed N/O-O/H relation (blue line in the left panel), while the red line shows the scaled relation which is corrected for the systematic difference between two N/O-O/H models. We use the scaled relation to measure the metallicities to maintain comparable results with optical measurements. For the solar metallicity, we adopt 12 + log(O/H) = 8.6977.

Extended Data Fig. 2 Metal abundances of galaxies depends on the assumptions related to oxygen and nitrogen ratios relative to hydrogen.

Similar to Fig. 1 of the main article, but FIR metallicities are derived from the original calibration without scaling values to the theoretical model of N/O−O/H relation. The MZR of SDSS star-forming galaxies is shown from calibration with a consistent assumption of N/O−O/H relation as FIR metallicity. The plus signs showing the MZR of star-forming SDSS galaxies at z = 0.05 are taken from literature71. The black line is a third-order polynomial fit to SDSS MZR data points.

Extended Data Fig. 3 Line maps and spectra of our sample targeted by SOFIA/FIFI-LS.

Each panel shows the moment 0 map of the spectral line with a yellow ellipse that demonstrates the region over which the spectrum is measured. In the bottom sub-panels, the extracted spectrum is shown along with the best-fit Gaussian function. The blue shaded region around the spectrum corresponds to the 1σ uncertainty of the spectrum. The gray shaded regions on each spectrum show the range of velocity in which the 0th moment line maps are calculated.

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Chartab, N., Cooray, A., Ma, J. et al. Low gas-phase metallicities of ultraluminous infrared galaxies are a result of dust obscuration. Nat Astron 6, 844–849 (2022). https://doi.org/10.1038/s41550-022-01679-y

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