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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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## 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.

## References

1. Maiolino, R. & Mannucci, F. De re metallica: the cosmic chemical evolution of galaxies. Astron. Astrophys. Rev. 27, 3 (2019).

2. Chartab, N. et al. The MOSDEF survey: environmental dependence of the gas-phase metallicity of galaxies at 1.4 ≤ z ≤ 2.6. Astrophys. J. 908, 120 (2021).

3. Sattari, Z. et al. Evidence for gas-phase metal deficiency in massive protocluster galaxies at z ~ 2.2. Astrophys. J. 910, 57 (2021).

4. Sanders, D. B. & Mirabel, I. F. Luminous infrared galaxies. Annu. Rev. Astron. Astrophys. 34, 749–792 (1996).

5. Genzel, R. et al. What powers ultraluminous IRAS galaxies? Astrophys. J. 498, 579–605 (1998).

6. Franceschini, A. et al. An XMM-Newton hard X-ray survey of ultraluminous infrared galaxies. Mon. Not. R. Astron. Soc. 343, 1181–1194 (2003).

7. Rigopoulou, D. et al. A large mid-infrared spectroscopic and near-infrared imaging survey of ultraluminous infrared galaxies: their nature and evolution. Astron. J. 118, 2625–2645 (1999).

8. Vega, O. et al. Modelling the spectral energy distribution of ULIRGs. II. The energetic environment and the dense interstellar medium. Astron. Astrophys. 484, 631–653 (2008).

9. Nardini, E. & Risaliti, G. Compton-thick active galactic nuclei inside local ultraluminous infrared galaxies. Mon. Not. R. Astron. Soc. 415, 619–628 (2011).

10. Yuan, T.-T., Kewley, L. J. & Sanders, D. B. The role of starburst-active galactic nucleus composites in luminous infrared galaxy mergers: insights from the new optical classification scheme. Astrophys. J. 709, 884–911 (2010).

11. Stierwalt, S. et al. Mid-infrared properties of nearby luminous infrared galaxies. I. Spitzer infrared spectrograph spectra for the GOALS sample. Astrophys. J. Supp. 206, 1 (2013).

12. Hani, M. H., Gosain, H., Ellison, S. L., Patton, D. R. & Torrey, P. Interacting galaxies in the IllustrisTNG simulations - II: star formation in the post-merger stage. Mon. Not. R. Astron. Soc. 493, 3716–3731 (2020).

13. Blumenthal, K. A. et al. Galaxy interactions in IllustrisTNG-100, I: the power and limitations of visual identification. Mon. Not. R. Astron. Soc. 492, 2075–2094 (2020).

14. Lonsdale, C. J., Farrah, D. & Smith, H. E. in Astrophysics Update 2 (ed. Mason, J. W.) Ch. 9, 285-336 (Springer, 2006).

15. Clements, D. L. et al. Optical imaging of ultraluminous IRAS galaxies: how many are mergers? Mon. Not. R. Astron. Soc. 279, 477–497 (1996).

16. Farrah, D. et al. HST/WFPC2 imaging of the QDOT ultraluminous infrared galaxy sample. Mon. Not. R. Astron. Soc. 326, 1333–1352 (2001).

17. Veilleux, S. et al. Optical and near-infrared imaging of the IRAS 1 Jy sample of ultraluminous infrared galaxies. II. The analysis. Astrophys. J. Suppl. Ser. 143, 315–376 (2002).

18. Veilleux, S. et al. A deep hubble space telescope H-Band imaging survey of massive gas-rich mergers. Astrophys. J. 643, 707–723 (2006).

19. Le Floc’h, E. et al. Infrared luminosity functions from the Chandra Deep Field-South: the Spitzer view on the history of dusty star formation at 0 z 1. Astrophys. J. 632, 169–190 (2005).

20. Rodighiero, G. et al. Mid- and far-infrared luminosity functions and galaxy evolution from multiwavelength Spitzer observations up to z ~ 2.5. Astron. Astrophys. 515, A8 (2010).

21. Magdis, G. E. et al. Herschel reveals a Tdust-unbiased selection of z ~ 2 ultraluminous infrared galaxies. Mon. Not. R. Astron. Soc. 409, 22–28 (2010).

22. Sajina, A. et al. Spitzer- and Herschel-based spectral energy distributions of 24 µm bright z ~ 0.3-3.0 starbursts and obscured quasars. Astrophys. J. 757, 13–22 (2012).

23. Geach, J. E. et al. A redline starburst: CO(2-1) observations of an Eddington-limited galaxy reveal star formation at its most extreme. Astrophys. J. 767, L17 (2013).

24. Ellison, S. L., Mendel, J. T., Patton, D. R. & Scudder, J. M. Galaxy pairs in the Sloan Digital Sky Survey - VIII. The observational properties of post-merger galaxies. Mon. Not. R. Astron. Soc. 435, 3627–3638 (2013).

25. Caputi, K. I. et al. The Optical Spectra of 24 µm Galaxies in the COSMOS Field. I. Spitzer MIPS bright sources in the zCOSMOS-Bright 10k Catalog. Astrophys. J. 680, 939–961 (2008).

26. Liang, Y. C. et al. The luminosity–metallicity relation of distant luminous infrared galaxies. Astron. Astrophys. 423, 867–880 (2004).

27. Rupke, D. S. N., Veilleux, S. & Baker, A. J. The oxygen abundances of luminous and ultraluminous infrared galaxies. Astrophys. J. 674, 172–193 (2008).

28. Roseboom, I. G. et al. FMOS near-IR spectroscopy of Herschel-selected galaxies: star formation rates, metallicity and dust attenuation at z ~ 1. Mon. Not. R. Astron. Soc. 426, 1782–1792 (2012).

29. Kilerci Eser, E., Goto, T. & Doi, Y. Ultraluminous infrared galaxies in the AKARI All-sky survey. Astrophys. J. 797, 54 (2014).

30. Herrera-Camus, R. et al. SHINING, a survey of far-infrared lines in nearby galaxies. II. Line-deficit models, AGN Impact, [C II]-SFR scaling relations, and massmetallicity relation in (U)LIRGs. Astrophys. J. 861, 95 (2018).

31. Montuori, M. et al. The dilution peak, metallicity evolution, and dating of galaxy interactions and mergers. Astron. Astrophys. 518, A56 (2010).

32. Rupke, D. S. N., Kewley, L. J. & Barnes, J. E. Galaxy mergers and the mass-metallicity relation: evidence for nuclear metal dilution and flattened gradients from numerical simulations. Astrophys. J. 710, L156–L160 (2010).

33. Torrey, P. et al. The metallicity evolution of interacting galaxies. Astrophys. J. 746, 108 (2012).

34. Kewley, L. J. et al. Metallicity gradients and gas flows in galaxy pairs. Astrophys. J. 721, L48–L52 (2010).

35. Veilleux, S. et al. Spitzer Quasar and Ulirg Evolution Study (QUEST). IV. Comparison of 1 Jy ultraluminous infrared galaxies with Palomar-Green quasars. Astrophys. J. Suppl. Ser. 182, 628–666 (2009).

36. Kaufman, M. J., Wolfire, M. G. & Hollenbach, D. J. [Si II], [Fe II], [C II], and H2 Emission from massive star-forming regions. Astrophys. J. 644, 283–299 (2006).

37. Fischer, J. et al. A far-infrared spectral sequence of galaxies: trends and models. Astrophys. J. 795, 117 (2014).

38. Liu, X.-W. et al. ISO LWS observations of planetary nebula fine-structure lines. Mon. Not. R. Astron. Soc. 323, 343–361 (2001).

39. Nagao, T., Maiolino, R., Marconi, A. & Matsuhara, H. Metallicity diagnostics with infrared fine-structure lines. Astron. Astrophys. 526, A149 (2011).

40. Pilbratt, G. L. et al. Herschel space observatory. An ESA facility for far-infrared and submillimetre astronomy. Astron. Astrophys. 518, L1 (2010).

41. Poglitsch, A. et al. The Photodetector Array Camera and Spectrometer (PACS) on the Herschel space observatory. Astron. Astrophys. 518, L2 (2010).

42. Griffin, M. J. et al. The Herschel-SPIRE instrument and its in-flight performance. Astron. Astrophys. 518, L3 (2010).

43. Pereira-Santaella, M., Rigopoulou, D., Farrah, D., Lebouteiller, V. & Li, J. Far-infrared metallicity diagnostics: application to local ultraluminous infrared galaxies. Mon. Not. R. Astron. Soc. 470, 1218–1232 (2017).

44. Ferna´ndez-Ontiveros, J. A., P´erez-Montero, E., V´ılchez, J. M., Amor´ın, R. & Spinoglio, L. Measuring chemical abundances with infrared nebular lines: HII-CHI-MISTRY-IR. Astron. Astrophys. 652, A23 (2021).

45. Tremonti, C. A. et al. The origin of the mass-metallicity relation: insights from 53,000 star-forming galaxies in the Sloan Digital Sky Survey. Astrophys. J. 613, 898–913 (2004).

46. Charlot, S. & Longhetti, M. Nebular emission from star-forming galaxies. Mon. Not. R. Astron. Soc. 323, 887–903 (2001).

47. Pilyugin, L. S. & Grebel, E. K. New calibrations for abundance determinations in H II regions. Mon. Not. R. Astron. Soc. 457, 3678–3692 (2016).

48. P´erez-Montero, E. & Contini, T. The impact of the nitrogen-to-oxygen ratio on ionized nebula diagnostics based on [NII] emission lines. Mon. Not. R. Astron. Soc. 398, 949–960 (2009).

49. Steidel, C. C. et al. Strong nebular line ratios in the spectra of z ~ 23 star forming galaxies: first results from KBSS-MOSFIRE. Astrophys. J. 795, 165 (2014).

50. Molla´, M., V´ılchez, J. M., Gavila´n, M. & D´ıaz, A. I. The nitrogen-to-oxygen evolution in galaxies: the role of the star formation rate. Mon. Not. R. Astron. Soc. 372, 1069–1080 (2006).

51. Kennicutt, R. C. J. Star formation in galaxies along the Hubble sequence. Annu. Rev. Astron. Astrophys. 36, 189–232 (1998).

52. Brinchmann, J. et al. The physical properties of star-forming galaxies in the low-redshift Universe. Mon. Not. R. Astron. Soc. 351, 1151–1179 (2004).

53. Kauffmann, G. et al. Stellar masses and star formation histories for 105 galaxies from the Sloan Digital Sky Survey. Mon. Not. R. Astron. Soc. 341, 33–53 (2003).

54. Baldwin, J. A., Phillips, M. M. & Terlevich, R. Classification parameters for the emission-line spectra of extragalactic objects. Publ. Astron. Soc. Pac. 93, 5–19 (1981).

55. Mannucci, F. et al. A fundamental relation between mass, star formation rate and metallicity in local and high-redshift galaxies. Mon. Not. R. Astron. Soc. 408, 2115–2127 (2010).

56. Battersby, C. et al. The Origins Space Telescope. Nat. Astron. 2, 596–599 (2018).

57. Chabrier, G. Galactic stellar and substellar initial mass function. Publ. Astron. Soc. Pac. 115, 763–795 (2003).

58. Komatsu, E. et al. Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: cosmological interpretation. Astrophys. J. Suppl. Ser. 192, 18 (2011).

59. Farrah, D. et al. Far-infrared fine-structure line diagnostics of ultraluminous infrared galaxies. Astrophys. J. 776, 38 (2013).

60. Sturm, E. et al. Massive molecular outflows and negative feedback in ULIRGs observed by Herschel-PACS. Astrophys. J. 733, L16 (2011).

61. Herrera-Camus, R. et al. SHINING, a survey of far-infrared lines in nearby galaxies. I. Survey description, observational trends, and line diagnostics. Astrophys. J. 861, 94 (2018).

62. D´ıaz-Santos, T. et al. A Herschel/PACS far-infrared line emission survey of local luminous infrared galaxies. Astrophys. J. 846, 32 (2017).

63. Saunders, W. et al. The PSCz catalogue. Mon. Not. R. Astron. Soc. 317, 55–63 (2000).

64. Armus, L. et al. Observations of ultraluminous infrared galaxies with the infrared spectrograph on the Spitzer space telescope. II. The IRAS bright galaxy sample. Astrophys. J. 656, 148–167 (2007).

65. Farrah, D. et al. High-resolution mid-infrared spectroscopy of ultraluminous infrared galaxies. Astrophys. J. 667, 149–169 (2007).

66. Desai, V. et al. PAH emission from ultraluminous infrared galaxies. Astrophys. J. 669, 810–820 (2007).

67. Temi, P., Hoffman, D., Ennico, K. & Le, J. SOFIA at full operation capability: technical performance. J. Astron. Instrum. 7, 1840011–1840186 (2018).

68. Fischer, C. et al. FIFI-LS: the field-imaging far-infrared line spectrometer on SOFIA. J. Astron. Instrum. 7, 1840003–1840556 (2018).

69. Fadda, D. & Chambers, E. T. SOSPEX, an interactive tool to explore SOFIA spectral cubes. Am. Astron. Soc. Meet. Abstr. 231, 150.11 (2018).

70. Peng, B. et al. Far-infrared line diagnostics: improving N/O abundance estimates for dusty galaxies. Astrophys. J. 908, 166 (2021).

71. Pilyugin, L. S., Grebel, E. K., Zinchenko, I. A., Nefedyev, Y. A. & Mattsson, L. On the influence of the environment on galactic chemical abundances. Mon. Not. R. Astron. Soc. 465, 1358–1374 (2017).

72. U, V. et al. Spectral energy distributions of local luminous and ultraluminous infrared galaxies. Astrophys. J. Suppl. Ser. 203, 9 (2012).

73. da Cunha, E. et al. Exploring the physical properties of local star-forming ULIRGs from the ultraviolet to the infrared. Astron. Astrophys. 523, A78 (2010).

74. Monreal-Ibero, A., Colina, L., Arribas, S. & Garc´ıa-Mar´ın, M. Search for tidal dwarf galaxy candidates in a sample of ultraluminous infrared galaxies. Astron. Astrophys. 472, 421–433 (2007).

75. Hou, L. G., Wu, Xue-Bing & Han, J. L. Ultra-luminous infrared galaxies in Sloan Digital Sky Survey data release 6. Astrophys. J. 704, 789–802 (2009).

76. Kauffmann, G. et al. The host galaxies of active galactic nuclei. Mon. Not. R. Astron. Soc. 346, 1055–1077 (2003).

77. Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009).

## 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).

## Author information

Authors

### 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.

### Corresponding author

Correspondence to Asantha Cooray.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review

### Peer review information

Nature Astronomy thanks Filippo Mannucci and Francois Hammer for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## 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 (2022). https://doi.org/10.1038/s41550-022-01679-y

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• DOI: https://doi.org/10.1038/s41550-022-01679-y