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Inefficient star formation in extremely metal poor galaxies

Abstract

The first galaxies contain stars born out of gas with few or no ‘metals’ (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation1,2, but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun2,3,4. Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities5,6,7, and while dust has been used to trace gas in low-metallicity galaxies5,8,9,10, low spatial resolution in the far-infrared has typically led to large uncertainties9,10. Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.

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Figure 1: False-colour, multi-wavelength images of our sample galaxies.
Figure 2: Infrared SEDs of individual regions were fitted to derive dust masses.
Figure 3: Seven metal poor star-forming clumps show extremely low star formation efficiencies.

References

  1. 1

    Ostriker, E. C., McKee, C. F. & Leroy, A. K. Regulation of star formation rates in multiphase galactic disks: a thermal/dynamical equilibrium model. Astrophys. J. 721, 975–994 (2010)

    CAS  ADS  Article  Google Scholar 

  2. 2

    Krumholz, M. R. The star formation law in molecule-poor galaxies. Mon. Not. R. Astron. Soc. 436, 2747–2762 (2013)

    CAS  ADS  Article  Google Scholar 

  3. 3

    Bigiel, F. et al. The star formation law in nearby galaxies on sub-kpc scales. Astron. J. 136, 2846–2871 (2008)

    CAS  ADS  Article  Google Scholar 

  4. 4

    Bolatto, A. D. et al. The state of the gas and the relation between gas and star formation at low metallicity: the small Magellanic cloud. Astrophys. J. 741, 12–30 (2011)

    ADS  Article  Google Scholar 

  5. 5

    Elmegreen, B. G. et al. Carbon monoxide in clouds at low metallicity in the dwarf irregular galaxy WLM. Nature 495, 487–489 (2013)

    CAS  ADS  Article  Google Scholar 

  6. 6

    Bolatto, A. et al. The CO-to-H2 conversion factor. Annu. Rev. Astron. Astrophys. 51, 207–268 (2013)

    CAS  ADS  Article  Google Scholar 

  7. 7

    Leroy, A. K. et al. The CO-to-H2 conversion factor from infrared dust emission across the Local Group. Astrophys. J. 737, 12–24 (2011)

    ADS  Article  Google Scholar 

  8. 8

    Fisher, D. et al. The rarity of dust in metal-poor galaxies. Nature 505, 186–189 (2014)

    CAS  ADS  Article  Google Scholar 

  9. 9

    Hunt, L. K. et al. ALMA observations of cool dust in a low-metallicity starburst, SBS 0335–052. Astron. Astrophys. 561, A49 (2014)

    Article  Google Scholar 

  10. 10

    Rémy-Ruyer, A. et al. Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range. Astron. Astrophys. 563, A31 (2014)

    Article  Google Scholar 

  11. 11

    Pettini, M. & Pagel, B. [OIII]/[NII] as an abundance indicator at high redshift. Mon. Not. R. Astron. Soc. 348, L59–L63 (2004)

    CAS  ADS  Article  Google Scholar 

  12. 12

    Kniazev, A. Y. et al. Spectrophotometry of Sextans A and B: chemical abundances of H II regions and planetary nebulae. Astron. J. 130, 1558–1573 (2005)

    CAS  ADS  Article  Google Scholar 

  13. 13

    Bergvall, N. & Ronnback, J. ESO 146–G14, a retarded disc galaxy. Mon. Not. R. Astron. Soc. 273, 603–614 (1995)

    CAS  ADS  Article  Google Scholar 

  14. 14

    Wise, J. et al. The birth of a galaxy: primordial metal enrichment and stellar populations. Astrophys. J. 745, 50–59 (2012)

    ADS  Article  Google Scholar 

  15. 15

    Sandstrom, K. M. et al. The CO-to-H2 conversion factor and dust-to-gas ratio on kiloparsec scales in nearby galaxies. Astrophys. J. 777, 5–37 (2013)

    ADS  Article  Google Scholar 

  16. 16

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

    ADS  Article  Google Scholar 

  17. 17

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

    ADS  Article  Google Scholar 

  18. 18

    Ott, J. et al. VLA-ANGST: A high-resolution HI survey of nearby dwarf galaxies. Astron. J. 144, 123–195 (2012)

    ADS  Article  Google Scholar 

  19. 19

    Peters, S. P. C. et al. The shape of dark matter halos in edge-on galaxies: I. Overview of HI observations. Preprint at http://arxiv.org/abs/1303.2463 (2013)

  20. 20

    Draine, B. T. & Li, A. Infrared emission from interstellar dust. IV. The silicate-graphite-PAH model in the post-Spitzer era. Astrophys. J. 657, 810–837 (2007)

    CAS  ADS  Article  Google Scholar 

  21. 21

    Draine, B. T. et al. Andromeda’s dust. Astrophys. J. 780, 172–189 (2014)

    ADS  Article  Google Scholar 

  22. 22

    Westmoquette, M. S. et al. Piecing together the puzzle of NGC 5253: abundances, kinematics and WR stars. Astron. Astrophys. 550, A88 (2013)

    Article  Google Scholar 

  23. 23

    Leroy, A. et al. The star formation efficiency in nearby galaxies: measuring where gas forms stars effectively. Astrophys. J. 136, 2782–2845 (2008)

    CAS  Google Scholar 

  24. 24

    Daddi, E. et al. Different star formation laws for disks versus starbursts at low and high redshifts. Astrophys. J. 714, L118 (2010)

    CAS  ADS  Article  Google Scholar 

  25. 25

    Cormier, D. et al. The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey. A ground-based follow-up survey of CO(1–0), CO(2–1), and CO(3–2). Astron. Astrophys. 564, A121 (2014)

    Article  Google Scholar 

  26. 26

    Taylor, C. L., Kobulnicky, H. A. & Skillman, E. D. CO emission in low-luminosity, H I-rich galaxies. Astron. J. 116, 2746–2756 (1998)

    CAS  ADS  Article  Google Scholar 

  27. 27

    Hunt, L. et al. The Spitzer view of low-metallicity star formation. III. Fine-structure lines, aromatic features, and molecules. Astrophys. J. 712, 164–187 (2010)

    CAS  ADS  Article  Google Scholar 

  28. 28

    Kuhlen, M. et al. Dwarf galaxy formation with H2-regulated star formation. Astrophys. J. 749, 36–57 (2012)

    ADS  Article  Google Scholar 

  29. 29

    Traficante, A. et al. Data reduction pipeline for the Hi-GAL survey. Mon. Not. R. Astron. Soc. 416, 2932–2943 (2011)

    ADS  Article  Google Scholar 

  30. 30

    Piazzo, L. et al. Artifact removal for GLS map makers by means of post-processing. IEEE Trans. Image Process. 21, 3687–3696 (2012)

    ADS  MathSciNet  Article  Google Scholar 

  31. 31

    Morrissey, P. et al. The calibration and data products of GALEX. Astrophys. J. 173 (Supp.). 682–697 (2007)

    CAS  Article  Google Scholar 

  32. 32

    Dale, D. A. et al. The Spitzer Local Volume Legacy: survey description and infrared photometry. Astrophys. J. 703, 517–556 (2009)

    CAS  ADS  Article  Google Scholar 

  33. 33

    Engelbracht, C. W. et al. Metallicity effects on dust properties in starbursting galaxies. Astrophys. J. 678, 804–827 (2008)

    CAS  ADS  Article  Google Scholar 

  34. 34

    Bertin, E. et al. SExtractor: Software for source extraction. Astron. Astrophys. Suppl. 117, 393–404 (1996)

    ADS  Article  Google Scholar 

  35. 35

    Sauvage, M. Experiments in photometric measurements of extended sources. http://herschel.esac.esa.int/twiki/pub/Public/PacsCalibrationWeb/ExtSrcPhotom.pdf (2011)

  36. 36

    Ali, B. Surface brightness comparison of PACS blue array with IRAS and Spitzer/MIPS images. https://nhscsci.ipac.caltech.edu/pacs/docs/Photometer/PICC-NHSC-TN-029.pdf (2011)

  37. 37

    Paladini, R. et al. Assessment analysis of the extended emission calibration for the PACS red channel. https://nhscsci.ipac.caltech.edu/pacs/docs/Photometer/PICC-NHSC-TR-034.pdf (2012)

  38. 38

    Paladini, R. et al. PACS map-making tools: analysis and benchmarking. http://herschel.esac.esa.int/twiki/pub/Public/PacsCalibrationWeb/pacs_mapmaking_report_ex_sum_v3.pdf (2013)

  39. 39

    Elbaz, D. et al. GOODS-Herschel: an infrared main sequence for star-forming galaxies. Astron. Astrophys. 533, A119 (2011)

    Article  Google Scholar 

  40. 40

    Egami, E. et al. The Herschel Lensing Survey (HLS): Overview. Astron. Astrophys. 518, L12 (2010)

    ADS  Article  Google Scholar 

Download references

Acknowledgements

Y.S. acknowledges support for this work from the Natural Science Foundation of China (NSFC), grant 11373021, the Strategic Priority Research Program ‘The Emergence of Cosmological Structures’ of the Chinese Academy of Sciences (CAS), grant XDB09000000, and Nanjing University grant 985. Y.G. acknowledges support from the NSFC (grants 11173059 and 11390373) and from the CAS Program (grant XDB09000000). J.W. was supported by the National 973 programme (grant 2012CB821805) and by the NSFC (grant 11173013). Z.-Y.Z. acknowledges support from the European Research Council (ERC) in the form of advanced grant COSMICISM. Q.G. was supported by the NSFC (11273015 and 11133001) and by the National 973 programme (grant 2013CB834905). We thank F. Bigiel for making his data points available to plot contours in Fig. 3, S. P. C. Peters for making available his H i gas map of ESO 146-G14 to us, and L. Piazzo for help in Herschel data reduction. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. This work was supported in part by the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. It was also supported in part by a NASA Herschel grant (OT2_yshi_3) issued by JPL/Caltech.

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Contributions

Y.S. led the Herschel proposal, Herschel data reduction and the writing of the manuscript. L.A. helped develop Herschel observations and helped in the writing of the manuscript. G.H. and S.S. assisted in the Herschel proposal. All authors discussed and commented on the manuscript.

Corresponding author

Correspondence to Yong Shi.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Multi-wavelength images of the two galaxies.

a, Images of Sextans A in (left to right) the far-ultraviolet, H i gas, 70 µm, 160 µm and 250 µm dust emission. The large circle is the star-forming disk, small circles are star-forming clumps, and ellipses are diffuse regions. b, Images of ESO 146-G14: wavebands and disks/ellipses as in a.

Extended Data Table 1 PACS and SPIRE photometry for the selected regions
Extended Data Table 2 Spitzer photometry
Extended Data Table 3 Measured sky noises of our observations compared to predictions by HSPOT
Extended Data Table 4 Fitting results
Extended Data Table 5 Gas mass surface densities given by models of different dust types
Extended Data Table 6 Predicted CO and warm H2 line fluxes

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Shi, Y., Armus, L., Helou, G. et al. Inefficient star formation in extremely metal poor galaxies. Nature 514, 335–338 (2014). https://doi.org/10.1038/nature13820

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