Detection of an oxygen emission line from a high-redshift galaxy in the reionization epoch
Shining brightly in the early universe
Galaxies that formed early in the history of the universe were powerful sources of ultraviolet radiation. This radiation ionized the surrounding intergalactic medium during the “epoch of reionization.” Inoue et al. detected atomic emission lines from a galaxy at high redshift—seen as it was when the universe was only ~5% of its current age (see the Perspective by De Breuck). Data from optical, infrared, and submillimeter observatories determined its gas and dust content and the amount of ultraviolet radiation it emitted. Studying similar galaxies in such a manner will allow astronomers to determine how the first galaxies formed, evolved, and influenced their surroundings.
Abstract
The physical properties and elemental abundances of the interstellar medium in galaxies during cosmic reionization are important for understanding the role of galaxies in this process. We report the Atacama Large Millimeter/submillimeter Array detection of an oxygen emission line at a wavelength of 88 micrometers from a galaxy at an epoch about 700 million years after the Big Bang. The oxygen abundance of this galaxy is estimated at about one-tenth that of the Sun. The nondetection of far-infrared continuum emission indicates a deficiency of interstellar dust in the galaxy. A carbon emission line at a wavelength of 158 micrometers is also not detected, implying an unusually small amount of neutral gas. These properties might allow ionizing photons to escape into the intergalactic medium.
The physical and chemical conditions of the interstellar medium (ISM) in galaxies can be revealed with forbidden atomic emission lines from the warm-phase ISM, such as ionized hydrogen (H ii) regions and photodissociation regions (PDRs). A far-infrared (FIR) forbidden emission line, the [C ii] 158-μm line predominantly coming from PDRs, has already been detected in many high-z objects (1, 2). Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed the [C ii] line emission from young star-forming galaxies emitting a strong hydrogen Lyα line, so-called Lyα emitters (LAEs), at redshift z ~ 5 to 6 (3, 4). However, ALMA observations have also shown that LAEs at z > 6 have at least an order of magnitude lower luminosity of the [C ii] line than that expected from their star formation rate (SFR) (4–7), suggesting unusual ISM conditions in these high-z LAEs (8).
Herschel observations of nearby dwarf galaxies, on the other hand, have revealed that a forbidden oxygen line, [O iii] 88 μm, is much stronger than the [C ii] line in these chemically unevolved galaxies (9–11). The Infrared Space Observatory and the Japanese infrared astronomical satellite AKARI have detected the [O iii] line from the Large Magellanic Cloud and from many nearby galaxies (12, 13). However, the [O iii] line has rarely been discussed in a high-z context, because of the lack of instruments suitable to observe the redshifted line. Only a few detections from gravitationally lensed dusty starburst galaxies with active galactic nuclei at z ~ 3 to 4 have been reported (14, 15) prior to ALMA. On the other hand, simulations predict that ALMA will be able to detect the [O iii] line from star-forming galaxies with reasonable integration time even at z > 8 (16).
To examine the [O iii] 88-μm line in high-z LAEs, we performed ALMA observations of an LAE at z = 7.2, SXDF-NB1006-2, discovered with the Subaru Telescope (17). We have also obtained ALMA data of the [C ii] 158-μm line of this galaxy. [The observations and the data reduction are described in (18)]. The [O iii] line is detected with a significance of 5.3σ (Fig. 1A), and the obtained line flux is 6.2 × 10–21 W m–2; the corresponding luminosity is 3.8 × 1035 W (Table 1). The [C ii] line is not detected at the position of the [O iii] emission line, and we take the 3σ upper limit for the [C ii] line flux as <5.3 × 10–22 W m–2. However, we note a marginal signal (3.5σ) that displays a spatial offset (≈ 0.4′′ ≈ 2 kpc in the proper distance) from the [O iii] emission (fig. S4). The continuum is not detected in either of the ALMA bands, resulting in a 3σ upper limit of the total IR luminosity of <2.9 × 1037 W when assuming a dust temperature of 40 K and an emissivity index of 1.5.
Fig. 1 [O iii] 88-μm and Lyα emission images and spectra of SXDF-NB1006-2.
(A) The ALMA [O iii] 88-μm image (contours) overlaid on the Subaru narrow-band Lyα image (offsets from the position listed in Table 1). Contours are drawn at (−2, 2, 3, 4, 5) × σ, where σ = 0.0636 Jy beam–1 km s–1. Negative contours are shown by the dotted line. The ellipse at lower left represents the synthesized beam size of ALMA. (B) The ALMA [O iii] 88-μm spectrum with resolution of 20 km s–1 at the intensity peak position shown against the relative velocity with respect to the redshift z = 7.2120 (blue dashed line). The best-fit Gaussian profile for the [O iii] line is overlaid. The RMS noise level is shown by the dotted line. (C) The Lyα spectrum (17) shown as a function of the relative velocity compared to the [O iii] 88-μm line. The flux density is normalized by a unit of 10–18 erg s–1 cm–2 Å–1. The sky level on an arbitrary scale is shown by the dotted line. The velocity intervals where Earth’s atmospheric lines severely contaminate the spectrum are flagged (hatched boxes). The Lyα line shows a velocity shift Δv ≈ +110 km s–1 relative to the [O iii] line (red dashed line).
The spatial distribution of the ALMA [O iii] emission overlaps with that of the Subaru Lyα emission (Fig. 1A), as expected because both emission lines are produced in the same ionized gas. On the other hand, the Lyα emission is well resolved (the image resolution is 0.4′′) and spatially more extended than the [O iii] line. This is because Lyα photons suffer from resonant scattering by neutral hydrogen atoms in the gas surrounding the galaxy. The systemic redshift of the galaxy is estimated at z = 7.2120 ± 0.0003 from the [O iii] emission line at an observed wavelength of 725.603 μm. The Lyα line is located at ΔvLyα = +1.1 (±0.3) × 102 km s–1 relative to the systemic redshift (Fig. 1C and fig. S6). This velocity offset, caused by scattering of neutral hydrogen, is relatively small by comparison to those observed in galaxies at z ~ 2 to 3 (ΔvLyα ~ 300 km s–1), given the ultraviolet (UV) absolute magnitude of this galaxy (MUV = −21.53 magAB) (19–21). The observed small ΔvLyα of SXDF-NB1006-2 may indicate an H i column density of NH i < 1020 cm–2 (21, 22). SXDF-NB1006-2 is in the reionization era where only the intergalactic medium (IGM) with a high hydrogen neutral fraction may cause an observation of ΔvLyα ≈ +100 km s–1 (23), implying an even smaller H i column density in the ISM of this galaxy.
We performed spectral energy distribution (SED) modeling to derive physical quantities such as the SFR of SXDF-NB1006-2 (Table 1). In addition to broadband photometric data from the United Kingdom Infra-Red Telescope (UKIRT) J, H, and K bands, Spitzer 3.6-μm and 4.5-μm bands, and Subaru narrowband photometry NB1006 (table S3), we have also used the [O iii] line flux and the IR luminosity as constraints (Fig. 2) (18). The extremely blue rest-frame UV color of this galaxy [slope β < −2.6 (3σ) estimated from J − H, where the flux density Fλ ∝ λβ] gives an age of ~1 million years for the ongoing star formation episode. The nondetection of the dust IR emission suggests little dust and hence negligible attenuation. The observed strong [O iii] line flux favors an oxygen abundance of 5% to 100% that of the Sun, but rejects 2% and 200% of the solar abundance at >95% confidence. The obtained oxygen abundance is similar to those estimated in galaxies at z ~ 6 to 7, for which UV C iii] and C iv emission lines were detected (24, 25). Because ~1 million years is insufficient to produce the inferred oxygen abundance, the galaxy must have had previous star formation episodes. Therefore, the derived stellar mass of ~300 million solar masses is regarded as a lower limit. We obtain a ~50% escape fraction of hydrogen-ionizing photons to the IGM in the best-fit model. Such a high escape fraction, although still uncertain, may imply a low H i column density of ~1017 cm–2 (26) or porous structure in the ISM of the galaxy.
Fig. 2 Spectral energy distribution of SXDF-NB1006-2.
(A) Thumbnail images (4′′ × 4′′; north is up, east is to the left) in the Subaru/Suprime-Cam z′, NB1006, UKIRT/WFCAM J, H, and K bands, and Spitzer/IRAC 3.6-μm and 4.5-μm bands, from left to right. (B) Near-infrared photometric data with the best-fit model. The bottom horizontal axis shows the wavelength in the observer’s rest frame; the upper axis shows the wavelength in the source rest frame. The observations are marked by the circles. The horizontal error bars show the wavelength range of the band filters. The vertical error bars for detection bands represent ±1σ photometric uncertainties; the downward arrows for nondetections represent the 3σ upper limits. The z′ point in gray is not used for the model fit. The best-fit model spectrum is shown by the solid green line, and the corresponding magnitudes through the filters are indicated by asterisks. (C) The observed flux with the ±1σ uncertainty of the [O iii] line and the best-fit model prediction (asterisk). (D) The 3σ upper limit on the total infrared luminosity with a dust temperature of 40 K and an emissivity index of 1.5. Also shown is the best-fit model prediction (asterisk; zero IR luminosity due to absence of dust in the best-fit model).
The [O iii]/far-UV luminosity ratio of SXDF-NB1006-2 is similar to those of nearby dwarf galaxies with an oxygen abundance of 10% to 60% that of the Sun (Fig. 3A), which suggests that the oxygen abundance estimated from the SED modeling is reasonable and chemical enrichment in this young galaxy has already proceeded. On the other hand, the dust IR continuum and the [C ii] line of SXDF-NB1006-2 are very weak relative to those of the nearby dwarf galaxies (Fig. 3, B and C). The galaxies at z ~ 3 to 4, from which the [O iii] line was detected previously, are IR luminous dusty ones (14, 15). Their [O iii]/IR and [O iii]/[C ii] luminosity ratios are similar to those of nearby spiral galaxies (10) and are at least one order of magnitude smaller than those of SXDF-NB1006-2. The high [O iii]/IR ratio of SXDF-NB1006-2, despite a degree of chemical enrichment (or so-called metallicity) similar to that of the nearby dwarf galaxies, indicates a very small mass fraction of dust in elements heavier than helium (or dust-to-metal mass ratio) in SXDF-NB1006-2. The dust deficiency of this galaxy is in contrast to the discovery of a dusty galaxy at z ≈ 7.5 (27), suggesting a diversity of the dust content in the reionization epoch. Because the [C ii] line predominantly arises in gas where hydrogen is neutral, the nondetection of the [C ii] line in SXDF-NB1006-2 suggests that this young galaxy has little H i gas.
Fig. 3 Comparisons of SXDF-NB1006-2 and other galaxies detected in the [O iii] line.
The horizontal axis represents the oxygen abundance relative to the Sun on a logarithmic scale: [O/H] = log10(nO/nH) − log10(nO/nH)☉, where nO and nH are the number density of oxygen and hydrogen atoms, respectively, and the solar abundance is assumed to be 12 + log10(nO/nH)☉ = 8.69 (30). Circles with error bars represent data of nearby dwarf galaxies (9–11); inverted triangles with error bars are averages of nearby spiral galaxies (13). The arrows at the right-side axis show luminosity ratios of dusty galaxies at z ~ 3 to 4 whose oxygen abundances have not yet been measured (10, 14, 15). Data from SXDF-NB1006-2 are shown as five-pointed stars with error bars. (A) The [O iii]/far-UV (FUV) luminosity ratio. The FUV luminosity is νLν at about 1500 Å in the source rest frame. (B) The [O iii]/total infrared (IR) luminosity ratio. The IR wavelength range is 8 to 1000 μm in the source rest frame. Because the IR continuum of SXDF-NB1006-2 is not detected, we show a 3σ lower limit with a dust temperature of 40 K and an emissivity index of 1.5. (C) The [O iii]/[C ii] luminosity ratio. Because the [C ii] 158-μm line of SXDF-NB1006-2 is not detected, we show a 3σ lower limit.
We also compared the observed properties of SXDF-NB1006-2 with the galaxies at z = 7.2 in a cosmological hydrodynamic simulation of galaxy formation and evolution (18). This simulation yielded several galaxies with a UV luminosity similar to that of SXDF-NB1006-2 (fig. S10). Relative to these simulated galaxies, SXDF-NB1006-2 has the highest [O iii] line luminosity, a similar oxygen abundance, and a lower dust IR luminosity by at least a factor of 2 to 3. This indicates a factor of >2 to 3 smaller dust-to-metal mass ratio within the ISM of SXDF-NB1006-2 relative to that in the simulated galaxies, where we assumed the dust-to-metal mass ratio of 50% as in the Milky Way ISM (28). Therefore, the dust-to-metal ratio of SXDF-NB1006-2 is implied to be <20%. The dust-to-metal ratio is determined by two processes: (i) dust growth by accretion of atoms and molecules onto the existing grains in cold dense clouds, and (ii) dust destruction by supernova (SN) shock waves consequent upon star formation (28). The dust-poor nature of SXDF-NB1006-2 may be explained by rapid dust destruction due to its high SN rate or by slow accretion growth due to a lack of cold dense clouds in the ISM.
In the context of cosmic reionization studies, the most uncertain parameter is the product of the escape fraction of ionizing photons and the emission efficiency of these photons, fescξion (29). From the SED modeling, we have obtained log10[fescξion(Hz erg–1)] = 25.44−0.84+0.46 for SXDF-NB1006-2 (18). This ionizing photon emission efficiency is strong enough to reach (or even exceed) the cosmic ionizing photon emissivity at z ~ 7 estimated from various observational constraints on reionization (29) by an accumulation of galaxies that have already been detected (MUV < −17), although this does not rule out contributions of fainter, currently undetected galaxies to the ionizing emissivity. The ISM properties of SXDF-NB1006-2, with little dust and H i gas, may make this galaxy a prototypical example of a source of cosmic reionization.
| Right ascension (J2000) | 2h18m56.536s (±0.002s) |
|---|---|
| Declination (J2000) | −5°19′58.87′′ (±0.02′′) |
| Redshift of [O iii] 88-μm line | 7.2120 ± 0.0003 |
| Lyα velocity shift (km s−1) | +1.1 (±0.3) × 102 |
| [O iii] 88-μm luminosity (W)* | 3.8 (±0.8) × 1035 |
| [C ii] 158-μm luminosity (W)* | <3.2 × 1034 (3σ) |
| Total IR luminosity (W)* | <2.9 × 1037 (3σ) |
| Oxygen abundance [O/H] | |
| Star formation rate [log10(M☉ year−1)]† | |
| Star formation age [log10(years)] | |
| Dust attenuation (EB−V mag) | |
| Escape fraction of ionizing photons | |
| Stellar mass (log10 M☉)† |
Table 1 A summary of the observed and estimated properties of SXDF-NB1006-2.
*Assuming a concordance cosmology with present-day Hubble parameter H0 = 70 km s-1 Mpc-1, density parameter of matter ΩM = 0.3, and density parameter of the cosmological constant ΩΛ = 0.7. †M☉ represents the solar mass (1.989 × 1030 kg).
Acknowledgments
This paper makes use of the following ALMA data: ADS/JAO.ALMA# 2013.1.01010.S and 2012.1.00374.S, which are available at https://almascience.nao.ac.jp/alma-data/archive. ALMA is a partnership of the European Southern Observatory (ESO) (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, the National Radio Astronomy Observatory/Associated Universities Inc., and the National Astronomical Observatory of Japan (NAOJ). Based in part on data collected at Subaru Telescope, which is operated by NAOJ; data are available at http://smoka.nao.ac.jp/ under project codes S08B-019, S08B-051, and S09B-055, and also at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; data are available at www2.keck.hawaii.edu/koa/public/koa.php under the project code S331D. When some of the data reported here were acquired, UKIRT was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK; UKIDSS data are available at http://wsa.roe.ac.uk//dr10plus_release.html. Based in part on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA; data are available at www.cfa.harvard.edu/SEDS/data.html. Supported by JSPS KAKENHI grants 26287034 (A.K.I. and K.M.), 26247022 (I.S.), 25287050 (N.Y.), 24740112 (T.O.), 15H02073 (Y.T.), and 15K17616 (B.H.), and by a grant-in-aid for JSPS Fellows (H.U.), by a grant-in-aid for the Global COE Program “The Next Generation of Physics, Spun from Universality and Emergence” from MEXT of Japan (K.O.), the Kavli Institute Fellowship (at Kavli Institute for Cosmology, University of Cambridge) supported by the Kavli Foundation (K.O.), and the Swedish Research Council (project 2011-5349) and the Wenner-Gren Foundations (E.Z.).
Supplementary Material
Summary
Materials and Methods
Figs. S1 to S12
Tables S1 to S4
Resources
File (inoue.sm.pdf)
References and Notes
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Volume 352 | Issue 6293
24 June 2016
24 June 2016
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Copyright © 2016, American Association for the Advancement of Science.
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Received: 14 December 2015
Accepted: 16 May 2016
Published in print: 24 June 2016
Acknowledgments
This paper makes use of the following ALMA data: ADS/JAO.ALMA# 2013.1.01010.S and 2012.1.00374.S, which are available at https://almascience.nao.ac.jp/alma-data/archive. ALMA is a partnership of the European Southern Observatory (ESO) (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, the National Radio Astronomy Observatory/Associated Universities Inc., and the National Astronomical Observatory of Japan (NAOJ). Based in part on data collected at Subaru Telescope, which is operated by NAOJ; data are available at http://smoka.nao.ac.jp/ under project codes S08B-019, S08B-051, and S09B-055, and also at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; data are available at www2.keck.hawaii.edu/koa/public/koa.php under the project code S331D. When some of the data reported here were acquired, UKIRT was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK; UKIDSS data are available at http://wsa.roe.ac.uk//dr10plus_release.html. Based in part on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA; data are available at www.cfa.harvard.edu/SEDS/data.html. Supported by JSPS KAKENHI grants 26287034 (A.K.I. and K.M.), 26247022 (I.S.), 25287050 (N.Y.), 24740112 (T.O.), 15H02073 (Y.T.), and 15K17616 (B.H.), and by a grant-in-aid for JSPS Fellows (H.U.), by a grant-in-aid for the Global COE Program “The Next Generation of Physics, Spun from Universality and Emergence” from MEXT of Japan (K.O.), the Kavli Institute Fellowship (at Kavli Institute for Cosmology, University of Cambridge) supported by the Kavli Foundation (K.O.), and the Swedish Research Council (project 2011-5349) and the Wenner-Gren Foundations (E.Z.).
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