H2 emission arises outside photodissociation regions in ultraluminous infrared galaxies

Journal name:
Nature
Volume:
465,
Pages:
60–63
Date published:
DOI:
doi:10.1038/nature09037
Received
Accepted

Ultraluminous infrared galaxies are among the most luminous objects in the local Universe and are thought to be powered by intense star formation1, 2. It has been shown that in these objects the rotational spectral lines of molecular hydrogen observed at mid-infrared wavelengths are not affected by dust obscuration3, but left unresolved was the source of excitation for this emission. Here I report an analysis of archival Spitzer Space Telescope data on ultraluminous infrared galaxies and demonstrate that dust obscuration affects star formation indicators but not molecular hydrogen. I thereby establish that the emission of H2 is not co-spatial with the buried starburst activity and originates outside the obscured regions. This is unexpected in light of the standard view that H2 emission is directly associated with star-formation activity3, 4, 5. I propose the alternative view that H2 emission in these objects traces shocks in the surrounding material that are excited by interactions with nearby galaxies. Large-scale shocks cooling by means of H2 emission may accordingly be more common than previously thought. In the early Universe, a boost in H2 emission by this process may have accelerated the cooling of matter as it collapsed to form the first stars and galaxies, and would make these first structures more readily observable6.

At a glance

Figures

  1. Wavelengths of emission features present in ULIRG spectra and representative opacity curves.
    Figure 1: Wavelengths of emission features present in ULIRG spectra and representative opacity curves.

    If emission regions are embedded in dust or ice, those emission features that are near the peaks of opacity should be the ones most strongly affected. If H2 emission originates inside silicate dust obscuration (example opacities25, 26, 27 in black), then S(3) should be strongly extinguished, S(1) somewhat less so, and other lines still less. If PAH emission originates inside obscuration, the features centred at 8.5μm and 11.3μm should be more affected by silicates than are the other features, and PAH[6.2μm] is the only feature that may be affected by water ice (grey; smoothed data for the whole complex28 and laboratory data for water ice29, shown at the maximum strength according to the S[6.0μm] = 0.6S[9.7μm] relation; Supplementary Information).

  2. PAH features are affected by dust extinction.
    Figure 2: PAH features are affected by dust extinction.

    Evidence for the effect of dust extinction on PAH features comes from correlations of the observed ratios of PAH fluxes (a, b) and the PAH/F(IR) ratios (c, d) with the apparent strength of the silicate opacity feature. Spearman’s rank probabilities of the null hypothesis that the plotted values are uncorrelated are P[NH] = 10-4 (a), P[NH] = 10-5 (c) and P[NH] = 10-5 (d). In b, the observed PAH[11.3μm]/PAH[6.2μm] ratio (grey points) is uncorrelated with the absorption strength (P[NH] = 0.22); but when PAH[6.2μm] is corrected for water-ice absorption (black points), the correlation becomes apparent (P[NH] = 2×10-3), suggesting that the PAH emitting region is located behind both silicate absorption and water-ice absorption. In a and b, PAH ratios calculated for the comparison sample of nearby star-forming galaxies12, 30 are shown by an ellipse whose semi-axes are determined by the standard deviation of the corresponding measure. Although PAH ratios are known to vary as functions of physical conditions30, those seen in ULIRGs are consistent with those found in the comparison sample when extrapolated to low absorption. The dashed lines in ad illustrate how unobscured ratios would change in the presence of an increasing amount of cold dust between the emitter and the observer for representative opacity curves26, 27. In c and d, the model calculation assumes that all absorbed flux is re-emitted at longer wavelengths, but that the total flux does not change. PAHs constitute a greater fraction of the total luminosity output in low-luminosity galaxies (estimates shown with dotted ellipses) than they do in ULIRGs13. The grey shaded areas show the 1σ range in the vertical offset around the best linear fit for ULIRG data. Points denote detections, arrows denote 3σ upper limits, and 1σ standard errors are shown in each panel except b, where individual error bars are omitted for clarity and the median error is shown in the bottom left corner.

  3. H2 emission in ULIRGs is not affected by extinction and shows excess over the H2/PAH ratio observed in normal galaxies.
    Figure 3: H2 emission in ULIRGs is not affected by extinction and shows excess over the H2/PAH ratio observed in normal galaxies.

    The ratios of H2 fluxes (a, b) and the H2/F(IR) ratios (c) for ULIRGs from two samples (points7; crosses3) are uncorrelated with the apparent strength of the silicate absorption. In b, R[J1/J2,J3] is the ratio of the observed flux of the line S(J1) to that expected on the basis of S(J2) and S(J3) assuming all three lines come from a region with a single excitation temperature. Grey areas in a and b and black dashed lines in c show the expected trends of line ratios with apparent silicate strength if H2 emission is behind a screen of dust. Dark grey areas correspond to models with an ortho/para ratio of 3 and a realistic range of excitation temperatures, whereas light grey areas are an extension of the model to include ortho/para ratios between 1 and 3. Although correlations are expected if H2 is affected by silicate absorption, none are detected (P[NH] = 5–75%). In d, if H2 and PAH emission had the same spatial distribution, the H2/PAH ratios would be expected to decrease (dashed lines as in Fig. 2) because dust opacity at the wavelength of the H2 line is greater than that at the wavelength of the PAH feature; but in fact an increase is observed (P[NH] = 0.007 (top), 0.016 (bottom)). The H2/PAH ratios are described better by a model that combines an obscured component of H2 associated with star formation and an unobscured H2 component with luminosity equal to or somewhat greater than that of the obscured component (solid lines: L[H2,outer]/L[H2,inner] = 1 (top), 1.5 (bottom)). Arrows denote 3σ upper and lower limits, and 1σ standard errors are shown for all measurements. Ellipses show flux ratios for comparison galaxies (semi-axes are determined by the standard deviation of the corresponding measure).

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  1. Institute for Advanced Study, Einstein Drive, Princeton, New Jersey 08540, USA

    • Nadia L. Zakamska

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