PerspectiveAstronomy

Let There Be Dust

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Science  02 Sep 2011:
Vol. 333, Issue 6047, pp. 1227-1228
DOI: 10.1126/science.1208381

Most of the ordinary matter in the universe is hydrogen and helium. In galaxies such as ours, heavier elements make up only about 1% of the mass, and about half of this is tied up in small particles, termed dust grains, that range in size from a nanometer to a fraction of a micrometer. Interstellar dust contains an appreciable fraction of the carbon and most of the refractory elements, such as magnesium, silicon, and iron. Because these particles are comparable in size to the wavelength of light, they are very effective at absorbing it. As a result, the Milky Way is much fainter in the night sky than it would otherwise be. This absorbed light is reradiated, but because the dust in the interstellar medium is so cold—about 20° above absolute zero—it is radiated at very long wavelengths, at around 200 µm. Such radiation can be observed only from space, and the European Space Agency's Herschel Space Observatory was designed to do just that. On page 1258 of this issue, Matsuura et al. (1) present Herschel observations showing that substantial amounts of dust are created in the aftermath of a supernova, the titanic explosion that terminates the life of a massive star.

From dust to stars.

Interstellar dust is important in the evolution of galaxies. (Left) A Hubble Space Telescope image of a disk galaxy at a redshift of about 1.1, when the universe was about 5 billion years old. (Right) Emission from carbon monoxide is overlaid in green. The molecular gas contains dust and is forming stars at a rate about 100 times that in the Milky Way (14).

CREDIT: R. GENZEL/BERKELEY

Dust is very important in determining the conditions in the interstellar medium (2). When dust absorbs ultraviolet radiation, it emits electrons that provide the dominant heating mechanism for atomic gases. Molecular hydrogen (H2), the most abundant molecule in the universe, is formed primarily on the surfaces of dust grains. By absorbing the ultraviolet radiation emitted by stars, dust enables molecules to survive in the harsh environment of space. Stars form only in regions that are so shielded, so dust is essential for the formation of new stars. Absorption of radiation imparts momentum to the dust grains, which can drive the dust and the associated gas away from a newly formed star or even from an entire galaxy. Dust thus plays a critical role in the evolution of galaxies (see the figure).

Dust has a complex life cycle. Evolved stars produce outflows: those outflows from cool oxygen-rich red giants produce silicate grains, and outflows from cool carbon stars produce carbonaceous grains (3). After the red giant stage of evolution, stars shed their envelopes and become hot white dwarfs. The envelopes are observed as planetary nebulae and contain large amounts of dust. These sources of dust convert preexisting heavy elements into dust; the elements themselves are created in supernovae. Observations of the youngest known supernova remnant in our galaxy, Cas A, suggest that only a small amount [∼0.1 solar mass (M)] of the heavy elements produced in the explosion went into dust grains, but confusion with emission from interstellar dust made it impossible to look for cold dust (temperature ∼20 K) in the remnant (4, 5). Supernova 1987A in the Large Magellanic Cloud (LMC), a companion galaxy of the Milky Way, is much younger than Cas A. Previous observations of this object were insensitive to cold dust, and the inferred dust mass in the supernova ejecta was less than about 0.001 M (6).

The production of interstellar dust is only one part of the story: Dust is also subject to destruction by shock waves, primarily those associated with supernova remnants (7, 8,). Indeed, so efficient is dust destruction by shocks that most of the dust in our galaxy must be due to accretion of refractory elements onto preexisting grain nuclei (3, 911). Dust produced inside a supernova remnant moves at velocities in excess of 1000 km s−1 and is also subject to destruction by shocks as it comes to rest in the interstellar medium.

Young galaxies pose the greatest challenge to our understanding of the creation and destruction of dust. For example, one of the most distant quasars known (Sloan Digital Sky Survey J1148+5251), which we observe only 900 million years after the Big Bang, has about 10 times as much dust as the entire Milky Way (12). If supernovae produce dust efficiently, they would do so at an earlier age than evolved stars, making it easier to understand how so much dust could be produced so early in the evolution of the universe (12).

Matsuura et al. present the first direct evidence that substantial amounts (0.4 to 0.7 M) of dust can be created in supernovae. They took advantage of Herschel's ability to observe cold dust at far infrared wavelengths to detect a much larger amount of dust in supernova 1987A than previously known. In contrast to Cas A, which is much closer, there is no confusing background dust emission, and as a result they can associate the observed dust emission with the supernova remnant. In modeling the emission, they find that most of the abundant refractory elements produced in the supernova must now be in the form of dust.

Matsuura et al. point out that supernovae could inject as much as 0.001 M of dust into the LMC per year. Provided this dust can survive the transition to the interstellar medium, it could account for an appreciable fraction of the dust observed there; it would not eliminate the need for grain growth in the interstellar medium, however. The same conclusion is stronger for the MilkyWay: With a supernova rate estimated to be about 2 per century (13), the injection rate of dust would be about 0.01 M per year, which is small compared to the rate needed to account for the observed amount of dust.

Matsuura et al.'s observation that most of the refractory material in the ejecta of a supernova has condensed into dust is a major step forward. The next steps will be to see if this holds true for other supernova remnants and to determine the actual composition and size distribution of this dust. However, the effect of supernova dust on the interstellar medium in galaxies depends on whether it can survive the shocks that will bring it to rest. Determining the fate of supernova dust—for example, by studying the x-ray emission from the elements released by the grains destroyed in the shocked ejecta—is a challenge for the future.

References and Notes

  1. Image provided courtesy of R. Genzel based on observations reported in (144).
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