The rarity of dust in metal-poor galaxies

Observations of local galaxy I Zw 18 imply that the dust mass in star-forming, metal-poor environments is much lower than expected, and, therefore, that the amount of dust in young galaxies of the early Universe, such as redshift-6.6 galaxy Himiko, is probably a factor of about 100 less than previou...

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Veröffentlicht in:Nature (London) 2014-01, Vol.505 (7482), p.186-189
Hauptverfasser: Fisher, David B., Bolatto, Alberto D., Herrera-Camus, Rodrigo, Draine, Bruce T., Donaldson, Jessica, Walter, Fabian, Sandstrom, Karin M., Leroy, Adam K., Cannon, John, Gordon, Karl
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Sprache:eng
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Zusammenfassung:Observations of local galaxy I Zw 18 imply that the dust mass in star-forming, metal-poor environments is much lower than expected, and, therefore, that the amount of dust in young galaxies of the early Universe, such as redshift-6.6 galaxy Himiko, is probably a factor of about 100 less than previously thought. The early galaxy gathers no dust 'Normal' galaxies observed at redshifts greater than 6, when the Universe was less than 1 billion years old, have so far shown no evidence of the cold dust that accompanies star formation the local Universe. Himiko ( z = 6.6) is typical of such galaxies, estimated to have a low fraction of elements heavier than helium (metallicity), and a dust-to-stellar mass ratio constrained to be less than 0.05%. Its gas mass cannot be determined at this time so to understand the physical conditions under which stars form in these primitive systems it is necessary to study local analogues. One such is I Zw 18, a nearby galaxy with one of the lowest gas-phase heavy-element abundances known. Here David Fisher et al . report new, deep observations from the Herschel Space Observatory that show that I Zw 18 has an extremely low dust mass and dust-to-gas ratio. By applying the dust mass, dust-to-stellar mass ratio and dust-to-gas mass ratio of I Zw 18 to Himiko, the authors calculate that the dust mass for such galaxies is around 50,000 solar masses, roughly 100 times smaller than conventional predictions. If most high- z galaxies are like Himiko, then the prospects for detecting the gas and dust in them are remote. Galaxies observed at redshift z  > 6, when the Universe was less than a billion years old, thus far very rarely show evidence 1 , 2 , 3 of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is around one per cent. A prototypical example is the galaxy Himiko ( z = 6.6), which—a mere 840 million years after the Big Bang—is forming stars at a rate of 30–100 solar masses per year, yielding a mass assembly time of about 150 × 10 6 years. Himiko is thought to have a low fraction (2–3 per cent of the Sun’s) of elements heavier than helium (low metallicity), and although its gas mass cannot yet be determined its dust-to-stellar mass ratio is constrained 3 to be less than 0.05 per cent. The local dwarf galaxy I Zwicky 18, which has a metallicity about 4 per cent that of the Sun’s 4 and is forming stars less rapidly (assembly time about 1.6 × 10 9 years) than Himiko but still vigorous
ISSN:0028-0836
1476-4687
DOI:10.1038/nature12765