Dusty Galaxy in the Early Universe

A small galaxy 700 million years after the universe’s birth has a dust reservoir that makes it look like a much older galaxy.

Astronomers have for the first time directly detected dust in a galaxy in the early universe. And I do mean early: the galaxy A1689-zD1 shines at us from only 700 million years after the Big Bang, giving the galaxy a redshift of 7.5. Surprisingly, compared with the galaxy’s gas, the amount of dust A1689-zD1 contains makes it look like a far more “mature” star-forming galaxy, such as the Milky Way.

Abell 1689

The galaxy cluster Abell 1689 is essentially a huge concentration of mass, and its gravitational field bends and magnifies light coming toward us from more distant objects beyond it. One such lensed object, the galaxy A1689-zD1 (boxed), is a dusty galaxy seen when the universe was just 700 million years old.
Credit: NASA / ESA / L. Bradley and H. Ford (both Johns Hopkins) / R. Bouwens and G. Illingworth (both UC Santa Cruz)

Darach Watson (University of Copenhagen, Denmark) and colleagues took advantage of the Atacama Large Millimeter/submillimeter Array (ALMA) to explore A1689-zD1, which had previously turned up in images by the Hubble and Spitzer Space Telescopes. ALMA’s capabilities, combined with optical observations from the Very Large Telescope (VLT), enabled them to wheedle out the galaxy’s properties, the team reports March 2nd in Nature.

The galaxy is only bright enough to study because its light is lensed by a galaxy cluster lying between it and us. The gravity of the cluster, Abell 1689, bends and magnifies A1689-zD1’s light as the light passes the cluster on its way to us. As a result, A1689-zD1 is magnified by more than a factor of nine.

The optical light the VLT detects actually began as ultraviolet radiation, emitted by young, massive stars in the galaxy and then stretched to visible wavelengths by the universe’s expansion. Thus, astronomers can use the light to estimate the galaxy’s stellar mass and rate of star formation. Based on the optical spectra taken by the VLT, the team calculates that the galaxy has roughly 1.7 billion solar masses’ worth of stars, a hundredth the Milky Way’s stellar mass. By today’s standards, that would make it a dwarf galaxy, but most galaxies in the early universe are smaller, clumpy ones.

The optical spectrum also implies that the galaxy is giving birth to just under 3 solar masses a year in stars; the dust emission detected by ALMA suggests a slightly higher rate, around 9 solar masses per year. The Milky Way puts out roughly a Sun a year, so A1689-zD1’s star-formation rate is at least a few times higher than the Milky Way’s — not unusual for this cosmic era.

What’s interesting is just how much dust ALMA detects. Watson’s team calculates that the galaxy contains about 40 million solar masses of dust, or (throwing in an estimate of the galaxy’s gas content) somewhere between half and a few times the ratio of dust to gas in the Milky Way.

That’s quite a bit of dust, and the ratio suggests the galaxy is in an “evolved” state — it’s already burned through roughly half of its gas, only 150 million years after the universe’s galaxies started churning out stars in earnest. Essentially, it looks like a more massive galaxy in today’s universe, yet it existed 13 billion years ago. (Dwarfs today tend to have lower dust-to-gas ratios, because they’ve delayed their star formation.)

This high dust ratio is surprising, but it’s not as shocking as you might think. Remember, A1689-zD1 is only a hundredth as massive as the Milky Way, and all other things being equal, the little galaxy therefore has only a hundredth as much dust, too. It could have built up that dust by forming stars at a moderate rate for the last 150 million years, or perhaps it had an extreme “starburst” phase earlier and is now calming down. Per unit area, its star-formation rate is on par with many starburst galaxies but is at most a tenth the rate of the most vigorous starbursting galaxies, the authors note.

The galaxy’s evolved dust-to-gas ratio suggests that, like today’s galaxies, most of the dust grains formed in the winds from swollen stars near the ends of their lives, or when massive stars went supernova, says Daniel Schaerer (Geneva Observatory, Switzerland). Giant stars don’t live long, he explains, “hence no need to have a very old galaxy to see dust.”

Schaerer and others had previously calculated upper limits for the dust content of several galaxies in this cosmic epoch and found they could have “normal” (pronounced “like those today”) dust-to-gas ratios. Before that, some astronomers had claimed that early galaxies would have a lot less dust compared with their stellar mass.

A1689-zD1 is only one galaxy. But it’s an important discovery, because it convincingly shows that ALMA can fairly easily detect dust emission from young galaxies, whereas previous instruments had seen only upper limits. Astronomers will thus be able to explore dust and, therefore, star formation in the early universe.

 

Reference: D. Watson et al. "A dusty, normal galaxy in the epoch of reionization." Nature. March 2, 2015.