Mature galaxy in the young Universe

A collaboration of astronomers, lead by Darach Watson from the University of Copenhagen, have discovered one of the most distant galaxies to date named A1689-zD1. The galaxy is at a redshift of 7.5, which means it is about 13 billion light years away and the light we detect is from when the Universe was only about 700 million years old. However, A1689-zD1’s distance is not the only thing that makes it interesting.

The discovery of A1689-zD1 was made possible by the existence of a gravitational lens that magnified the light of the galaxy by a factor of 9.3. Gravitational lenses are created when massive foreground objects – like galaxy clusters – distort space-time around them. When this happens light no longer travels in a straight line, but is bent around the intervening massive object, as if the light had passed through a convex lens. Depending on the object’s position behind the lens it can appear multiply imaged (Einstein cross) or is smudged into an arc of light around the lens (Einstein arc).

Without gravitational lensing it would be very difficult to observe galaxies at the distance of A1689-zD1. This is because the light emitted from A1689-zD1 is redshifted quite a bit by the time it reaches an observer on Earth. Redshifting of light occurs because the Universe is expanding, and as light travels through space it is literally stretched along with space.

When light is stretched its wavelength becomes longer. Light that has longer wavelengths is redder. Light from very distant galaxies is fainter than that from similar galaxies closer to us at first order due to the inverse-square law (but at such extreme distances other cosmological dimming effects dominate). Therefore, the light sent out from A1689-zD1 is very red and faint because it has had to travel such a large distance to reach us.

Hubble Space Telescope image of the rich galaxy cluster Abell 1689. The enormous concentration of mass from the cluster acts as a lens to bend light from more distant objects, increasing their apparent brightness. The insert box shows the lens background galaxy A1689-zD1. (Credit: NASA; ESA; L. Bradley (JHU); R. Bouwens (U. California, Santa Cruz); H. Ford (JHU); and G. Illingworth (U. California, Santa Cruz))

The other reason A1689-zD1 is important is because of its properties and how they were determined. Watson’s team observed A1689-zD1 using two instruments. The first instrument was the X-shooter spectrograph, which is part of the Very Large Telescope in Chile. A spectrograph spreads a galaxy’s light into a spectrum, similar to how a prism spreads white light into a rainbow. The amount of light in each part of the spectrum reveals several characteristics about the galaxy. Sometimes astronomer note a chunk of the spectrum is missing, which occurs when material is absorbing light emitted at those wavelengths.

Schematic of an absorption spectrum, showing how intervening gas can “remove” specific transition lines from the resulting spectrum.

Galaxies that are vigorously forming stars have such a chunk missing from their spectrum because young stars emit light at a wavelength that is just right to be absorbed by the neutral hydrogen in the galaxy. When hydrogen absorbs light, this boosts its energy and the hydrogen atoms undergoes what is called an energy transition. The strongest of these transitions is called the Lyman-α transition. The wavelength where this transition occurs is where light will be missing from the galaxy’s spectrum and is called the Lyman-α break. Because the galaxy’s light is redshifted based on how far it has travelled, astronomers can measure the wavelength the break occurs at in the galaxy and therefore determine how far away it is.

The spectrum of a galaxy is also very important because it allows astronomers to determine the age of the galaxy, the mass of the galaxy, and estimate its star-formation rate, which is how many stars it forms each year. Watson’s team found thatA1689-zD1 is quite a typical galaxy; it is not forming stars at an alarming rate and its mass is on the light side (much less than our own Milky Way galaxy).

The other tool used to study A1689-zD1 was the Atacama Large Submillimeter Array (ALMA), which can be used to measure the dust content of the distant galaxy. When stars evolve and die, they eject their material into space and enrich the galaxy with new elements. After many cycles of star formation, the galaxy will become enriched with dust and hence dust is a characteristic of a mature star-forming galaxy.  When Watson’s team observed A1689-zD1 with ALMA, the galaxy appeared very bright, indicating the it contains a significant amount of dust.

But now there was a conundrum; the amount of dust detected within the galaxy is much larger than expected for its age (since many cycles of star formation are needed to build up the dust content of a galaxy). This indicates that the galaxy must have been forming stars for a long time, which is tricky because the Universe itself is not that old at this point – recall that the light we detect from A1689-zD was when the Universe was only about 700 million years old). Alternatively, it must have under gone a very intense period of star-formation, where its star formation rate was very high for a short period of time and during this short timeframe the stars in the galaxy could have produced a lot of dust. This is not the first time astronomers have discovered objects that appear to be much older than expected at high redshift. Last year, Caroline Straatman of Leiden University found a population of very mature compact galaxies at a redshift of 4, when the Universe was 1.6 billion years old, which means they too must have been vigorously forming stars while the Universe was very young.

This shows how little we still know about the early universe and the first galaxies. Understanding the formation and evolution of these distant galaxies is one of the main goals of astronomers, but is only possible if these objects can be detected. Hopefully, in the next decade more advanced telescopes will allow astronomers to look back to these epochs and answer fundamental questions about the galaxies which populated the Universe 13 billion years ago.

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[Rebecca Allan]

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