On the edge of time and space
Galaxy HDF850.1 can be seen as it was 12.5 billion years ago
In observations with optical light telescopes, such as the Hubble Space Telescope, HDF850.1 is completely invisible. From other surveys, however, scientists knew that the object is one of the most productive star-forming galaxies in the observable universe – a Milky Way system with the extremely high stellar birth rate of 1,000 suns per year. A team of astronomers led by Fabian Walter of the Max Planck Institute for Astronomy has now managed for the first time to determine the distance of the galaxy HDF850.1. The light reaching us from it today started its voyage when the universe was less than 10% of its current age, i.e. 12.5 billion years ago. The galaxy appears to be part of a proto-galaxy cluster that formed within the first billion years of cosmic history after Big Bang.
The galaxy HDF850.1 was discovered in 1998. It is famous for producing new stars at a rate that is near-incredible even on astronomical scales: a combined mass of a thousand Suns per year. For comparison: an ordinary galaxy such as our own produces no more than one solar mass’s worth of new stars per year. Yet for more than a decade, HDF850.1 has remained strangely elusive – its location in space, specifically: its distance from Earth the subject of many studies, but ultimately unknown. How was that possible?
The “Hubble Deep Field”, where HDF850.1 is located, is a region in the sky that affords an almost unparalleled view into the deepest reaches of space. It was first studied extensively using the Hubble Space Telescope. Yet observations using visible light only reveal part of the cosmic picture, and astronomers were quick to follow-up at different wavelengths. In the late 1990s, astronomers using the James Clerk Maxwell Telescope on Hawai'i surveyed the region using submillimetre radiation. This type of radiation, with wavelengths between a few tenths of a millimetre and a millimetre, is particularly suitable for detecting cool structures such as clouds of gas and dust.
The researchers found a surprise: HDF850.1, the brightest source of submillimetre emissions in the field by far; a galaxy that evidently produced more stars than all the other galaxies in the Hubble Deep Field together – and which was completely invisible in the observations of the Hubble Space Telescope!
The galaxy’s invisibility is no great mystery. Star formation takes place within dense clouds of gas and dust. These are opaque to visible light, hiding the galaxy from sight; submillimetre radiation can pass through those clouds unhindered, showing what is inside. But the lack of data from all but a very narrow range of the spectrum made it very difficult to determine the galaxy's distance and, in consequence, its place in cosmic history.
Now, a group of researchers led by Fabian Walter of the Max Planck Institute for Astronomy has managed to solve the mystery. Taking advantage of recent upgrades to the IRAM interferometer on the Plateau de Bure, which combines six radio antennas that then act as a gigantic millimetre telescope, they identified the characteristic features (“spectral lines”) necessary for an accurate distance measurement.
The result is somewhat surprising: The galaxy is at a distance of 12.5 billion light-years from Earth (z ~ 5.2). We see it as it was 12.5 billion years ago, at a time when the universe itself was only 1.2 billion years old! HDF850.1’s intense star-forming activity thus belongs to a very early period of cosmic history, when the universe was less than 10% of its current age.
A combination with observations obtained at the National Science Foundation's Karl Jansky Very Large Array (VLA) then revealed that a large fraction of the galaxy's mass is in the form of molecules – the raw material for future stars. The fraction is much higher than what is found in galaxies in the local universe.
Once the distance was known, the researchers were also able to put the galaxy into context. Using additional data from published and unpublished surveys, they were able to show that the galaxy is part of what appears to be an early form of galaxy cluster – one of only two such clusters known to date.
The new work highlights the importance of ALMA, a new compound telescope for submillimetre and millimetre observations currently being built by an international consortium in the Atacama Desert of Chile. ALMA is set to cover the submillimetre- and millimetre range of wavelengths at an unprecedented level of detail, and should allow for distance determinations of a great number of further galaxies that remain invisible at optical wavelengths.
What is the Hubble Deep Field, and what is so special about it?
The Hubble Deep Field (HDF) is a region of the sky in the constellation of Ursa Major (the Great Bear), less than one percent the apparent size of the full moon. It contains no bright nearby sources such as stars or galaxies, and within our home galaxy, the Milky Way, there is very little matter (such as gas or dust) impeding the view into the distance (“low galactic extinction”). As the Hubble Space Telescope orbits the Earth, the HDF remains continually within view. These properties make the HDF a near-ideal region for in-depth studies of distant galaxies; within the first HDF survey by the Hubble Space Telescope in late 1995, more than 3000 distant galaxies were found, with the most distant at more than 12 billion light-years from Earth (z ~ 4). Observations of the HDF have been a treasure trove of information for studying the evolution of galaxies throughout cosmic history.
How are distances measured for these distant galaxies, and what is the connection with cosmic history?
For very distant celestial objects, there is a straightforward way of measuring their distance from Earth fairly accurately: Since the Big Bang, the universe has been expanding continually, with all distant galaxies moving ever further apart from each other. One consequence of this is the so-called cosmological red-shift: an object looks all the more reddish the more distant that object is from Earth (put more precisely: the more distant an object, the greater the factor by which its light is shifted towards lower frequencies).
In astronomy, knowing the distance means more than just being able to pinpoint an object's location in three-dimensional space. Astronomers inevitably look into the past: We never see the Sun as it is now, only as it was 8 minutes ago, simply because it takes 8 minutes for light from the Sun to reach Earth. We always see the Andromeda galaxy as it was 2.5 million years ago, because that's the time it takes for the galaxy's light to reach us Earthlings. Knowing an object's distance means knowing which part of cosmic history you are looking at. That information, in turn, is crucial if you want to reconstruct the events of cosmic history: When did the first galaxies form? Did early galaxies produce more stars than modern ones? Is the evolution of a galaxy related to the development of its central supermassive black hole?
Why was it so difficult to determine HDF850.1’s distance, and how was this goal accomplished?
Ordinarily, the brightness of an object gives at least a rough estimate for the distance - the dimmer an object, the farther away it is. Not so in the submillimetre range where HDF850.1 was first observed: At such wavelengths, the combined effect of the cosmological redshift (objects appearing the redder the greater their distance), the specific form of the spectrum of such galaxies, and the natural dimming with distance combine in such a way as to render the brightness practically distance-independent. Since HDF850.1. was observed only at submillimetre wavelengths, there was no clue as to its distance.
Without any prior clues, looking for specific spectral lines is like looking for a needle in a haystack. To make things more difficult, those receivers in the radio, millimetre or submillimetre part of the spectrum that allow for the identification of specific frequencies typically only work over rather narrow frequency range – searching for specific spectral lines would have taken impractically many different observations.
The work of Walter et al. only became possible because of a recent upgrade of the IRAM interferometer, when receivers able to cover a broader wavelength range were installed. Walter and his team used those for 100 hours of observations of a region in the Hubble Deep Field that happened to contain HDF850.1, using ten different frequency settings. They tentatively identified two lines associated with rotational oscillation of carbon monoxide molecules, CO(6-5) and CO(5-4), and tested their identification in two ways: if their identification was correct, then the ionized carbon line [CII] should lie within their range with the IRAM interferometer, at a frequency of 307 GHz – and indeed, there it was. To clinch the identification, the team used the Jansky Very Large Array, a giant compound radio telescope, to observe the CO(2-1) line exactly where their identification predicted, at 37.3 GHz.
HOR / MP