Astronomers pinpoint elusive galaxy after decade-long hunt
– and find it's not alone
An international team of astronomers led by Fabian Walter of the Max Planck Institute for Astronomy has managed for the first time to determine the distance of the galaxy HDF850.1, well-known among astronomers as being one of the most productive star-forming galaxies in the observable universe. The galaxy is at a distance of 12.5 billion light years. Hence, we see it as it was 12.5 billion years ago, when the universe was less than 10% of its current age. Even more of a surprise, HDF850.1 turns out to be part of a group of around a dozen protogalaxies that formed within the first billion years of cosmic history – only one of two such primordial clusters known to date. The work is being published in the journal Nature.
Figure 1: The region of the Hubble Deep Field where HDF850.1 is located. The cross indicates the submillimeter galaxy's position. For observations with ordinary, visible light telescopes such as the Hubble Space Telescope, the galaxy is completely invisible.
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 the past fourteen years, 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 submillimeter radiation. This type of radiation, with wavelengths between a few tenths of a millimeter and a millimeter, is particularly suitable for detecting cool clouds of gas and dust.
The researchers were taken by surprise when they realized that HDF850.1 was the brightest source of submillimeter emission in the field by far, a galaxy that was evidently forming as many stars as all the other galaxies in the Hubble Deep Field combined – and which was completely invisible in the observations of the Hubble Space Telescope!
"The galaxy's invisibility is no great mystery. Stars form in dense clouds of gas and dust. These dense clouds are opaque to visible light, hiding the galaxy from sight. Submillimeter radiation passes through the dense dust 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 redshift, and thus its place in cosmic history," explains MPIA's Fabian Walter.
Now, an international 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 in the French Alps, which combines six radio antennas that then act as a gigantic millimeter telescope, they identified the characteristic features ("spectral lines") necessary for an accurate distance determination. "It is the availability of more powerful and sensitive instruments recently installed on the IRAM interferometer that allowed us to detect these weak lines in HDF850.1, and finally find what we had been unsuccessfully looking for, during the past 14 years," explains Pierre Cox, Director of IRAM.
The result is a surprise: 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 1.1 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 future, more powerful interferometers operating at millimeter and submillimeter wavelengths. Both NOEMA, the future extension of the Plateau de Bure interferometer, and ALMA, a new interferometer array currently being built by an international consortium in the Atacama desert in Chile, will cover these wavelengths in unprecedented detail. They should allow for distance determinations and more detailed study of many more galaxies, invisible at optical wavelengths, that were actively forming stars in the early universe.
The work described here will be published as F. Walter et al., "The Intense Starburst HDF850.1 in a Galaxy Overdensity at z = 5.2 in the Hubble Deep Field" in the June 14th, 2012, issue of the journal Nature.
The authors are Fabian Walter (Max Planck Institute for Astronomy [MPIA] and National Radio Astronomy Observatory [NRAO], Socorro), Roberto Decarli (MPIA), Chris Carilli (NRAO and Cambridge University), Frank Bertoldi (University of Bonn), Pierre Cox (IRAM), Elisabete Da Cunha (MPIA), Emanuele Daddi (CEA Saclay), Mark Dickinson (NOAO, Tucson), Dennis Downes (IRAM), David Elbaz (CEA Saclay), Richard Ellis (Caltech), Jacqueline Hodge (MPIA), Roberto Neri (IRAM), Dominik Riechers (Caltech), Axel Weiss (Max Planck Institute for Radio Astronomy [MPIfR]), Eric Bell (University of Michigan, Ann Arbor), Helmut Dannerbauer (University od Vienna), Melanie Krips (IRAM), Mark Krumholz (UCSC), Lindley Lentati (Cambridge University), Roberto Maiolino (INAF-Osservatorio Astronomico di Roma and Cambridge University), Karl Menten (MPIfR), Hans-Walter Rix (MPIA), Brant Robertson (University of Arizona), Hyron Spinrad (UC Berkeley), Dan Stark (University of Arizona), and Daniel Stern (Jet Propulsion Laboratory).
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 determined 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 releated to the development of its central supermassive black hole?
On cosmological scales, there are different definitions for distance. In this release, we use light-travel distance: If it took 12.5 billion years for light from a distant object to reach us, we say the object is 12.5 billion years away. Another common distance definition, used in Hubble's law, is linked to the concept of cosmic time and takes into account the expansion of the universe. By that measure, HDF850.1. is 26 billion light-years away.
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 submillimeter 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 submillimeter 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, millimeter or submillimeter part of the spectrum that allow for the identification of specific frequencies typically only work over rather narrow frequency range. So blindly searching for specific spectral lines would normally take too much time.
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 in New Mexico, USA to observe the CO(2-1) line exactly where their identification predicted, at 37.3 GHz.
Figure 2: View of the Northern target area for the "Great Observatories Origins Deep Survey" (GOODS-N). The position of the Hubble Deep Field and, within that field, the position of the submillimeter galaxy HDF850.1, are shown separately. HDF850.1 is invisible for observations using ordinary, visible light.
Figure 3: The Hubble Deep Field, with the position of the submillimeter galaxy HDF850.1 marked with contour lines. The lines represent the date of submillimeter observations of the galaxy; in visible light, it cannot be observed at all.
Figure 4: Four antennas of the IRAM telescope on the Plateau de Bure. With this compound telescope, Walter et. al. identified the first of the spectral lines of the galaxy HDF850.1 that would allow them to determine the galaxy's distance.
Figure 5: Radio telescopes of the IRAM Interferometer on the Plateau de Bure in the French Alps. With this compound telescope, Walter et. al. identified the first of the spectral lines of the galaxy HDF850.1 that would allow them to determine the galaxy's distance.