A galaxy is not enough

Astronomers discover that behind the most productive star systems more hide

April 18, 2013

Using the telescope network ALMA, astronomers have been able to determine the positions of more than 100 of the most prolific galaxies with unprecedented accuracy. The exact positions could help to solve the mystery of the supposedly extremely high star formation rates: Accordingly the amount of radiation released is not from one but from several galaxies - each of which have more moderate star formation rate and in accordance with theoretical models.

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Zooming in on distant galaxies: the images show star systems which the astronomers obtained with the ALMA antenna system in the submillimeter range (orange, red). Overlaid are infrared photos of the Spitzer Space Telescope.

Submillimeter galaxies, discovered in the late 1990s, produce so many new stars as to be responsible for a significant fraction of the total energy output of all galaxies over the course of all of cosmic history. A side effect of having many (and many massive) stars is the production of lots of dust. And indeed, in the most extreme cases, these fertile galaxies are so deeply shrouded in dust they are effectively hidden from sight for astronomers observing in visible light. That is why, in order for a full census of these objects, and to reliably gauge their star formation activity, astronomers need to resort to submillimeter observations. Additional information can come from observations using infrared radiation or radio waves.

Previous submillimeter surveys of these distant objects suffered from a lack of detail. But now, a team led by Ian Smail (Durham University, UK) has completed a large, yet highly detailed survey of more than 100 such objects using the international compound telescope ALMA (Atacama Large Millimeter/Submillimeter Array), located in Chile, at a resolution more than a factor 10 better than previous surveys. The observations, targeting a region known as the Extended Chandra Deep Field South in the Southern constellation Fornax, made use of 15 of ALMA's antennas, which were combined to act as a single, large telescope.

The new survey's high-resolution images for a large number of galaxies have helped to solve one apparent mystery concerning submillimeter galaxies. Alexander Karim (Argelander Institute for Astronomy, Bonn and Durham University, UK) explains: “We previously thought the brightest of these galaxies were forming stars more than a thousand times more vigorously than our own galaxy, the Milky Way, putting them at risk of blowing themselves apart. But instead of single, hyperactive galaxies, the ALMA images revealed multiple, smaller galaxies, each forming stars at a more reasonable rate.” Karim, formerly a PhD student at the Max Planck Institute for Astronomy, is a member of the survey team and lead author of the paper reporting this key result of the survey.

The newly published survey also promises to put future studies of submillimeter galaxies onto a solid footing. Jacqueline Hodge of the Max Planck Institute for Astronomy, lead author of the survey paper, explains: “Astronomers use many different kinds of light to examine celestial objects. But this only works if you know precise positions – only then can you say 'Yes, this blob here in my infrared image represents the same object as that blob there in my submillimeter image'. Our survey shows that previous attempts to identify the infrared and radio counterparts of submillimeter galaxies were considerably error-prone, leading to incorrect identifications in about a third of all cases. With our precise submillimeter position measurements, such errors can be avoided.”

The work by Smail, Hodge, Karim and their colleagues has prepared the stage for the next logical step: Examinations at even higher resolution, deploying the full power of the completed ALMA array with all of its 66 antennas, could help elucidate the nature of submillimeter galaxies. The most plausible scenario is that submillimeter galaxies are created when large galaxies collide, their mutual gravitational pull triggering an intense phase of star formation. High-resolution images could show aspects of those galaxies' shapes, and possibly traces of the collision process itself.

HOR / MP

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