Monster Starbursts Seen by New Radio Telescope


With new instruments coming online, the most distant (and youngest) regions of our universe are finally being explored in depth.

This week, the Atacama Large Millimeter/Submillimeter Array, or ALMA, is being dedicated, and they’re celebrating with an amazing new study of some of the most distant prolifically star-forming galaxies.

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The millimeter and submillimeter wave bands can be thought of as very short wavelength radio astronomy, or very long wavelength infrared. For decades, it’s been a hard region of the electromagnetic spectrum to probe since water vapor in our atmosphere causes a lot of absorption and distortion, and the challenges that go into the technology are non-trivial.

But after 30 years of plotting and planning, an international collaboration has succeeded in creating a sensitive instrument at a high dry mountain site that can fully open up this part of the universe.

It turns out, thanks to a lucky coincidence of physics, that the millimeter band is a great place to study distant, star-forming galaxies. You can see galaxies over a wide range of redshifts, or distances, in this band with little bias. That is, you just don’t see the brightest sources from further away; you get the whole history of galaxy formation in one go. Well, theoretically, at least. Previously existing millimeter wave telescopes were not very sensitive, and so only could see a small part of the population.

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Now ALMA is coming online, and a study led by Joaquin Vieira using just 16 of the array’s planned 66 antennas have already doubled the number of star-forming galaxies seen at redshifts greater than 4, or from the first 1.5 billion years of the universe’s history. Also, each observation took only 2 minutes to create images that would have taken older telescopes hours to produce, if they even could.

So, technologically, this is pretty mind-blowing and demonstrates the capability of ALMA, even in its earliest stages. Scientifically, this is a boon for astronomers wanting to understand galaxy formation in the early universe, as this is one of the most important research questions of the day.

These galaxies are all gravitationally lensed by some foreground object, such as an elliptical galaxy, and that helps us to see the furthest objects.

In total, 47 galaxies were imaged, and spectra were collected from 26 of these. A spectrum is taken when the light from the object is separated by wavelength, not unlike a prism making a rainbow out of visible light.

ALMA’s scientific prowess stems in part from very fine spectral resolution, allowing for detailed studies of emission lines from molecular clouds. The other part of its strength, the incredible sensitivity, allows astronomers to see much fainter lines than ever before.

The spectra collected from these galaxies highlight an important analysis, the exact determination of redshift, or distance, to distant galaxies.

The most accurate way to determine redshift is to measure several spectra lines and how much they have been shifted to longer wavelengths by the expansion of the universe over great distances. However, astronomers often rely on more indirect techniques for very distant objects when no lines can be seen. Also, optical and infrared telescopes cannot easily survey these star-forming distant galaxies because they are obscured by their own dust. The millimeter and submillimeter regime is the way to go to collect precise redshifts of this population.

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And what lovely spectra did they find! At least one spectral line was detected in 23 of the 26 galaxies by ALMA over, again, a very short observing time. Most of the spectral emission lines come from CO, or carbon monoxide, clouds in the galaxies, with a few coming from water vapor. Such strong CO lines are often a signpost of intense star formation happening in a dust enshrouded galaxy.

This plot of the spectra all lined up stunned me when it came up in our press conference yesterday, as it brought to my mind the concept of a “z-machine,” or a telescope that could quickly and accurately collect distances from a large number of galaxies in a short period of time. The distance is one of the most important parameters since all the physics that we derive from astronomical observations depend on accurate measurements of distance.

ALMA has now demonstrated, to my mind, that it can fulfill the promises that have long been made about its efficiency, sensitivity and accuracy. This is a telescope that will allow astronomers to advance science forward with great speed over the next 30 or so years of operation. For that, it is well worth the cost that it has taken to build and the time that it has taken to perfect the instrument.

Image: Blue is the Hubble Space Telescope image of a foreground galaxy. Red is the emission from the galaxy of interest as seen by ALMA. Credit: Vieira et al., ALMA (ESO, NAOJ, NRAO), NASA, NRAO/AUI/NSF

This research will appear in Nature on 13 March 2013 and the preprint can be found on ArXiV. 

Thanks to the folks at the National Radio Astronomy Observatory that have invited me to come along on this inauguration trip.

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