LOFAR discovers largest carbon atoms outside our Milky Way

The starburst galaxy M82, the size of the carbon atoms and the observed spectral line. 

Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

An international team of astronomers under the guidance of graduate student Leah Morabito of Leiden Observatory has for the first time discovered the largest carbon atoms outside our Milky Way with the LOFAR radio telescope.

In the future astronomers will be able to measure how cold and dense the gas around these atoms is that influences star formation and the evolution of a galaxy. The results are published in the journal Astrophysical Journal Letters on October 28th.

"Carbon atoms are about half a million times smaller than the average thickness of a human hair, but they can be a billion times larger in the cold and sparse gas. The outermost electron is then orbiting the nucleus at a much larger distance," explains first author Morabito.

The outermost electron can be captured by an atom that is missing an electron. A spectral line will then be visible in the light spectrum. All spectral lines form the chemical fingerprint of an atom such as carbon.

Astronomers predicted in the 70's that the carbon spectral line would be detectable outside our galaxy. This first observation took 40 years to be made.

The line is hard to detect because it is too faint when the gas that is surrounding the atoms is too warm or too dense.

The cold, sparse gas is present in starburst galaxies, galaxies in which stars form at a high rate. For this reason the carbon spectral line is easier to detect in galaxies of this type.

Most radio telescopes observe at frequencies at which the carbon line can not be detected. Other telescopes are not sensitive enough to detect the spectral lines of the carbon atoms at low frequencies.

The LOFAR radio telescope, that stretches from the northeast of the Netherlands across Europe, is perfect for these kind of observations because of its frequency range and sensitivity.

Co-author Raymond Oonk from Leiden Observatory en ASTRON: "LOFAR is an unique telescope. This telescope opens up a new window on the universe."

The carbon atoms are present in the heart of the starburst galaxy M82, where 10 times more stars are being born in the same period as in our Milky Way.

The cold and sparse gas in this area impacts star formation, and the evolution of M82. "Since the co-discovery of the hydrogen 21-cm line by Dutch, American and Australian astronomers, we have been looking for a way to determine additional properties of the cold gas such as its temperature and density. It is fantastic that we now have found a way thanks to this carbon line.

We can now collect more and better observations, and compare them to predictions from theoretical models," says co-author Huub Röttgering (Leiden Observatory).

More information: "Discovery of Carbon Radio Recombination Lines in M82," Leah K. Morabito et al., Astrophysical Journal Letters, 28 October 2014 DOI: 10.1088/2041-8205/795/2/L33 . http://arxiv.org/abs/1410.1544

Silicon-capped hydrocarbons: Mysterious molecules in space

This graph shows absorption wavelength as a function of the number of carbon atoms in the silicon-terminated carbon chains SiC_(2n+1)H, for the extremely strong pi-pi electronic transitions. 

When the chain contains 13 or more carbon atoms, not significantly longer than carbon chains already known to exist in space, these strong transitions overlap with the spectral region occupied by the elusive diffuse interstellar bands. 

Credit: D. Kokkin, ASU

Over the vast, empty reaches of interstellar space, countless small molecules tumble quietly though the cold vacuum.

Forged in the fusion furnaces of ancient stars and ejected into space when those stars exploded, these lonely molecules account for a significant amount of all the carbon, hydrogen, silicon and other atoms in the universe.

In fact, some 20 percent of all the carbon in the universe is thought to exist as some form of interstellar molecule.

Many astronomers hypothesize that these interstellar molecules are also responsible for an observed phenomenon on Earth known as the "diffuse interstellar bands," spectrographic proof that something out there in the universe is absorbing certain distinct colours of light from stars before it reaches the Earth.

But since we don't know the exact chemical composition and atomic arrangements of these mysterious molecules, it remains unproven whether they are, in fact, responsible for the diffuse interstellar bands.

Now in a paper appearing this week in The Journal of Chemical Physics, from AIP Publishing, a group of scientists led by researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. has offered a tantalising new possibility: these mysterious molecules may be silicon-capped hydrocarbons like SiC3H, SiC4H and SiC5H, and they present data and theoretical arguments to back that hypothesis.

At the same time, the group cautions that history has shown that while many possibilities have been proposed as the source of diffuse interstellar bands, none has been proven definitively.

"There have been a number of explanations over the years, and they cover the gamut," said Michael McCarthy a senior physicist at the Harvard-Smithsonian Center for Astrophysics (CfA) who led the study.

Molecules in Space and How We Know They're There
Astronomers have long known that interstellar molecules containing carbon atoms exist and that by their nature they will absorb light shining on them from stars and other luminous bodies.

Because of this, a number of scientists have previously proposed that some type of interstellar molecules are the source of diffuse interstellar bands, the hundreds of dark absorption lines seen in color spectrograms taken from Earth.

In showing nothing, these dark bands reveal everything. The missing colours correspond to photons of given wavelengths that were absorbed as they travelled through the vast reaches of space before reaching us.

More than that, if these photons were filtered by falling on space-based molecules, the wavelengths reveal the exact energies it took to excite the electronic structures of those absorbing molecules in a defined way.

Armed with that information, scientists here on Earth should be able to use spectroscopy to identify those interstellar molecules, by demonstrating which molecules in the laboratory have the same absorptive "fingerprints."

But despite decades of effort, the identity of the molecules that account for the diffuse interstellar bands remains a mystery.

Nobody has been able to reproduce the exact same absorption spectra in laboratories here on Earth.

"Not a single one has been definitively assigned to a specific molecule," said Neil Reilly, a former postdoctoral fellow at Harvard-Smithsonian Center for Astrophysics (CfA) and a co-author of the new paper.

Now Reilly, McCarthy and their colleagues are pointing to an unusual set of molecules, silicon-terminated carbon chain radicals, as a possible source of these mysterious bands.

As they report in their new paper, the team first created silicon-containing carbon chains SiC3H, SiC4H and SiC5H in the laboratory using a jet-cooled silane-acetylene discharge.

They then analysed their spectra and carried out theoretical calculations to predict that longer chains in this family might account for some portion of the diffuse interstellar bands.

However, McCarthy cautioned that the work has not yet revealed the smoking gun source of the diffuse interstellar bands.

To prove that these larger silicon capped hydrocarbon molecules are such a source, more work needs to be done in the laboratory to define the exact types of transitions these molecules undergo, and these would have to be directly related to astronomical observations.

But the study provides a tantalising possibility for finding the elusive source of some of the mystery absorption bands, and it reveals more of the rich molecular diversity of space.

"The interstellar medium is a fascinating environment," McCarthy said. "Many of the things that are quite abundant there are really unknown on Earth."

More information: The Journal of Chemical Physics, July 29, 2014. DOI: 10.1063/1.4883521