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Galaxy’s cannibalistic snacking habits revealed

Last Updated on Thursday, 21 May 2015 18:04
Published on Wednesday, 20 May 2015 23:01

A team of Australian and Spanish astronomers have caught a greedy galaxy gobbling on its neighbours and leaving crumbs of evidence about its dietary past. Their results have been published in Monthly Notices of the Royal Astronomical Society.

Galaxies grow by churning loose gas from their surroundings into new stars, or by swallowing neighbouring galaxies whole. However, they normally leave very few traces of their cannibalistic habits.

Fig3 NGC1512 composite smallA composite image of the galaxy NGC 1512, located 38 million light years away in the direction of the constellation of Horologium, in the southern hemisphere of the sky. The image shows the regions of unusual chemical enrichment that demonstrate that NGC 1512 has absorbed other galaxies earlier in its history. Credit: Angel R. Lopez-Sanchez (AAO / MQU), & Baerbel Koribalski (CSIRO / CASS). Click for a full size image

The study not only reveals a spiral galaxy devouring a nearby compact dwarf galaxy, but shows evidence of its past galactic snacks in unprecedented detail.

Ángel R. López-Sánchez, an astrophysicist at Australian Astronomical Observatory (AAO) and Macquarie University, and his collaborators have been studying the galaxy NGC 1512 to see if its chemical story matches its physical appearance.

The team of researchers used the unique capabilities of the 3.9-metre Anglo-Australian Telescope (AAT), near Coonabarabran, New South Wales, to measure the level of chemical enrichment in the gas across the entire face of NGC 1512.

Chemical enrichment occurs when stars churn the hydrogen and helium from the Big Bang into heavier elements through nuclear reactions at their cores. These new elements are released back into space when the stars die, enriching the surrounding gas with chemicals like oxygen, which the team measured.

"We were expecting to find fresh gas or gas enriched at the same level as that of the galaxy being consumed, but were surprised to find the gases were actually the remnants of galaxies swallowed earlier," Dr López-Sánchez said.

"The diffuse gas in the outer regions of NGC 1512 is not the pristine gas created in the Big Bang but is gas that has already been processed by previous generations of stars."

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) Australia Telescope Compact Array (ATCA), a powerful 6-km diameter radio interferometer (an array of radio antennas that effectively act as a larger single instrument) located in eastern Australia, was used to detect large amounts of cold hydrogen gas that extends way beyond the stellar disk of the spiral galaxy NGC 1512.

"The dense pockets of hydrogen gas in the outer disk of NGC 1512 accurately pinpoint regions of active star formation", said CSIRO's Dr Baerbel Koribalski, a member of the research collaboration.

When this finding was examined in combination with radio and ultraviolet observations, the scientists concluded that the rich gas being processed into new stars did not come from the inner regions of the galaxy either. Instead, the gas was likely absorbed by the galaxy over its lifetime as NGC 1512 accreted other, smaller galaxies around it.

Dr Tobias Westmeier, from the International Centre for Radio Astronomy Research in Perth, said that while galaxy cannibalism has been known for many years, this is the first time that it has been observed in such fine detail.

"By using observations from both ground and space based telescopes we were able to piece together a detailed history for this galaxy and better understand how interactions and mergers with other galaxies have affected its evolution and the rate at which it formed stars" he said.

The team’s successful and novel approach to investigating how galaxies grow is being used in a new program to further refine the best models of galaxy evolution. For this work the astronomers used spectroscopic data from the AAT at Siding Spring Observatory in Australia to measure the chemical distribution around the galaxies. They identified the diffuse gas around the dual galaxy system using ATCA radio observations. In addition, they identified regions of new star formation with data from the Galaxy Evolution Explorer (GALEX) orbiting space telescope.

"The unique combination of these data provide a very powerful tool to disentangle the nature and evolution of galaxies" said Dr López-Sánchez. "We will observe several more galaxies using the same proven techniques to improve our understanding of the past behaviour of galaxies in the local Universe."


Further information

The new work is published in Á. R. López-Sánchez et al., "Ionized gas in the XUV disc of the NGC1512/1510 system", Monthly Notices of the Royal Astronomical Society, vol. 450, pp. 3381-3409, 2015, published by Oxford University Press.

Media contacts

Dr Amanda Bauer
Australian Astronomical Observatory (AAO)
Tel: +61 2 9372 4852
Mob: +61 447 029 368
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Pete Wheeler
The International Centre for Radio Astronomy Research (ICRAR)
Tel: +61 8 6488 7758
Mob: +61 0423 982 018
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Science contacts

Dr Ángel R. López-Sánchez
Australian Astronomical Observatory / Macquarie University
Tel: +61 2 9372 4898
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Dr Tobias Westmeier
The International Centre for Radio Astronomy Research / University of
Western Australia
Tel: +61 8 6488 4592
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Dr Baerbel Koribalski
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Tel: +61 293 724 361
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Notes for editors

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @RoyalAstroSoc.