From ALMA: “ALMA Uses ‘Double Vision’ to Study Galaxy’s Gaseous Ingredients”

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01 July 2016
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Composite ALMA and optical image showing glowing emission from carbon monoxide (red) in galaxy PKS0439_008_04. The more distant quasar, PKS0439-433 (central blue feature), revealed through absorption in the visible spectrum the presence of an extended, diffuse halo of molecular gas surrounding the galaxy. The optical data are from the Irénée du Pont Telescope at the Las Campanas Observatory. Credit: Neeleman et al.; ALMA (ESO/NAOJ/NRAO); B. Saxton (NRAO/AUI/NSF); H.-W. Chen, Carnegie Observatories.

Las Campanas Dupont telescope exterior,Atacama Desert, Chile
Las Campanas Dupont telescope interior
Carnegie Las Campanas Dupont telescope, Atacama Desert, Chile

Researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) combined two techniques for the first time to achieve the most comprehensive study to date of molecular gas in a galaxy. They observed the emission of the very nearby galaxy PKS0439_008_04 and, at the same time, the absorption of a far quasar on a direct line of sight with the galaxy.

Astronomers usually study molecular gas —the fuel for star formation— using one of two methods: either by investigating a galaxy’s “emission” or its “absorption.” The former technique involves looking at the natural radio emission from the gas that permeates the galaxy and its expansive halo. The latter method examines the light from more distant objects, like a bright quasar, that gets absorbed as it passes through the galaxy.

This work constitutes the most comprehensive study of molecular gas in a galaxy to date. “Previously, quasar absorption studies ‘blinded’ telescopes to the less intense and much more diffuse emission signal from the galaxy,” said Marcel Neeleman, with the University of California’s Lick Observatory and lead author on a paper published in Astrophysical Journal Letters. In this study, astronomers were able to directly detect the faint emission signal from carbon monoxide in the galaxy.

By comparing this emission to the absorption signal from the quasar, the researchers found that the strongest absorption did not occur in the disk of the galaxy. Instead, most of the quasar’s light was absorbed by the diffuse gas surrounding the galaxy—gas that would be nearly impossible to detect through other means.

The astronomers’ results suggest that far from the galaxy, there is a significant amount of molecular gas that is not actually part of the galaxy, but bound to it. “We may be witnessing the recycling of material that, in a few billion years, will again trigger a burst of star formation,” concluded Neeleman.

The nearby galaxy, known as PKS0439_008_04, is approximately 1.4 billion light-years from Earth. The more distant quasar, PKS0439-433, is approximately 7.3 billion light-years from Earth. Both are in the direction of the constellation Caelum.

Additional information

These observation results were published as Neeleman et al. First Connection between Cold Gas in Emission and Absorption: CO Emission from a Galaxy-Quasar Pair, in the Astrophysical Journal of Letters, April 2016. (Preprint:
The team is composed of Marcel Neeleman [1], J. Xavier Prochaska [1], Martin A. Zwaan [2], Nissim Kanekar [3], Lise Christensen [4], Miroslava Dessauges-Zavadsky [5], Johan P.U. Fynbo [4], Eelco van Kampen [2], Palle Møller [2] and Tayyaba Zafar [6].

[1] Department of Astronomy & Astrophysics, UCO/Lick Ob- servatory, University of California, Santa Cruz, USA.

[2] European Southern Observatory, Garching bei München, Germany.

[3] Swarnajayanti Fellow; National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune, India.

[4] Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.

[5] Observatoire de Genève, Université de Genève, Sauverny, Switzerland.

[6] Australian Astronomical Observatory, North Ryde, Australia.

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The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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