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The Cosmic Snake reveals huge molecular clouds

16 September 2019

An international team including scientists from the Paris Observatory - PSL has detected clouds of molecules in a galaxy 8 billion light years away. This exploit yields information about their physical properties, which are very different from those of clouds which we know in our immediate galactic environment. This work will be published in the journal Nature Astronomy dated September 16th 2019.

Molecular clouds, the cradle of star formation, are well known in the Milky Way. But are they the same in distant galaxies that form more stars? Until now it was very difficult to isolate clouds in distant galaxies, for lack of spatial resolution. Astronomers then came up with the idea of benefiting from a natural telescope - the gravitational lens phenomenon - coupled with the use of ALMA (Atacama Large Millimeter / submillimiter Array), an interferometer of 50 millimeter-scale radio antennas that reconstruct the entire image of a galaxy instantly. Thanks to the alignment of a massive object between the observer and the distant object, the gravitational lenses produce a magnifying effect, and considerably enlarge the size of the distant object studied. This resolution, further improved thanks to the ALMA interferometer (resolution of 0.2 ’’) made it possible to characterize the clouds individually in a distant galaxy, nicknamed the Cosmic Snake, located at 8 billion light-years (see Figure 1).

Molecular gas distribution in the galaxy of the Cosmic Snake, distorted and amplified by a gravitational lens. In the center is the image obtained with the Hubble Space Telescope, showing the 4 images, delimited by the red curve, critical line of the amplification corresponding to the redshift 1.036 of the galaxy. From these 4 images, the image of the undistorted galaxy is reconstructed at the top left. The yellow contours indicate the intensity of the CO (4-3) emission line observed with ALMA. The size of the ALMA beam (ellipse in yellow) is 0.22 ’’ x 0.18 ’’. The image on the right represents the zoom of the Snake’s southernmost image, and the identified molecular clouds, in space (X, Y) and velocity (Z).

These observations revealed that the molecular clouds of distant galaxies had a mass, density and turbulence 10 to 100 times higher than the clouds of nearby galaxies. Yet this galaxy is "normal" for its epoch, and is not subject to a starburst. The international team has also discovered that the level of star formation efficiency of the Cosmic Snake’s molecular clouds is particularly high, favored by the large internal cloud turbulence. In nearby galaxies, a cloud forms about 5% of its mass in stars. In distant galaxies, this number jumps to 30%.

Figure 2: Mass of molecular gas as a function of cloud radius identified in the Cosmic Snake galaxy (magenta dots), local quiescent galaxies (gray dots), local starbursts galaxies (solid black circles), and galaxy SDP81 at z = 3.042 (blue dots). The dashed magenta and blue lines indicate the potential range of molecular gas masses in the Cosmic Snake and SDP81, respectively, as determined with two extreme CO / H2 conversion factors, the factor calibrated in the Milky Way (4.36) and that in the starbursts (1.0). The black lines indicate molecular gas surface densities of 100 Msol / pc2 (continuous line), 1000 Msol / pc2 (dashes) and 2600 Msol / pc2 (dotted line) respectively. High redshift clouds have much higher gas surface densities than typical local clouds.

Figure 2 compares the cloud surface densities in near galaxies, normal or starbursts, with those of distant galaxies, which are much larger.

Reference

  • Molecular clouds in the Cosmic Snake, a normal star-forming galaxy 8 billion years ago, Dessauges-Zavadsky, M., Richard, J., Combes F. et al 2019, Nature Astronomy

Scientific contact

  • Francoise Combes
    Observatoire de Paris - LERMA, CNRS, Collège de France, PSL Université, Sorbonne Université
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