ESO ALMA: Violent origins of disc galaxies probed

Each of the colourful objects in this image illustrates one of 30 merging galaxies. 

The contours in the individual galaxies indicate the dispersion of carbon monoxide while the color represents the motion of gas. 

Gas that is moving away from us appears red while the blue colour shows gas that is approaching. 

The contours together with the transition from red to blue indicate a gaseous disc that is rotating about the center of the galaxy. Credit: ALMA (ESO /NAOJ /NRAO) /SMA /CARMA /IRAM /J. Ueda et al.

For decades scientists have believed that galaxy mergers usually result in the formation of elliptical galaxies.

Now, for the the first time, researchers using the Atacama Large Millimeter/sub-millimeter Array (ALMA) and a host of other radio telescopes have found direct evidence that merging galaxies can instead form disc galaxies, and that this outcome is in fact quite common.

This surprising result could explain why there are so many spiral galaxies like the Milky Way in the Universe.

An international research group led by Junko Ueda, a Japan Society for the Promotion of Science postdoctoral fellow, has made surprising observations that most galaxy collisions in the nearby Universe, within 40 million light-years from Earth, result in so-called disc galaxies.

Disc galaxies, including spiral galaxies like the Milky Way and lenticular galaxies, are defined by pancake-shaped regions of dust and gas, and are distinct from the category of elliptical galaxies.

It has, for some time, been widely accepted that merging disc galaxies would eventually form an elliptically shaped galaxy.

During these violent interactions the galaxies do not only gain mass as they merge or cannibalise each-other, but they are also changing their shape throughout cosmic time, and therefore changing type along the way.

Computer simulations from the 1970s predicted that mergers between two comparable disc galaxies would result in an elliptical galaxy.

The simulations predict that most galaxies today are elliptical, clashing with observations that over 70% of galaxies are in fact disc galaxies.

However, more recent simulations have suggested that collisions could also form disc galaxies.


This artist’s impression shows the merger between two galaxies leading to the formation of a disc galaxy. 

Upon merging, the shape of the galaxies is disturbed by their mutual gravitational interaction and results in a galaxy with a disc structure.

Gas that is moving away from us appears red while the blue colour shows gas that is approaching. 

The contours together with the transition from red to blue indicate a gaseous disc that is rotating about the centre of the galaxy.

The movie shows the collision of two disc galaxies, but the actual shape of galaxies prior to the collision in this study is not known. Credit: NAOJ

To identify the final shapes of galaxies after mergers observationally, the group studied the distribution of gas in 37 galaxies that are in their final stages of merging.

The Atacama Large Millimeter/sub-millimeter Array (ALMA) and several other radio telescopes were used to observe emission from carbon monoxide (CO), an indicator of molecular gas.

The team's research is the largest study of molecular gas in galaxies to date and provides unique insight into how the Milky Way might have formed.

Their study revealed that almost all of the mergers show pancake-shaped areas of molecular gas, and hence are disc galaxies in the making.

Ueda explains: "For the first time there is observational evidence for merging galaxies that could result in disc galaxies. This is a large and unexpected step towards understanding the mystery of the birth of disc galaxies."

Nonetheless, there is a lot more to discover. Ueda added: "We have to start focusing on the formation of stars in these gas discs."

"Furthermore, we need to look farther out in the more distant Universe. We know that the majority of galaxies in the more distant Universe also have discs."

"We however do not yet know whether galaxy mergers are also responsible for these, or whether they are formed by cold gas gradually falling into the galaxy."

"Maybe we have found a general mechanism that applies throughout the history of the Universe."

More information: Research paper on Astro-Ph: arxiv.org/abs/1407.6873

Atacama Array (ALMA) detects star forming molecular gas

An artist’s conception of the environment around GRB 020819B based on ALMA observations. 

Image Credit: NAOJ

Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of researchers reports the first-ever detection of molecular gas, the fuel for star formation, in two galaxies that were previously rocked by gamma ray bursts (GRBs), the brightest explosions in the Universe.

These new observations revealed that the molecular gas was concentrated toward the centers of the galaxies, while the GRBs occurred in unusual environments that were surprisingly bereft of gas yet rich in dust.

The researchers speculate that the dearth of molecular gas around the GRBs was due to strong ultraviolet (UV) radiation from young, massive stars, which can break apart the molecules of gas while leaving the dust relatively undisturbed.

The GRBs, dubbed GRB 020819B and GRB 051022, are located approximately 4.3 billion and 6.9 billion light-years away from Earth, respectively.

Astronomer Bunyo Hatsukade, assistant professor at the Chile Observatory of the National Astronomical Observatory of Japan (NAOJ), led the research group that studied the GRB host galaxies. The results are published in the journal Nature.

ALMA's unprecedented sensitivity made it possible to make the first detection ever of carbon monoxide (CO) gas in a GRB host galaxy.

ALMA's unparalleled high resolution also revealed GRB 020819B occurred in a galaxy where the molecular gas was concentrated at the nuclear region while dust was concentrated at the site of the GRB.

The ratio of dust to molecular gas at the GRB site is ten or more times higher than in normal environments. It is the first time that the spatial distribution of molecular gas and dust in the GRB host galaxies is revealed.

Currently, GRBs are classified as either long- or short-duration.

• A long-duration GRB, which lasts two seconds or longer, is believed to be generated by the supernova explosion of a star 40 or more times the mass of our Sun. 
• Short GRBs last less than two seconds and are associated with the collision and merger of neutron stars.