Gas Outflow From Sculptor Galaxy (NGC 253) Hints At Scarcity Of High Mass Galaxies

This comparison picture of the nearby bright spiral galaxy NGC 253, also known as the Sculptor Galaxy, shows the infrared view from ESO’s VISTA Telescope (left) and a detailed new view of the cool gas outflows at millimetre wavelengths from ALMA (right).
Credit:ESO/ALMA (ESO/NAOJ/NRAO)/J. Emerson/VISTA

The Atacama Large Millimeter/submillimeter Array (ALMA) have observed massive molecular gas outflows ejected by the Sculptor Galaxy (NGC 253). This may explain how starburst galaxies behave and why there is a scarcity of very massive galaxies in the Universe.

Starburst galaxies are galaxies that have a very high rate of star formation compared to regular galaxies. They produce stars so fast that their available gas content is depleted in a shorter time span. Starburst galaxies like the Sculptor Galaxy are defined by the rate at which they convert gas into stars, the available quantity of gas available, and the timescale on which SFR (star formation rate) will consume the available gas with the age or rotation period of the galaxy.

With the available data supplied by ALMA, scientist can study and explain why there are so few massive galaxies around. And if the ejected gas theory holds true for most of these galaxies, they also want to find out what ultimately happens to to these gas outflows.

Sculptor Galaxy Gas Outflow Observed

New observations from the ALMA telescope in Chile have given astronomers the best view yet of how vigorous star formation can blast gas out of a galaxy and starve future generations of stars of the fuel they need to form and grow. The dramatic images show enormous outflows of molecular gas ejected by star-forming regions in the nearby Sculptor Galaxy. These new results help to explain the strange paucity of very massive galaxies in the Universe. The study is published in the journal Nature on 25 July 2013.

Galaxies — systems like our own Milky Way that contain up to hundreds of billions of stars — are the basic building blocks of the cosmos. One ambitious goal of contemporary astronomy is to understand the ways in which galaxies grow and evolve, a key question being star formation: what determines the number of new stars that will form in a galaxy?

The Sculptor Galaxy, also known as NGC 253, is a spiral galaxy located in the southern constellation of Sculptor. At a distance of around 11.5 million light-years from our Solar System it is one of our closer intergalactic neighbours, and one of the closest starburst galaxies [1] visible from the southern hemisphere. Using the Atacama Large Millimeter/submillimeter Array (ALMA) astronomers have discovered billowing columns of cold, dense gas fleeing from the centre of the galactic disc.

“With ALMA’s superb resolution and sensitivity, we can clearly see for the first time massive concentrations of cold gas being jettisoned by expanding shells of intense pressure created by young stars,” said Alberto Bolatto of the University of Maryland, USA lead author of the paper. “The amount of gas we measure gives us very good evidence that some growing galaxies spew out more gas than they take in. We may be seeing a present-day example of a very common occurrence in the early Universe.”

Video: Sculptor Galaxy (NGC 253) Gas Outflows Observed

These results may help to explain why astronomers have found surprisingly few high-mass galaxies throughout the cosmos. Computer models show that older, redder galaxies should have considerably more mass and a larger number of stars than we currently observe. It seems that the galactic winds or outflow of gas are so strong that they deprive the galaxy of the fuel for the formation of the next generation of stars [2].

“These features trace an arc that is almost perfectly aligned with the edges of the previously observed hot, ionised gas outflow,” noted Fabian Walter, a lead investigator at the Max Planck Institute for Astronomy in Heidelberg, Germany, and a co-author of the paper. “We can now see the step-by-step progression of starburst to outflow.”

The researchers determined that vast quantities of molecular gas — nearly ten times the mass of our Sun each year and possibly much more — were being ejected from the galaxy at velocities between 150 000 and almost 1 000 000 kilometres per hour [3]. The total amount of gas ejected would add up to more gas than actually went into forming the galaxy’s stars in the same time. At this rate, the galaxy could run out of gas in as few as 60 million years.

“For me, this is a prime example of how new instruments shape the future of astronomy. We have been studying the starburst region of NGC 253 and other nearby starburst galaxies for almost ten years. But before ALMA, we had no chance to see such details,” says Walter. The study used an early configuration of ALMA with only 16 antennas. “It’s exciting to think what the complete ALMA with 66 antennas will show for this kind of outflow!” Walter adds.

More studies with the full ALMA array will help determine the ultimate fate of the gas carried away by the wind, which will reveal whether the starburst-driven winds are recycling or truly removing star forming material.