Starved galaxies make no more stars

The world’s highest and most advanced observatory is already making great astronomical discoveries even though it is less than half complete. ALMA – the Atacama Large Millimetre Array – appears to have helped solve the riddle of how some galaxies switch from being active sites of star-birth to red elliptical galaxies that are effectively dead. It is a fate that may await our own Milky Way.

The author at ALMA
The author at ALMA in Chile (Credit: Paul Sutherland)

Astronomers had, over past years, been drawing up an answer which suggested that violent collisions that caused spiral galaxies to merge were to blame. In this chaotic scenario, gas and dust are pushed into clumps of rapid star creation, called starbursts, and then into the supermassive black hole lying at the heart of the merged galaxies. Powerful jets flare from this cosmic crime scene, causing the whole region around the black hole to shine brilliantly as a quasar. The jets eventually blow away all the gas that might have formed new stars so that the galaxy remaining becomes “infertile”.

That was the theory, but astronomers were having trouble amassing enough evidence to prove it was a correct explanation because they failed to observe enough examples of the essential, jet stage. However, in its first three months of work at the end of last year, ALMA was able to confirm nearly two dozen examples of quasars many billions of light-years away, where the dramatic and powerful jets were active.

ALMA, which Skymania visited in Chile in early October as it began operations, provided valuable information for astronomers thanks to what it failed to observe rather than any positive observation. Its array of detectors, which you can read about in my on-the-spot report for space website SEN, were directed to look for dust warmed by active star-forming regions in several galaxies. But nothing showed up for half of the galaxies observed and the rest were extremely dim, suggesting there was little dust left in them.

Dr Carol Lonsdale of the North American ALMA Science Center at the National Radio Astronomy Observatory (NRAO), Virginia, said this week: “Despite ALMA’s great sensitiviy to detecting starbursts, we saw nothing, or next to nothing – which is exactly what we hoped it would see.”

She told the American Astronomical Society’s conference in Austin, Texas: “ALMA’s results reveal to us that there is little-to-no starbursting going on in these young, active galaxies. The galaxy evolution model says this is thanks to their central black holes whose jets are starving them of star-forming gas. On its first run out of the gate, ALMA confirmed a critical phase in the timeline of galaxy evolution.”

With no star-forming gas remaining as an essential ingredient, merging galaxies will be unable to make new stars. so when its final generation of massive and brilliant, but short-lived, blue stars dies out, the long-lived, lower mass, redder stars become dominant, giving the galaxy a new reddish hue.

Lonsdale’s team picked their candidate galaxies for study by comparing objects recorded by NASA’s Wide-field Infrared Survey Explorer (WISE) spaceprobe in its survey of the sky with another survey carried out by the NRAO’s Very Large Array – just redubbed the Karl G Jansky VLA after the father of radio astronomy. Because ALMA observed as longer wavelengths than WISE, it was able to tell the difference between dust warmed by starburst activity and dust heated by material falling onto the central black hole.

ALMA will look closely at another 26 quasars recorded by WISE to give Lonsdale’s team more evidence before they publish their results later this year. She said: “ALMA revealed to us this rare stage of galaxy starvation, and now we want to use the VLA to focus on delineating the outflows that robbed these galaxies of their fuel. Together, the two most sensitive radio telescope arrays in the world will help us truly understand the fate of spiral galaxies like our own Milky Way.”

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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