Astronomers using the partially completed ALMA observatory have found compelling evidence for how star-forming galaxies evolve into ‘red and dead’ elliptical galaxies, catching a large group of galaxies right in the middle of this change. According to lead investigator Dr. Carol Lonsdale of the North American ALMA Science Center at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, despite ALMA’s great sensitiviy to detecting starbursts, they saw nothing, or next to nothing, which is exactly what they hoped it would see. For these observations, ALMA was tuned to look for dust warmed by active star-forming regions. However, half of Lonsdale’s two dozen galaxies didn’t show up at all in ALMA’s observations, and the other half were extremely dim, indicating that there was very little of the tell-tale dust present.
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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.Once their star-forming gas has been blown away, merging galaxies will be unable to make new stars. A New Method for Finding Candidate Starving Galaxies to support this gas-starvation theory, astronomers needed to see it at work in lots of merging galaxies with high power jets. The place to observe enough of them is among the quasars, active galaxies found in the Universe’s past, several billion light-years away. Their selective hunt for these specific quasars started with NASA’s Wide-field Infrared Survey Explorer (WISE) spacecraft. Then the team compared its selections with NRAO’s VLA Sky Survey of 1.8 million radio objects and chose the overlapping results as the most suitable targets for their search for starburst activities with ALMA. Observing at longer infrared wavelengths than WISE, ALMA enabled Lonsdale’s team to discriminate between dust warmed by starburst activity and dust heated by material falling onto the central black hole. ALMA has 26 more WISE quasars to probe before Lonsdale and her international team publish their results later this year.