The Milky Way may once have resembled a plant more than a stellar vortex.
Over their unfathomably long lifetimes, spiral galaxies like the Milky Way are thought to transform into lenticular “lens” galaxies and then into elliptical blobs. But the analysis of neighboring galaxies shows that our galaxy and others like it were once lenticular astronomer Alistair Graham reports in July Monthly Notices of the Royal Astronomical Society . If true, Graham’s proposed update to the evolutionary sequence of galaxies would rewrite the history of the Milky Way.
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“Lenticulars have always been a kind of abandoned stepchild of morphology [галактики],” says astronomer Christopher Conselis of the University of Manchester in England, who was not involved in the study. But this paper focuses on them, he says, as a key aspect of how galaxies change.
Lens glasses got their name because their entire star halo, when viewed from the edge, protrudes in the middle and thins to the sides, like a lens. These galaxies exhibit an inexplicable combination of properties, which makes their supposed place in the middle of the galactic evolutionary sequence rather suspect.
“We’ve known for a while that this was almost certainly wrong,” Conselis says. What’s particularly puzzling is that lenslets, despite their spiral disks, don’t have much gas, which prevents them from forming new stars. Spiral galaxies do have plenty of star-forming gas, and scientists aren’t sure why lenticular galaxies don’t.
Graham of Swinburne University of Technology in Hawthorne, Australia, found new clues to this mystery of galaxy evolution by looking at black holes.
At the center of most galaxies is a supermassive black hole, and when galaxies merge, these black holes also merge. This makes the mass of a galaxy’s black hole a kind of record of its past collisions. If a galaxy grew large by absorbing its neighbors rather than sucking in the surrounding gas, its black hole should be massive compared to the swarm of stars that surround it.
Using images from the Hubble and Spitzer space telescopes, Graham compared the black hole and the stellar masses of about 100 nearby galaxies. For galaxies of the same shape, he saw that the mass of the black hole and the mass of the star are related in a predictable way—except for lenticular galaxies.
When Graham took a closer look at the lensoculars, he realized that they were actually two different groups lumped together: those with a lot of interstellar dust and those without. This division, which he previously reported in May Monthly Notices of the Royal Astronomical Society , could be a superficial aesthetic difference. But the masses of black holes in galaxies suggest otherwise.
Dust-poor and dust-rich lenticular galaxies have completely different ratios of black hole masses to stellar masses, suggesting different histories and explaining the apparently scattered behavior of lenticular galaxies. Dusty galaxies tend to have a larger supermassive black hole than those in both spirals and dust-poor lenses. Dust-poor lenticulars are typically small in both black hole and stellar mass.
This led Graham to conclude that spiral galaxies are actually between the two types of lenticulars, from an evolutionary point of view. His new analysis suggests that dust-poor lenses become spirals after capturing small “satellite galaxies” and other minor mergers — increasing their black hole masses — and capturing nearby gas.
He suggests that when spirals collide with other substantial galaxies, they become dust-rich lenses—and indeed, he adds, every dust-rich lens in his data set has previously been recognized as the remnant of a spiral galaxy merger. Collisions between these dust-rich lenses are then enough to finally eat away the stellar disks of galaxies and destroy their dust, forming loose elliptical galaxies.
Black holes are good at tracking the evolution of galaxies, Conselis says, but the new sequence could be controversial. One problem, he said, is that lenticular galaxies in the nearby Universe are typically so light that they would need to merge dozens or even hundreds of times—much more than the expected average of about three every 10 billion years—to form a large spiral . galaxy.
But things could have been different in the early universe, he adds. There could have been more massive lenses a long time ago. Finding out is possible with the James Webb Space Telescope, which can see incredibly faint infrared light, allowing scientists to see further — and further into the past — than ever before.
“If you could look into the more distant universe, you could potentially see some of these galaxies when they’re first forming or when they’re evolving,” Conselis says. “We could really test this idea.”
