The Milky Way may have looked like a whirlpool or a legume in the past.
Spiral galaxies such as the Milky Way have lived for unimaginably long periods. It is generally believed that You can also find out more about morph into lentil-shaped “lenticular” galaxies and then into elliptical blobs (SN: 4/23/18). However, an analysis of nearby galaxy suggests that Our galaxy and other galaxies like it were once lenticularAlister Graham, an astronomer, reports in the July Monthly Notices of Royal Astronomical Society. If correct, Graham’s proposed update to the evolutionary sequence of galaxies would rewrite the history of the Milky Way.
“Lenticulars have always been sort of the abandoned stepchild of [galaxy] morphology,” says astronomer Christopher Conselice of the University of Manchester in England, who wasn’t involved in the study. This paper brings them to the forefront, says Conselice, as a major factor in how galaxies evolve.
Lenticulars are named for the way that their entire halo, when viewed from one side, bulges at the center and then thins toward the edges, similar to a lentil. These galaxies exhibit a confusing mix of properties that’s made their presumed place in the middle of galaxy evolution sequences rather suspect.
“We’ve known for a while that that’s almost certainly not correct,” Conselice says. Particularly puzzling is that lenticulars, despite their spiral-like disks, don’t have lots of gas, which hinders them from producing new stars. Spiral galaxies do have lots of star-forming gas, and scientists aren’t sure why lenticular galaxies don’t.
Black holes have provided new clues in the mystery of galaxy evolution. Graham of Swinburne University of Technology, Hawthorn (Australia), has found these by studying black holes.
The black holes in the center of most galaxies are supermassive. When galaxies merge so do these black holes. This makes the mass of a galaxy’s black hole a kind of record of its past collisions. If a black hole is large because a galaxy has grown by eating its neighbors instead of sucking in gas from the surrounding area, it should be so if that’s what happened.
Graham used images taken by the Hubble and Spitzer telescopes to compare the stellar and black hole masses of 100 galaxies in the vicinity. For galaxies of the same shape, he saw that black hole mass and stellar mass tend to be linked in a predictable way — except for the lenticular galaxies.
Graham realized that the lenticulars had been grouped together based on whether they contained interstellar particles or not. He reported this division in the May issue. Monthly Notices of Royal Astronomical SocietyThis could have been an aesthetic difference. But the galaxies’ black hole masses suggest otherwise.
The dust-rich and dust-poor galaxies have completely different relationships between black hole masses and stellar mass, indicating different histories and explaining lenticular galaxy behavior. Dusty galaxies are more likely to have a supermassive, massive black hole than spirals or dusty lenticulars. Dust-poor spirals and lenticulars tend to be on the smaller side when it comes to black hole mass as well as stellar mass.
Graham came to the conclusion that spiral galaxies actually fall between two types of lenticulars. His new analysis suggests that dust-poor lenticulars become spirals after capturing small “satellite galaxies” and other minor mergers — bumping up their black hole masses — and scooping up nearby gas.
When spirals collide with other substantial galaxies, he proposes, they become dust-rich lenticulars — and indeed, he adds, every dust-rich lenticular in his dataset was previously recognized as the remnant of a spiral galaxy merger. Collisions between these dust-rich lenticulars are then enough to finally erode the galaxies’ discs of stars and destroy their dust, producing blobby elliptical galaxies.
Conselice says black holes can be a good indicator of galaxy development, but that the new sequence might cause controversy. One issue, he says, is that lenticular galaxies in the nearby universe are usually such lightweights that they would need to merge tens or even hundreds of times — far more than the expected average of around three over 10 billion years — to form a large spiral galaxy.
He continues: “But things might have looked different in the early Universe.” It is possible that there were more lenticulars long ago. James Webb Space Telescope could help us figure this out. It can see infrared wavelengths that are so faint. Allowing scientists to see further away — and further back in time — than ever before (SN: 12/16/22).
“If you could look in the more distant universe, you could potentially see some of these galaxies when they’re first forming, or when they’re evolving,” Conselice says. “We could potentially really test this idea.”
