Astronomers have identified 53 “super spirals” — spiral galaxies that are huge and incredibly luminous — as part of a project exploring archived observations.
The Milky Way is an average sort of spiral galaxy. Its disk spans roughly 100,000 light-years (although maybe more like 160,000 light-years, if recently detected, distant ripples are part of it). Each year it produces the equivalent of a Sun’s mass in new stars.
Although the Milky Way is one of the largest galaxies in the Local Group, across the cosmos the biggest, brightest galaxies are usually not spirals but ellipticals, gargantuan football-shaped collections of old stars.
But Patrick Ogle (Caltech) and his team have found 53 distant spirals that are bigger, brighter, and starbirthing-mightier than expected from their nearby spiral brethren. These newfound galaxies, which the team calls superluminous spirals or “super spirals” for short, have disks between 180,000 and 440,000 light-years wide, and they’re churning out stars at a rate of 5 to 65 Suns each year.
For galaxies this big and this late in cosmic history, that’s a lot of stars.
The team was sorting through the NASA/IPAC Extragalactic Database (a.k.a. “NED”) — a repository of observations, research papers, and other information (even includes a great glossary) — to see what kind of big-picture insights they could glean from its archive. They picked 1,616 galaxies out of NED that had redshifts less than 0.3 (meaning their light left them less than 3.5 billion years ago) and luminosities more than eight times that of the Milky Way. Through a process of “how much/what kind of data do we have on this galaxy” elimination, the team whittled down the group to a couple hundred and discovered that although the majority were ellipticals, 53 of these bright galaxies were super spirals.
No, that’s not a large number: these giants are clearly rare. In fact, they’re so rare in a given volume of space that cosmological simulations don’t operate on a large enough scale to produce many of them, meaning astronomers can’t use simulations to predict how many there should be or how they form.
There’s not much to go on from their appearances, either. Four of the galaxies look like they’re midway through merging with another galaxy, and at least 10 seem to be in clusters or groups. But the others look fairly isolated. So it’s not clear yet if super spirals form via mergers (since gas-rich mergers preserve disks) or via pipelines of cold gas that would feed the growing disk from the inside out. Astronomers will need to study how star formation and neutral gas are distributed in the spirals in order to narrow in on the origins, the team suggests in the February 1st Astrophysical Journal.
What’s interesting isn’t merely the galaxies’ size, but that they’re forming so many stars. Galaxies this massive would normally be well on the way to becoming “red and dead,” their stellar populations old and rosy. Somehow, super spirals managed not to choke off or heat up the cold gas supplies they need in order to make stars. The authors offer various theories for why that is, but the short answer is: we have no idea.
But we do know that there are dying, equally massive disk galaxies later on in the universe. These super spirals could be the precursors, the team suggests.
Reference: P. Ogle et al. "Superluminous Spiral Galaxies." Astrophysical Journal. February 1, 2016.