Red vs. Blue: Astronomers Nail Down the Origins of Rare Loner Dwarf Galaxies

Blue Ultra Diffuse Galaxy

In this image, the fall of a blue ultra-diffuse galaxy into a galaxy system and its subsequent ejection as a red ultra-diffuse galaxy, is depicted. Credit: MIT

The results provide a blueprint for finding such systems in the universe’s quieter, emptier regions.

By definition, dwarf galaxies are small and dim, with just a fraction of the stars found in the Milky Way and other galaxies. There are, however, giants among the dwarfs: Ultra-diffuse galaxies, or UDGs, are dwarf systems that contain relatively few stars but are scattered over vast regions. Because they are so diffuse, these systems are difficult to detect, though most have been found tucked within clusters of larger, brighter galaxies.

Now astronomers from MIT, the University of California at Riverside, and elsewhere have used detailed simulations to detect “quenched” UDGs — a rare type of dwarf galaxy that has stopped generating stars. They identified several such systems in their simulations and found the galaxies were not in clusters, but rather exiled in voids — quiet, nearly empty regions of the universe.

This isolation goes against astronomers’ predictions of how quenched UDGs should form. So, the team used the same simulations to rewind the dwarf systems’ evolution and see exactly how they came to be.

The researchers found that quenched UDGs likely coalesced within halos of dark matter with unusually high angular momentum. Like a cotton candy machine, this extreme environment may have spun out dwarf galaxies that were anomalously stretched out.

These UDGs then evolved within galaxy clusters, like most UDGs. But interactions within the cluster likely ejected the dwarfs into the void, giving them wide, boomerang-like trajectories known as “backsplash” orbits. In the process, the galaxies’ gas was stripped away, leaving the galaxies “quenched” and unable to produce new stars.

The simulations showed that such UDGs should be more common than what has been observed. The researchers say their results, published today in Nature Astronomy, provide a blueprint for astronomers to go looking for these dwarfish giants in the universe’s voids.

“We always strive to get a complete consensus of the galaxies that we have in the universe,” says Mark Vogelsberger, associate professor of physics at MIT. “This study is adding a new population of galaxies that the simulation actually predicts. And we now have to look for them in the real universe.”

Vogelsberger co-led the study with Laura Sales of UC Riverside and José A. Benavides of the Institute of Theoretical and Experimental Astronomy in Argentina.

Red vs. blue

The team’s search for quenched UDGs began with a simple survey for UDG satellites — ultra-diffuse systems that reside outside galaxy clusters. Astronomers predict that UDGs within clusters should be quenched, as they would be surrounded by other galaxies that would essentially rub out the UDG’s already-diffuse gas and shut off star production. Quenched UDGs in clusters should then consist mainly of old stars and appear red in color.

If UDGs exist outside clusters, in the void, they are expected to continue churning out stars, as there would be no competing gas from other galaxies to quench them. UDGs in the void, therefore, are predicted to be rich with new stars, and to appear blue.

When the team surveyed previous detections of UDG satellites, outside clusters, they found most were blue as expected — but a few were red.

“That’s what caught our attention,” Sales says. “And we thought, ‘What are they doing there? How did they form?’ There was no good explanation.”

Galactic cube

To find one, the researchers looked to TNG50, a detailed cosmological simulation of galaxy formation developed by Vogelsberger and others at MIT and elsewhere. The simulation runs on some of the most powerful supercomputers in the world and is designed to evolve a large volume of the universe, from conditions resembling those shortly after the Big Bang to the present day.

The simulation is based on fundamental principles of physics and the complex interactions between matter and gas, and its results have been shown in many scenarios to agree with what astronomers have observed in the actual universe. TNG50 has therefore been used as an accurate model for how and where many types of galaxies evolve through time.  

In their new study, Vogelsberger, Sales, and Benavides used TNG50 to first see if they could spot quenched UDGs outside galaxy clusters. They started with a cube of the early universe measuring about 150 million light-years wide, and ran the simulation forward, up through the present day. Then they searched the simulation specifically for UDGs in voids, and found most of the ones they detected were blue, as expected. But a surprising number — about 25 percent — were red, or quenched.

They zeroed in on these red satellite dwarfs and used the same simulation, this time as a sort of time machine to see how, when, and where these galaxies originated. They found that the systems were initially part of clusters but were somehow thrown out into the void, on a more elliptical, “backsplash” orbit.

“These orbits are almost like those of comets in our solar system,” Sales says. “Some go out and orbit back around, and others may come in once and then never again. For quenched UDGs, because their orbits are so elliptical, they haven’t had time to come back, even over the entire age of the universe. They are still out there in the field.”

The simulations also showed that the quenched UDGs’ red color arose from their ejection — a violent process that stripped away the galaxies’ star-forming gas, leaving it quenched and red. Running the simulations further back in time, the team observed that the tiny systems, like all galaxies, originated in halos of dark matter, where gas coalesces into galactic disks. But for quenched UDGs, the halos appeared to spin faster than normal, generating stretched out, ultra-diffuse galaxies.

Now that the researchers have a better understanding of where and how quenched UDGs arose, they hope astronomers can use their results to tune telescopes, to identify more such isolated red dwarfs — which the simulations suggest must be lurking in larger numbers than what astronomers have so far detected.

“It’s quite surprising that the simulations can really produce all these very small objects,” Vogelsberger says. “We predict there should be more of this kind of galaxy out there. This makes our work quite exciting.”

For more on this reseearch, see Astronomers Uncover Origin of Elusive Ultradiffuse Galaxies.

Reference: “Quiescent ultra-diffuse galaxies in the field originating from backsplash orbits” by José A. Benavides, Laura V. Sales, Mario. G. Abadi, Annalisa Pillepich, Dylan Nelson, Federico Marinacci, Michael Cooper, Ruediger Pakmor, Paul Torrey, Mark Vogelsberger and Lars Hernquist, 6 September 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01458-1

5 Comments on "Red vs. Blue: Astronomers Nail Down the Origins of Rare Loner Dwarf Galaxies"

  1. this is a scientific prediction a
    running the simulation further back in time
    when there were halos of dark matter that spin normal velocity speeds where big bang gas coalesces and generates into tiny systems of ordinary spiral galactic discs
    but for halos of dark matter that spin faster than normal velocity speeds where big bang gas coalesces generate into tiny systems of stretched out ultra diffuse galactic discs
    mullach abu this big bang gas could be hydrogen or helium or both
    so ultra difuse could be hydrogen only at low energy level
    and normal could be hyrdogen and helium at low medium and high energy levels
    ultra diffuse galaxies udgs
    ulra diffuse galaxies surrounded by other galaxies within galaxy clusters should be quenched rub out the udgs already diffuse gas
    and shut off stellar star production
    a violent process that stripped away the galaxies ejected star forming gas leaving it quenched and red
    quenched udgs in clusters should then consist mainly of old stars and appear red in colour
    mullach abu thinks the opposite
    only when they meet and greet another member of the cluster and
    lose all their gas to the other member
    or they are robbed of all their gas or they are rubbed out of their gas
    will they then become quenched as there is no gas left to make new stars and the old stars remain lit like red embers in a fire dont you remember when u were fit and toned and ready to take on the universe and now u are burned out and old
    red ultra diffuse dwarf galaxies ruddgs about 25% in simulation
    isolated red ultra diffuse dwarf galaxies must be loitering and lurking out there in large numbers than what astronomers have so far detected

  2. fundamental principle of physics and the complex interactions between matter and gas in the simulated actual active universe and an accurate model for how and where many types of galaxies evolve through time
    this is a scientific prediction b a blueprint for finding systems in the universes quieter emptier regions
    ultra diffuse galaxies that reside outside galaxy clusters the local group of galaxies in the void outside
    they are expected to continue churning out stars as there would be no competing gas from other galaxies to quench them ????????????
    udgs in the void therefore are predicted to be rich with new stars and to appear blue
    mullach abu thinks maybe but adds
    ??????????/ no competing gas from other galaxies to quench them
    sure isnt that the reason galaxies exist to absorb the gas from other regions to produce stars
    absorb the gas from inside their own structure and make type i stars
    absorb the gas from outside their own structure and make type ii stars
    absorb the gas from other galactic structures and make type iii stars
    so why would this not happen inside the local galaxy group as well
    ultra diffuse galaxies might meet gas in the intergalactic void outside the local galaxy clusters and when it does new stellar stars are formed new blue ones aflame fire flickering
    blue ultra diffuse dwarf galaxies buddgs inter galactic gas ones giants among the dwarfs
    and when they dont meet this intergalactic gas in the void between local clusters they have three and a half possibilities
    if after their exit from the local group they contain a small amount of their own ultra diffuse dwarf galactic gas then they have a possibility of igniting into combustion starlight and becoming bright and light again
    blue ultra diffuse dwarf galaxies buddgs self igniting gas ones
    if after their exit from the local group they contain a small amount of their own ultra diffuse dwarf galactic gas and a bit of the gas from the colliding galaxy then they have a possibility of igniting into combustion starlight and becoming bright and light again
    blue ultra diffuse dwarf galaxies buddgs self igniting and colliding galaxy gas ones
    but if after their exit from the local group they contain no amount of gas
    their own gas
    the colliding galaxies gas
    and the intergalactic void gas
    then they cant make stars and so they remain red ultra diffuse dwarf galaxies
    or are you saying your prediction is right
    Astronomers predict that UDGs within clusters should be quenched, as they would be surrounded by other galaxies that would essentially rub out the UDG’s already-diffuse gas and shut off star production. Quenched UDGs in clusters should then consist mainly of old stars and appear red in color.
    that the ultra diffuse dwarf galaxies internal gas and stellar matter system ultra diffuse that it may be has been gassed out of existence by another galaxies gas system
    and that that gas is non combustible in the ultra diffuse dwarf galaxy

  3. a normandian campbell asks me to think again
    i think therefore i am
    what is there to ponder

  4. Good article made prefect space sense. Better than 1970s space classes thanks Great teacher great writer

Leave a comment

Email address is optional. If provided, your email will not be published or shared.