Lasers Recreate the Conditions Inside Galaxy Clusters

Galaxies don’t exist in a vacuum. Ok, maybe they do (mostly, since even interstellar space has some matter in it). But galaxies aren’t normally solitary objects. Multiple galaxies interacting gravitationally can form clusters. These clusters can interact with each other, forming superclusters. Our own galaxy is part of a group of galaxies called the Local Group. This Local Group is part of the Virgo Supercluster, which is in turn a part of a group of superclusters called the Laniakea Supercluster.

Mixed in with all of these galaxies is a lot of heat, with extremely high temperatures comparable to the core of our Sun, around 10 million Kelvin (27 million degrees Fahrenheit). This temperature is so hot that hydrogen atoms cannot exist, and instead of gas a plasma forms of protons and electrons. This is a problem for physicists though, who say it shouldn’t be that hot.

As Gianluca Gregori, a professor of physics at University of Oxford and one author of a new paper detailing an experiment to recreate the conditions inside a galaxy cluster, puts it: “The reason why the gas inside the galaxy cluster should have cooled down is simply due to the fact that the cluster has existed for a very long time (for a time which is comparable to the age of the Universe). So, if we assume thermal conduction works in the normal way, we would have expected the initial hot core to have dissipated its heat by now. But observations shows it has not.”

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Astronomers see an Enormous Shockwave, 60 Times Bigger Than the Milky Way

Astronomers have a thing for big explosions and collisions, and it always seems like they are trying to one-up themselves in finding a bigger, brighter one.  There’s a new entrant to that category – an event so big it created a burst of particles over 1 billion years ago that is still visible today and is 60 times bigger than the entire Milky Way.

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By Measuring Light From Individual Stars Between Galaxy Clusters, Astronomers Find Clues About Dark Matter

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416. This is one of six clusters that was studied by the Hubble Frontier Fields programme, which yielded the deepest images of gravitational lensing ever made. Scientists used intracluster light (visible in blue) to study the distribution of dark matter within the cluster.

Astronomers have been able to measure an extremely faint glow of light within galaxy clusters, and that measurement came with a surprise: it traced the amount of invisible dark matter, something that scientists have been trying to pin down for decades.

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Matter makes up exactly 31.5±1.3% of the Universe

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416. This is one of six clusters that was studied by the Hubble Frontier Fields programme, which yielded the deepest images of gravitational lensing ever made. Scientists used intracluster light (visible in blue) to study the distribution of dark matter within the cluster.

Weighing the universe is a tricky task, but a team of astronomers have used a clever technique to measure how many galaxy clusters are in the cosmos, and from there come up with a total amount of matter. The answer: 31.5±1.3% of all the energy in the universe.

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The Universe is the Same, Everywhere We Look. Even More than Cosmologists Predicted

Several superclusters revealed by the 2dF Galaxy Redshift Survey.
Several superclusters revealed by the 2dF Galaxy Redshift Survey.

No matter which direction you look in the Universe, the view is basically the same if you look far enough. Our local neighborhood is populated with bright nebulae, star clusters, and dark clouds of gas and dust. There are more stars toward the center of the Milky Way than there are in other directions. But across millions, and billions, of light-years, galaxies cluster evenly in all directions, and everything starts to look the same. In astronomy, we say the Universe is homogeneous and isotropic. Put another way, the Universe is smooth.

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Complete and Total Mayhem in a Distant Galaxy Collision

The filamentary structures observed by LOFAR at the center of Abell 2255, here reported in red. These radio emissions are due to trails of particles and magnetic fields released by the galaxies during their motion inside the cluster (credits: Botteon et al. (2020) – LOFAR – SDSS).

A cluster of galaxies is nothing trivial. The shocks, the turbulence, the energy, as all of that matter and energy merges and interacts. And we can watch all the chaos and mayhem as it happens.

A team of astronomers are looking at the galaxy cluster Abell 2255 with the European Low-Frequency Array (LOFAR) radio telescope, and their images are showing some never-before-seen details in this actively merging cluster.

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Astronomers See a Pileup of 14 Separate Galaxies in the Early Universe

Artist impression of 14 galaxies detected by ALMA as they appear in the very early, very distant universe. These galaxies are in the process of merging and will eventually form the core of a massive galaxy cluster. Credit: NRAO/AUI/NSF; S. Dagnello

Looking deep into the observable Universe – and hence, back to the earliest periods of time – is an immensely fascinating thing. In so doing, astronomers are able to see the earliest galaxies in the Universe and learn more about how they evolved over time. From this, they are not only able to see how large-scale structures (like galaxies and galaxy clusters) formed, but also the role played by dark matter.

Most recently, an international team of scientists used the Atacama Large Millimeter-submillimeter Array (ALMA) to observe the Universe when it was just 1.4 billion years old. What they observed was a “protocluster”, a series of 14 galaxies located 12.4 billion light-years away that were about to merge. This would result in the formation of a massive galaxy cluster, one of the largest objects in the known Universe.

The study which described their findings, titled “A massive core for a cluster of galaxies at a redshift of 4.3“, recently appeared in the journal Nature. The study was led by Tim Miller – an astronomer from Dalhousie University, Halifax, and Yale University – and included members from NASA’s Jet Propulsion Laboratory, the European Southern Observatory (ESO), Canada’s National Research Council, the Harvard-Smithsonian Center for Astrophysics, the National Radio Astronomy Observatory, and multiple universities and research institutions.

ALMA image of 14 galaxies forming a protocluster known as SPT2349-56. These galaxies are in the process of merging and will eventually form the core of a truly massive galaxy cluster. Credit: ALMA (ESO/NAOJ/NRAO); B. Saxton (NRAO/AUI/NSF)

As they indicate in their study, this protocluster (designated SPT2349-56) was first observed by the National Science Foundation’s South Pole Telescope. Using the Atacama Pathfinder Experiment (APEX), the team conducted follow-up observations that confirmed that it was an extremely distant galactic source, which was then observed with ALMA. Using ALMA’s superior resolution and sensitivity, they were able to distinguish the individual galaxies.

What they found was that these galaxies were forming stars at rate 1,000 times faster than our galaxy, and were crammed inside a region of space that was about three times the size of the Milky Way. Using the ALMA data, the team was also able to create sophisticated computer simulations that demonstrated how this current collection of galaxies will likely grow and evolve over billion of years.

These simulations indicated that once these galaxies merge, the resulting galaxy cluster will rival some of the most massive clusters we see in the Universe today. As Scott Chapman, and astrophysicist at Dalhousie University and a co-author on the study, explained:

“Having caught a massive galaxy cluster in throes of formation is spectacular in and of itself. But, the fact that this is happening so early in the history of the universe poses a formidable challenge to our present-day understanding of the way structures form in the universe.”

Zooming in to the galaxies discovered by ALMA that are evolving into a galaxy cluster. Credit: ALMA (ESO/NAOJ/NRAO), T. Miller & S. Chapman et al.; Herschel; South Pole Telescope; (NRAO/AUI/NSF) B. Saxton

The current scientific consensus among astrophysicists states that a few million years after the Big Bang, normal matter and dark matter began to form larger concentrations, eventually giving rise to galaxy clusters. These objects are the largest structures in the Universe, containing trillions of stars, thousands of galaxies, immense amounts of dark matter and massive black holes.

However, current theories and computer models have suggested that protoclusters – like the one observed by ALMA – should have taken much longer to evolve. Finding one that dates to just 1.4 billion years after the Big Bang was therefore quite the surprise. As Tim Miller, who is currently a doctoral candidate at Yale University, indicated:

“How this assembly of galaxies got so big so fast is a bit of a mystery, it wasn’t built up gradually over billions of years, as astronomers might expect. This discovery provides an incredible opportunity to study how galaxy clusters and their massive galaxies came together in these extreme environments.”

Looking to the future, Chapman and his colleagues hope to conduct further studies of SPT2349-56 to see how this protoclusters eventually became a galaxy cluster. “ALMA gave us, for the first time, a clear starting point to predict the evolution of a galaxy cluster,” he said. “Over time, the 14 galaxies we observed will stop forming stars and will collide and coalesce into a single gigantic galaxy.”

The study of this and other protoclusters will be made possible thanks to instruments like ALMA, but also next-generation observatories like the Square Kilometer Array (SKA). Equipped with more sensitive arrays and more advanced computer models, astronomers may be able to create a truly accurate timeline of how our Universe became what it is today.

Further Reading: NRAO, Nature

Are Little Blue Dots in the Hubble Frontier Fields Precursors to Globular Clusters?

The massive galaxy cluster Abell 370 as seen by Hubble Space Telescope in the final Frontier Fields observations. Credit: NASA/ESA/HFF

In 2012, the Hubble Space Telescope Frontier Fields program (aka. Hubble Deep Fields Initiative 2012) officially kicked off. The purpose of this project was to study the faintest and most distant galaxies in the Universe using the gravitational lensing technique, thus advancing our knowledge of early galaxy formation. By 2017, the Frontier Field program wrapped up, and the hard work of analyzing all the data it collected began.

One of the more interesting finds within the Frontier Fields data has been the discovery of low mass galaxies with high star formation rates. After examining the “parallel fields” for Abell 2744 and MACS J0416.1-2403 – two galaxy clusters studied by the program – a pair of astronomers noted the presence of what they refer to as “Little Blue Dots” (LBDs), a finding which has implications for galaxy formation and globular clusters.

The study which details their findings recently appeared online under the title “Little Blue Dots in the Hubble Space Telescope Frontier Fields: Precursors to Globular Clusters?“. The study team consisted of Dr. Debra Meloy Elmegreen – a professor of astronomy at Vassar College – and Dr. Bruce G. Elmegreen, an astronomer with the IBM Research Division at the T.J. Watson Research Center in Yorktown Heights.

The galaxy clusters Abell 2744, MACS J0416.1-2403, MACS J0717.5+3745, MACS J1149.5+2223, Abell S1063, Abell 370. Credit: NASA, ESA, STScI, and the HFF team

To put it simply, the Frontier Fields program used the Hubble Space Telescope to observe six massive galaxy clusters at optical and near-infrared wavelengths – with its Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3), respectively. These massive galaxies were used to magnify and stretch images of remote galaxies located behind them which were otherwise too faint for Hubble to see directly (aka. gravitational lensing).

While one of these Hubble cameras would look at a galaxy cluster, the other would simultaneously view an adjacent patch of sky. These adjacent patches are known as “parallel fields”, otherwise faint regions that provide some of the deepest looks into the early Universe. As Dr. Bruce Elmegreen told Universe Today via email:

“The purpose of the HFF program is to take deep images of 6 regions of the sky where there are clusters of galaxies, because these clusters magnify background galaxies through the gravitational lens effect. In this way, we can see further than just with direct imaging of the sky alone. Many galaxies have been studied using this magnification technique. The clusters of galaxies are important because they are big mass concentrations which make strong gravitational lenses.”

This six galaxy clusters used for the sake of the project included Abell 2744, MACS J0416.1-2403 and their parallel fields, the latter of which were the focal point in this study. These and the other clusters were used to find galaxies that existed just 600 to 900 million years after the Big Bang. These galaxies and their respective parallels had already been cataloged using computer algorithms that automatically found galaxies in the images and determined their properties.

Images of the MACS J0416.1–2403 and Abell 2744 galaxy clusters, taken as part of the Hubble Frontier Fields program. Credit: NASA/ESA/HST Frontier Fields team (STScI)

As the research duo go on to explain in their study, recent large-scale deep surveys have enabled studies of smaller galaxies at higher redshifts. These include “green peas” – luminous, compact and low mass galaxies with high specific star formation rates – and even lower-mass “blueberries”, small starburst galaxies that are a faint extension of the green peas that also show intense rates of star formation.

Using the aforementioned catalogues, and examining the parallel fields for Abell 2744 and MACS J0416.1-2403, the team went looking for other examples of low-mass galaxies with high star formation rates. The purpose of this was to measure the properties of these dwarf galaxies, and to see if any of their positions accorded with where globular clusters are known to have formed.

What they found was what they referred to as “Little Blue Dots” (LBSs), which are even lower-mass versions of “blueberries”. As Dr. Debra Elmegreen told Universe Today via email:

“When I was examining the images (there are about 3400 galaxies detected in each field), I noticed occasional galaxies that appeared as little blue dots, which was very intriguing because of Bruce’s previous theoretical work on dwarf galaxies. The published catalogs included redshifts and star formation rates and masses for each galaxy, and it turns out the little blue dots are low mass galaxies with very high star formation rates for their mass.”

Close-up view of a HFF parallel field, showing a LBD (upper right). Credit: Debra & Bruce Elmegreen

These galaxies didn’t show structure, so Debra and Bruce stacked the images of galaxies into 3 different ranges of redshift (which worked out to about 20 galaxies each) to create deeper images. “Still they showed no structure or faint extended outer disk,” said Debra, “so they are at the limit of resolution, with average sizes of 100-200 parsecs (about 300-600 light years) and masses of a few million times the mass of our sun.”

In the end, they determined that within these LBDs, star formation rates were very high. They also noted that these dwarf galaxies were very young, being less than 1% the age of the Universe at the time that they were observed. “So the tiny galaxies just formed,: said Bruce, “and their star formation rates are high enough to account for the globular clusters, maybe one in each LBD, when the star bursts in them wind down after a few tens of million years.”

Debra and Bruce Elmegreen are no strangers to high redshift galaxies. Back in 2012, Bruce published a paper that suggested that the globular clusters that orbit the Milky Way (and most other galaxies) formed in dwarf galaxies during the early Universe. These dwarf galaxies would have since been acquired by larger galaxies like our own, and the clusters are essentially their remnants.

Globular clusters are essentially massive star clusters that orbit around the Milky Way Halo. They are typically around 1 million Solar masses and are made up of stars that are very old – somewhere on the order of 10 to 13 billion years. Beyond the Milky Way, many appear in common orbits and in the Andromeda Galaxy, some even appear connected by a stream of stars.

The three groups of LBDs, where the dwarf galaxies were stacked based on their redshift. Credit: Debra & Bruce Elmegreen

As Bruce explained, his is a compelling argument for the theory that globular clusters formed from dwarf galaxies in the early Universe:

“This suggests that the metal-poor globular clusters are the dense remnants of little galaxies that got captured by bigger galaxies, like the Milky Way, and ripped apart by tidal forces. This idea for the origin of halo globular clusters goes back several decades… It would be only the metal-poor one that are like this, which are about half the total, because dwarf galaxies are metal poor compared to big galaxies, and they were also more metal poor in the early universe.”

This study has many implications for our understanding of how the Universe evolved, which was the chief aim of the Hubble Frontier Fields program. By examining objects in the early Universe, and determining their properties, scientists are able to determine how the structures that we are familiar with today – i.e. stars, galaxies, clusters, etc. – truly came from.

These same studies also allow scientists to make educated guesses about where the Universe is going and what will become of those same structures millions or even billions of years from now. In short, knowing where we’ve been lets us predict where we are headed!

Further Reading: arXiv

A Snapshot of a Galactic Crash

This image combines NASA/ESA Hubble Space Telescope observations with data from the Chandra X-ray Observatory. As well as the electric blue ram pressure stripping streaks seen emanating from ESO 137-001, a giant gas stream can be seen extending towards the bottom of the frame, only visible in the X-ray part of the spectrum. Credit: NASA, ESA, CXC

Some galaxies shine with a red ghostly glow. Once these galaxies stop forming new stars, they can only host long-lived stars with low masses and red optical colors. Astronomers often call these ghostly galaxies “red and dead.” But the basics behind why some form so quickly is still a mystery.

“It is one of the major tasks of modern astronomy to find out how and why galaxies in clusters evolve from blue to red over a very short period of time,” said lead author Michele Fumagalli from Durham University in a news release. “Catching a galaxy right when it switches from one to the other allows us to investigate how this happens.”

And that’s exactly what Fumagalli and colleagues did.

The team used ESO’s Multi Unit Spectroscopic Explorer (MUSE) instrument mounted on the 8-meter Very Large Telescope. With this instrument, astronomers collect 90,000 spectra every time they look at an object, allowing them to gain a detailed map of the object’s motion through space.

This chart shows the location of the distant galaxy ESO 137-001 in the constellation of Triangulum Australe (The Southern Triangle). This is a rich area of the sky close to the Milky Way, but this galaxy is faint and needs a large telescope to be visible. Credit: ESO, IAU and Sky & Telescope
The location of the distant galaxy ESO 137-001. Credit: ESO / IAU / Sky & Telescope

The target, ESO 137-001, is a spiral galaxy 200 million light-years away in the constellation better known as the Southern Triangle. But more importantly, it’s currently hurtling toward the Norma Cluster and embarking on a grand galactic collision.

ESO 137-001 is being stripped of most of its gas due to a process called ram-pressure stripping. As the galaxy falls into the galaxy cluster, it feels a headwind, much as a runner feels a wind on even the stillest day. At times this can compress the gas enough to spark star formation, but if it’s too intense then the gas is stripped away, leaving a galaxy that’s empty of the material needed to form new stars.

So the galaxy is in the midst of a brilliant transformation, changing from a blue gas-rich galaxy to a red gas-poor galaxy.

The observations show that the outskirts of the galaxy are already completely devoid of gas. Here the stars and matter are more thinly spread, and gravity has a relatively week hold over the gas. So it’s easier to push the gas away.

In fact, dragging behind the galaxy are 200,000 light-year-long streams of gas that have already been lost, making the galaxy look like a jellyfish trailing its tentacles through space. In these streamers, the gas is turbulent enough to compress small pockets of gas and therefore actually ignite star formation.

The center of the galaxy, however, is not yet devoid of gas because the gravitational pull is strong enough to hold out much longer. But it will only take time until all of the galactic gas is swept away, leaving ESO 137-001 red and dead.

Surprisingly the new MUSE observations show the gas trailing behind continues to rotate in the same way that the galaxy does. Furthermore, the rotation of stars at the center of the galaxy remains unhindered by the great fall.

Astronomers remain unsure why as this is only a snapshot of one galactic crash, but soon MUSE and other instruments will pry more out of the cosmic shadows.

The results will be published in the journal Monthly Notices of the Royal Astronomical Society and are available online.

Hubble Spots the Ghostly Light From Dead Galaxies

Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe. Credit: NASA.

In a patch of sky 3.5 billion light-years away there are hazy elliptical galaxies, colorful spirals, blue arcs and distorted shapes seen clumping together. It’s the result of a vast cosmic collision that took place over the course of 350 million years.

The mess is a treasure trove of information for astronomers, allowing them to piece together the history of a cosmic pile-up of multiple galaxy clusters.

But now astronomers are digging through the nearby darkness. They’re eyeing the remnant stars that were cast adrift in intergalactic space. These stars should emit a faint glow known as intracluster light that — until now — has mostly remained a subject of speculation.

Mireia Montes and Ignacio Trujillo, both from the University of La Laguna, Spain, have used the Hubble Space Telescope to observe the aforementioned cluster, Abel 2744, in exquisite detail. The cluster has already earned the nickname Pandora’s Cluster for its violent past.

The team looked at both visible and near-infrared color images of the cluster, and then split these color images by brightness. This allowed Montes and Trujillo to pinpoint the color of the cluster’s faintest glow and therefore glean the ghost stars’ age, chemical content, and total mass.

Compared to stars within the cluster’s galaxies, the ghost stars emit bluer light and are therefore rich in heavier elements like oxygen, carbon, and nitrogen. So the scattered stars must be second- or third-generation stars enriched by previous supernovae. But they’re still between three and nine billion years younger than the stars within the cluster’s galaxies.

The team estimates that the combined light of about 100 billion outcast stars contributes approximately six percent of the cluster’s brightness.

But how did the stars get thrown from their respective galaxies in the first place? This new forensic evidence suggests that violent collisions tore apart between four and six Milky Way-size galaxies, scattering their stars into intergalactic space.

“The Hubble data revealing the ghost light are important steps forward in understanding the evolution of galaxy clusters,” said Trujillo in a news release. “It is also amazingly beautiful in that we found the telltale glow by utilizing Hubble’s unique capabilities.”

Abell 2744 is only one target in Hubble’s Frontier Fields program, which will map five more galaxy clusters in superb detail.

The results have been published in the Astrophysical Journal and are available online.