Artist's illustration of the Andromeda galaxy and the Milky Way, the two largest galaxies in the Local Group. Credit: NASA
In about 4 billion years, scientists estimate that the Andromeda and the Milky Way galaxies are expected to collide, based on data from the Hubble Space Telescope. And when they merge, they will give rise to a super-galaxy that some are already calling Milkomeda or Milkdromeda (I know, awful isn’t it?) While this may sound like a cataclysmic event, these sorts of galactic collisions are quite common on a cosmic timescale.
As an international group of researchers from Japan and California have found, galactic “hookups” were quite common during the early universe. Using data from the Hubble Space Telescope and the Subaru Telescope at in Mauna Kea, Hawaii, they have discovered that 1.2 billion years after the Big Bang, galactic clumps grew to become large galaxies by merging. As part of the Hubble Space Telescope (HST) “Cosmic Evolution Survey (COSMOS)”, this information could tell us a great about the formation of the early universe.
This artist's rending shows "before" and "after" images of a changing look quasar. Credit: Yale University.
Last week, astronomers at Yale University reported seeing something unusual: a seemingly stedfast beacon from the far reaches of the Universe went quiet. This relic light source, a quasar located in the region of our sky known as the celestial equator, unexpectedly became 6-7 times dimmer over the first decade of the 21st century. Thanks to this dramatic change in luminosity, astronomers now have an unprecedented opportunity to study both the life cycle of quasars and the galaxies that they once called home.
A quasar arises from a distant (and therefore, very old) galaxy that once contained a central, rotating supermassive black hole – what astronomers call an active galactic nucleus. This spinning beast ravenously swallowed up large amounts of ambient gas and dust, kicking up surrounding material and sending it streaming out of the galaxy at blistering speeds. Quasars shine because these ancient jets achieved tremendous energies, thereby giving rise to a torrent of light so powerful that astronomers are still able to detect it here on Earth, billions of years later.
In their hey-day, some active galactic nuclei were also energetic enough to excite electrons farther away from the central black hole. But even in the very early Universe, electrons couldn’t withstand that kind of excitement forever; the laws of physics don’t allow it. Eventually, each electron would drop back down to its rest state, releasing a photon of corresponding energy. This cycle of excitation happened over and over and over again, in regular and predictable patterns. Modern astronomers can visualize those transitions – and the energies that caused them – by examining a quasar’s optical spectrum for characteristic emission lines at certain wavelengths.
A simple example of an atomic spectrum, showing emission lines at particular wavelengths. Broad humps correspond to brighter emission lines, while lines that arise from narrow, lower-intensity emissions appear dimmer. Credit: NASA
Not all quasars are created equal, however. While the spectra of some quasars reveal many bright, broad emission lines at different energies, other quasars’ spectra consist of only the dim, narrow variety. Until now, some astronomers thought that these variations in emission lines among quasars were simply due to differences in their orientation as seen from Earth; that is, the more face-on a quasar was relative to us, the broader the emission lines astronomers would be able to see.
But all of that has now been thrown into question, thanks to our friend J015957.64+003310.5, the quasar revealed by the team of astronomers at Yale. Indeed, it is now plausible that a quasar’s pattern of emission lines simply changes over its lifetime. After gathering ten years of spectral observations from the quasar, the researchers observed its original change in brightness in 2010. In July 2014, they confirmed that it was still just as dim, disproving hypotheses that suggested the effect was simply due to intervening gas or dust. “We’ve looked at hundreds of thousands of quasars at this point, and now we’ve found one that has switched off,” explained C. Megan Urry, the study’s co-author.
How would that happen, you ask? After observing the comparable dearth of broad emission lines in its spectrum, Urry and her colleagues believe that long ago, the black hole at the heart of the quasar simply went on a diet. After all, an active galactic nucleus that consumed less material would generate less energy, giving rise to fainter particle jets and fewer excited atoms. “The power source just went dim,” said Stephanie LaMassa, the study’s principal investigator.
LaMassa continued, “Because the life cycle of a quasar is one of the big unknowns, catching one as it changes, within a human lifetime, is amazing.” And since the life cycle of quasars is dependent on the life cycle of supermassive black holes, this discovery may help astronomers to explain how those that lie at the center of most galaxies evolve over time – including Sagittarius A*, the supermassive black hole at the center of our own Milky Way.
“Even though astronomers have been studying quasars for more than 50 years, it’s exciting that someone like me, who has studied black holes for almost a decade, can find something completely new,” added LaMassa.
The team’s research will be published in an upcoming issue of The Astrophysical Journal. A pre-print of the paper is available here.
New research suggests that Dark Matter may exist in clumps distributed throughout our universe. Credit: Max-Planck Institute for Astrophysics
We know dark matter exists. We know this because without it and dark energy, our Universe would be missing 95.4% of its mass. What’s more, scientists would be hard pressed to explain what accounts for the gravitational effects they routinely see at work in the cosmos.
For decades, scientists have sought to prove its existence by smashing protons together in the Large Hadron Collider. Unfortunately, these efforts have not provided any concrete evidence.
Hence, it might be time to rethink dark matter. And physicists David M. Jacobs, Glenn D. Starkman, and Bryan Lynn of Case Western Reserve University have a theory that does just that, even if it does sound a bit strange.
In their new study, they argue that instead of dark matter consisting of elementary particles that are invisible and do not emit or absorb light and electromagnetic radiation, it takes the form of chunks of matter that vary widely in terms of mass and size.
As it stands, there are many leading candidates for what dark matter could be, which range from Weakly-Interacting Massive Particles (aka WIMPs) to axions. These candidates are attractive, particularly WIMPs, because the existence of such particles might help confirm supersymmetry theory – which in turn could help lead to a working Theory of Everything (ToE).
According to supersymmetry, dark-matter particles known as neutralinos (aka WIMPs) annihilate each other, creating a cascade of particles and radiation. Credit: Sky & Telescope / Gregg Dinderman.
But so far, no evidence has been obtained that definitively proves the existence of either. Beyond being necessary in order for General Relativity to work, this invisible mass seems content to remain invisible to detection.
According to Jacobs, Starkman, and Lynn, this could indicate that dark matter exists within the realm of normal matter. In particular, they consider the possibility that dark matter consists of macroscopic objects – which they dub “Macros” – that can be characterized in units of grams and square centimeters respectively.
Macros are not only significantly larger than WIMPS and axions, but could potentially be assembled out of particles in the Standard Model of particle physics – such as quarks and leptons from the early universe – instead of requiring new physics to explain their existence. WIMPS and axions remain possible candidates for dark matter, but Jacobs and Starkman argue that there’s a reason to search elsewhere.
“The possibility that dark matter could be macroscopic and even emerge from the Standard Model is an old but exciting one,” Starkman told Universe Today, via email. “It is the most economical possibility, and in the face of our failure so far to find dark matter candidates in our dark matter detectors, or to make them in our accelerators, it is one that deserves our renewed attention.”
After eliminating most ordinary matter – including failed Jupiters, white dwarfs, neutron stars, stellar black holes, the black holes in centers of galaxies, and neutrinos with a lot of mass – as possible candidates, physicists turned their focus on the exotics.
Ongoing experiments at the Large Hadron Collider have so far failed to produce evidence of WIMPs. Credit: CERN/LHC/GridPP
Nevertheless, matter that was somewhere in between ordinary and exotic – relatives of neutron stars or large nuclei – was left on the table, Starkman said. “We say relatives because they probably have a considerable admixture of strange quarks, which are made in accelerators and ordinarily have extremely short lives,” he said.
Although strange quarks are highly unstable, Starkman points out that neutrons are also highly unstable. But in helium, bound with stable protons, neutrons remain stable.
“That opens the possibility that stable strange nuclear matter was made in the early Universe and dark matter is nothing more than chunks of strange nuclear matter or other bound states of quarks, or of baryons, which are themselves made of quarks,” said Starkman.
Such dark matter would fit the Standard Model.
This is perhaps the most appealing aspect of the Macros theory: the notion that dark matter, which our cosmological model of the Universe depends upon, can be proven without the need for additional particles.
Still, the idea that the universe is filled with a chunky, invisible mass rather than countless invisible particles does make the universe seem a bit stranger, doesn’t it?
3D map of the cosmic web at a distance of 10.8 billion light years from Earth. The map was generated from imprints of hydrogen gas observed in the spectrum of 24 background galaxies, which are located behind the volume being mapped. This is the first time that large-scale structures in such a distant part of the Universe have been mapped directly. The coloring represents the density of hydrogen gas tracing the cosmic web, with brighter colors representing higher density.
Credit: Casey Stark (UC Berkeley) and Khee-Gan Lee (MPIA)
On the largest scales, networks of gaseous filaments span hundreds of millions of light-years, connecting massive galaxy clusters. But this gas is so rarified, it’s impossible to see directly.
For years, astronomers have used quasars — brilliant galactic centers fueled by supermassive black holes rapidly accreting material — to map the otherwise invisible matter.
But now, for the first time, a team of astronomers led by Khee-Gan Lee, a post-doc at the Max Planck Institute for Astronomy, has managed to create a three-dimensional map of the large-scale structure of the Universe using distant galaxies. And the advantages are numerous.
The science has always gone a little something like this: as the bright light from a distant quasar travels toward Earth, it encounters the intervening clouds of hydrogen gas and is partially absorbed. This leaves dark absorption lines in the quasar’s spectrum.
Artist’s impression illustrating how a distant quasar’s or galaxy’s spectrum becomes clouded with absorption lines from intervening hydrogen gas. Credit: Khee-Gan Lee (MPIA) and Casey Stark (UC Berkeley)
If the Universe were static, the dark absorption lines would always be located at the same spot (121 nanometers for the so-called Lyman-alpha line) in the quasar’s spectrum. But because the Universe is expanding, the distant quasar is flying away from the Earth at a rapid speed. This stretches the quasar’s light, such that each intervening hydrogen gas cloud imprints its absorption signature on a different region of the quasar’s spectrum, leaving a forest of lines.
Therefore detailed measurements of multiple quasars’ spectra close together can actually reveal the three-dimensional nature of the intervening hydrogen clouds. But galaxies are nearly 100 times more numerous than quasars. So in theory they should provide a much more detailed map.
The only problem is that galaxies are also about 15 times fainter than quasars. So astronomers thought they were simply not bright enough to see well in the distant universe. But Lee carried out calculations that suggested otherwise.
“I was surprised to find that existing large telescopes should already be able to collect sufficient light from these faint galaxies to map the foreground absorption, albeit at a lower resolution than would be feasible with future telescopes,” said Lee in a news release. “Still, this would provide an unprecedented view of the cosmic web which has never been mapped at such vast distances.”
Lee and his colleagues used the 10-meter Keck I telescope on Mauna Kea, Hawaii to take a look a closer look at the distant galaxies and the forest of hydrogen absorption embedded in their spectra. But even the weather in Hawaii can turn ugly.
“We were pretty disappointed as the weather was terrible and we only managed to collect a few hours of good data,” said coauthor Joseph Hennawi, also from the Max Planck Institute for Astronomy. “But judging by the data quality as it came off the telescope, it was already clear to me that the experiment was going to work.”
The team was only able to collect data for four hours. But it was still unprecedented. They looked at 24 distant galaxies, which provided sufficient coverage of a small patch of the sky and allowed them to combine the information into a three-dimensional map.
The map reveals the large-scale structure of the Universe when it was only a quarter of its current age. But the team hopes to soon parse the map for more information about the structure’s function — following the flows of cosmic gas as it funneled away from voids and onto distant galaxies. It will provide a unique historical record on how the galaxy clusters and voids grew from inhomogeneities in the Big Bang.
The results have been published in the Astrophysical Journal and are available online.
This image shows observations of a newly discovered galaxy core dubbed GOODS-N-774, taken by the NASA/ESA Hubble Space Telescope's Wide Field Camera 3 and Advanced Camera for Surveys. The core is marked by the box inset, overlaid on a section of the Hubble GOODS-N, or GOODS North, field (Great Observatories Origins Deep Survey). Credit:
NASA, ESA, and E. Nelson (Yale University, USA)
Astronomers have spotted, for the first time, a dense galactic core blazing with the light of millions of newborn stars in the early universe.
The finding sheds light on how elliptical galaxies, the large, gas-poor gatherings of older stars, may have first formed in the early universe. It’s a question that has eluded astronomers for decades.
The research team first uncovered the compact galactic core, dubbed GOODS-N-774, in images from the Hubble Space Telescope. Later observations from the Spitzer Space Telescope, the Herschel Space Observatory, and the W.M. Keck Observatory helped make this a true scientific finding.
The core formed 11 billion years ago, when the universe was less than 3 billion years old. Although only a fraction of the size of the Milky Way, at that time it already contained above twice as many stars as our own galaxy.
Theoretical simulations suggest that giant elliptical galaxies form from the inside out, with a large core marking the very first stages of formation. But most searches for these forming cores have come up empty handed, making this a first observation and a phenomenal find.
“We really hadn’t seen a formation process that could create things that are this dense,” explained lead author Erica Nelson from Yale University in a press release. “We suspect that this core-formation process is a phenomenon unique to the early universe because the early universe, as a whole, was more compact. Today, the universe is so diffuse that it cannot create such objects anymore.”
Alongside determining the galaxy’s size from the Hubble images, the team dug into archived far-infrared images from Spitzer and Herschel to calculate how fast the compact galaxy is creating stars. It seems to be producing 300 stars per year, a rate 30 times greater than the Milky Way.
The frenzied star formation likely occurs because the galactic core is forming deep inside a gravitational well of dark matter. Its unusually high mass constantly pulls gas in, compressing it and sparking star formation.
But these bursts of star formation create dust, which blocks the visible light. This helps explain why astronomers haven’t seen such a distant core before, as they may have been easily missed in previous surveys.
The team thinks that shortly after the early time period we can see, the core stopped forming stars. It likely then merged with other smaller galaxies, until it transformed into a much greater galaxy, similar to the more massive and sedate elliptical galaxies we see today.
“I think our discovery settles the question of whether this mode of building galaxies actually happened or not,” said coauthor Pieter van Dokkum from Yale University. “The question now is, how often did this occur?”
The team suspects that other galactic cores are abundant, but hidden behind their own dust. Future infrared telescopes, such as the James Webb Space Telescope, should be able to find more of these early objects.
Artist concept of matter swirling around a black hole. (NASA/Dana Berry/SkyWorks Digital)
Black holes one billion times the Sun’s mass or more lie at the heart of many galaxies, driving their evolution. Although common today, evidence of supermassive black holes existing since the infancy of the Universe, one billion years or so after the Big Bang, has puzzled astronomers for years.
How could these giants have grown so massive in the relatively short amount of time they had to form? A new study led by Tal Alexander from the Weizmann Institute of Science and Priyamvada Natarajn from Yale University, may provide a solution.
Black holes are often mistaken to be monstrous creatures that suck in dust and gas at an enormous rate. But this couldn’t be further from the truth (in fact the words “suck” and “black hole” in the same sentence makes me cringe). Although they typically accumulate bright accretion disks — swirling disks of gas and dust that make them visible across the observable Universe — these very disks actually limit the speed of growth.
First, as matter in an accretion disk gets close to the black hole, traffic jams occur that slow down any other infalling material. Second, as matter collides within these traffic jams, it heats up, generating energy radiation that actually drives gas and dust away from the black hole.
A star or a gas stream can actually be on a stable orbit around the black hole, much as a planet orbits around a star. So it is quite a challenge for astronomers to think of ways that would make a black hole grow to supermassive proportions.
Luckily, Alexander and Natarajan may have found a way to do this: by placing the black hole within a cluster of thousands of stars, they’re able to operate without the restrictions of an accretion disk.
Black holes are generally thought to form when massive stars, weighing tens of solar masses, explode after their nuclear fuel is spent. Without the nuclear furnace at its core pushing against gravity, the star collapses. While the inner layers fall inward to form a black hole of only about 10 solar masses, the outer layers fall faster, hitting the inner layers, and rebounding in a huge supernova explosion. At least that’s the simple version.
The erratic path of the black hole through the gas (black line) is randomized by the surrounding stars (yellow circles). Meanwhile, dense cold gas (green arrows) flows toward the center of the cluster (red cross). Credit: Weizmann Institute of Science.
The team began with a model of a black hole, created from this stellar blast, embedded within a cluster of thousands of stars. A continuous flow of dense, cold, opaque gas fell into the black hole. But here’s the trick: the gravitational pull of many nearby stars caused it to zigzag randomly, preventing it from forming an accretion disk.
Without an accretion disk, not only is matter more able to fall into the black hole from all sides, but it isn’t slowed down in the accretion disk itself.
All in all, the model suggests that a black hole 10 times the mass of the Sun could grow to more than 10 billion times the mass of the Sun by one billion years after the Big Bang.
GOODS field containing distant dwarf galaxies forming stars at an incredible rate. Image Credit: ESO
Massive galaxies in the early Universe formed stars at a much faster clip than they do today — creating the equivalent of a thousand new suns per year. This rate reached its peak 3 billion years after the Big Bang, and by 6 billion years, galaxies had created most of their stars.
New observations from the Hubble Space Telescope show that even dwarf galaxies — the small, low mass clusters of several billion stars — produced stars at a rapid rate, playing a bigger role than expected in the early history of the Universe.
Today, we tend to see dwarf galaxies clinging to larger galaxies, or sometimes engulfed within, rather than existing as blazing collections of stars alone. But astronomers have suspected that dwarfs in the early Universe could turn over stars quickly. The trouble is, most images aren’t sharp enough to reveal the faint, faraway galaxies we need to observe.
“We already suspected that dwarf starbursting galaxies would contribute to the early wave of star formation, but this is the first time we’ve been able to measure the effect they actually had,” said lead author Hakim Atek of the École Polytechnique Fédérale de Lausanne (EPFL) in a press release. “They appear to have had a surprisingly significant role to play during the epoch where the Universe formed most of its stars.”
Previous studies of starburst galaxies in the early Universe were biased toward massive galaxies, leaving out the huge number of dwarf galaxies that existed in this era. But the highly sensitive capabilities of Hubble’s Wide Field Camera 3 have now allowed astronomers to peer at low-mass dwarf galaxies in the distant Universe.
This image represents the data that comes from using the NASA/ESA Hubble Space Telescope’s highly-sensitive Wide Field Camera 3 in its grism spectroscopy mode. Image Credit: NASA / ESA
Atek and colleagues looked at 1000 galaxies from roughly three billion years to 10 billion years after the Big Bang. They dug through their data, in search of the H-alpha line: a deep-red visible spectral line, which occurs when a hydrogen electron falls from its third to second lowest energy level.
In star forming regions, the surrounding gas is continually ionized by radiation from the newly formed stars. Once the gas is ionized, the nucleus and removed electron can recombine to form a new hydrogen atom with the electron typically in a higher energy state. This electron will then cascade back to the ground state, a process that produces H-alpha emission about half the time.
So the H-alpha line is an effective probe of star formation and the brightness of the H-alpha line (which is much easier to detect than the faint, almost invisible, continuum) is an effective probe of the star formation rate. From this single line, Attek and colleagues found that the rate at which stars are turning on in early dwarfs is surprisingly high.
“These galaxies are forming stars so quickly that they could actually double their entire mass of stars in only 150 million years — this sort of gain in stellar mass would take most normal galaxies 1-3 billion years,” said co-author Jean-Paul Kneib, also of EPFL.
The team doesn’t yet know why these small galaxies are producing such a vast number of stars. In general, bursts of star formation are thought to follow somewhat chaotic events like galactic mergers or the shock of a supernova. But by continuing to study these dwarf galaxies, astronomers hope to shed light on galactic evolution and help paint a consistent picture of events in the early Universe.
The paper has been published today in the Astrophysical Journal and may be viewed here. The latest Hubblecast (below) also covers this exciting result.
FMOS spectra in the J-band (left panel) and H-band (right panel), each of which filters light so that only specific wavelengths can pass through. The horizontal axis refers to the wavelength direction while the vertical axis indicates individual spectra observed through each fiber. Small blue circles indicate the detection of emission lines (left: H? and [OIII]; right: H?, [NII]). The inset box shows the intensity of the emission lines for one galaxy. The vertical bands indicate the masked regions where bright sky (OH) emissions are prevented from entering science fibers placed on high-redshift galaxies. (Credit: FMOS-COSMOS)
Nobody likes a sloppy COSMOS (Cosmological Evolution Survey) and astronomers utilizing the Fiber-Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope have put order into chaos through their studies. The survey has found that some nine billion years ago galaxies were capable of producing new stars in a fashion as orderly as game of checkers. Despite their young cosmological age, the galaxies show signs containing high amounts of dust enriched by heavier elements – a mature state.
“These findings center on a major question: What was the universe like when it was maximally forming its stars?” says John Silverman, the principal investigator of the FMOS-COSMOS project at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU).
These “universal” questions are just what the COSMOS team seeks to answer. Their research goals are to enlighten the scales of cosmic time in relationship with the environment, formation and evolution of massive galactic structures. When studying individual galaxies, they may be able to tell if their rate of growth can be attributed to large-scale environments. Information of this type can clarify what factors the early Universe structure may have contributed to the current form of local galaxies. One of the data sets the team is focusing on is using the FMOS on the Subaru Telescope to chart out the distribution of more than a thousand galaxies which formed over nine billion years ago – a time when the Universe was hitting its star-formation peak.
“One key to generating fruitful results is collaboration between COSMOS researchers to maximize optimal use of FMOS.” Silverman continues, “In this project, researchers from Kavli IPMU in Japan and the Institute for Astronomy at the University of Hawaii (principal investigator: David Sanders) formed an effective collaboration to implement their goal.” The observations spanned 10 clear nights starting in March 2012.
Why choose spectroscopy? This advanced fiber optics technology speaks for itself, collecting light over an area of sky equal in size to that of the Moon. The FMOS focuses on the near-infrared, filtering out unwanted emissions caused by warm temperatures and can acquire spectra from 400 galaxies simultaneously with a wide field of coverage of 30 arc minutes at prime-focus. By employing such a wide field of view, astronomers can squeeze in a wide range of objects in their local environments. This enables researchers to maximize information on star-forming regions, cluster formation, and cosmology.
As David Sanders, the principal investigator of the FMOS-COSMOS project at IfA, puts it, “FMOS has clearly revolutionized our ability to study how galaxies form and evolve across cosmic time. It is currently the most powerful instrument we have to study the large numbers of objects needed to understand galaxies of all sizes, shapes and masses — from the largest ellipticals to the smallest dwarfs. We are extremely fortunate that the Kavli IPMU-IfA collaboration is giving us this unique opportunity to study the distant universe in such exquisite detail.”
FMOS will soon be famous by revealing its true potential. It has been collecting copious amounts of data in a high spectral resolution mode and at a very successful rate. So far it has accomplished nearly half of its goal – to examine over a thousand galaxies with redshifts to map the large-scale structure. The current survey consists of mapping an area of sky which spans a square degree in high-resolution mode and future plans for FMOS will involve enlarging the area. This expanded coverage will complement other instruments on alternative telescopes which have a wider spectral imaging system or a higher resolution which is limited to a smaller area. These combined findings may one day result in showing us some of the very first structures that eventually evolved into the massive galaxy clusters we see today!
Galaxy z8_GND_5296 (seen in the inset) is the earliest galaxy that astronomers have measured the distance to accurately. It formed approximately 700 million years after the Big Bang, and is forming stars at an incredibly rapid rate. [Credit: V. Tilvi (Texas A&M), S. Finkelstein (UT Austin), the CANDELS team, and HST/NASA]
“The farthest galaxy yet seen!” Haven’t we heard that one before? (See here and here, for example.) While it’s true that astronomers keep pushing farther back in time with better instruments, there are fundamental challenges both in observing and measuring the distances to the earliest galaxies in the cosmos.
That’s why this new observation of a galaxy that formed about 700 million years after the Big Bang is significant. While scores of galaxies have been identified that formed in that era, astronomers have only measured accurate distances for five of them. This galaxy marks the sixth, and it is the farthest of the bunch. Perhaps even more important than the distance measurement, researchers determined that this galaxy gave birth to new stars at more than 100 times the rate the Milky Way does today. That indicates early galaxies may have been more aggressive with star-formation than previously believed. Continue reading “Taking Measure: A ‘New’ Most Distant Galaxy”
This image shows "slices" of the Universe at different times throughout its history (present day, and at 4 and 11 billion years ago). Each slice goes further back in time, showing how galaxies of each type appear. The shape is that of the Hubble tuning fork diagram, which describes and separates galaxies according to their morphology. Credit: NASA, ESA, M. Kornmesser
What we’re gonna’ do here is go back. Way back into time. Back to when the only thing that existed was… galaxies? When astronomers employed the power of Hubble’s CANDELS survey to observe different galaxy types from the distant past, they expected to see a variety of spiral, elliptical, lenticular and peculiar structures, but what they didn’t expect was that things were a whole lot more “peculiar” a long time ago!
Known as the Hubble Sequence, astronomers use this classified system for listing galaxy sizes, shapes and colors. It also arranges galaxies according to their morphology and star-forming activity. Up to the present, the Hubble Sequence covered about 80% of the Universe’s history, but the latest information shows that the sequence was valid as much as 11 billion years ago! Out of what we currently know, there are two dominant galaxy types – spiral and elliptical – with the lenticular structure as a median. Of course, this is constrained to the regions of space which we can readily observe, but how true did the sequence hold back when the Universe theoretically began?
“This is a key question: when and over what timescale did the Hubble Sequence form?” says BoMee Lee of the University of Massachusetts, USA, lead author of a new paper exploring the sequence. “To do this you need to peer at distant galaxies and compare them to their closer relatives, to see if they too can be described in the same way.”
Using the Hubble Space Telescope, astronomers took on the sequence challenge to peer back 11 billion years in time to study galaxy structure. Up until now, researchers could confirm the sequence was valid as long ago as 8 billion years, but these new studies pushed CANDELS, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, to the outer limits. It is simply the largest project ever, and soaked up 902 assigned orbits of observing time. Using the WFC3 and ACS cameras, the team examined structures that existed less than one billion years after the Big Bang. While earlier studies had aimed for lower-mass galaxies in this era, no study had really taken on serious observation of mature structures – ones similar to our own galaxy. Now the new CANDELS observations show us that all galaxies, regardless of size, fit into a totally different classification!
“This is the only comprehensive study to date of the visual appearance of the large, massive galaxies that existed so far back in time,” says co-author Arjen van der Wel of the Max Planck Institute for Astronomy in Heidelberg, Germany. “The galaxies look remarkably mature, which is not predicted by galaxy formation models to be the case that early on in the history of the Universe.”
Just what did this study see that’s so different? Just the power of two. Galaxies were either complex, with blue star forming regions and irregular structures, or they were like our nearby neighbors: massive red galaxies that exhibit no new star-formation. In the early Universe, galaxies like the Milky Way were uncommon. With so little to study, it was nearly impossible to get a large enough sample to sufficiently catalog their characteristics. Early research could only peer back in visible light, a format which emphasized star formation and revealed the red-shifted ultraviolet emission of the galaxies. This information was inconclusive because galaxy structure appeared disrupted and unlike the formations we see near to us. Through the use of infra-red, astronomers could observe the now red-shifted massive galaxies in their visible rest frame. Thanks to CANDELS lighting the way, astronomers were able to thoroughly sample a significantly larger amount of mature galaxies in detail.
“The huge CANDELS dataset was a great resource for us to use in order to consistently study ancient galaxies in the early Universe,” concludes Lee. “And the resolution and sensitivity of Hubble’s WFC3 is second to none in the infrared wavelengths needed to carry out this study. The Hubble Sequence underpins a lot of what we know about how galaxies form and evolve — finding it to be in place this far back is a significant discovery.”
