First Images of the Cosmic Web Reveal Unsuspected Presence of Billions of Dwarf Galaxies

In the Universe, galaxies are distributed along extremely tenuous filaments of gas millions of light years long separated by voids, forming the cosmic web.
The MUSE instrument on the Very Large Telescope has captured an image of several filaments in the early Universe…
… revealing the unexpected presence of billions of dwarf galaxies in the filaments

Figure 1: cosmological simulation of the distant Universe. The image shows the light emitted by hydrogen atoms in the cosmic web in a region roughly 15 million light years across. In addition to the very weak emission from intergalactic gas, a number of point sources can be seen: these are galaxies in the process of forming their first stars. Credit: Jeremy Blaizot / projet SPHINX

Although the filaments of gas in which galaxies are born have long been predicted by cosmological models, we have so far had no real images of such objects. Now for the first time, several filaments of the ‘cosmic web’ have been directly observed using the MUSE[1] instrument installed on ESO’s Very Large Telescope in Chile. These observations of the early Universe, 1 to 2 billion years after the Big Bang, point to the existence of a multitude of hitherto unsuspected dwarf galaxies. Carried out by an international collaboration led by the Centre de Recherche Astrophysique de Lyon (CNRS/Université Lyon 1/ENS de Lyon), also involving the Lagrange laboratory (CNRS/Université Côte d’Azur/Observatoire de la Côte d’Azur),[2] the study is published in the journal Astronomy & Astrophysics.

Figure 2: the 2250 galaxies in the ‘cone’ of the Universe observed by MUSE are shown here according to the age of the Universe (in billions of years). The period of the early Universe (0.8 to 2.2 billion years after the Big Bang) explored in this study is shown in red. The 22 regions with galaxy over-density are indicated by grey rectangles. The 5 regions where filaments have been identified most prominently are shown in blue. Credit: Roland Bacon / David Mary

The filamentary structure of hydrogen gas in which galaxies form, known as the cosmic web, is one of the major predictions of the model of the Big Bang and of galaxy formation [figure 1]. Until now, all that was known about the web was limited to a few specific regions, particularly in the direction of quasars, whose powerful radiation acts like car headlights, revealing gas clouds along the line of sight. However, these regions are poorly representative of the whole network of filaments where most galaxies, including our own, were born. Direct observation of the faint light emitted by the gas making up the filaments was a holy grail which has now been attained by an international team headed by Roland Bacon, CNRS researcher at the Centre de Recherche Astrophysique de Lyon (CNRS/Université Lyon 1/ENS de Lyon).

Figure 3: one of the hydrogen filaments (in blue) discovered by MUSE in the Hubble Ultra-Deep Field. It is located in the constellation Fornax at a distance of 11.5 billion light years, and stretches across 15 million light years. The image in the background is from Hubble. Credit: Roland Bacon, David Mary, ESO and NASA

The team took the bold step of pointing ESO’s Very Large Telescope, equipped with the MUSE instrument coupled to the telescope’s adaptive optics system, at a single region of the sky for over 140 hours. Together, the two instruments form one of the most powerful systems in the world.[3] The region selected forms part of the Hubble Ultra-Deep Field, which was until now the deepest image of the cosmos ever obtained. However, Hubble has now been surpassed, since 40% of the galaxies discovered by MUSE have no counterpart in the Hubble images.

Figure 4: cosmological simulation of a filament made up of hundreds of thousands of small galaxies. The image on the left shows the emissions produced by all the galaxies as it might be observed in situ. The image on the right shows the filament as it would be seen by MUSE. Even with a very long exposure time, the vast majority of the galaxies cannot be detected individually. However, the light from all these small galaxies is detected as a diffuse background, rather like the Milky Way when seen with the naked eye. Credit: Thibault Garel and Roland Bacon

After meticulous planning, it took eight months to carry out this exceptional observing campaign. This was followed by a year of data processing and analysis, which for the first time revealed light from the hydrogen filaments, as well as images of several filaments as they were one to two billion years after the Big Bang, a key period for understanding how galaxies formed from the gas in the cosmic web [figures 2 et 3]. However, the biggest surprise for the team was when simulations showed that the light from the gas came from a hitherto invisible population of billions of dwarf galaxies spawning a host of stars [figure 4].[4] Although these galaxies are too faint to be detected individually with current instruments, their existence will have major consequences for galaxy formation models, with implications that scientists are only just beginning to explore.

Notes
MUSE, which stands for Multi Unit Spectroscopic Explorer, is a 3D spectrograph designed to explore the distant Universe. The construction of the instrument was led by the Centre de Recherche Astrophysique de Lyon (CNRS/Université Claude Bernard-Lyon 1/ENS de Lyon).
Other French laboratories involved: Laboratoire d’Astrophysique de Marseille (CNRS/Aix-Marseille Université/CNES), Institut de Recherche en Astrophysique et Planétologie (CNRS/Université Toulouse III – Paul Sabatier/CNES).

See ESO press release.
Until now, theory predicted that the light came from the diffuse cosmic ultraviolet background radiation (very weak background radiation produced by all the galaxies and stars) which, by heating the gas in the filaments, causes them to glow.

JADES will go deeper than the Hubble Deep Fields

Astronomers announced this month that a new deep-field survey called JADES will be carried out with the James Webb Space Telescope, Hubble’s much-anticipated successor. The Webb is due to launch later this year. Astronomers announced a new deeper-than-ever sky survey this month (January 15, 2021), to be conducted with the James Webb Space Telescope, the Hubble telescope’s successor, scheduled for launch in October of this year. The new survey is abbreviated JADES, which is short for James Webb Space Telescope Advanced Deep Extragalactic Survey. The survey will be like the Hubble Deep Fields, but deeper still. Its main goal is to see far away in space – and thus far back into the very young universe – and image it just at the end of the so-called Cosmic Dark Ages, that is, at the time when gas in the universe went from being opaque to transparent. This is also the time when the very first stars were forming – very large, massive and bright stars – in a veritable firestorm of star birth when the young universe was less than 5% of its current age. The Webb telescope will be located near the second Lagrange point – a relatively stable region of space, gravitationally speaking, known as L2 – some 930,000 miles (1.5 million km) from Earth. To conduct the new survey, the Webb telescope will be staring at a small point of space for nearly 800 hours (approximately 33 days) to be able to see fainter objects than those ever seen before and thus to find the first generation of galaxies. Astronomers want to know, among other things, how fast did these galaxies form, and how fast did their stars form? They also want to look for the very first supermassive black holes, which are thought to lie at the hearts of nearly all large galaxies, including our Milky Way.

The Hubble Ultra Deep Field (in its eXtreme version) is the deepest view of the universe yet obtained … and will be, until JADES takes over. It stretches approximately 13 billion light-years and includes approximately 10,000 galaxies. It took 11.3 days for the Hubble Space Telescope to collect these ancient photons. Try downloading the largest version and zoom in on different sections. We’re seeing these galaxies as they were billions of years ago. How might they look today? Image via NASA/ ESA/ S. Beckwith (STSci)/ HUDF team.

The long-anticipated launch of the James Webb Space Telescope has been postponed a number of times for a variety of reasons, most recently because of effects of the Covid-19 pandemic. It is the formal successor to the Hubble Space Telescope, but is equipped with instrumentation able to image further into the infrared part of the electromagnetic spectrum than Hubble could.

This capability also makes it a worthy successor to the infrared Spitzer Space Telescope which recently went into retirement.

What makes the infrared part of the spectrum so important for surveys like JADES? If you look really deep, you will also look back in time, and the farther back in time you look, the more redshifted the galaxies are (the farther away they are, the faster they move away from us, and the more their light has been shifted towards the red part of the spectrum). This means that the light we want to observe, originally in the optical (visible) part of the electromagnetic spectrum, might not even show much in the optical part anymore. Instead, it’s been shifted to longer wavelengths, into the infrared regime.

In other words, the use of infrared cameras is necessary to be able to see the light from the first generation of galaxies. Daniel Eisenstein, a professor of astronomy at Harvard University, said:

Galaxies, we think, begin building up in the first billion years after the Big Bang, and sort of reach adolescence at 1 to 2 billion years. We’re trying to investigate those early periods. We must do this with an infrared-optimized telescope because the expansion of the universe causes light to increase in wavelength as it traverses the vast distance to reach us. So even though the stars are emitting light primarily in optical and ultraviolet wavelengths, that light is shifted quite relentlessly out into the infrared. Only Webb can get to the depth and sensitivity that’s needed to study these early galaxies.

In fact, the James Webb Space Telescope was built specifically for this purpose. Up to now, infrared images are much less resolved – less clear – than optical images, because of their longer wavelength. With its much larger collecting area, the Webb will be able to image, in infrared, at the same resolution – detail – that Hubble could obtain in the optical part of the spectrum.

Get ready for a whole new set of mind-blowing images of the universe, this time in the infrared, from Webb!

After having successfully deployed its solar panels – precisely as it’s supposed to do once it’s in space – the Webb telescope is shown here ready for the final tests on December 17, 2020, at NASA’s Goddard Space Flight Center. Then it will be packed up and transported to French Guyana, to be launched on October 31, 2021, via an Ariane V rocket. Image via NASA/ Chris Gunn.

The use of deep field surveys is a young science, for two reasons. First, astronomers didn’t have the right instrumentation before Hubble to do them. Second, it’s also because no one initially knew the result of staring into a piece of empty space for a long time. Such a long stare into the unknown would require valuable observation time, and if this long observation didn’t produce any results, it would be considered a waste.

But in 1995, Robert Williams, then the director of the Space Telescope Science Institute (STSci), which administrates the Hubble telescope, decided to use his “director’s discretionary time” to point the Hubble toward a very small and absolutely empty-looking part of the sky in the direction of the constellation Ursa Major the Great Bear. There were no stars visible from our Milky Way (or extremely few), no nearby galaxies visible in the field, and no visible gas clouds. Hubble collected photons for 10 consecutive days, and the result, the Hubble Deep Field, was a success and a paradigm changer: A patch of sky about as small as the eye of George Washington on an American quarter (25-cent coin) held out at arm’s length, showed a 10 billion-light-years-long tunnel back in time with a plethora of galaxies – around 3,000 of them – at different evolutionary stages along the way. The field of observational cosmology was born.

This was done again in 1998 with the Hubble telescope pointed to the southern sky (Hubble Deep Field South), and the result was the same. Thus we learned that the universe is uniform over large scales.

Next was the installation of a new, powerful camera on Hubble (the Advanced Camera for Surveys) in 2002. The incredible Hubble Ultra Deep Field was acquired in 2004, in a similarly small patch of sky near the constellation Orion, about 1/10 of a full moon diameter (2.4 x 3.4 arc minutes, in contrast to the original Hubble Deep Fields north and south, which were 2.6 x 2.6 arc minutes). And so our reach was extended even deeper into space, and even further back in time, showing light from 10 thousand galaxies along a 13-billion-light-years-long tunnel of space. If you’ll remember that the universe is about 13.77 billion years old, you’ll see this is getting us really close to the beginning!

The Hubble Ultra Deep Field was the most sensitive astronomical image ever made at wavelengths of visible (optical) light until 2012, when an even more refined version was released, called the Hubble eXtreme Deep Field, which reached even farther: 13.2 billion years back in time.

The JADES survey will be observed in two batches, one on the northern sky and one on the southern in two famous fields called GOODS North and South (abbreviated from Great Observatories Origins Deep Survey).

Marcia Rieke, a professor of astronomy at the University of Arizona who co-leads the JADES Team with Pierre Ferruit of the European Space Agency (ESA), explained:

We chose these fields because they have such a great wealth of supporting information. They’ve been studied at many other wavelengths, so they were the logical ones to do.

Look closely. Every single speck of light in this image is a distant galaxy (except for the very few ones with spikes which are foreground stars). This telescopic field of view is part of the GOODS South field. It’s one of the directions in space that’ll be observed in JADES, a new survey that aims to study the very first galaxies to appear in the infancy of the universe. Image via NASA/ Hubble Space Telescope/ James Webb Space Telescope site.

The GOODS fields have been observed with several of the most famous telescopes, covering a great wavelength range from infrared through optical to X-ray. They are not fully as deep (the observations don’t reach as far back) as the Ultra Deep Field, but cover a larger area of the sky (4-5 times larger) and are the most data-rich areas of the sky in terms of depth combined with wavelength coverage. By the way, the first deep field, HDF-N, is located in the GOODS north image, and the Ultra deep field/eXtreme (don’t you love these names?) is located in the GOODS south field.

There are a large number of ambitious science goals for the JADES program pertaining to the composition of the first galaxies, including the first generation of supermassive black holes. How these came about at such an early time is a mystery. As well, the transition of gas from neutral and opaque to transparent and ionized, something astronomers call the epoch of reionization, is not well understood. JADES team member Andrew Bunker, professor of astrophysics at the University of Oxford in the United Kingdom, who is also part of the ESA team behind the Webb telescope, said:

This transition is a fundamental phase change in the nature of the universe. We want to understand what caused it. It could be that it’s the light from very early galaxies and the first burst of star formation … It is kind of one of the Holy Grails, to find the so-called Population III stars that formed from the hydrogen and helium of the Big Bang.

People have been trying to do this for many decades and results have been inconclusive so far.

New Clues About How Ancient Galaxies Lit up the Universe

NASA's Spitzer Space Telescope has revealed that some of the universe's earliest galaxies were brighter than expected. The excess light is a byproduct of the galaxies releasing incredibly high amounts of ionizing radiation. The finding offers clues to the cause of the Epoch of Reionization, a major cosmic event that transformed the universe from being mostly opaque to the brilliant starscape seen today. In a new study, researchers report on observations of some of the first galaxies to form in the universe, less than 1 billion years after the big bang (or a little more than 13 billion years ago). The data show that in a few specific wavelengths of infrared light, the galaxies are considerably brighter than scientists anticipated. The study is the first to confirm this phenomenon for a large sampling of galaxies from this period, showing that these were not special cases of excessive brightness, but that even average galaxies present at that time were much brighter in these wavelengths than galaxies we see today. No one knows for sure when the first stars in our universe burst to life. But evidence suggests that between about 100 million and 200 million years after the big bang, the universe was filled mostly with neutral hydrogen gas that had perhaps just begun to coalesce into stars, which then began to form the first galaxies. By about 1 billion years after the big bang, the universe had become a sparkling firmament. Something else had changed, too: Electrons of the omnipresent neutral hydrogen gas had been stripped away in a process known as ionization. The Epoch of Reionization - the changeover from a universe full of neutral hydrogen to one filled with ionized hydrogen - is well documented.

Before this universe-wide transformation, long-wavelength forms of light, such as radio waves and visible light, traversed the universe more or less unencumbered. But shorter wavelengths of light - including ultraviolet light, X-rays and gamma rays - were stopped short by neutral hydrogen atoms. These collisions would strip the neutral hydrogen atoms of their electrons, ionizing them.

But what could have possibly produced enough ionizing radiation to affect all the hydrogen in the universe? Was it individual stars? Giant galaxies? If either were the culprit, those early cosmic colonizers would have been different than most modern stars and galaxies, which typically don't release high amounts of ionizing radiation. Then again, perhaps something else entirely caused the event, such as quasars - galaxies with incredibly bright centers powered by huge amounts of material orbiting supermassive black holes.

"It's one of the biggest open questions in observational cosmology," said Stephane De Barros, lead author of the study and a postdoctoral researcher at the University of Geneva in Switzerland. "We know it happened, but what caused it? These new findings could be a big clue."

Looking for Light

To peer back in time to the era just before the Epoch of Reionization ended, Spitzer stared at two regions of the sky for more than 200 hours each, allowing the space telescope to collect light that had traveled for more than 13 billion years to reach us.

As some of the longest science observations ever carried out by Spitzer, they were part of an observing campaign called GREATS, short for GOODS Re-ionization Era wide-Area Treasury from Spitzer. GOODS (itself an acronym: Great Observatories Origins Deep Survey) is another campaign that performed the first observations of some GREATS targets. The study, published in the Monthly Notices of the Royal Astronomical Society, also used archival data from NASA's Hubble Space Telescope.

Using these ultra-deep observations by Spitzer, the team of astronomers observed 135 distant galaxies and found that they were all particularly bright in two specific wavelengths of infrared light produced by ionizing radiation interacting with hydrogen and oxygen gases within the galaxies. This implies that these galaxies were dominated by young, massive stars composed mostly of hydrogen and helium. They contain very small amounts of "heavy" elements (like nitrogen, carbon and oxygen) compared to stars found in average modern galaxies.

These stars were not the first stars to form in the universe (those would have been composed of hydrogen and helium only) but were still members of a very early generation of stars. The Epoch of Reionization wasn't an instantaneous event, so while the new results are not enough to close the book on this cosmic event, they do provide new details about how the universe evolved at this time and how the transition played out.

"We did not expect that Spitzer, with a mirror no larger than a Hula-Hoop, would be capable of seeing galaxies so close to the dawn of time," said Michael Werner, Spitzer's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "But nature is full of surprises, and the unexpected brightness of these early galaxies, together with Spitzer's superb performance, puts them within range of our small but powerful observatory."

NASA's James Webb Space Telescope, set to launch in 2021, will study the universe in many of the same wavelengths observed by Spitzer. But where Spitzer's primary mirror is only 85 centimeters (33.4 inches) in diameter, Webb's is 6.5 meters (21 feet) - about 7.5 times larger - enabling Webb to study these galaxies in far greater detail. In fact, Webb will try to detect light from the first stars and galaxies in the universe. The new study shows that due to their brightness in those infrared wavelengths, the galaxies observed by Spitzer will be easier for Webb to study than previously thought.

"These results by Spitzer are certainly another step in solving the mystery of cosmic reionization," said Pascal Oesch, an assistant professor at the University of Geneva and a co-author on the study. "We now know that the physical conditions in these early galaxies were very different than in typical galaxies today. It will be the job of the James Webb Space Telescope to work out the detailed reasons why."

CALIFA renews the classification of galaxies

The objects within galaxies have two basic types of motions: orbiting around the galaxy centre in a regular organized disc, or in orbits oriented at random without a clear direction of rotaiton. If we imagined that galaxies behave in the same way as the Solar System we could think that as the objects move further away from the centre their orbital velocities decrease. However this is not necessarily the case for galaxies, as there are several factors which affect the rotational velocity of these objects, such as the dimensions of the galaxy, the gravitational pull of other galaxies, and the quantity of dark matter in a given galaxy. An international team of astrophysicists,among them IAC and Universidad de La Laguna researcher Jesus Falcon Barroso, coordinator of the CALIFa (Calar Alto Legacy Integral Field Area survey) at the IAC who is one of the authors of the article published in Nature Astronomy, has collected tat from 600 galaxies in the neighbourbhood of the Milky Way with the Potsday Multiple Aperture Spectrophotometer (PMAS) on the 3.6m telescope at the Calar Alto Observatory (Almeria, Spain). As part of this catalogue the scientists have made velocity maps of 300 galaxies showing the movements of their stars.

In this way they have defined three different groups among the sets of stellar orbits, which they have called "cold orfbits" "warm orbits" and "hot orbits", the latter typical of stars with random motions. When they analyzed tghe data they showed that circular orbits are frequent in lower mass galaxies, while the "hot orbits" are more often found in galaxies with higher mass. In addition tghey have found quite a number of "warm orbits", greater than that previously expected for this type of galaxies.

Using these maps of stellar motions one can obtain a lot of informatioin about the history of formation of these galaxies. They evolve and grow over thousands of millions of years, merging with other galaxies. Those which have absorbed other smaller galaxies generally have thin rotating discs, while when two galaxies with similar masses merge an elliptical galaxy is formed, in which the orbits are arranged in random directions.

Measuring the orbits in the galaxies analyzed allows us to distinguish between disc galaxies (with colder orbits) and elliptical galaxies (with hotter orbits) even when this difference cannot be detected when using images alone. This implies that by measuring the stellar orbits the researchers will be able to determine if the galaxy we observe is the result of internal evolution of an isolated object, a relatively calm series of mergers with smaller objects, or the product of a violent merger.

CALIFA, which with its 300 galaxy simple has become one of the biggest archives of data on galaxy dynamics up to now is the "first study to propose a scheme of galaxy classification base don the orbital distribution of their stars, which is different from the classical Hubble diagram , based on morphological classification" explains Falcon Barroso. This researcher also acknowledges that the results of this study " present some problems for current theories of formation and evolution of galaxies"

This new classification has been carefully prepared to produce a representative simple which will help astronomers to make models of the evolution of galaxies, and show whether their simulations produce valid predictions.

Galaxy Orbits in the Local Supercluster

A team of astronomers from Maryland, Hawaii, Israel, and France has produced the most detailed map ever of the orbits of galaxies in our extended local neighborhood, showing the past motions of almost 1,400 galaxies within 100 million light-years of the Milky Way. The team reconstructed the galaxies' motions from 13 billion years in the past to the present day. The main gravitational attractor in the mapped area is the Virgo Cluster, with 600 trillion times the mass of the Sun, 50 million light-years from us. Over a thousand galaxies have already fallen into the Virgo Cluster, while in the future all galaxies that are currently within 40 million light-years of the cluster will be captured. Our Milky Way galaxy lies just outside this capture zone. However the Milky Way and Andromeda galaxies, each with 2 trillion times the mass of the Sun, are destined to collide and merge in 5 billion years.

"For the first time, we are not only visualizing the detailed structure of our local supercluster of galaxies but we are seeing how the structure developed over the history of the universe. An analogy is the study of the current geography of the Earth from the movement of plate tectonics," said co-author Brent Tully, from the University of Hawaii Institute for Astronomy.

These dramatic merger events are only part of a larger show. There are two overarching flow patterns within this volume of the universe. All galaxies in one hemisphere of the region - including our own Milky Way - are streaming toward a single flat sheet. In addition, essentially every galaxy over the whole volume is flowing, as a leaf would in a river, toward gravitational attractors at far greater distances.

Representations of the orbits can be seen in a video and, alternatively, with an interactive model.

With the interactive model, a viewer can pan, zoom, rotate, and pause/activate the time evolution of movement along orbits. The orbits are shown in a reference frame that removes the overall expansion of the universe. What we are seeing are the deviations from cosmic expansion caused by the interactions of local sources of gravity.