SPICE connects stellar feedback in the first galaxies and cosmic reionisation

These images show the four most massive galaxies from each of the three simulation models. The top two rows show "bursty" feedback, the middle two rows show "smooth" feedback, and the bottom two rows show a mixture of the two, where each galaxy is shown both face-on (top) and edge-on (bottom). The galaxies in each column occupy the exact same environment in every simulation. The effect of different stellar feedback models is evident as the bursty feedback model produces elliptical-like galaxies whereas the smooth and intermediate models predominantly produce spiral-like galaxies. © MPA

The first billion years saw the transformation of a cold neutral Universe to a hot and ionised one. This Epoch of Reionisation is thought to come about from stellar radiation from the first galaxies. Understanding the nature of the galaxies that drove reionisation remains a key question. Scientists at MPA have designed a novel suite of simulations to systematically understand how different modes of energy and mass injection from stars affect the first galaxies. According to these new models, subtle differences in the behaviour of stellar feedback drive profound differences in the morphologies of galaxies and the speed at which they ionise the universe. Combining these findings with the latest observations will help constrain feedback models in the first billion years of the Universe.

Some 300,000 years after the Big Bang, for the first time, protons and free electrons formed hydrogen and helium atoms – the Universe was mostly neutral. In the following billion years, however, the vast majority of hydrogen became ionised. This transition from a neutral to an ionised state is known as the "epoch of reionisation". For this to happen, ionising photons had to escape from galaxies. Massive stars emit ionising radiation and at the end of their lives, they die in supernova explosions, releasing mass and energy into gas around them. These processes are termed "stellar feedback", and change the physical structure of the so-called interstellar medium. How this stellar feedback changes the properties of the first galaxies and in particular the gas within these galaxies is poorly understood, but it is key to interpret the recent flood of observations of the early Universe from the James Webb Space telescope.

In the last decade, advancements in high performance computing and theoretical models have allowed scientists to reproduce observations of the first billion years of the Universe. Even though these sophisticated simulations were able to combine radiation and hydrodynamics, due to the computational costs, they typically explore only one of the many possible models of stellar feedback. A team of researchers at MPA led by Aniket Bhagwat, therefore, developed a new suite of simulations called SPICE to systematically test different models and, in particular, to look at connections between stellar feedback and the properties of galaxies that drive reionisation, which could be tested through observations.

The SPICE simulations follow the hydrodynamical evolution of gas, including key processes like star formation and supernova explosions, while tracking the propagation of radiation emitted by those stars. In particular, they include three different modes of stellar feedback: "bursty", where supernovae explode in clustered bursts; "smooth", where supernova explosions are spread out in time; or a mix of the two. And indeed, the simulations show that differences in the strength and behaviour of feedback can have dramatic effects on the morphology of the galaxies. Bursty feedback preferentially produces red and passive galaxies (elliptical-like), whereas smooth feedback produces mostly galaxies that are blue and star forming (spiral-like).

Evolution of ionized hydrogen

The animations show the process of reionisation in SPICE for the three different feedback models over the first billion years of the universe (from redshift ~24 to 5). The stars emit energetic photons that first ionize their surroundings in the form of bubbles. As more photons escape, these ionised bubbles expand to eventually coalesce and complete the process of reionisation. This process is much faster for the bursty feedback model.

What implications does this have for the process of reionisation? The researchers connected the morphologies of the galaxies to the fraction of ionising photons that manage to leave the galaxy; this quantity is called the “escape fraction”. They find that elliptical-like galaxies show a much higher escape fraction than spiral-like ones (by a factor of 20-50). Why does this happen? The bursts of energy due to the combined supernova explosions are able to substantially disturb the gas and punch low-density holes in the interstellar medium, through which the ionising photons can escape easily. Without bursts, the feedback is unable to disturb the gas enough to allow photons to escape en masse. Therefore, bursty feedback models allow for faster reionisation.

Overall, the new SPICE simulations demonstrate for the first time how sensitive cosmic reionisation and galaxy morphologies are to the mode of stellar feedback, marking a step forward in our understanding of the first billion years of the Universe.

Temperature evolution of the simulations

These animations show how the temperature changes in the simulations. Bright red bubbles show hot gas ejected from supernova explosions. Bright white bubbles show regions where ionising photons escape galaxies and ionise their surroundings. The different stellar feedback models produce widely different final states of the simulated universes.

Simulations and data analysis were carried out on the RAVEN and COBRA supercomputers at MPCDF.

Source: Max Planck Institute for Astrophysics/News

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Author:

Aniket Bhagwat
PhD student
2270
mrmgehlm@mpa-garching.mpg.de

For all authors: Aniket Bhagwat, Tiago Costa, Benedetta Ciardi, Ruediger Pakmor, Enrico Garaldi

Original publication:

Aniket Bhagwat, Tiago Costa, Benedetta Ciardi, Ruediger Pakmor, Enrico Garaldi
SPICE: the connection between cosmic reionisation and stellar feedback in the first galaxies.

To be submitted to MNRAS

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NASA’s Webb Will Use Quasars to Unlock the Secrets of the Early Universe

This is an artist's concept of a galaxy with a brilliant quasar at its center. A quasar is a very bright, distant and active supermassive black hole that is millions to billions of times the mass of the Sun. Among the brightest objects in the universe, a quasar’s light outshines that of all the stars in its host galaxy combined. Quasars feed on infalling matter and unleash torrents of winds and radiation, shaping the galaxies in which they reside. Using the unique capabilities of Webb, scientists will study six of the most distant and luminous quasars in the universe. Credits: NASA, ESA and J. Olmsted (STScI). Hi-res image

Quasars are very bright, distant and active supermassive black holes that are millions to billions of times the mass of the Sun. Typically located at the centers of galaxies, they feed on infalling matter and unleash fantastic torrents of radiation. Among the brightest objects in the universe, a quasar’s light outshines that of all the stars in its host galaxy combined, and its jets and winds shape the galaxy in which it resides.

Shortly after its launch later this year, a team of scientists will train NASA’s James Webb Space Telescope on six of the most distant and luminous quasars. They will study the properties of these quasars and their host galaxies, and how they are interconnected during the first stages of galaxy evolution in the very early universe. The team will also use the quasars to examine the gas in the space between galaxies, particularly during the period of cosmic reionization, which ended when the universe was very young. They will accomplish this using Webb’s extreme sensitivity to low levels of light and its superb angular resolution.

Webb: Visiting the Young Universe

As Webb peers deep into the universe, it will actually look back in time. Light from these distant quasars began its journey to Webb when the universe was very young and took billions of years to arrive. We will see things as they were long ago, not as they are today.

“All these quasars we are studying existed very early, when the universe was less than 800 million years old, or less than 6 percent of its current age. So these observations give us the opportunity to study galaxy evolution and supermassive black hole formation and evolution at these very early times,” explained team member Santiago Arribas, a research professor at the Department of Astrophysics of the Center for Astrobiology in Madrid, Spain. Arribas is also a member of Webb’s Near-Infrared Spectrograph (NIRSpec) Instrument Science Team.

The light from these very distant objects has been stretched by the expansion of space. This is known as cosmological redshift. The farther the light has to travel, the more it is redshifted. In fact, the visible light emitted at the early universe is stretched so dramatically that it is shifted out into the infrared when it arrives to us. With its suite of infrared-tuned instruments, Webb is uniquely suited to studying this kind of light.

Studying Quasars, Their Host Galaxies and Environments, and Their Powerful Outflows

The quasars the team will study are not only among the most distant in the universe, but also among the brightest. These quasars typically have the highest black hole masses, and they also have the highest accretion rates — the rates at which material falls into the black holes.

“We’re interested in observing the most luminous quasars because the very high amount of energy that they’re generating down at their cores should lead to the largest impact on the host galaxy by the mechanisms such as quasar outflow and heating,” said Chris Willott, a research scientist at the Herzberg Astronomy and Astrophysics Research Centre of the National Research Council of Canada (NRC) in Victoria, British Columbia. Willott is also the Canadian Space Agency’s Webb project scientist. “We want to observe these quasars at the moment when they’re having the largest impact on their host galaxies.”

An enormous amount of energy is liberated when matter is accreted by the supermassive black hole. This energy heats and pushes the surrounding gas outward, generating strong outflows that tear across interstellar space like a tsunami, wreaking havoc on the host galaxy.

Outflows play an important role in galaxy evolution. Gas fuels the formation of stars, so when gas is removed due to outflows, the star-formation rate decreases. In some cases, outflows are so powerful and expel such large amounts of gas that they can completely halt star formation within the host galaxy. Scientists also think that outflows are the main mechanism by which gas, dust and elements are redistributed over large distances within the galaxy or can even be expelled into the space between galaxies – the intergalactic medium. This may provoke fundamental changes in the properties of both the host galaxy and the intergalactic medium.

Examining Properties of Intergalactic Space During the Era of Reionization

More than 13 billion years ago, when the universe was very young, the view was far from clear. Neutral gas between galaxies made the universe opaque to some types of light. Over hundreds of millions of years, the neutral gas in the intergalactic medium became charged or ionized, making it transparent to ultraviolet light. This period is called the Era of Reionization. But what led to the reionization that created the “clear” conditions detected in much of the universe today? Webb will peer deep into space to gather more information about this major transition in the history of the universe. The observations will help us understand the Era of Reionization, which is one of the key frontiers in astrophysics.

The team will use quasars as background light sources to study the gas between us and the quasar. That gas absorbs the quasar’s light at specific wavelengths. Through a technique called imaging spectroscopy, they will look for absorption lines in the intervening gas. The brighter the quasar is, the stronger those absorption line features will be in the spectrum. By determining whether the gas is neutral or ionized, scientists will learn how neutral the universe is and how much of this reionization process has occurred at that particular point in time.

“If you want to study the universe, you need very bright background sources. A quasar is the perfect object in the distant universe, because it’s luminous enough that we can see it very well,” said team member Camilla Pacifici, who is affiliated with the Canadian Space Agency but works as an instrument scientist at the Space Telescope Science Institute in Baltimore. “We want to study the early universe because the universe evolves, and we want to know how it got started.”

The team will analyze the light coming from the quasars with NIRSpec to look for what astronomers call “metals,” which are elements heavier than hydrogen and helium. These elements were formed in the first stars and the first galaxies and expelled by outflows. The gas moves out of the galaxies it was originally in and into the intergalactic medium. The team plans to measure the generation of these first “metals,” as well as the way they’re being pushed out into the intergalactic medium by these early outflows.

The Power of Webb

Webb is an extremely sensitive telescope able to detect very low levels of light. This is important, because even though the quasars are intrinsically very bright, the ones this team is going to observe are among the most distant objects in the universe. In fact, they are so distant that the signals Webb will receive are very, very low. Only with Webb’s exquisite sensitivity can this science be accomplished. Webb also provides excellent angular resolution, making it possible to disentangle the light of the quasar from its host galaxy.

The quasar programs described here are Guaranteed Time Observations involving the spectroscopic capabilities of NIRSpec.

The James Webb Space Telescope will be the world's premier space science observatory when it launches in 2021. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

For more information about Webb, visit www.nasa.gov/webb.

Media contacts:

Ann Jenkins / Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4366
jenkins@stsci.edu / cpulliam@stsci.edu

Laura Betz
NASA's Goddard Space Flight Center, Greenbelt, Md.
laura.e.betz@nasa.gov

Editor: Lynn Jenner

Source: NASA/Black Holes

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Discovery of a luminous galaxy reionizing the local intergalactic medium 13 billion years ago

This timeline shows the evolution of the universe since the Big Bang. The new observations, labeled "Hubble 2012," explored the deepest reaches of the cosmos that has ever been studied with Hubble, going back about 13.4 billion years -- around when the first galaxies were being formed. Previously, the universe was in the so-called Dark Ages, before there were any stars to light up the cosmos. During reionization, ultraviolet light from the newly formed stars and galaxies ionized the neutral hydrogen that permeated the universe. The dashed line labeled Hubble 2009 represents the last set of observations of HUDF. Credit: NASA/ESA

Discovery of a luminous galaxy reionizing the local intergalactic medium 13 billion years ago

Astronomers have discovered a luminous galaxy caught in the act of reionizing its surrounding gas only 800 million years after the Big Bang. The research, led by Romain Meyer, PhD student at UCL in London, UK, has been presented today at the virtual annual meeting of the European Astronomical Society (EAS). 

Studying the first galaxies that formed 13 billion years ago is essential to understanding our cosmic origins. One of the current hot topics in extragalactic astronomy is ‘cosmic reionization’, the process in which the intergalactic gas was ionized (atoms stripped of their electrons). Cosmic reionization is similar to an unsolved murder: we have clear evidence for it, but who did it, how and when? We now have strong evidence that hydrogen reionization was completed about 13 billion years ago, in the first billion years of the Universe, with bubbles of ionized gas slowly growing and overlapping. The objects capable of creating such ionized hydrogen bubbles have however remained mysterious until now: the discovery of a luminous galaxy in which 60-100% of ionizing photons escape, is likely responsible for ionizing its local bubble. This suggests the case is closer to being solved.

The two main suspects for cosmic reionization are usually 1) a population of numerous faint galaxies leaking ~10% of their energetic photons, and 2) an ‘oligarchy’ of luminous galaxies with a much larger percentage (>50%) of photons escaping each galaxy. In either case, these first galaxies were very different from those today: galaxies in the local Universe are very inefficient leakers, with only <2-3% of ionizing photons escaping their host. To understand which galaxies governed cosmic reionization, astronomers must measure the so-called escape fractions of galaxies in the reionization era. 

The detection of light from excited hydrogen atoms (the so-called Lyman-alpha line) can be used to infer the fraction of escaping photons. On the one hand, such detections are rare because reionization-era galaxies are surrounded by neutral gas which absorbs that signature hydrogen emission. On the other hand, if this hydrogen signal is detected it represents a ‘smoking gun’ for a large ionized bubble, meaning we have caught a galaxy reionizing its surroundings. The size of the bubble and the galaxy’s luminosity determines whether it is solely responsible for creating this ionized bubble or if unseen accomplices are necessary.

The galaxy A370p_z1 in the Hubble imaging and a zoom-in in each filter. The non-detection in the first three filers, followed by detections in all the redder filter is a typical signature of distant galaxies Credit: NASA, ESA, Z. Levay (STSci). Hi-res image

The discovery of a luminous galaxy 800 million years after the Big Bang supports the scenario where an ‘oligarchy’ of bright leakers emits most of the ionizing photons. “It is the first time we can point to an object responsible for creating an ionized bubble, without the need for a contribution from unseen galaxies,” comments Meyer. “Additional observations with the upcoming James Webb Space Telescope will enable us to study further what is likely one of the best suspects for the unsolved case of cosmic reionization.”

Source: European Astronomical Society (EAS)

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