A Stellar Clump Long Ago And Far Away

By  Judith E Braffman-Miller 


It is now generally agreed that the Universe was likely born in the hot Big Bang almost 14 billion years ago--and in only the tiniest fraction of a second after its birth, quantum fluctuations were blown up to macroscopic scales as the result of a short burst of exponential growth called inflation. Inflation was responsible for the remarkable degree of homogeneity observed in the early Universe, as well as for the small amount of structure present at that very ancient time. In astronomy, long ago is the same as far away. The more distant a shining celestial object is in Space, the more ancient it is in Time. When the Universe was a mere baby at only 3 billion years of age, the galactic structures that then existed possessed very different properties than those they show today. In May 2015, a team of astronomers announced their remarkable discovery of a newborn massive star-forming clump in the disk of an ancient galaxy long ago and far, far away!

The giant star-forming clump is less than 10 million years old, and it is the very first time that such a neonatal star-forming region has been observed. This important discovery sheds new light on how stars were born within distant, ancient galaxies, and the physical properties displayed by this primordial object show that very young clumps in such galaxies survive as a result of stellar winds and supernova feedback. The clumps can "live" for a few hundred million years, and this weakens the predictions of several theoretical models. The stellar clumps' long "lifetimes" could facilitate their migration toward the inner domain of their host galaxies, thus contributing to the total mass of the galactic bulge and the growth of its central black hole. The results are published in the May 7, 2015 issue of the journal Nature, under the title An extremely young massive clump forming by gravitational collapse in a primordial galaxy.

The galaxies that inhabited the ancient Universe had more irregular shapes than modern galaxies, and their disks were considerably more gas-rich. Astronomers realize that ancient galaxies gave birth to fiery, brilliant baby stars far more rapidly than their modern counterparts, and that stellar birth occurred within enormous, considerably more massive, and luminous star-forming regions than those commonly seen in local galaxies today. However, the way that these enormous stellar clumps formed is not well understood, and their mysterious evolution throughout the long and complicated history of the Universe is still a subject of hot debate. In order to shed light on these important issues, a team of French astronomers carried out their own observations using the Hubble Space Telescope (HST) in its imaging and spectroscopic mode, and the Subaru telescope (in spectroscopic mode) installed in Hawaii. The apparent faintness of the very distant, ancient galaxies demands the use of extremely powerful observing facilities. The astronomers discovered the signature of the giant clump composed of a population of brilliantly luminous baby stars, and caught them while they were still in the process of stellar birth, within a remote galaxy located at a distance of 11 billion light years from Earth.

As part of this observing campaign, carried out with HST and the Subaru Telescope, researchers from the Service d'Astrophysique-Laboratoire AIM of the Institute of Research into the Fundamental Laws of the Universe, CEA (French Alternative Energies and Atomic Energy Commission), led by Dr. Anita Zanella, discovered the first cry of this massive star-birthing clump.

The clump is less than 10 million years old, and has not yet evolved sufficiently for its constituent stars to be directly observed. Therefore, its detection came as the happy result of the radiation emitted from the gas being ionized by these brilliant baby stars. This enormous star-forming clump has a gas mass that is approximately equal to one billion times that of our Star, the Sun. Giving birth to its fiery baby stars at a rate of 30 solar-masses per year, it contributes as much as half of the total star formation of its host galaxy. In addition, its growth rate in mass and its efficiency in converting molecular gas into new baby stars are 10 times greater than the typical values observed at this particular epoch of the Universe's history. This observation confirms the presence of a gigantic burst of star-formation in this region.

Long Ago And Far. Far Away!

Most scientists think that the Universe started out as an unimaginably tiny Patch, that was even smaller than a proton, and then--in the tiniest fraction of a second--expanded exponentially to achieve macroscopic size. As small as that unbelievably tiny Patch was, it was so extremely hot and dense that literally everything that we are and all that we ever will know, emerged from it. Something, scientists have not as yet determined what, made that exquisitely tiny Patch experience runaway inflation.

Although proof of the period of inflation has not yet been detected, recent observations and measurements suggest that it is the most probable explanation (currently being considered) that could have caused our Universe to evolve in the way that it apparently has. In the smallest fraction of a second, inflation is believed to have blown up like a magnificent balloon or bubble every region of our Patch of space by a factor of at least 10 to the 27th power. Before inflation caused our Patch to experience this runaway expansion, the region of the Universe that we can observe today--the visible or observable Universe--was a featureless, smooth little tidbit much tinier than an elementary particle. At this time, our Universe was a stew--more precisely a plasma--of elementary particles. Fast-moving photons (particles of light) slowly but surely lost energy and slowed down (cooled off) as the Universe continued to expand. The energy showered into this expansion. In the almost 14 billion years after inflation, our Universe has expanded by another factor of 10 to the 27th power.

Following on the heels of this early period of exponential expansion was a more gradual, normal evolution during which the Universe continued to expand and cool off. During this cooling phase, the very first protons and neutrons (atomic nuclei) condensed out of the strange stew, and then the lightest of all atomic elements hydrogen, helium, and beryllium froze out. When the Universe was at last 400,000 years old, it had cooled off to the point that the ambient photons in the Universe were no longer sufficiently energetic to keep the hydrogen ionized. This caused one of the first great phase transitions in the state of elemental hydrogen during our Universe's history. Hydrogen, which up to this point existed in the fully ionized form as free protons, recombined with the available electrons to create neutral hydrogen. Simultaneously, the ambient radiation--now at too low energy to ionize hydrogen--was able to dance freely away to wander through the ancient Universe. This tattle-tale background radiation was with us at the beginning, and is with us still. It is much, much colder now, but it provides us with one of the first and strongest pieces of evidence for the existence of the hot Big Bang. This era has earned the title of recombination. Ionization refers to the process by which an atom or molecule acquires a positive or negative charge by gaining or losing electrons to form ions. Ionization usually goes along with other chemical changes.

The mysterious birth of stars and galaxies brought about several important alterations in the rapidly evolving ancient Universe. For one thing, the most ancient generation of stars gave the Universe its very first source of elements heavier than beryillium. Even though conditions in the Universe's earliest era were not suitable for the production of the heavier elements--all elements heavier than helium are metals in the terminology astronomers use--such elements could readily be manufactured in the nuclear-fusing cores of massive stars that progressively fused heavier and heavier atomic elements out of lighter ones--and then later blasted this freshly manufactured batch of heavier elements out into space when they exploded as supernovae. The second important alteration caused by the first generation of stars was the introduction of high-energy photons into the Universe. The last time that the Universe had contained such high-energy photons was during the first 400,000 years of its existence, when it was still searing-hot and hydrogen still existed in an ionized state.

These extremely high-energy photons began what is thought to have been a lengthy process whereby hydrogen in the Universe was reionized. This period of reionization likely started several hundred million years after the Big Bang and concluded about 600 million years later. Scientists are very interested in gaining an understanding of the properties of the first galaxies because they probably played a starring role in this profound drama.

The first generation of small galaxies was probably already in existence 400 million years after the Big Bang. Following this initial phase of galactic birth, galaxies then experienced an extended phase of merging and coalescing with other galaxies--whereby they grew from several thousand solar-masses to billions of solar-masses. This buildup process continued until the Universe was approximately two billion years old. Then, as the result of some still undetermined feedback process--commonly speculated to be active galactic nucleus (AGN) feedback--it is thought that this buildup at last stopped, and gas accretion and star formation within the most massive galaxies halted--and galaxies went on to experience a very different form of evolution. This type of galactic evolution continues on to the present day.

What is really out there? What is it made of? Without this knowledge, it is not possible to arrive at any real conclusions about how the Universe evolved.

A Stellar Clump Long Ago And Far, Far Away

In order to supplement the interpretion of their unprecedented results, Dr. Zanella and her team also developed a set of hydrodynamical simulations at very high resolution using the supercomputer of the Tres Grand Centre de Calcul of CEA and the Grand equipement national de calculis purpose (GENCI). GENCI's purpose is to implement and ensure the coordination of the major equipment of the French high-performance computing centers by providing funding and by assuming ownership.

Dr. Zanella and her team's simulations reveal that gas-rich galaxies, gravitational instabilities, and turbulence within the interstellar medium caused the gas to fragment and collapse. This resulted in the birth of a myriad of sparkling, fiery baby stars, concentrated in regions whose size could have extended as much as several hundred light-years. Secreted within these stellar clumps, the rapid star-birthing process experienced a dramatic increase during the very first few million years. The star-birthing efficiency in these regions was also larger than the typical value measured at galactic scales. After approximately 15 million years, the feedback and ferocious stellar winds rushing out from the baby massive stars and the explosion of the first supernovae blasts became powerful enough to counterbalance the gravitational collapse of the gas. Therefore, star-birth gradually declined, while still maintaining an efficiency higher than that characterizing the other disk regions.

This discovery of a very young star-birthing clump in a distant, ancient galaxy has important implications for the understanding of galactic formation on cosmological scales. The massive, baby clump discovered by the astronomers of CEA reveals rare physical attributes, only observable during the approximately 15 million years after the gravitational collapse of the gas and the formation of such concentrations of fiery, new stars. Among the 60 galaxies observed as part of this study, it is the only object displaying such characteristics. Giant clumps seen in the other distant, ancient galaxies correspond with much more evolved stellar regions. Therefore, the rarity of the phenomenon observed in this study suggests that the "lifetime" of such clumps in the distant Universe could extend to at least 500 million years. This newly derived constraint excludes certain theoretical scenarios that predict the rapid disruption of the giant star-birthing clumps due to their rapid destruction by the feedback and action of the rushing winds originating from newborn stars, and it strengthens the idea that these enormous clumps can survive long enough to evolve within the galactic disk where they were born. They then could travel inward to the core of the galaxy, and play a starring role in the growth of both its bulge and its central supermassive black hole.

In order for scientists to properly describe the role that these giant clumps play in galactic evolution, an even more precise determination of their physical properties must be made--such as their dynamical mass and size. Increasing the statistical sample of such young star-birthing regions will also be an important part of the upcoming work led by this research group. In particular, such studies will require the use of observing facilities like the Atacama Large Millimeter Array (ALMA) network of radio telescopes installed in northern Chile, as well as the James Webb Space Telesope (JWST) whose launch is scheduled for the end of 2018.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

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