Starlit “Fossils” And The Mysteries Of Massive Galactic Evolution

Massive, starlit galaxies are thought to have attained their majestic sizes as the result of collisions and mergers between small, amorphous protogalaxies inhabiting the ancient Universe. The earliest galaxies created a multitude of dazzling newborn stars and, even though they were much smaller in size than our own large, barred-spiral Milky Way Galaxy, they were just as brilliant because of these active episodes of stellar formation fury. In September 2015, an international scientific team, led by astronomers at the Swiss Federal Institute of Technology in Zurich, announced that they had observed massive dead galaxies in the Universe, 4 billion years after the Big Bang, using the Subaru Telescope’s Multi-Object Infrared Camera and Spectrograph (MOIRCS). The Subaru Telescope is an 8.2 meter flagship telescope of the National Astronomical Observatory of Japan. It is located at the Mauna Kea Observatory in Hawaii.

The astronomers discovered that the stellar content of these ancient galaxies is strikingly similar to that of massive elliptical galaxies observed locally in the Universe today. Indeed, the scientists identified progenitors of these dead galaxies when they were young, and first giving birth to stars at a more ancient cosmic epoch–thus revealing the formation and evolution of massive galaxies across 11 billion years of cosmic time–in our 13.8 billion year old Universe.

In our Milky Way’s general galactic neighborhood, massive galaxies populated by billions of stars are often dead, barren, elliptical galaxies. These immense galaxies host a population of mostly red, elderly stars, and they show no signs of baby star-birth. A number of questions still remain to be answered pertaining to how, when, and for how long stellar formation occurred in such enormous galaxies before they perished, unable to produce additional baby stars whose stellar fires would light up their galactic hosts. Furthermore, it is still unknown what occurred since then to form the dead elliptical galaxies that are observed in the Universe today.

In order to answer these important and profound questions, the team of astronomers made use of the “fossil” records left as relics imprinted by stars in the spectra of remote, dead galaxies–thus providing precious clues concerning their metal content, age, and element abundances. All atomic elements that are heavier than hydrogen and helium are termed metals in the jargon of astronomers. Hydrogen (the lightest and most abundant atomic element in the Universe) and helium were both born in the Big Bang fireball (Big Bang nucleosynthesis), while all of the other, heavier atomic elements listed in the familiar Periodic Table were manufactured in the nuclear-fusing fires of the stars–or, alternatively, in the incandescent explosive rage of their supernovae deaths (stellar nucleosynthesis).

Massive, dead, and distant galaxies, dwelling in the Milky Way’s local neighborhood, are approximately 10 billion years old, and they are abundantly endowed with heavy metal elements. These heavy metals, which all have an atomic number that is a multiple of 4, can be used to determine the duration of star formation. Examples of these elements include oxygen, neon, magnesium, silicon, sulfur, calcium, and titanium.

Bottom Up

The most popular theory of galactic formation, the so-called bottom-up theory, indicates that large, massive galaxies were uncommon denizens of the ancient Universe, and that they only eventually reached their enormous sizes as a result of collisions and mergers between much smaller protogalaxies floating around throughout the primordial Cosmos.

There are over 100 billion galaxies inhabiting our visible Universe. The visible Universe, or observable Universe, is that relatively small portion of the entire incredibly vast Cosmos that we are able to observe. Most of the Universe lies far beyond what we can observe. This is because the light traveling out from those unimaginably distant regions–far beyond our visibility–has not had enough time to wander to us since our Universe was born in the Big Bang almost 14 billion years ago, as a result of the expansion of space.

Before the very first generation of stars ignited, and set our Universe on fabulous fire with their brilliant blasts of light, the Cosmos was an enormous swath of featureless, terrible darkness. In our primordial Cosmos, opaque clouds primarily composed of pristine hydrogen gas gathered together along massive filaments of the so-called “Cosmic Web”, which is composed of invisible dark matter. Even though the identity of the mysterious dark matter is unknown, scientists believe that it is not composed of so-called “ordinary” atomic matter, which is the stuff of stars, planets, moons, and people, and literally all of the atomic elements listed in the Periodic Table. “Ordinary” atomic matter–which is really extraordinary–accounts for a mere 4.6% of the Universe, while the transparent and invisible ghostly dark matter accounts for 24% of it. Most of the Universe–71.4% of it–is composed of the weird dark energy. Dark energy is a bizarre substance, often considered to be a property of space itself, that is causing the Universe to speed up in its expansion.

According to the system of galaxy classification, spiral galaxies, like our own Milky Way, are composed of rotating, flat disks that are lit up with the fires of myriad stars, and also contain an abundance of dust and gas. In addition a spiral galaxy displays a central concentration of stars, termed a bulge, which is encircled by a much fainter halo of stars, many of which inhabit globular clusters. Spirals got their name from their spiral arms that extend out from the center of the disk.

Elliptical galaxies are another galactic type. Ellipticals sport an approximately ellipsoidal shape and a smooth, almost featureless, brightness profile. Unlike their flat spiral cousins, that possess both organization and structure, ellipticals display little in the way of structure, and their stellar population possesses more or less random orbits around their centers.

Lenticular galaxies are intermediate between spirals and ellipticals–that is, they share kinematic properties with both spirals and ellipticals. Indeed, lenticulars are sometimes called armless spiral galaxies. This is because they host a bulge, but have no spiral arms. Spirals and lenticulars are both classified as disk galaxies, and are characterized by a flattened, circular region composed of gas and dust. Lenticulars are defined by their possession of pancake-shaped regions of gas and dust that set them apart from their elliptical galactic cousins.

In the ancient Cosmos, dense regions composed of the mysterious, ghostly, and invisible dark matter gravitationally snatched up wandering clouds of pristine, mostly hydrogen, gas. Dark matter cannot interact with “ordinary” atomic matter or electromagnetic radiation except through the force of gravity. Fortunately, because it does interact with atomic matter, and it warps, bends, and distorts the path of traveling light (gravitational lensing), it reveals its presence to the curious eyes of astronomers who are trying to solve our Universe’s ancient, profound, and very well-kept secrets. Gravitational lensing is a prediction of Albert Einstein’s Theory of General Relativity (1915), whereby his calculations demonstrated that gravity could warp light and therefore exert lens-like effects.

Eerie, transparent, and phantom-like, the strange dark matter pulled at the ancient clouds of pristine gas with its relentless, powerful, and irresistible gravitational grip. These pristine, primordial gas clouds became the weird nurseries of the Universe’s first generation of fiery baby stars–the very first of their stellar kind to light up what was once a dark and featureless expanse with their glaring blasts of fabulous light, chasing away the ancient darkness. The massive dark matter filaments of the vast Cosmic Web grabbed atomic prey until the wandering clouds of gas formed blobs that gently somersaulted down, down, down into the mysterious centers of these eerie, ghostly, and invisible halos of the dark matter–strung out along the bizarre filaments of this great Cosmic Web.

Slowly, swirling, the clouds of ancient gases, along with the eerie, phantom-like dark matter, danced around together throughout the primordial Universe, at last combining together in such a way that they would eventually create the distinct, familiar structures that can be observed today. Regions of greater density within the filaments composed of the dark matter, weaving the Cosmic Web, served as the seeds from which the galaxies were born. The gravitational tugs of those ancient seeds slowly, surely, pulled the ancient gases into increasingly tighter clouds. Many scientists think that these clouds of ancient gases began to merge together to form protogalaxies of various sizes–both large and small–and these whirled around together, thus creating ever larger galactic structures. The protogalaxies danced around together, ultimately evolving into the gigantic, majestic large galaxies–such as our own Milky Way–that can be observed today. The protogalaxies bumped into each other and formed ever-larger structures. The early Universe was much smaller and more crowded than what we are used to today. As a result, collisions and mergers between the ancient protogalactic structures were frequent occurrences.

Starlit “Fossils”And The Mysteries Of Massive Galactic Evolution

The team of international astronomers from the Swiss Federal Institute of Technology used MOIRCS’s ability to watch multiple celestial objects simultaneously. The astronomers efficiently observed a sample of 24 dim, distant galaxies, and created a composite spectrum that would have required 200 hours of the Subaru Telescope’s time to obtain one lone spectrum of the same quality.

The astronomers’ careful analysis of the composite spectrum revealed the age of the 24 galaxies–they were already 1 billion years old when observed 4 billion years after the Big Bang. The galaxies also contain 1.7 times more metals relative to their amount of hydrogen. This research represents the very first time that the heavy element abundance in stars could be measured in such very remote, dead galaxies, and this showed that the duration of star-birth in these galaxies was shorter than 1 billion years. These results indicate that the dead, massive galaxies have evolved to what they are today without any additional star formation.

What do dead, massive galaxies look like when they are forming stars? That is the question. In order to answer this, the team studied the progenitors of their sample of galaxies based on the spectral analysis. They found that these progenitor galaxies had to be forming stars 1 billion years prior to the epoch observed for the dead galaxies. The astronomers observed similarly massive star-birthing galaxies at the right epoch–and also with the star formation rate that they had expected to see based on the spectra. If these active galaxies would have continued to give birth to bright new baby stars at the same rate, they would have grown more massive than what is observed in the Universe today. Therefore, the astronomers concluded, these galaxies would stop producing stars very soon, and would then go on to grow old gracefully.

This research is important because it establishes a consistent portrayal of the history of massive galaxies over a span of 11 billion years. “We would like to explore galaxy evolution in more detail by carrying out an object-by-object study and by extending the method to an even earlier epoch,” Dr. Masato Onodera explained in a September 24, 2015 Subaru Telescope Press Release. Dr. Onodera leads the international team of astronomers who conducted this research.

The study is published in the August 1, 2015 issue of The Astrophysical Journal, under the title: The Ages, Metallicities, and Element Abundance Ratios of Massive Quenched Galaxies at z~1.6.