Learning About Our Baby Milky Way From Galaxies Far, Far Away
By Judith E Braffman-Miller
Our Milky Way Galaxy is only one of billions of other galaxies in the Universe. Nevertheless, it is very precious to us because it is our planet’s home–and when we stare up at the night sky, far away from the lights of a city, we can observe it as a starlit smile from horizon to horizon, telling us that we are a small, but special, part of something immense, majestic, and mighty. Long before our Sun and its retinue of planets, moons, and smaller objects were born, our Galaxy had to form in the mysterious primordial swath of Space that emerged in the wild exponential inflation of the Big Bang almost 14 billion years ago. But, how did our Milky Way form, long before our Sun was here, before our Earth was here, and before observers were around to witness this magnificent beginning? In March 2017, astronomers announced that by observing the early stages of Milky Way-like galaxies inhabiting the distant, ancient Universe, they were able to peer back in Time, to discover how our Galaxy formed long, long ago.
For decades, astronomers have discovered remote galaxies in the ancient Universe by spotting the characteristic way their gas absorbs light traveling out from a bright background quasar. However, efforts to detect the light emitted by these same ancient galaxies have mostly met with failure. Now, a team of astronomers using the Atacama Large Millimeter Array (ALMA) in Chile, has observed emissions from a duo of distant galaxies first discovered by their quasar absorption signatures–and the results were not what they had expected.
First, the emissions caused by star-birth within the distant galaxies were separated by a surprisingly great distance from the dense gas revealed by the background, tattle-tale quasar absorption. This indicated that the ancient galaxies are embedded in an extended halo of hydrogen gas. The estimated stellar birth rates were also surprisingly high.
“We had expected we would see faint emissions right on top of the quasar, and instead we saw bright galaxies at large separations from the quasar,” explained Dr. J. Xavier Prochaska in a March 23, 2017 University of California at Santa Cruz (UCSC) Press Release. Dr. Prochaska is a professor of astronomy and astrophysics at UCSC, and coauthor of a paper describing the new findings published in the March 24, 2017 issue of the journal Science.
Quasars are exceptionally bright active galactic nuclei (AGN) that are the swirling accretion disks surrounding supermassive black holes in the early Universe. As unfortunate material–shredded stars, perhaps, or clouds of doomed gas–tumble down into the waiting maw of the supermassive beast, destined to become its dinner–the material becomes hotter, and hotter, and hotter, as well as brighter, and brighter, and brighter. Supermassive black holes dwell in the secretive hearts of perhaps every large galaxy in the Universe–including our own Milky Way–and they sport masses of millions to billions of suns. In our Galaxy’s early youth, it’s resident supermassive black hole may have been surrounded by a brilliantly glaring quasar, hurling its powerful and intense light out into the space between galaxies.
A Starlit Spiral In Space
Our Milky Way Galaxy hosts about two hundred billion stars, as well as an untold number of other objects. When observed with binoculars on a clear, dark night, it is a breathtaking sight, with literally thousands of stars doing a sparkling and mesmerizing dance in Space in each field of view.
Our Galaxy has been studied extensively by astronomers, and currently our ideas about its nature are built upon a more solid foundation than when our ancient ancestors created lovely myths and stories to explain this celestial starlit band’s existence.
Before our Milky Way formed, the Cosmos was brimming with mostly hydrogen gas–as well as smaller amounts of helium. The gas was ultimately converted into blazing stars, their retinues of planets, and conscious beings, such as ourselves, on Earth–and probably elsewhere. However, before all of this could happen, the Galaxy had to exist.
The Milky Way Galaxy in which we currently dwell is a very different place than the frigid gas from which it emerged billions of years ago. In its early days, our Galaxy was a spherical mass of hydrogen. However, today astronomers using radio telescopes have been able to determine that our Milky Way is a spiral galaxy–just one of countless others. Nevertheless, because we are inside our Galaxy, astronomers cannot observe it as a whole. The good news is that we can peer out into space and observe galaxies which we believe may be similar to our own.
Disk galaxies, which also include galaxies with magnificent, beautiful starlit spiral arms, like our Milky Way, as well as those with somewhat less well-defined characteristics (lenticular galaxies)–are all defined by their possession of pancake-shaped domains of dust and gas that separate them from their elliptical gallactic kin.
According to galaxy classification, spiral galaxies, like our own, are composed of rotating, flat disks populated by stars, gas, and dust, as well as a central collection of stars called a bulge. These are surrounded by a much more faint halo of stars, a number of which are denizens of globular clusters. Spirals are named for their possession of spiral arms that reach out from the center into the disk. In contrast, elliptical galaxies possess an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. Ellipticals, in contrast to their highly structured and well-organized spiral kin, are more three-dimensional, and have little in the way of structure. Indeed, the stellar denizens of ellipticals bob around in, more or less, random orbits surrounding their centers. Lenticulars are intermediate between spirals and ellipticals, and they share kinematic attributes with both galaxy types. Lenticulars are frequently referred to as “armless spiral galaxies.” This is because they possess a bulge–but no spiral arms.
According to the bottom-up theory of galaxy formation, large galaxies eventually attained their gigantic and majestic sizes as a result of collisions and mergers between relatively small protogalaxies bouncing around in the baby Universe. The most ancient galaxies furiously produced blazing, brilliant newborn stars.
Before the first stars ignited, lighting up the vast swath of darkness that was the primordial Universe, opaque clouds of primarily hydrogen gas collected along the heavy filaments of the Cosmic Web. The filaments of the massive Cosmic Web are generally thought to be composed of dark matter–an unidentified substance that is not the atomic matter that we are used to in our familiar world. Atomic matter accounts for literally all of the elements listed in the Periodic Table, but this so-called “ordinary” form of matter is extraordinary because it is the stuff of stars, planets, moons, and people. “Ordinary” atomic (baryonic) matter accounts for a relatively small 4.6% of the Cosmos, while the mysterious dark matter is much more abundant at about 24% of it. The lion’s share of the Cosmos is composed of the weird dark energy, that is even more mysterious than the dark matter. Dark energy accounts for about 71.4% of the Universe, and it is commonly thought to be a property of Space itself–and it is just as important as it is mysterious, because it is causing our Universe to accelerate in its expansion.
In the very ancient Universe, dense areas composed of dark matter grabbed at floating clouds of pristine gas with a relentless and powerful gravitational grasp. Dark matter does not interact with “ordinary” atomic matter or electromagnetic radiation except through the force of gravity. However, because it does dance with baryonic matter by way of its gravity, and it warps and bends the path that light takes as it wanders through the Cosmos (gravitational lensing), it gives away its phantom-like presence–despite its haunting, eerie transparency. Gravitational lensing is a phenomenon proposed by Albert Einstein when he realized that his calculations indicated that gravity could warp the path that light takes through the Universe, and thus have lens-like effects.
Transparent and invisible, the ghostly dark matter clutched at clouds of pristine gas. These clouds of primeval gases evolved into the nurseries of the first fiery newborn stars to scream out their victorious birth cries, as they hurled their fierce light and the possibility of life out into what had previously been a barren blackness. The massive filaments composed of the dark matter spun the Cosmic Web throughout Spacetime–and this web tugged and tugged on its atomic prey until the clouds of newly formed gas tumbled down to nest within the invisible halos of the mysterious dark matter. The gas clouds floated down into the very hearts of these ghostly, transparent halos composed of the transparent non-atomic material.
Gradually, majestically, the writhing sea of primeval gases and the ghostly dark matter, flowed throughout the ancient Cosmos. The two forms of matter performed an ancient dance together, ultimately combining to create the distinct and familiar large-scale structures that can be observed today. The regions of greater-than-average density within the filaments of dark matter, weaving the magnificent Cosmic Web, traveled throughout the newborn Cosmos and became the “seeds” from which the galaxies were born and grew. The relentless gravitational tugs of those ancient “seeds” gradually pulled the pristine gases into ever more tightly bound blobs. Many astronomers propose that these blobs of gas began to collect together, and that the resulting protogalaxies, both large and small, danced a wild dance creating ever larger and larger galactic building blocks. The protogalaxies did their bewitching dance, moving ever closer and closer together because of their mutual gravitational attraction, merging to create ever larger and larger structures destined to grow into the enormous, majestic galaxies inhabiting the Universe today. Like tiny chunks of wet sand, being mashed together in the small hands of a playful child sitting in a sandbox, the protogalaxies bumped into one another and stuck together to create a cosmic sand castle.
The early Universe was much smaller than what we are used to today–and, as a result, it was very crowded. Therefore, the protogalaxies frequently bumped into one another in this relatively small and crowded environment–sticking together to create larger and larger galactic castles in Space and Time.
Learning About Our Baby Milky Way From Galaxies Far, Far Away
The team of astronomers, using ALMA, observed that the neutral hydrogen gas– revealed by its absorption of quasar light–is probably a portion of a large halo or extended disk of gas surrounding the distant galaxy, explained first author of the new study, Dr. Marcel Neeleman, in the March 23, 2017 UCSC Press Release.
“It’s not where the star formation is, and to see so much gas that far from the star-forming region means there is a large amount of neutral hydrogen around the galaxy. We don’t know if it’s a large, extended disk of gas that’s falling in, or if it’s just a really dense halo of gas around the galaxy,” Dr. Neeleman continued to explain.
One of the observed distant galaxies shows an emission spectrum that indicates the presence of a rotating disk, according to Dr. Prochaska. “These galaxies appear to be massive, dusty, and rapidly star-forming systems, with large, extended layers of gas. These observations give us terrific insight into how galaxies like our Milky Way looked 13 billion years ago,” Dr. Prochaska added in the March 23, 2017 UCSC Press Release.
Dr. Prochaska noted that the new research paper represents the victorious conclusion of a long quest that he first embarked on back in 2003, with his doctoral thesis advisor at the University of California, San Diego (UCSD), the late Dr. Arthur M. Wolfe (also a coauthor on the current paper). Dr. Wolfe (1939-2014) pioneered the use of quasar spectra to study concentrations of neutral hydrogen gas in the remote Universe, termed the damped Lyman-alpha (DLA) systems. This is because those particular systems showed characteristic absorption features that the hydrogen gas imprints on the traveling light emanating from the background quasar. Dr. Wolfe also understood the potential for ALMA to spot emissions from these systems long before the radio observatory’s completion in 2011.
“We’ve been wanting to do this for 14 years. The ‘holy grail’ has been to identify and study the galaxies that host the hydrogen gas we see in quasar spectra, and it took a facility with ALMA’s capability to do it,” Dr. Prochaska noted in the March 23, 2017 UCSC Press Release.
The team of astronomers used ALMA to hunt for far-infrared emission signatures originating from the galaxies that they already knew could be distinguished from the bright light of the quasars. Ionized carbon emits a bright spectral line at a characteristic wavelength in the infrared (158 microns), which astronomers can use as a tracer of galactic structure in the ancient, distant Universe.
In astronomy, long ago and far away are synonymous. The more distant an object is in Space, the more ancient it is in Time. This is because the speed of light sets something of a universal speed limit–no known signal can travel faster than light in a vacuum, and the more distant a celestial object is, the longer it has taken for its light to travel to where it can be detected by observers on Earth. This results from the expansion of Spacetime.
The ALMA astronomers also observed emissions from dust in the far infrared portion of the electromagnetic spectrum. This enabled them to estimate the rate of star-birth.
ALMA’s configurable array of radio antennas helped the astronomers focus their search for galaxy emissions in the region around each of a duo of DLAs, situated at a distance of approximately 13 billion light-years. From that very remote distance, the traveling light now finally reaching telescopes gives astronomers a precious peek into an early stage of galaxy formation, that occurred about 1 billion years after the Big Bang.
“This is the epoch when galaxies were really starting to take off in terms of star formation–sort of an adolescent growth spurt before reaching the peak of star formation about 2 billion years later,” Dr. Prochaska explained in the March 23, 2017 UCSC Press Release.
The observations strongly suggest that both galaxies are giving birth to baby stars at moderately high rates, with a star-formation rate greater than 100 suns per year for one galaxy, and about 25 suns per year for the other. The separation from the quasar is about 137,000 light-years for one galaxy and about 59,000 light-years for the other.
According to Dr. Neeleman, astronomers have assumed that spotting light emanating from galaxies that host DLAs is difficult because it is well-hidden in the powerful light coming from brilliant background quasars. However, when taking into consideration the large separation of these galaxies from their tattle-tale quasars, it may be that they are instead shrouded by an obscuring veil of dust. With ALMA, the astronomers were able to detect the starlight absorbed and then reradiated at longer electromagnetic wavelengths by the blanket of dust.
Dr. Prochaska is looking forward to obtaining similar observations of a much larger sample of galaxies over the next few years.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. 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.