“When a galaxy enters a rich, massive cluster, it has to contend with two murderous factors. A single major merger can use up all the gas in both progenitor galaxies, leading to a red-and-dead elliptical galaxy. Even without one, the intracluster medium is rich in matter, and speeding through it can strip out a galaxy’s gas. Without that gaseous presence, new stars can no longer form.”
Here in our Local Group, our Milky Way forms stars at a low but steady rate, and will likely continue to do so for billions of years. It’s only our impending major merger with Andromeda that will use up all of our gas, and turn us into a giant elliptical without the capacity to form new stars. If we were more isolated, we could continue to form stars for trillions of years: many times the age of the Universe.
“The cosmic story that led to the Milky Way is one of constant evolution. We likely formed from hundreds or even thousands of smaller, early-stage galaxies that merged together. The spiral arms likely formed and were destroyed many times by interactions, only to re-form from the rotating, gas-rich nature of an evolving galaxy. Star formation occurred inside in waves, often triggered by minor mergers or gravitational interactions. And these waves of star-formation brought along increases in supernova rates and heavy metal enrichment. (Which sounds like everyone’s favorite after-school activity.)
These continuous changes are still occurring, and will come to a conclusion billions of years in the future, when all the galaxies of the Local Group have merged together. Every single galaxy has its own unique cosmic story, and the Milky Way is just one typical example. As grown up as we are, we’re still evolving.”
We normally think of events in the past of having occurred at a specific time. Star formation began in the Universe when it was 50-to-100 million years old. The first galaxies formed some ~200 million years after that. The Universe became transparent to visible light 550 million years after the Big Bang, and star formation reached its maximum between 2 and 3 billion years after the Big Bang.
But when did the Milky Way form?
That’s a silly question, as it turns out, because what we know as the Milky Way has been constantly evolving and growing over time. Had we come along billions of years ago, or were we to come along billions of years in the future, our galaxy would be unrecognizable to us.
“Sometime in the distant past, likely when the Universe was less than 2% its current age, the very first galaxy of all formed when massive star clusters merged together, resulting in an unprecedented burst of star formation. The high-energy light from these stars struggles to escape, but the longer-wavelength light can penetrate farther through neutral atoms. The expansion of the Universe redshifts all the light, stretching it far beyond anything Hubble could potentially observe, but next-generation infrared telescopes should be able to catch it. And if we observe the right part of the sky, with the right instruments, for a sufficiently long time to reveal the right details about these objects, we’ll push back the cosmic frontier of the first galaxies even farther.
Somewhere, the most distant, first galaxy of all is out there, waiting to be discovered. As the 2020s approach, we can feel confident that we’ll not only shatter the current cosmic record-holder, but we know exactly how we’ll do it.”
13.8 billion years ago, our Universe as-we-know-it began with the hot Big Bang. There were no stars or galaxies back then; there weren’t even bound structures of any type. Everything was too energetic, and would immediately be destroyed by the unfathomably high temperatures and energies that every particle possessed. Yet, with time, the Universe expanded and cooled. Protons, nuclei, and neutral atoms formed; overdense regions gravitationally pulled-in mass and matter; stars were born, lived, died, and new stars were born in their aftermath. At some point, the first large star clusters merged together, passing a critical threshold and forming the first galaxy in the Universe.
The Brightest Galaxy In The Universe Is Surprisingly Young And Tiny
“In 2015, a new record was set for the brightest known galaxy, thanks to observations with the WISE telescope. Supermassive black holes power Extremely Luminous Infrared Galaxies. The brightest ones shine 10,000+ times as bright as our Milky Way.Although the Universe is just 10% of its current age and the galaxy is even smaller than ours, it outshines them all.”
I want you to close your eyes and imagine the Milky Way: a typical galaxy. Now, imagine a different galaxy, the brightest one you can think up. What does it look like? How do you imagine it?
Do you imagine something large, massive, with hundreds or even thousands of times as many stars? Do you imagine something that’s built itself up over billions of years? Well if that’s what you imagined, prepare to be shocked! The brightest ones of all are young, ultra-distant, and even smaller than our own galaxy!
“So how many earth observable galaxies have dropped out of sight? That is, how many galaxies (with the highest redshift) have disappeared from our point of view?”
When we look out at the distant reaches of space, there are some 2 trillion galaxies observable within our Universe. But our Universe is expanding, the expansion is accelerating, and light can only travel at the speed of light. Does that mean that galaxies are dropping out of sight?
There are two ways to look at this: from the point of view of not being able to see galaxies that we can presently see, and from the point of view of whether we can see the light those galaxies are emitting today, 13.8 billion years after the Big Bang? If we take the first definition, not only is the answer “zero,” but there will be trillions more galaxies revealed to us over time. But if we take the second, we find that most of the galaxies we can see today are already gone.
“Finally, beyond a certain distance, the Universe hasn’t formed enough stars to reionize space and make it 100% transparent.
We only perceive galaxies in a few serendipitous directions, where copious star-formation occurred.
In 2016, we fortuitously discovered GN-z11 at a redshift of 11.1: from 13.4 billion years ago.
But recent, indirect evidence suggests stars formed at even greater redshifts and earlier times.“
It was only a couple of years ago that we set the current record for where the most distant galaxy is: from 13.4 billion years ago, when the Universe was just 3% its current age. This record is unlikely to be broken with our current set of observatories, as discovering a galaxy this distant required a whole bunch of unlikely, serendipitous phenomena to line up at once. But in 2020, the James Webb Space Telescope will launch: an observatory optimized for finding exactly the kinds of galaxy that push past the limits of what Hubble can do. We fully expect to not only break the record for most distant galaxy ever discovered, but to learn, for the first time, exactly where and when the first galaxies in the Universe truly formed.
“In 2011, we found the first evidence for unpolluted, pristine gas, but it hadn’t yet collapsed to form stars.
But even bigger news came in 2015, when the galaxy COSMOS Redshift 7 (CR7) was discovered.
From 13 billion years ago, helium lines were observed, without any carbon or oxygen lines.
The hope was that CR7 contained stars made of hydrogen and helium alone.”
In 2015, the galaxy COSMOS Redshift 7 (CR7) was discovered. Its distance is so great that its light is arriving after a 13 billion year journey, meaning it comes from when the Universe was only 6% its current age. Earlier observations, taken with Hubble, showed the presence of hydrogen and helium, but not of heavier elements like carbon, nitrogen or oxygen. Could this be the first example of Population III stars: the first stars made from the pristine material left over from the Big Bang? For years, astronomers thought it just might be, but new observations in the radio from ALMA have crushed those dreams. Looking for the long-wavelength signature of singly-ionized carbon, they’ve found it everywhere, surrounding all the individual, independent components of galaxy CR7.
The Milky Way Is Still Growing, Surprising Scientists
“It’s no big secret that galaxies grow over time. The force of gravity is powerful enough to pull smaller galaxies, gas clouds, and star clusters into larger ones, even over distances of millions of light years. Our own Milky Way has likely devoured hundreds of smaller galaxies over its lifetime, and continues to absorb the dwarf satellites which surround us. But there’s a steadier, more subtle way that galaxies grow: by continuing to form stars from the gas already inside. While most of the stars that form will do so in the plane or central bulge of a spiral galaxy like our own, a new study has shown that galaxies also grow outward over time, meaning that their physical extent increases in space. The implication is that our own galaxy is increasing in size by 500 meters per second: growing by a light year every 600,000 years.”
Imagine a galaxy all by its lonesome out there in the Universe. It’s full of stars, with gas, dust, plasma, and dark matter permeating all throughout it. What’s going to happen to the galaxy over time? You might think that it will continue to form new stars in its spiral arms, while older stars burn out and eventually die. All of that is true, but there’s a subtle but important effect that really adds up over cosmic time: the physical extent of where stars can be found grows as even isolated galaxies age. The Milky Way itself is growing at a rate of 500 m/s, typical of spiral galaxies around this size. It means that by time the Universe is three times as old as it presently is, Milky Way-like galaxies will have grown to be twice as large as they presently are.
“2.) Ejected from galactic mergers. When two galaxies smash together, they usually merge entirely, but sometimes there is ejected material. Sufficient amounts could create a baryons-only galaxy.”
Last week, astronomers announced the discovery of the ultra-diffuse galaxy NGC 1052-DF2 (or DF2), which appears to be completely free of dark matter. Other similar galaxies have been seen before, but all contain more, not less, dark matter than you’d have expected on average. This first galaxy ever seen without the gravitation-altering effects of dark matter was touted to defy theory, but it does no such thing. In fact, there are many explanations that lead directly to galaxies such as DF2 as an inevitable consequence, including one that was put forth in a predictive fashion as much as 20 years ago.