What caused this galactic genocide 11 Billion years ago?
Today our view of galaxies in our universe shows that these galaxies are full of a wide
mix of stars.
Most galaxies that we know of, much like our Milky way, contain old stars, middle-aged
stars and young stars.
They contain stars of all ranges of colours from Blue to Red and everything in between.
But there was something strange that happened 11 Billion years ago that changed the very
structure of these galaxies.
A mass dying off!!
What was this?
Why did this happen?
Join us as we talk more about it in this video.
And just like our Milky way, most of the Galaxies we know today are still star forming and star
rich galaxies.
These galaxies form stars at a slow but Steady rate, with neutral hot gas contracting to
create new episodes of star-formation, with the occasional galaxy undergoing a starburst,
where huge quantities of new stars are formed at once.
But there are a large number of galaxies, especially towards the centers of large galaxy
clusters, containing only old stars, practically no new ones.
Its stars are almost all yellow, orange or red, practically none white or blue.
Some of them, in extreme cases, have not formed new stars for more than 12 billion years.
It is as if some of the most massive and star-rich galaxies in the young universe suddenly "turn
off" their star formation, never to turn them on again.
A few of these galaxies were discovered "turning off" for the last time 11 billion years ago
or more, according to a startling new study.
A massive cosmic massacre appears to have occurred just 3 billion years after the Big
Bang, and astronomers are still baffled as to how it happened.
There were no stars in the early stages of the hot Big Bang.
It takes a long time for matter to clump, cluster, and gravitate so that enough mass
can gather in one place to form stars.
The first galaxies appear a few hundred million years after the Big Bang, and the rate of
star formation increases as more matter falls onto the overdense regions, star clusters
and then proto-galaxies merge together, resulting in larger collections of mass, and the earliest
generations of stars inject heavy elements into the mix, allowing for faster cooling
and faster star formation.
For the next two billion years or so, the universe continues to form stars at an increasing
rate, but then it reaches a peak and then gradually declines.
By the time we get to the present, we can see that many of our modern-day galaxies are
"red and dead," which means that there haven't been any new major star-forming events in
billions of years.
After that amount of time, all the hot, young, blue stars will have burned through their
fuel, putting an end to their lives and leaving only the cooler, redder, lower-mass stars
behind.
We can learn from both the stellar populations that remain in nearby galaxies and observations
of ultra-distant galaxies from the early universe that nearly half of the most massive galaxies
that existed 11 billion years ago have already stopped forming stars permanently.
This is a puzzle in and of itself.
In theory, at least in terms of what we would naively expect, there are three main reasons
why star formation would not simply stop inside these massive galaxies, never to start again:
1.)
Despite the fact that massive star formation can energise, ionise, and expel neutral matter
from these galaxies, we fully expect the dark matter halos around them to keep that matter
gravitationally bound, where it will eventually fall back onto the galaxy and form new stars.
2.)
Even in an expanding universe, material from the intergalactic medium will fall onto these
galaxies over time and cause them to grow, eventually leading to the restart of star
formation once a critical density is reached.
3.)These galaxies do not exist in isolation, but rather in close proximity.
Minor and major mergers, for example, should result in subsequent bursts of star formation
throughout their history, resulting in new populations of stars.
It is clear from what we have seen that this does not happen for a large proportion of
these early, massive, star-rich galaxies.
Of course, the burning question on astronomers' minds is: Why?
Ideally, we would be able to utilise our most powerful observatories to gaze back at these
very early galaxies just when star production peaks and then ceases.
We'd be able to see what was going on in terms of the stellar populations that were forming,
the winds that came from these hot, young stars as well as the galaxy's centre black
hole, and the surrounding matter in terms of velocity, ionisation, and density.
We could now comprehend how and why these galaxies become "red and dead" by conducting
firsthand observations of them at the earliest times and in amazing detail.
Unfortunately, the current generation of observatories is not up to the task, as they lack the resolution
and light-gathering power required to discern what is happening inside those distant galaxies.
However, for a subset of these galaxies, these observations are suddenly made possible due
to the amplification effects of severe gravitational lensing.
If there is enough mass between our line of sight and the background galaxy we are attempting
to study, the light can be warped, stretched out into many images, and, most crucially,
magnified.
In an ideal alignment, these background objects can appear tens or hundreds of times brighter
than they would without the intervening mass, which acts as a gravitational lens.
Imaging studies aimed at measuring the regions of space that should be gravitationally lensed
by huge foreground clusters have been one of the most intriguing developments in observational
cosmology.
Hubble's Frontier Fields campaign was one of the first and most effective instances,
although it has been recently surpassed in many aspects by a more contemporary effort,
REQUIEM.
The REsolving QUIEscent Magnified (REQUIEM) galaxy survey, combining views from Hubble
(in the optical and near-infrared) and ALMA (in the radio), in particular, allows us to
first find massive, star-forming galaxies at these early epochs, and then characterise
them and their gas populations.
Previously, the only option to measure the presence of gas in galaxies this distant was
to use a highly imprecise proxy: carbon monoxide tracing, which frequently required stacking
data from multiple locations to derive any form of average abundance.
Individual measurements of the presence (or absence) of dust in each individual galaxy
at these large cosmic distances could finally be obtained with magnifications up to a factor
of 30 for a population of six galaxies in the REQUIEM galaxy survey.
Because gravitational lenses are solely gravitational effects, they magnify all wavelengths of light
identically, whereas dust emits light differentially at different wavelengths depending on its
temperature.
The gravitational lensing effects enabled us to collect these dust emission data for
these distant, lensed galaxies, and scientists then utilised a well-established process to
convert those results into abundances of cold molecular gas: the stuff that produces new
stars.
The researchers were then able to use their methods to calculate the amount of gas — specifically,
neutral molecular hydrogen gas, which is required for the birth of new stars — present in
the vicinity of these six galaxies, and they discovered something remarkable.
One of these galaxies has around 4.6 percent of the gas of a normal galaxy at that distance,
another has 0.6 percent, and the remaining four have undetectably small amounts.
The gas and dust within these galaxies and their surroundings have been nearly entirely
removed.Furthermore, all six of these galaxies are now forming relatively few new stars,
with each one forming new stars at a pace lower than than the modern Milky Way, which
creates less than one solar mass worth of new stars every year.
This discovery is notable for two reasons: (1) the tremendous technical feat that enabled
it, and (2) the absence of cold, interstellar gas has never been recorded this early in
the universe.
The star formation rate and total gas presence in five of these six galaxies are less than
one percent of what is typical, and these galaxies are not anomalies, but rather represent
around half of the most massive galaxies from when the universe was just about three billion
years old.When we extrapolate the results of our study to the entire population of big
galaxies that existed at the time, we discover that over half of them had already stopped
generating stars by the time the universe was three billion years old — that is, around
11 billion years ago.
The ramifications are staggering.
Previously, it was expected that gas would be absorbed gradually by the host galaxy over
time, but this cannot be the case.
These large galaxies not only lose their gas reservoir, the stuff required to build future
generations of stars, but it looks that they will never regain it: not from gas falling
back onto them, not from future mergers or infall, nothing.
As evidence accumulates, it does not appear to support the expected scenario of star formation
occurring in repeated bursts and eventually turning back on for nearly all of these galaxies
in some capacity.
Instead, it appears that galaxies "burn up" all of their potentially star-forming material
in large quantities, becoming gas impoverished very early on and remaining that way — as
far as we can tell — for the rest of cosmic history.
So, what is the secret to the stopping of star formation in these young galaxies?
It is because their gas stores are low and are not replenished.
Previously, it was considered that after long periods of star formation, some sort of inefficiency
may occur that simply forced it to halt, such as a feedback effect or some type of back-reaction.
These theories may play a role in some galaxies, but they cannot account for what we see.
The gas reservoirs definitely drive star formation in these galaxies, and star formation halts
when these reservoirs are emptied and do not replenish themselves.
The obvious next inquiry is, "Why does this happen?"
Is it because stellar winds from large, hot, blue stars blast away this neutral matter?
Could feedback from active supermassive black holes in galaxies' centres be to blame?
May there even be impacts happening in these galaxies' halos, or could mergers (which would
have to resolve pretty quickly) be what's ejecting this gas and putting an end to star
formation for good?
All of these are open questions that will be answered with future observations and better
measurements.
We've gotten to the point where we can accurately describe the genesis and attributes of "red
and dead" galaxies; now we need to figure out how they got this way, and how they stay
that way.
Future generations of observatories will undoubtedly allow us to investigate more of these early
galaxies and their surroundings, as well as test our hypotheses regarding the relationship
between the existence or lack of gas and dust.
At the moment, the combination of gravitational lensing and multi-wavelength deep observations
has just revealed that a great galactic genocide occurred very early in the universe's history
— bringing us one step closer to unravelling the mystery of how the universe not only grows
up, but also how some of its properties stop growing entirely and simply die away.
So what do you think about this amazing phenomenon?
Let us know what you think in the comment section below.
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