Understanding the Formation and Structure of Galaxies
Abstract:
This article explains how galaxies formed and grew over time, starting after the Big Bang. It describes how the difference in the early universe’s density grew into large structures like galaxies and clusters, connected by the threads of the “Cosmic Web." Dark matter, an invisible type of matter, played a key role by pulling normal matter together to form galaxies and holding them in place. This article also discusses the “Epoch of Reionization," a time when the first stars and galaxies lit up the universe and made it possible for light to travel freely.
The process of star formation is explained as gas and dust clouds collapsing under gravity, heating up to form young stars. Galaxies are classified into three main types: spiral, elliptical, and irregular, each with unique shapes and features. Observations and modern telescopes, like the James Webb Space Telescope, have revealed surprisingly massive galaxies in the early universe, challenging previous ideas. By exploring the history of galaxies, the role of dark matter, and how stars are born, this article helps us understand how the universe became the beautiful, organized place we see today.
Keywords- stellar nurseries, quantum fluctuations, neutral hydrogen, supermassive black hole, James Webb Space Telescope.
I. Introduction to Galaxies
A Galaxy is a huge collection of gas, dust, and billions of stars and their solar systems, held together by gravity. The known universe contains 200 M galaxies. Milky Way, our galaxy is approximately 12 M years old with enormous spiral arms and a central nucleus. A typical galaxy could contain 100 billion stars. They are stellar nurseries, the location where stars are formed in the parish. The stars in a galaxy are formed in nebulas, which are dust and gas concentrations.
In 1924, astronomer Edwin Hubble observed the universe using the most sophisticated telescope available at the time. The 100-inch hooker on Mt. WILSON near Los Angeles. He made the finest history in the history of astronomy when he discovered that the universe contains a large number of galaxies rather than just one.
Galaxies like the Whirlpool Galaxy and M87 are important markers in the universe's long journey that started with the Big Bang. Following the Big Bang, the universe was at a very hot and dense plasma state. It gradually expanded and cooled, and due to gravity, matter started clumping together into the first stars and galaxies. Considered over billions of years, these galaxies merged through invisible threads of dark matter and gas, forming the Cosmic Web: an enormous structure made by filaments and voids that stretch across the universe. This web acts as a skeleton that hosts galaxies, clusters, and stars with a great exhibition of chaos from the Big Bang churning into the organized beauty of the cosmos we see today.
II. The Big Bang and Cosmic Web Formation
The Big Bang marks the beginning of the universe, where all matter and energy are concentrated in a hot, dense point. After the rapid expansion (inflation), small quantum fluctuations in this dense plasma were stretched to large scales. This resulted in the creation of slight over- and under-dense regions in the universe. As the universe expanded, these ripples became density variations-some areas were slightly denser (over-dense), and others were less dense (under-dense).?
Gravity played a crucial role next. It acted like a magnifying glass, amplifying these density differences over time. Over-dense regions pulled more matter toward them, eventually forming clusters of galaxies and stars. Under-dense regions had less matter and became voids, where there's relatively less cosmic stuff. The cosmic web forms, with galaxies along dense filaments and voids in between. Imagine these denser regions as clusters of galaxies, connected like threads in a giant spider web (the cosmic web). In between these threads are vast spaces (voids) where there are fewer galaxies.
III. Cosmic Microwave Background and Quantum Fluctuation
This was a very exciting time in the early universe, shortly after the Big Bang. At first, it was a pitch-black place, no stars, no galaxies, light nowhere. Everything was just a composition of hot, thick energy and a lot of particles moving around. As time went on, these particles began to condense into atoms, for the first time allowing light to propagate freely. This is called "recombination" and changed the universe from one that was utter blackness to shining bright. That light is what we see today as the Cosmic Microwave Background, the oldest light in the universe. It marked the beginning of a radiant, expanding cosmos full of possibilities.
The CMB is the afterglow of the Big Bang, emitted when the universe cooled enough (~380,000 years after the Big Bang) for protons and electrons to combine into neutral hydrogen, making the universe transparent. Tiny fluctuations in the temperature of the CMB correspond to regions of slightly different densities, which are the fingerprints of quantum fluctuations during inflation. These small differences grew over time to form galaxies, clusters, and the large-scale structure.
Analogy: Think of the CMB as the surface of a cooling pot of soup. Tiny bubbles form on the surface, representing slight temperature variations. Over time, these bubbles might develop into larger, more noticeable structures (like the galaxies we see today).
As the universe kept expanding and cooling, these fluctuations stretched out across the universe. Eventually, the universe cooled enough for light (photons) to travel freely, creating the Cosmic Microwave Background (CMB). The slight differences in energy from quantum fluctuations were imprinted on the CMB as tiny temperature differences.
IV. Galaxy Morphology and the Epoch of Reionization
Less than a billion years after the Big Bang, a cosmological phase transition occurred in the universe. During this event, the ultraviolet photons emitted by the first stars, galaxies, and growing black holes carved out ionized regions around them. After a sufficient number of ionizing sources have formed, the ionized fraction of the gas in the universe rapidly increases until hydrogen becomes fully ionized. This period, during which the cosmic gas went from neutral to ionized, is known as the universe's Epoch of Reionization. This era marks the transition from a universe filled with neutral hydrogen to one where the gas becomes ionized due to the first luminous objects, including stars, galaxies, and quasars.
V. The Impact of Dark Matter in Galaxy Formation
1. Introduction to dark matter
In Astronomy, dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter plays a vital role in the formation of galaxies. It doesn't emit, absorb, or reflect light, making it invisible to see and detect and even harder to research.
After the Big Bang, when the universe cooled down, over densities of dark matter, or dark matter halos, worked as gravitational pullers or wells, attracting all the normal matter towards them and forming galaxies and other structures. Basically, dark matter works like a "glue" that holds together a galaxy.
2. Significance in galaxy formation?
The galaxy rotation problem is the discrepancy between observed rotational curves and theoretical predictions. Stars and gases in the farther part of the galaxy have actual orbital velocities higher than calculated and expected. This indicates the presence of matter, which contributes to the actual centrifugal force being applied to the object, increasing its orbital velocity.
Without dark matter halos, the baryonic matter would not be attracted towards each other, which is the most important part of galaxy formation. Besides, it contributes a lot of centrifugal force for the objects in the later part of the galaxy. Imagine a scenario where the dark matter present in the galaxy suddenly disappeared. All its effects will vanish. So, the centrifugal force applied by the dark matter would also perish. The force applied by the supermassive black hole at the center of the galaxy is not enough to help in its current velocity. Slowly, the objects would lose their momentum, and the galaxy would start to break apart. This shows how important dark matter is in holding the galaxy together and in its formation.?
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VI. Formation of Stars and Galaxies
Stars and galaxies form from the great clouds of gas and dust in space. These clouds are 1,000 to 10 million times the mass of the sun and create high-density pockets due to the clumping of gas. The building of density in a few of these clumps is enough for their galaxy to strengthen and start to collapse. As the clump collapses, friction heats the material, forming a protostar (a very young star). This heating continues until the core of the star becomes so hot that hydrogen atoms begin to fuse into helium atoms, releasing tremendous amounts of energy in the process. The nuclear fusion then supplies the outward pressure needed to balance the inward pull of gravity, stabilizing the star. A balance is then reached, and complete collapse does not take place, forming a bright, stable star.
The evolution history of molecular clouds regarding their development into stars and even into galaxies shares some parallels with some atomic principles. While lower-energy orbitals fill before higher ones do, the Aufbau principle states that regions within molecular clouds with the highest density fill first, attracting more gas and dust due to stronger gravitational attraction. Similarly, Hund's rule suggests that the regions are filled singly until full, much as the clusters within the cloud break up and then start to merge into bigger structures. The Pauli Exclusion principle makes sure that the particles remain distinguishable, just as many different types of star clusters may form from a molecular cloud. Putting all of these strings together gives an idea of how gravity pulls particles together and how the energy released by fusion maintains and keeps a star stable. Entire galaxies go through similar star formation, rotation, and collision processes that let us see the growth and change over billions of stars.?
When astronomers got their first glimpses of galaxies in the early universe from NASA's James Webb Space Telescope, they were expecting to find galactic pipsqueaks-small galaxies that contain fewer stars and are much less massive than typical large galaxies like the Milky Way, but instead, they found what appeared to be a bevy of Olympic bodybuilders. Some galaxies appeared to have grown so massive, so quickly, that the simulations couldn't account for them.
VII. Types of Galaxies
Galaxies come in various shapes, primarily spiral, elliptical, and irregular. Spiral galaxies have a flat disk structure with a central bulge and arms, while elliptical galaxies are more three-dimensional, lacking a disk. These shapes depend on factors like angular momentum and the merging histories of galaxies. Mergers between galaxies can result in dramatic transformations, such as two spirals merging into an elliptical galaxy
1. Elliptical
The most important galaxy types were classified in the Hubble sequence: elliptical galaxies, spiral galaxies, and lenticular galaxies. They are typified by the general ellipsoidal shape they take on, a smooth and almost featureless appearance, and a predominance of older, low-mass stars. They are often starved of star-making gases and hence normally have little birth of stars, giving them a red color due to their older stellar populations. Elliptical galaxies span a wide range in size, from ~ 10^9 M dwarf ellipticals to brilliant ~ 10^14 M supergiants that normally reside near the centers of large galaxy clusters. With their low content of the interstellar medium, there are no young stars or open star clusters, but all ellipticals are enveloped by large systems of globular clusters in two distinct populations: metal-rich red clusters and metal-poor blue clusters. Further, each giant elliptical galaxy harbors a supermassive black hole at its center, and the mass of the black hole is directly connected with the mass of the galaxy itself through scaling relations like the M-sigma relation.
Initially, Hubble had postulated that the elliptical galaxies could evolve into spiral galaxies, but this was later proved wrong. Instead, the latest thoughts are that elliptical galaxies may develop disk-like characteristics: gas accretion and mergers with other galaxies, forming a disk around an originally ellipsoidal structure. The dynamical properties are comparable to those of the bulges of disk galaxies, which suggests they may form through similar physical processes, though this is an issue open for debate. Their luminosity profiles and structural parameters follow Sersic's law and specific scaling relations, which unify them completely with certain disk galaxy bulges. They share most basic properties with common bulges but are found in the most crowded of environments, namely galaxy clusters and compact groups. The motions of their stars are fairly random rather than organized into rotations as in flat spiral galaxies.
2. Spiral
Among all the observed galaxies, the most common type of galaxy is a spiral galaxy. A spiral galaxy typically has a rotating disc with spiral ‘arms’ that curve out from the dense central region. Roughly two-thirds of all the spiral galaxies are observed; the arms are connected to a bar-like structure that passes through the center. Such galaxies are called barred spiral galaxies. Spirals are further classified based on how tightly their arms are wound. A “type A spiral galaxy” has its arms tightly wounded, a "type B spiral galaxy” has its arms a little loosely wounded comparatively, and a "type C spiral galaxy” has its arms very loosely wounded. The Milky Way was once considered an ordinary spiral galaxy. However, in the 1960s, astronomers first started to suspect it to be a barred spiral galaxy. Their suspicions were confirmed by the Spitzer Space Telescope in the year 2005.?
Due to high mass density and high rate of star formation, the spiral arms consist of a lot of young, blue stars, which makes the arms so bright. The bulge of Sa galaxies is usually composed of stars that are old or red stars with low metal content. Bulges of Sa and SBa galaxies tend to be large, while the bulges of Sc and SBc are much smaller in size. Bulges are said to host a supermassive black hole at their center. Our galaxy also hosts Sagittarius A*, a supermassive black hole at its center.
The table below shows the abbreviations that are used for spiral galaxy types.
Conclusion
Galaxies are some of the oldest astronomical structures in the universe. Some galaxies have existed since the beginning of the universe. Studying the oldest of the galaxies, helps us understand more about the beginning of the universe and the evolution of the universe. Now, we can observe these galaxies with more advanced telescopes like the JWST.?
Their diverse forms show a dynamic history of the universe, filled with collisions, mergers, and explosions. Studying their formation and rotation curves led to the discovery of dark matter, which is a big mystery in itself. As research progresses, we move closer to discovering and uncovering the mysteries that the universe holds, and an appreciation for the universe’s vastness and complexity.
References
Credits?
fig. 1: Harvard and Smithsonian, the MIT and the Max Planck Institute of Astrophysics (https://www.cfa.harvard.edu/news/astronomers-reveal-remarkable-simulations-early-universe-dark-ages-through-first-light)
fig. 2: Wikipedia - Galaxy rotation curve (https://en.wikipedia.org/wiki/Galaxy_rotation_curve)
fig. 3: Galaxies - Hubble Site (https://hubblesite.org/science/galaxies)