28.03.01 · astronomy / galaxies

Galaxies and the Milky Way

shipped3 tiersLean: none

Anchor (Master): primary sources: Hubble 1926, Shapley 1918, Rubin 1978; secondary: Binney and Tremaine 2008

Intuition Beginner

A galaxy is a vast collection of stars, gas, dust, and dark matter bound together by gravity. Our own galaxy, the Milky Way, contains between 100 and 400 billion stars, along with enormous clouds of gas and dust, all orbiting a common centre. On a clear, dark night far from city lights, you can see the Milky Way as a luminous band stretching across the sky, the combined light of billions of stars too faint and distant to see individually.

Galaxies come in a remarkable variety of shapes and sizes. Edwin Hubble, working in the 1920s, developed a classification system based on their appearance that is still used today. Elliptical galaxies are smooth, featureless collections of stars, ranging from nearly spherical to highly elongated. They contain mostly old, red stars and very little gas or dust, which means they form few new stars.

Spiral galaxies, like the Milky Way, have a central bulge of old stars surrounded by a flat disk containing spiral arms rich in gas, dust, and young blue stars. Spiral galaxies are further divided into barred spirals, which have a straight bar of stars cutting across the centre, and unbarred spirals, which do not. The Milky Way is now known to be a barred spiral. Irregular galaxies have no regular shape and are often chaotic collections of gas and stars, frequently the result of gravitational interactions or mergers between galaxies.

The range of galaxy sizes is enormous. Dwarf galaxies may contain only a few billion stars, while the largest elliptical galaxies contain trillions. The largest known galaxy, IC 1101, is about 6 million light-years in diameter, roughly 60 times the size of the Milky Way. On the other end of the scale, ultra-compact dwarf galaxies are only about 200 light-years across but contain tens of millions of stars packed into an incredibly small volume.

The Milky Way is a large barred spiral galaxy. If you could view it from outside, you would see a disk about 100,000 light-years in diameter with a central bar and several spiral arms emanating from its ends. The Sun is located in one of these arms, about 26,000 light-years from the centre, orbiting the galactic centre at a speed of about 220 kilometres per second. At this speed, it takes the Sun roughly 230 million years to complete one orbit, a period called a galactic year.

The galaxy's spiral arms are not fixed structures but density waves, regions where stars and gas pile up as they orbit, like traffic congestion on a highway. As gas enters the density wave, it is compressed, triggering new star formation. The youngest, brightest stars trace the spiral arms, while older stars fill the spaces between. This density wave theory explains why spiral arms persist despite the differential rotation of the galaxy, which would otherwise wind them up over time.

At the very centre of the Milky Way lies a supermassive black hole called Sagittarius A* (pronounced "A-star"), with a mass of about 4 million times that of the Sun. Despite this enormous mass, the black hole itself is small, with a diameter roughly equal to the orbit of Mercury. Its gravitational influence is significant only in the innermost region of the galaxy. The overall structure and dynamics of the Milky Way are dominated not by the central black hole but by the collective gravity of all its stars, gas, and an invisible component called dark matter.

The galactic centre is one of the most studied regions in astronomy. Despite being obscured by dust at visible wavelengths, radio, infrared, and X-ray telescopes have revealed a dense star cluster surrounding the black hole, with stars orbiting at extraordinary speeds. One star, designated S2, orbits Sagittarius A* with a period of only 16 years and reaches speeds of 7,650 km/s at closest approach, providing precise measurements of the black hole's mass.

Dark matter is one of the great mysteries of modern astronomy. In the 1970s, Vera Rubin and her colleagues measured the rotation speeds of spiral galaxies and found that the outer parts of galaxies were rotating just as fast as the inner parts. According to Kepler's laws and Newtonian gravity, the outer regions should rotate more slowly because there is less visible mass interior to them. The observation that they do not slow down implies the existence of additional, unseen mass, dark matter, that extends far beyond the visible disk of the galaxy.

The nature of dark matter remains unknown, but its gravitational effects are observed throughout the universe. It does not emit, absorb, or reflect light, making it invisible to all forms of electromagnetic radiation. It is detected only through its gravitational influence on visible matter and light.

Galaxies are not isolated. They exist in groups, clusters, and superclusters, connected by filaments of gas and dark matter in a vast cosmic web. The Milky Way is part of a small group called the Local Group, which also includes the Andromeda galaxy (our nearest large galactic neighbour, about 2.5 million light-years away), the Triangulum galaxy, and several dozen smaller galaxies.

Galaxy interactions are common throughout the universe. When galaxies pass close to each other, their mutual gravity can distort their shapes, trigger bursts of star formation, and eventually lead to mergers. The Antennae Galaxies (NGC 4038/4039), about 60 million light-years away, show two spiral galaxies in the process of colliding, with long tidal tails of stars flung out by the interaction. These mergers were more common in the early universe, when galaxies were closer together, and they played a major role in building up the massive galaxies we see today.

The environment in which a galaxy lives profoundly affects its evolution. Isolated field galaxies, far from neighbours, tend to be gas-rich spirals with ongoing star formation. Galaxies in dense clusters are more likely to be ellipticals or lenticulars with little gas, their star formation quenched by the hostile cluster environment.

The Milky Way, in its relatively quiet corner of the Local Group, has been able to maintain steady star formation for billions of years, contributing to the conditions that made life on Earth possible. Understanding our galaxy's place in the broader landscape of galaxy types and environments helps astronomers predict which other galaxies might host habitable worlds. The study of galaxies connects the smallest scales of star and planet formation to the largest scales of cosmology and the structure of the universe itself.

Visual Beginner

The Hubble classification system organises galaxies by shape.

Type	Shape	Star formation	Gas/dust	Example
E0-E7 (Elliptical)	Round to elongated	Very low	Very little	M87 (E0)
S0 (Lenticular)	Disk with bulge, no arms	Low	Little	NGC 2787
Sa-Sb-Sc (Spiral)	Bulge + disk + arms	Moderate to high	Moderate to much	Andromeda (Sb)
SBa-SBb-SBc (Barred spiral)	Bar + arms	Moderate to high	Moderate to much	Milky Way (SBbc)
Irr (Irregular)	No regular shape	High	Abundant	Large Magellanic Cloud

The Milky Way's key parameters: diameter approximately 100,000 light-years, containing 100-400 billion stars, mass approximately 1.5 trillion solar masses (including dark matter), with the Sun located 26,000 light-years from the centre.

Worked example Beginner

Vera Rubin's observation of flat rotation curves provides a direct example of dark matter's effect. For a spiral galaxy, the expected rotation speed at distance $r$ from the centre depends on the mass enclosed within that radius.

If all the mass were in the visible stars and gas, the rotation curve would follow Kepler's law: $v (r) = GM (r) / r$ , declining at larger radii as most of the mass is enclosed. But Rubin observed that $v (r)$ remains approximately constant out to the largest measurable distances.

The implication is that $M (r)$ must increase linearly with $r$ , meaning there is additional mass that does not emit light: $M (r) = v^{2} r / G$ . For a typical spiral galaxy with a flat rotation curve at $v = 200$ km/s, the enclosed mass at $r = 50$ kpc (kiloparsecs) is $M = (200 \times 1 0^{3})^{2} \times 50 \times 3.086 \times 1 0^{19} / (6.674 \times 1 0^{- 11}) \approx 9.3 \times 1 0^{41}$ kg, or about $4.7 \times 1 0^{11}$ solar masses. At this radius, the visible mass (stars and gas) accounts for only about 10-20 percent of the total. The remaining 80-90 percent is dark matter, distributed in a roughly spherical halo extending far beyond the visible disk.

This calculation revolutionised our understanding of the universe. Dark matter outweighs visible matter by a factor of about five, and its nature remains one of the biggest unsolved problems in physics.

Check your understanding Beginner

Formal definition Intermediate+

Galaxy classification

Edwin Hubble's 1926 classification scheme, often presented as a tuning-fork diagram, organises galaxies into three main categories. Elliptical galaxies (E0 through E7, where the number is $10 (1 - b / a)$ with $a$ and $b$ the semi-major and semi-minor axes) are smooth, featureless systems dominated by old stellar populations with little gas or dust. Lenticular galaxies (S0) are transition objects with a disk and bulge but no spiral arms. Spiral galaxies (Sa through Sc) have a central bulge and disk with well-defined spiral arms; the sequence from Sa to Sc represents decreasing bulge-to-disk ratio, increasingly open spiral arms, and increasing gas content. Barred spirals (SBa through SBc) have the same sequence with a central bar. Irregular galaxies (Irr) lack any regular structure.

The Milky Way's structure

The Milky Way consists of several components. The thin disk has a scale height of about 300 pc and contains most of the young stars, gas, and dust, including the spiral arms. The thick disk extends to about 1 kpc above and below the plane and contains older stars with different chemical compositions. The stellar halo is a diffuse spherical distribution of old, metal-poor stars and globular clusters extending to at least 100 kpc. The dark matter halo extends to 200-300 kpc or more and dominates the total mass.

Galactic rotation curves

For a galaxy with mass distribution $ρ (r)$ , the circular velocity at radius $r$ is determined by the enclosed mass:

$v_{c} (r) = \frac{GM ( r )}{r} = \frac{4 π G}{r} \int_{0}^{r} ρ (r^{'}) r^{'2} d r^{'}$

For an exponential disk with surface density $Σ (r) = Σ_{0} e^{- r / R_{d}}$ , where $R_{d}$ is the disk scale length, the rotation curve rises steeply in the inner regions and then declines. The observed flat rotation curves require an additional mass component: a dark matter halo with density profile $ρ \propto r^{- 2}$ at intermediate radii (the isothermal halo model) or more realistic profiles such as the NFW profile ( $ρ \propto 1/ [r (r + r_{s})^{2}]$ ) from cosmological simulations.

The Tully-Fisher relation

The Tully-Fisher relation is an empirical correlation between the luminosity of a spiral galaxy and its maximum rotation velocity: $L \propto v_{ma x}^{α}$ , where $α \approx 3 - 4$ depending on the wavelength band. This relation is useful as a distance indicator and reflects a fundamental connection between the baryonic and dark matter content of galaxies. The baryonic Tully-Fisher relation, which includes the mass of gas in addition to stars, provides an even tighter correlation.

Key result: the evidence for dark matter Intermediate+

The evidence for dark matter in galaxies comes from multiple independent lines of observation that converge on the same conclusion: the visible matter in galaxies accounts for only about 15 percent of the total mass.

Galaxy rotation curves

As described above, the flat rotation curves of spiral galaxies require roughly spherical dark matter halos extending well beyond the visible disks. The dark matter mass-to-light ratio in the outer regions of typical spirals is 10-30 solar units, meaning that for every solar luminosity of starlight, there are 10-30 solar masses of dark matter.

Galaxy cluster dynamics

Fritz Zwicky, working in 1933, measured the velocities of galaxies in the Coma Cluster and found they were moving too fast for the cluster to be gravitationally bound by its visible mass alone. He estimated that the cluster needed about 400 times more mass than was visible to hold it together. Although Zwicky's estimate was too large (he underestimated the distance to the cluster), his basic conclusion was correct: galaxy clusters contain far more mass than can be accounted for by their stars and gas alone.

Gravitational lensing

General relativity predicts that massive objects bend light from background sources, an effect called gravitational lensing. Strong lensing produces multiple images or arcs; weak lensing produces subtle distortions in the shapes of background galaxies. By measuring these distortions, astronomers can map the distribution of mass, both visible and dark, in galaxy clusters and individual galaxies. Lensing observations consistently confirm the presence of dark matter halos and provide independent measurements of their masses and shapes.

The bullet cluster

The Bullet Cluster (1E 0657-558), a pair of colliding galaxy clusters, provides one of the most compelling pieces of evidence for dark matter. X-ray observations show the hot gas (the dominant visible mass component) in the two clusters has been slowed by the collision and is concentrated between them. But gravitational lensing reveals that most of the mass is in two separate clumps that have passed through each other, coincident with the galaxy distributions rather than the gas. This is exactly what would be expected if the mass were in collisionless dark matter particles that passed through each other, and is very difficult to explain with modified gravity theories.

Exercises Intermediate+

Exercise 3 (medium, short answer).

Explain how gravitational lensing provides evidence for dark matter.

Hint

Consider what lensing measures and how it differs from measurements of visible matter.

Answer

Gravitational lensing measures the total mass distribution along the line of sight, regardless of whether that mass emits light. By mapping the lensing distortions of background galaxies, astronomers can reconstruct the mass distribution in galaxy clusters and individual galaxies. These lensing maps consistently show more mass than can be accounted for by visible stars and gas alone, confirming the presence of dark matter. In galaxy clusters, lensing reveals extended mass halos that match the predictions of dark matter models. The Bullet Cluster is particularly compelling because the lensing mass peaks do not coincide with the visible gas, ruling out explanations based on modified gravity alone.

Exercise 4 (hard, short answer).

Describe how the Tully-Fisher relation connects galaxy properties to dark matter, and explain why this relation challenges theories that attempt to eliminate dark matter.

Hint

Consider what the Tully-Fisher relation links and why this link requires a connection between visible and invisible mass.

Answer

The Tully-Fisher relation connects a galaxy's luminosity (proportional to its stellar mass) to its rotation velocity (determined by its total mass, including dark matter). The tightness of this correlation implies a consistent relationship between the baryonic (visible) mass and the dark matter mass across a wide range of galaxy masses. If dark matter did not exist and rotation curves were explained by modifying gravity (as in MOND, Modified Newtonian Dynamics), the Tully-Fisher relation would arise from the modified dynamics but would not naturally explain why the relation holds across such a wide range of galaxy types and environments. The baryonic Tully-Fisher relation, which includes gas mass, is even tighter, suggesting a fundamental coupling between baryons and dark matter that is naturally explained by the standard dark matter model but is more challenging for modified gravity alternatives.

Advanced results Master

Galaxy formation and evolution

Galaxies form and evolve through a combination of hierarchical structure formation, gas accretion, star formation, feedback processes, and mergers. In the standard cosmological model, dark matter halos form first through gravitational collapse, and galaxies assemble within them as gas cools and condenses at the halo centre.

The hierarchical model predicts that small dark matter halos form first and subsequently merge to form larger structures. Dwarf galaxies, which are the most numerous type of galaxy in the universe, are thought to be the building blocks from which larger galaxies assembled. The Milky Way is surrounded by dozens of dwarf satellite galaxies, many of which are being tidally disrupted and absorbed, a process called galactic cannibalism. The Sagittarius Dwarf Spheroidal Galaxy is currently being torn apart by the Milky Way's gravity, leaving a stream of stars that wraps around the entire sky.

The star formation history of galaxies depends on both their mass and environment. Massive galaxies in dense environments tend to have formed most of their stars early in the universe's history (at redshifts $z > 2$ , more than 10 billion years ago) and have been relatively inactive since, a phenomenon called downsizing. Less massive galaxies continue forming stars to the present day.

Environmental effects play a major role in galaxy evolution. Ram pressure stripping removes gas from galaxies as they move through the hot intracluster medium, quenching star formation. Galaxy harassment, repeated high-speed encounters in clusters, can disturb galaxy morphology. Strangulation occurs when a galaxy's supply of fresh gas is cut off, causing it to slowly consume its remaining gas and fade. These mechanisms explain why galaxies in dense clusters are predominantly ellipticals and lenticulars (the morphology-density relation), while isolated galaxies are more often spirals.

Galaxy mergers are transformative events. When two galaxies of comparable mass merge (a major merger), the result is typically an elliptical galaxy. The violent relaxation of stellar orbits during the merger destroys the ordered rotation of the original disks. Minor mergers (where one galaxy is much smaller) can add material without completely disrupting the larger galaxy, building up the thick disk and stellar halo. The Milky Way has undergone several minor mergers, as revealed by stellar streams and chemically distinct populations identified by Gaia and spectroscopic surveys.

Star formation and the interstellar medium

Star formation in galaxies occurs in molecular clouds, dense regions of the interstellar medium composed primarily of molecular hydrogen (H $_{2}$ ) with temperatures of 10-20 K. The rate of star formation is linked to the surface density of molecular gas through the Kennicutt-Schmidt law: $Σ_{S F R} \propto Σ_{g a s}^{N}$ , where $N \approx 1.4 - 2.0$ and $Σ_{S F R}$ and $Σ_{g a s}$ are the star formation rate surface density and gas surface density, respectively. This empirical relation holds over many orders of magnitude in gas surface density, from dwarf galaxies to the most luminous starbursts.

Feedback processes regulate star formation. Massive stars produce UV radiation that ionises and heats the surrounding gas, stellar winds that inject energy and momentum into the interstellar medium, and supernovae that create hot bubbles and drive turbulence. On galactic scales, these processes can drive galactic winds that expel gas from the galaxy, limiting the amount of material available for future star formation. Supermassive black hole feedback, through AGN-driven winds and jets, can heat or expel gas on even larger scales, quenching star formation in massive galaxies.

The interstellar medium of galaxies exists in multiple phases. The cold molecular medium (10-20 K, densities above 100 particles per cubic centimetre) hosts star formation. The warm neutral medium (about 8,000 K, densities about 0.5 per cubic centimetre) fills much of the galactic disk. The warm ionised medium (about 8,000 K, ionised by stellar UV) traces recent star formation. The hot ionised medium (above $1 0^{6}$ K, created by supernova shocks) fills much of the volume above and below the disk. These phases exist in a dynamic pressure equilibrium that is constantly disturbed by stellar feedback.

Active galactic nuclei and quasars

Some galaxies harbour active galactic nuclei (AGN), extraordinarily luminous central regions powered by accretion of matter onto supermassive black holes. Quasars are the most luminous AGN, outshining their host galaxies by factors of 100 or more. They were first identified in 1963 by Maarten Schmidt, who recognised that their spectra showed highly redshifted emission lines, indicating they were extremely distant and therefore incredibly luminous.

AGN are classified by their observed properties into several categories, including Seyfert galaxies (lower-luminosity AGN in spiral hosts), radio galaxies (powerful radio emitters with relativistic jets), blazars (AGN whose jets point nearly at Earth), and quasars (the most luminous class). The unified model proposes that these different types are the same underlying object, a supermassive black hole accreting matter from a surrounding disk, viewed from different angles and at different accretion rates.

The black hole at the centre of the Milky Way, Sagittarius A*, is currently quiescent, accreting very little matter. But evidence from the Fermi bubbles, two enormous gamma-ray-emitting structures extending 25,000 light-years above and below the galactic plane, suggests that the Milky Way's black hole was much more active in the past, perhaps as recently as a few million years ago. The Event Horizon Telescope produced the first image of Sagittarius A* in 2022, revealing a bright ring of emission surrounding the dark shadow of the black hole, consistent with the predictions of general relativity.

The $M$ - $σ$ relation, a tight correlation between the mass of a galaxy's central supermassive black hole and the velocity dispersion of its stellar bulge, suggests that black hole growth and galaxy evolution are linked. AGN feedback is thought to be the mechanism: as the black hole accretes and produces outflows, it heats and expels gas, regulating both its own growth and the star formation in its host galaxy. This co-evolution of galaxies and their black holes is a central theme of modern astrophysics.

The Milky Way as a galactic ecosystem

The Milky Way is not a static object but a dynamic, evolving system. Stars form in molecular clouds, live out their lives, and return material to the interstellar medium through stellar winds and supernovae. This cycle of matter and energy, called the galactic ecosystem, has operated for the entire 13 billion year history of the galaxy.

The chemical evolution of the Milky Way reflects this ongoing cycle. Each generation of stars produces heavier elements and returns them to the interstellar medium. As a result, younger stars tend to have higher metallicities (abundances of elements heavier than helium) than older stars. The Sun, at 4.6 billion years old, has a metallicity typical of stars its age. The oldest stars in the galaxy, found in the halo, have metallicities less than 1 percent of the Sun's, reflecting the composition of the gas from which they formed when the universe was still young.

The Milky Way's star formation rate is currently about 1-2 solar masses per year, concentrated in the spiral arms where molecular clouds are densest. This is modest compared to starburst galaxies, which can form stars at rates of 100 solar masses per year or more, but it is enough to sustain the galactic ecosystem and produce occasional supernovae that enrich the interstellar medium and shape its structure.

The Gaia mission has revealed that the Milky Way's disk is not a simple structure. It contains distinct populations: the thin disk (formed over the last 8-10 billion years), the thick disk (an older, more turbulent component), and the metal-poor halo (containing the oldest stars and globular clusters). Gaia has also identified several distinct episodes of accretion, where dwarf galaxies were absorbed into the Milky Way, contributing stars with distinct chemical compositions and orbital properties. The Gaia-Enceladus/Sausage merger, which occurred about 10 billion years ago, was a major event that contributed a significant fraction of the halo's stars and may have thickened the disk.

Galaxy clusters and large-scale structure

Galaxies are not uniformly distributed throughout the universe. They are organised into a hierarchy of structures: groups (a few to a few dozen galaxies), clusters (hundreds to thousands of galaxies), superclusters (groups and clusters connected by filaments), and the cosmic web, the largest known structure in the universe, consisting of filaments, walls, and voids.

The Virgo Cluster, the nearest large galaxy cluster, lies about 54 million light-years away and contains over 1,300 galaxies. Its gravitational influence extends to the Local Group, and the Milky Way is moving toward Virgo at about 300 km/s. On even larger scales, the Laniakea Supercluster, identified in 2014 by a team led by Brent Tully, encompasses the Virgo Cluster, the Local Group, and roughly 100,000 other galaxies across a region 500 million light-years wide, all flowing toward a common gravitational attractor called the Great Attractor.

Galaxy clusters are the most massive gravitationally bound structures in the universe, with total masses of $1 0^{14}$ to $1 0^{15}$ solar masses. Most of this mass is dark matter (about 85 percent), with hot intracluster gas (about 12 percent) and stars in galaxies (about 3 percent) making up the remainder. The hot gas, at temperatures of $1 0^{7}$ to $1 0^{8}$ K, emits X-rays and is observed by X-ray telescopes such as Chandra and XMM-Newton. The Sunyaev-Zeldovich effect, a distortion of the cosmic microwave background caused by scattering of CMB photons off the hot electrons in the intracluster medium, provides an independent way to detect and study clusters, one that is independent of redshift and has been exploited by surveys such as the South Pole Telescope and the Atacama Cosmology Telescope.

The study of large-scale structure connects galaxy observations to cosmology: the distribution of galaxies traces the distribution of dark matter, which in turn reflects the initial conditions of the universe as revealed by the cosmic microwave background. Redshift surveys such as 2dFGRS and SDSS have mapped the three-dimensional distribution of millions of galaxies, revealing the cosmic web in stunning detail and allowing precise measurements of cosmological parameters.

The Andromeda-Milky Way collision

The Andromeda galaxy (M31) is approaching the Milky Way at about 110 km/s. In approximately 4.5 billion years, the two galaxies will begin to merge, a process that will take several billion years to complete. The resulting merged galaxy, sometimes nicknamed Milkomeda, will be a large elliptical galaxy. Despite the dramatic name, individual stars are so far apart that direct stellar collisions will be extremely rare. The main effects will be the disruption of the spiral structures of both galaxies, bursts of star formation triggered by the compression of gas clouds, and the rearrangement of stellar orbits in the merged potential. The Sun may be ejected to a more distant orbit, but it will have evolved into a red giant by then, making the fate of Earth a separate concern.

The merger will be a complex dynamical event. Tidal tails of stars will be drawn out during the first passage. The gas disks of both galaxies will collide, triggering waves of star formation. The central supermassive black holes of both galaxies will spiral toward the common centre, eventually merging with a burst of gravitational waves detectable by future space-based gravitational wave observatories. The Triangulum galaxy (M33), the third-largest member of the Local Group, will likely be captured during or after the merger, adding its stars to the growing elliptical galaxy.

Dwarf galaxies and the missing satellites problem

Dwarf galaxies are the most common type of galaxy in the universe, yet they pose a significant challenge to the standard cosmological model. Cold dark matter simulations predict that the Milky Way should be surrounded by hundreds of small dark matter halos, but only a few dozen dwarf satellite galaxies have been observed. This discrepancy is known as the missing satellites problem.

The leading resolution involves reionisation: when the first stars and galaxies formed, their UV radiation heated the intergalactic medium, preventing gas from cooling and condensing into the smallest dark matter halos. Only the more massive halos, above about $1 0^{9}$ solar masses, retained enough gas to form stars. This solution is supported by the observation that the known dwarf satellites occupy only the more massive predicted subhalos, and by the discovery of ultrafaint dwarf galaxies in SDSS and DES data, which are gradually reducing the gap between prediction and observation.

A related problem is the too-big-to-fail problem: the most massive dark matter subhalos in simulations are denser than the observed dwarf galaxies, meaning they should have formed stars efficiently. The resolution may involve supernova feedback that drives repeated outflows, reducing the central densities of the subhalos, or warmer dark matter that suppresses the formation of the densest subhalos.

Connections Master

Connections to cosmology

Galaxies are the building blocks of the large-scale universe, and their properties and distribution encode information about the cosmological parameters. The galaxy luminosity function (the number of galaxies per unit volume as a function of luminosity) constrains models of galaxy formation. The two-point correlation function of galaxies (how galaxy clustering depends on separation) traces the dark matter power spectrum.

The evolution of the galaxy population with redshift, observed through deep galaxy surveys, tests predictions of structure formation models. Observations show that the star formation rate density of the universe peaked about 10 billion years ago (at redshift $z \approx 2$ ) and has been declining since, a period sometimes called "cosmic noon." This decline is driven by the progressive quenching of star formation in massive galaxies, the depletion of gas reservoirs, and the feedback from AGN and supernovae. Understanding why the universe has become less active is a central question in galaxy evolution.

Galaxies also serve as tracers for measuring the expansion history of the universe. Baryon acoustic oscillations, subtle fluctuations in the galaxy distribution imprinted by sound waves in the early universe, provide a standard ruler that can be used to measure cosmic distances at different redshifts. Galaxy redshift surveys such as SDSS, DESI, and Euclid use this technique to constrain dark energy and test whether the cosmological constant accurately describes the observed acceleration.

Connections to mathematics and simulation

The study of galaxy dynamics and evolution relies heavily on mathematical modelling and numerical simulation. N-body simulations, which follow the gravitational interactions of millions or billions of particles representing dark matter, gas, and stars, are the primary tool for understanding how galaxies form and evolve in the cosmological context. Leading simulation projects such as IllustrisTNG, EAGLE, and FIRE have produced realistic galaxy populations that can be compared to observations.

Simulating galaxy formation is computationally demanding because of the enormous range of scales involved. A galaxy spans tens of kiloparsecs, but individual star formation events occur on scales of less than a parsec. Bridging this gap requires subgrid models, approximations of processes like star formation, feedback, and radiative transfer that cannot be resolved directly. The development of more accurate subgrid models and the increasing power of supercomputers are gradually improving the realism of these simulations.

Analytical models also play an important role. The Press-Schechter formalism predicts the abundance of dark matter halos as a function of mass and redshift. Semi-analytic models use physically motivated prescriptions for gas cooling, star formation, feedback, and mergers to predict galaxy properties across cosmic time, providing a faster alternative to full hydrodynamic simulations.

Connections to particle physics

Dark matter, the dominant mass component of galaxies, is a particle physics mystery. Candidates include weakly interacting massive particles (WIMPs), axions, and sterile neutrinos. Direct detection experiments (searching for dark matter particles scattering off atomic nuclei in underground detectors), indirect detection experiments (searching for dark matter annihilation products in cosmic rays and gamma rays), and collider experiments (attempting to produce dark matter particles in high-energy collisions) are all pursuing the nature of dark matter. The answer will connect galaxy astronomy to fundamental physics at the deepest level.

The excess of gamma rays from the Galactic Centre observed by the Fermi Gamma-ray Space Telescope has been proposed as a possible signature of dark matter annihilation, though this interpretation remains controversial because millisecond pulsars could produce the same signal. The absence of dark matter detections in the most sensitive experiments to date has constrained the simplest WIMP models, pushing theorists to consider lighter dark matter candidates, more complex dark sectors, or alternative particle physics frameworks.

Connections to the history and philosophy of science

The discovery that the Milky Way is just one galaxy among billions, made by Hubble in the 1920s, was another step in the Copernican displacement of humanity from the centre of the cosmos. Earlier, the Shapley-Curtis debate of 1920 had grappled with whether spiral nebulae were within the Milky Way or separate galaxies. Shapley argued they were within it; Curtis argued they were separate objects. Hubble's resolution of the debate with Cepheid distances to Andromeda showed that Curtis was correct (though Shapley's estimate of the Milky Way's size was also more accurate than Curtis's). This episode illustrates how scientific debates are resolved not by rhetoric but by new observations and measurements.

The dark matter problem has philosophical dimensions as well. The fact that 85 percent of the matter in the universe is invisible and of unknown nature is a profound challenge to our understanding of the physical world. Some physicists argue that the discrepancy between observed dynamics and visible mass might be resolved by modifying the laws of gravity rather than invoking unseen matter. MOND (Modified Newtonian Dynamics), proposed by Mordehai Milgrom in 1983, modifies Newton's second law at low accelerations and can reproduce flat rotation curves without dark matter. While MOND has had some success on galaxy scales, it struggles with galaxy clusters and the cosmic microwave background, where dark matter provides a more natural explanation. The competition between the dark matter paradigm and modified gravity theories is a modern example of a fundamental scientific debate, where the resolution will likely come from increasingly precise observations.

Connections to technology and data science

Modern galaxy surveys produce enormous datasets that require sophisticated computational methods to analyse. The Sloan Digital Sky Survey (SDSS) has catalogued hundreds of millions of galaxies, and upcoming surveys like the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will catalogue billions. Machine learning techniques, including convolutional neural networks for galaxy classification and anomaly detection, are becoming essential tools for extracting information from these datasets. Galaxy astronomy is at the forefront of the intersection between physical science and data science.

Citizen science projects have also played a role. Galaxy Zoo, launched in 2007, invited volunteers to classify galaxies from SDSS images. Over 100,000 volunteers participated, producing classifications that led to the discovery of new galaxy types (such as green pea galaxies) and large, homogeneous catalogues that would have been impossible to produce with professional astronomers alone. The success of Galaxy Zoo demonstrated the potential of crowdsourced science and inspired similar projects across many disciplines.

Connections to chemistry and the origin of elements

The chemical composition of galaxies reflects their star formation history. Each generation of stars synthesises heavier elements and returns them to the interstellar medium. The metallicity of a galaxy (its abundance of elements heavier than helium) therefore increases with time and with the total amount of star formation. The mass-metallicity relation, which correlates galaxy mass with metallicity, arises because more massive galaxies form more stars and retain their processed material more effectively against outflows driven by supernovae and AGN.

The alpha-enhancement of elliptical galaxies (higher ratios of alpha-process elements like oxygen and magnesium to iron-peak elements) indicates that they formed most of their stars rapidly, before Type Ia supernovae had time to contribute significant iron. Spiral galaxies like the Milky Way show roughly solar alpha-to-iron ratios, indicating a more extended star formation history. These chemical fingerprints connect galaxy evolution to the nuclear physics of stellar interiors and supernovae.

Historical and philosophical context Master

The Great Debate of 1920

On April 26, 1920, Harlow Shapley and Heber Curtis held a debate at the National Academy of Sciences in Washington, D.C., on the scale of the universe and the nature of spiral nebulae. Shapley, who had used globular clusters to determine that the Milky Way was much larger than previously thought (about 300,000 light-years, an overestimate), argued that spiral nebulae were nearby gas clouds within the Milky Way. Curtis, who had studied spiral nebulae and noted that they were concentrated away from the plane of the Milky Way (suggesting they were separate objects), argued they were distant galaxies comparable to the Milky Way.

The debate was inconclusive at the time, because both sides had pieces of the truth mixed with errors. Shapley was right about the large size of the Milky Way but wrong about the spiral nebulae. Curtis was right about the spiral nebulae being separate galaxies but underestimated the Milky Way's size. The issue was resolved in 1924-1925 when Hubble used the 100-inch Hooker Telescope at Mount Wilson to identify Cepheid variable stars in the Andromeda nebula, establishing its distance as far beyond the Milky Way's boundary and proving it was a separate galaxy.

Edwin Hubble and the classification of galaxies

Hubble's 1926 paper "Extra-galactic Nebulae" established the first systematic classification of galaxies and presented the tuning-fork diagram. Hubble initially interpreted the diagram as an evolutionary sequence, with elliptical galaxies evolving into spirals, a view that is now known to be incorrect. Galaxies do not evolve from one Hubble type to another in a simple sequence; rather, their morphology is determined by their formation history, merger history, and environment.

Hubble's classification system, despite its limitations, remains in use because it captures real physical differences between galaxy types. Elliptical galaxies tend to be found in dense environments and have old stellar populations, while spirals are more isolated and have ongoing star formation. This morphology-density relation reflects the role of environment in galaxy evolution.

Vera Rubin and dark matter

Vera Rubin (1928-2016) began her career studying galaxy clustering but is best known for her work on galaxy rotation curves, begun in the late 1960s in collaboration with Kent Ford. Using a sensitive spectrograph Ford had built, Rubin measured the rotation speeds of stars and gas in spiral galaxies out to large radii. In every galaxy she studied, the rotation curve remained flat or rising in the outer regions, rather than declining as expected.

Rubin's work was not the first evidence for dark matter (Zwicky's cluster observations preceded it by decades), but it provided the most direct and compelling evidence that dark matter was not a peculiarity of galaxy clusters but a universal feature of galaxies. Her observations were initially met with scepticism but were quickly confirmed by other groups. By the late 1970s, the reality of dark matter in galaxies was widely accepted. Rubin was elected to the National Academy of Sciences in 1981 and received the National Medal of Science in 1993, though many considered her worthy of the Nobel Prize, which she did not receive.

The discovery of the Milky Way's structure

Understanding the structure of our own galaxy is uniquely challenging because we are embedded within it. William Herschel (1738-1822) made the first attempt to map the Milky Way by counting stars in different directions, concluding it was a flattened disk with the Sun near the centre. Harlow Shapley (1918) used the distribution of globular clusters to show that the Sun was far from the centre, located in the galactic suburbs. The discovery of the central bar came much later, from infrared observations in the 1990s and 2000s that could penetrate the dust obscuring the inner galaxy.

The Gaia mission, launched by the European Space Agency in 2013, has revolutionised our knowledge of the Milky Way by measuring precise positions, distances, and motions for over 1.8 billion stars. Gaia data has revealed the spiral arm structure in unprecedented detail, identified stellar streams from past mergers, measured the galaxy's rotation curve with new precision, and discovered that the Milky Way's disk is warped, bending up on one side and down on the other. These discoveries continue to reshape our understanding of our galactic home.

The Hubble Deep Field and galaxy evolution

The Hubble Deep Field (1995), the Hubble Ultra Deep Field (2004), and the Hubble eXtreme Deep Field (2012) provided the deepest images of the universe ever obtained, revealing galaxies as they existed when the universe was only a few hundred million years old. These images showed that early galaxies were smaller, more irregular, and more vigorously forming stars than present-day galaxies, supporting the hierarchical model of galaxy assembly.

The James Webb Space Telescope (JWST), launched in 2021, has pushed these observations to even earlier cosmic times, detecting galaxies at redshifts $z > 10$ , when the universe was less than 400 million years old. Some of these early galaxies appear more massive and more evolved than expected, challenging current models of galaxy formation and suggesting that the early universe may have been more efficient at forming stars than previously thought. Resolving this tension is one of the active frontiers of extragalactic astronomy.

The cosmic distance ladder

Measuring distances to galaxies requires a hierarchy of methods, each calibrating the next, called the cosmic distance ladder. The first rung is parallax, measuring the apparent shift of nearby stars against the background as Earth orbits the Sun (the method used by Gaia). The second rung uses standard candles like Cepheid variables, whose true luminosity can be determined from their pulsation period. The third rung uses Type Ia supernovae, which are bright enough to be seen in distant galaxies. The fourth rung uses redshift and Hubble's law to estimate distances to the most remote galaxies.

Each rung of the ladder introduces uncertainties, and the calibration between rungs is a major source of systematic error in cosmological measurements. The Hubble constant tension, the discrepancy between the value of $H_{0}$ measured from nearby supernovae (about 73 km/s/Mpc) and that inferred from the cosmic microwave background (about 67 km/s/Mpc), may indicate new physics or may reflect unrecognised systematic errors in one or more rungs of the distance ladder. Resolving this tension is one of the most pressing problems in cosmology.

Bibliography Master

Primary sources

Hubble, E. (1926). "Extra-galactic Nebulae." Astrophysical Journal, 64, 321-369. The foundational galaxy classification paper.
Rubin, V.C. and Ford, W.K. (1970). "Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions." Astrophysical Journal, 159, 379-403.
Rubin, V.C., Ford, W.K., and Thonnard, N. (1978). "Extended Rotation Curves of High-Luminosity Spiral Galaxies." Astrophysical Journal, 225, L107-L111.

Secondary sources

Binney, J. and Tremaine, S. (2008). Galactic Dynamics (2nd ed.). Princeton University Press. The standard graduate-level reference on galaxy dynamics.
Sparke, L.S. and Gallagher, J.S. (2007). Galaxies in the Universe (2nd ed.). Cambridge University Press.
Binney, J. and Merrifield, M. (1998). Galactic Astronomy. Princeton University Press.
Freeman, K. and McNamara, G. (2006). In Search of Dark Matter. Springer.
Bertin, G. (2000). Dynamics of Galaxies. Cambridge University Press.
Conselice, C.J. (2014). "The Evolution of Galaxy Structure." Annual Review of Astronomy and Astrophysics, 52, 1-58.
Navarro, J.F., Frenk, C.S., and White, S.D.M. (1997). "A Universal Density Profile from Hierarchical Clustering." Astrophysical Journal, 490, 493-508.
Kennicutt, R.C. (1998). "The Global Schmidt Law in Star-Forming Galaxies." Astrophysical Journal, 498, 541-552.
Tully, R.B. et al. (2014). "The Laniakea Supercluster of Galaxies." Nature, 513, 71-73.
Clowe, D. et al. (2006). "A Direct Empirical Proof of the Existence of Dark Matter." Astrophysical Journal, 648, L109-L113.
Gaia Collaboration (2018). "Gaia Data Release 2." Astronomy and Astrophysics, 616, A1.
The Event Horizon Telescope Collaboration (2022). "First Sagittarius A* Results." Astrophysical Journal Letters, 930, L12.
Silk, J. and Mamon, G.A. (2012). "The Current Status of Galaxy Formation." Research in Astronomy and Astrophysics, 12, 917-946.
Kormendy, J. and Ho, L.C. (2013). "Coevolution of Supermassive Black Holes and Host Galaxies." Annual Review of Astronomy and Astrophysics, 51, 511-653.