Elliptical Galaxies: Formation and Features
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 03 / Article 06
Elliptical galaxies
Smooth starlight can conceal a complicated past: early bursts of star formation, later mergers, changing stellar orbits, and gas that keeps cycling through the galaxy.
What lies behind the smooth glow?
An elliptical galaxy can look almost simple beside a spiral: a bright center fading into a rounded envelope, with no sweeping arms to guide the eye. Its stars are still moving through a complex gravitational system.
The previous article followed patterns in disks. Ellipticals invite a different kind of investigation, using stellar motions, chemical signatures, faint outer structures, and gas to reconstruct how they grew.
There is no single route shared by every elliptical. Their present appearance combines the history of their stars with the later assembly of the galaxy around them.
What makes a galaxy elliptical?
An elliptical galaxy has a broadly smooth, centrally concentrated stellar appearance, usually without prominent spiral arms or an obvious large disk. That description allows considerable variation in luminosity, internal motion, and hidden structure.[1]
The familiar E0–E7 labels describe flattening on the sky. They compare the image’s short axis, b, with its long axis, a, using approximately E = 10(1 − b/a). Three examples show what those labels mean.
E0 · round in projection
The image has equal long and short axes. This does not establish that the galaxy is a sphere.
E3 · moderately flattened
The short axis is about 70% of the long axis in the projected view.
E7 · strongly flattened
The projected short axis is about 30% of the long axis. The outline alone does not determine the intrinsic shape.
The contours and axes illustrate apparent flattening at equal displayed major-axis size. These are not three-dimensional models or evolutionary stages.
Viewing angle matters: a flattened system seen nearly face-on can look round. Recovering its intrinsic shape requires assumptions and, where possible, information from stellar motions.[2]
Deeper images can reveal shells, faint tails, dust, or embedded stellar components. “Elliptical” summarizes the dominant appearance; it does not mean every part of the galaxy is featureless.[1]
How do stellar orbits support an elliptical?
The stars in a galaxy generally pass through the system without physically colliding. Their collective gravity determines their orbits. In an elliptical, those orbits can cover a wide range of directions and shapes.
Velocity dispersion measures the spread in stellar velocities. When this spread supplies much of the dynamical support, astronomers call the system dispersion-supported or pressure-supported. The term describes orbital motion, rather than stars pushing against one another like particles in a dense gas.[1]
Fast rotators
These systems show substantial ordered stellar rotation relative to their velocity dispersion. Some galaxies that look elliptical belong to this category.
Slow rotators
These have less ordered rotation by the classification criteria, which also account for apparent flattening. Slow rotation does not mean the individual stars move slowly or that all net spin is absent.
Integral-field spectroscopy measures a spectrum across many positions in a galaxy, building a map of velocity and dispersion. Such maps reveal distinctions that a smooth optical image can hide.[3]
What did the ATLAS3D survey find?
Its nearby sample contained 260 early-type galaxies, a category including both ellipticals and lenticulars. About 86% of that combined sample were fast rotators.
Among the galaxies classified morphologically as ellipticals, about 66% were fast rotators. These are sample-specific results, but they clearly show why “elliptical” and “non-rotating” should not be treated as synonyms.[3]
What happens during violent relaxation?
When a merger rapidly changes the collective gravitational field, it can redistribute stellar orbital energies. This process is called violent relaxation. It can help create a smoother stellar distribution without repeated physical star–star collisions.
The relevant timescale is related to how long stars take to cross the system, so it varies with size and mass. Relaxation can also be incomplete, leaving substructure and memory of the encounter.[4]
How do elliptical galaxies form and grow?
Mergers are an important route to building spheroidal systems. A major encounter can rearrange stellar orbits, move gas inward, and substantially change a disk’s structure. Its outcome depends on gas supply, mass ratio, orbital geometry, and later accretion.
A gas-rich merger can form new stars while leaving some disk structure intact or permitting a disk to grow again. A gas-poor merger mainly combines and redistributes existing stars. A major merger therefore does not guarantee one particular elliptical remnant.[6]
When did the stars form?
Many may have been born early, in the main progenitor or in separate galaxies that later joined it.
When did the galaxy assemble?
Later mergers can bring those old populations together and extend the galaxy’s outer envelope.
When did star formation decline?
The supply and cooling of gas set another part of the history, which need not coincide with final assembly.
Simulations often describe a combination of in-situ growth—stars forming within the main progenitor—and ex-situ growth—stars forming elsewhere and arriving through accretion. Their relative contributions depend on mass and history.[5]
The growth of the largest central galaxies
Models of brightest cluster galaxies show how stars formed early in many progenitors can become part of one much larger galaxy through later mergers. Minor accretion can add material to the outskirts while the central population remains old.
This separates the age of the stars from the age of the final assembly. It also explains why an old spectrum need not imply that the galaxy has remained unchanged since those stars were born.[7]
Why do many ellipticals look old and red?
When a galaxy forms few new stars, its short-lived, hot, blue stars disappear without being replaced in large numbers. Cooler and longer-lived stars then contribute more strongly to its integrated light.
Many massive nearby ellipticals contain predominantly old populations. Their spectra also indicate chemically enriched stars, but age and metallicity vary with galaxy mass, position within the galaxy, and individual history.[8]
Metallicity is the abundance of elements heavier than helium. A star’s composition mainly records the gas from which it formed. A stellar population does not become metal-rich simply because it gets older.
Earlier stellar generations enrich surrounding gas through mass loss and explosions. Later stars can then form from that enriched material. Spectra let astronomers investigate this sequence, although mixtures of ages, abundances, and dust complicate the interpretation.[10]
Color alone cannot supply a complete history. Astronomers compare absorption lines, broad wavelength coverage, and population models to estimate ages and chemical properties, including whether a small younger component is hidden within an older population.[10]
What happens to the gas after star formation declines?
Quenching means a sustained reduction in star formation. The practical requirement is that less gas reaches the cold, dense state from which stars form. Gas may be heated, displaced, consumed, or prevented from cooling efficiently.
Many massive ellipticals have extended X-ray-emitting hot atmospheres. Some also contain cooler gas, dust, or renewed star formation. Aging stars continuously return material to their surroundings, so the gas budget remains active even when stellar births are uncommon.[11]
Gas can change phase
Hot gas radiates energy. Under suitable conditions, part of the atmosphere can cool into denser material and feed central activity. Whether it does so depends on its thermodynamic state and surroundings.[11]
Jets can oppose cooling
Radio jets from an accreting central black hole can inflate cavities and transfer mechanical energy into the atmosphere. This helps explain why some hot halos cool into stars much less rapidly than an unheated model would predict.[22]
The balance can change with time. Cooling, accretion, and feedback interact, and fresh material can arrive through later encounters. An active nucleus is therefore part of a continuing gas cycle, rather than proof that every elliptical was permanently emptied by one final event.[11], [22]
Shape and star formation must be measured separately. A galaxy can become quiescent while retaining considerable rotation, and a spheroidal system can still contain a small supply of cold gas.
What can light profiles and faint structures reveal?
A galaxy’s surface-brightness profile describes how its projected light changes with distance from the center. The effective radius, Re, encloses half of the total projected light in the adopted profile.
The flexible Sérsic profile often describes the broad distribution. Its index n controls the profile’s shape. The familiar de Vaucouleurs form corresponds to n = 4, but ellipticals do not all have that value.[12]
The Sérsic profile in its usual normalization
I(R) = Ie exp{−bn[(R/Re)1/n − 1]}
I(R) is the surface brightness at projected radius R, and Ie is its value at Re. The constant bn is chosen so Re encloses half the profile’s total light.
This is a description of a light distribution. A good fit does not uniquely identify the events that built it.[12]
Isophotes are contours of equal surface brightness. Small departures from perfect ellipses can give them a slightly boxy or pointed, “disky” appearance. These details add clues about structure, but are imperfect guides to rotation or a unique merger history.[1], [3]
Shells and tidal debris
Accreted stars can remain organized into faint structures long after the bright center looks smooth. Deep images help recover these traces of earlier encounters.[1], [5]
A depleted stellar core
Some massive ellipticals have less central light than expected from an inward extension of the outer profile. This is a relative deficit, not an empty hole.
Sinking supermassive black holes can transfer orbital energy to central stars. After a black-hole binary forms, gravitational slingshot interactions can redistribute additional stars. Models of this process help explain depleted cores; ordinary physical star–star collisions are not the general explanation.[13]
What do the scaling relations mean?
The Faber–Jackson relation connects luminosity with stellar velocity dispersion. The Fundamental Plane relates effective radius, mean surface brightness within that radius, and velocity dispersion.
These empirical patterns constrain a galaxy’s mass distribution, stellar populations, and dynamical structure. Their scatter and changing slopes carry information too. They do not establish that every elliptical followed one identical merger history.[14]
How do ellipticals relate to clusters, dwarfs, and S0 galaxies?
Massive ellipticals are prominent in dense environments, including the centers of groups and clusters. Some brightest cluster galaxies have extended stellar envelopes that blend gradually into diffuse intracluster light.
A rich cluster does not make every encounter favorable for merging. Its high relative velocities often produce flybys. Earlier assembly in slower-moving groups, together with subsequent accretion onto central galaxies, helps connect the environments to the observed populations.[7], [15]
| Category | What the label describes | What should not be assumed |
|---|---|---|
| Elliptical | A broadly smooth, centrally concentrated stellar system without prominent spiral structure. | That it has no rotation, gas, embedded structure, or history beyond one major merger. |
| Lenticular, or S0 | A galaxy with a stellar disk but no prominent spiral arms. | That it must have formed by only one mechanism or have exactly zero star formation.[17] |
| Dwarf elliptical or dwarf spheroidal | Labels used for populations of low-luminosity, generally gas-poor stellar systems, with differing observational conventions. | That the labels are fully interchangeable or describe scaled-down copies of giant merger remnants.[16] |
Dwarf early-type galaxies can retain rotation, embedded disks, or varied stellar populations. Gas stripping and tidal effects may contribute to their evolution, with different processes important in different systems.[16]
S0 galaxies also show evidence of several pathways, including changes to the gas supply and mergers. The broader term early-type galaxy groups E and S0 morphologies together; “early” is a historical classification label, not a direct measurement of age.[17], [1]
What are observations revealing now?
M87
M87 combines a smooth, extended stellar body with a rich globular-cluster population and an active central black hole. Its jet makes energetic activity visible within a galaxy whose main stellar population looks comparatively quiet.
The example brings stellar structure and nuclear activity into the same picture.[18]
NGC 3610
This elliptical contains a conspicuous stellar disk. The remaining internal structure has been interpreted as evidence of relatively recent assembly.
It is a useful reminder that an elliptical classification can coexist with recognizable disk material.[19]
Were the first quiescent galaxies already like today’s giant ellipticals?
A high-redshift galaxy can be compact and form few new stars while still retaining ordered stellar rotation. Imaging, star formation measurements, and kinematics answer different questions about its state.
Quiescent galaxies with strong rotation
Of 15 quiescent galaxies studied at redshifts around 1.2–2.3, ten had orientations that allowed useful constraints on rotation. All ten were classified as fast rotators. The result shows that quenching need not erase organized stellar motion.[20]
A slow rotator before two billion years
JWST observations of XMM-VID1-2075 at redshift 3.449 found low stellar spin consistent with dispersion-dominated kinematics. Slow-rotating massive systems were therefore also developing when the universe was less than two billion years old.[21]
Neither observation makes all early galaxies alike. Together, they show why researchers need to measure motion as well as light when tracing the precursors of nearby ellipticals.
Deeper imaging reveals faint outskirts; spatially resolved spectra uncover orbital and chemical structure; X-ray and radio observations follow the gas and feedback. Combining these views is how a smooth-looking galaxy becomes a detailed record of formation and growth.[1], [11]
Old starlight. A history still being uncovered.
An elliptical can preserve early stellar populations while continuing to gain stars, exchange gas, and respond to its surroundings. Its shape, motion, and chemistry each carry a different part of that history.
Next, we explore irregular galaxies, where uneven structure and localized star formation offer another view of how galaxies evolve.
Sources and further reading
Research papers, author reviews, and observatory descriptions supporting the definitions, formation pathways, and examples. The shape illustrations depict projected axis ratios, not intrinsic three-dimensional forms.
- Cappellari (2016) — Structure and Kinematics of Early-Type Galaxies from Integral-Field SpectroscopyOrbital diversity and the relationship between apparent morphology and internal motion.
- Weijmans et al. (2014) — The ATLAS3D project – XXIV. The intrinsic shape distribution of early-type galaxiesProjected flattening, intrinsic shape, and the limits of interpreting an image alone.
- Emsellem et al. (2011) — The ATLAS3D project – III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of Fast and Slow RotatorsFast and slow rotation in a sample containing both elliptical and lenticular galaxies.
- Lynden-Bell (1967) — Statistical Mechanics of Violent Relaxation in Stellar SystemsRedistribution of stellar orbital energies by a rapidly changing collective gravitational field.
- Oser et al. (2010) — The Two Phases of Galaxy FormationA simulation framework distinguishing stars formed in the main progenitor from stars accreted later.
- Hopkins et al. (2009) — How Do Disks Survive Mergers?Why gas content and encounter properties affect disk survival and regrowth.
- De Lucia & Blaizot (2007) — The hierarchical formation of the brightest cluster galaxiesA modeled distinction between early stellar formation and later assembly of massive central galaxies.
- Thomas et al. (2005) — The epochs of early-type galaxy formation as a function of environmentObserved trends in stellar age, metallicity, and abundance ratios.
- Yan, Jerabkova & Kroupa (2019) — The star formation timescale of elliptical galaxies – Fitting [Mg/Fe] and total metallicity simultaneouslyWhy element ratios constrain star formation history without supplying a unique clock.
- Conroy (2013) — Modeling the Panchromatic Spectral Energy Distributions of GalaxiesExtracting stellar ages, chemical information, and star formation histories from light.
- Werner et al. (2019) — Hot Atmospheres, Cold Gas, AGN Feedback and the Evolution of Early Type Galaxies: a Topical PerspectiveHot atmospheres, cooler gas, stellar mass loss, and the regulation of cooling.
- Graham & Driver (2005) — A concise reference to (projected) Sérsic R^(1/n) quantities, including Concentration, Profile Slopes, Petrosian indices, and Kron MagnitudesThe Sérsic profile, effective radius, and definitions used to describe galaxy light.
- Frigo et al. (2021) — The two phases of core formation – orbital evolution in the centres of ellipticals with supermassive black hole binariesHow sinking black holes and binary interactions redistribute central stars.
- Cappellari et al. (2013) — The ATLAS3D project – XV. Benchmark for early-type galaxies scaling relations from 260 dynamical models: mass-to-light ratio, dark matter, Fundamental Plane and Mass PlaneScaling relations as constraints on mass, structure, and stellar populations.
- Makino & Hut (1997) — Merger Rate of Equal-Mass Spherical GalaxiesThe importance of encounter velocity for gravitational capture and merging.
- Lisker (2009) — Early-type dwarf galaxies in clusters: a mixed bag with various origins?The diversity of dwarf early-type galaxies and their possible formation channels.
- Deeley et al. (2020) — The SAMI Galaxy Survey: A Range in S0 Properties Indicating Multiple Formation PathwaysLenticular galaxies retain disks and need not share a single formation history.
- NASA / Hubble — Messier 87A giant elliptical in Virgo with an active nucleus and a rich globular-cluster population.
- NASA / Hubble — A young ellipticalThe conspicuous stellar disk within the elliptical galaxy NGC 3610.
- Slob et al. (2025) — Fast Rotators at Cosmic Noon: Stellar Kinematics for 15 Quiescent Galaxies from JWST-SUSPENSERotation in the ten galaxies whose orientations allowed useful kinematic constraints.
- Forrest et al. (2026) — A massive and evolved slow-rotating galaxy in the early UniverseJWST stellar kinematics of XMM-VID1-2075 at redshift 3.449; the revised paper was accepted in Nature Astronomy.
- McNamara & Nulsen (2012) — Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and ClustersJets, cavities, and the transfer of mechanical energy into hot atmospheres.
All articles in this chapter
- Dark Matter Halos: Galactic Foundations
- Hubble’s Galaxy Classification: Spiral, Elliptical, Irregular
- Collisions and Mergers: Drivers of Galactic Growth
- Galaxy Clusters and Superclusters
- Spiral Arms and Barred Galaxies
- Elliptical Galaxies: Formation and Features — you are here
- Irregular Galaxies: Chaos and Starbursts
- Evolutionary Paths: Secular vs. Merger-Driven
- Active Galactic Nuclei and Quasars
- Galactic Futures: Milkomeda and Beyond