Elliptical galaxies are smooth, rounded collections of stars that lack the dramatic spiral arms and dusty lanes seen in galaxies like our Milky Way. They range from nearly spherical blobs to elongated, football-shaped systems, and they account for a substantial fraction of the most massive galaxies in the observable universe. Despite their featureless appearance in a telescope, ellipticals turn out to be surprisingly complex once you look beneath the surface, with diverse formation histories, hidden reservoirs of hot gas, and intimate connections to the supermassive black holes at their centers.
How Elliptical Galaxies Look and How They Differ from Spirals
The defining visual trait of an elliptical galaxy is its smooth, diffuse glow. There are no well-defined spiral arms, no obvious lanes of dark dust cutting across the face, and no prominent disk of young blue stars. Instead, the light falls off gradually from a bright center to faint outer edges. Astronomers have long known that the brightness profile of an elliptical follows a specific mathematical curve, sometimes called the Sérsic profile, which captures how steeply the light concentration drops with distance from the center.1Astronomy & Astrophysics. The lensing properties of the Sersic model A spiral galaxy also has a central bulge that follows a similar profile, but the rest of the light comes from a thin disk. In an elliptical, the disk is either absent or so subtle that the galaxy looks three-dimensional rather than flat.
Ellipticals are classified by how elongated they appear on the sky, labeled from E0 (nearly circular) to E7 (highly elongated). This is just a projection effect, though: the true three-dimensional shape of a given elliptical can be hard to pin down, because we only see it from one angle. Some are genuinely close to spherical, while others are intrinsically flattened or triaxial, shaped more like a squashed watermelon than a basketball.
Two Flavors of Elliptical: Fast Rotators and Slow Rotators
One of the more surprising discoveries of the past two decades came from surveys that mapped the internal motions of stars across the faces of elliptical galaxies. It turns out that ellipticals are not all alike inside. They split into two broad kinematic classes: fast rotators, whose stars orbit in a coherent pattern much like a spinning top, and slow rotators, which show little or no organized rotation.2The Astrophysical Journal. No Evidence of a Dichotomy in the Elliptical Galaxy Population
Fast rotators are far more common. They tend to be moderately massive, and their internal properties form a smooth continuum with those of spiral galaxies, suggesting they may simply be spirals that ran out of gas and stopped forming new stars. Slow rotators, by contrast, are typically the most massive ellipticals and are found preferentially in the densest regions of galaxy clusters. Their stellar populations are old, metal-rich, and show chemical signatures indicating that star formation happened quickly and ended early.3Encyclopedia of Astrophysics. Early-Type Galaxies: Elliptical and S0 Galaxies, or Fast and Slow Rotators Slow rotators become common above a stellar mass roughly two hundred billion times that of the Sun, a threshold that separates the galaxy population into two fairly distinct regimes.
Old Stars, Red Colors, and the “Red and Dead” Reputation
Elliptical galaxies are famous for their reddish hue. That color comes from the stars inside them, which are overwhelmingly old. Unlike spiral galaxies, where pockets of gas collapse to form new blue stars on an ongoing basis, most ellipticals finished the bulk of their star formation billions of years ago. Evidence from both nearby ellipticals and those seen at intermediate distances consistently points to the same pattern: the stars in cluster ellipticals formed at very early cosmic times, while ellipticals in lower-density environments are on average one to two billion years younger in their stellar populations. Perhaps most strikingly, the most massive ellipticals tend to host the oldest stars, and their star-forming episodes were shorter than those of less massive ones.4Annual Review of Astronomy and Astrophysics. Stellar Population Diagnostics of Elliptical Galaxy Formation
This pattern, sometimes called “downsizing,” is counterintuitive. You might expect the biggest galaxies to take the longest to assemble, but the evidence says the opposite: the heavyweights formed their stars the fastest and shut down the earliest. Smaller ellipticals often had more extended star-forming histories. The explanation involves both the efficiency of early starbursts in massive systems and the powerful feedback from supermassive black holes, which we will get to shortly.
How Elliptical Galaxies Form
The leading explanation for how most ellipticals came into being centers on galaxy mergers. When two roughly equal-sized galaxies collide and merge, the gravitational chaos of the encounter scrambles the orderly orbits that define spiral arms and disks, leaving behind a puffed-up, pressure-supported system: an elliptical. Numerical simulations have shown that such mergers can reproduce the observed structural properties of ellipticals, including their brightness profiles and the relationships between their size, luminosity, and internal velocities.5Monthly Notices of the Royal Astronomical Society. Dissipationless mergers of elliptical galaxies and the evolution of the fundamental plane
Centaurus A, the nearest giant elliptical to Earth at roughly twelve million light-years away, offers a vivid example. Its prominent dust lane and disturbed halo features have long suggested a violent past, and hydrodynamical simulations indicate that a major merger around two billion years ago can account for its kinematics, its inner gas disk, and the ages and chemical compositions of stars in its outer halo.6Monthly Notices of the Royal Astronomical Society. A recent major merger tale for the closest giant elliptical galaxy Centaurus A Models with a relatively low gas fraction in the merging galaxies produce the smooth brightness profile characteristic of ellipticals.
Not all mergers are equal, though. “Wet” mergers between gas-rich spirals can trigger intense bursts of star formation and funnel gas toward the center, while “dry” mergers between gas-poor galaxies simply rearrange existing stars. The most massive slow-rotating ellipticals are thought to have grown substantially through dry mergers at relatively late cosmic times, piling up stellar mass without forming many new stars.
How Black Holes Keep Ellipticals “Dead”
If you simply merged two gas-rich galaxies and let gravity do its thing, the resulting object would cool its gas, form stars, and potentially rebuild a disk. Something has to prevent that from happening, and the prime suspect is the supermassive black hole at the galaxy’s center. When gas falls toward such a black hole, the black hole’s accretion process can launch powerful jets and winds that heat the surrounding gas, preventing it from condensing into new stars. This is known as AGN feedback.
Simulations have demonstrated that even a modest fraction of the energy released by gas falling onto a black hole is enough to quench star formation in the merger remnant and push the galaxy onto the “red sequence,” the population of passive, red galaxies dominated by old stars.7Monthly Notices of the Royal Astronomical Society. Is AGN feedback necessary to form red elliptical galaxies? The feedback does not just work once; it operates throughout the galaxy’s life. Three-dimensional hydrodynamic simulations show that mechanical outflows from the black hole can solve the so-called “cooling flow problem,” in which hot gas in and around the galaxy should theoretically be cooling and forming stars but observationally does not. The outflows inflate buoyant bubbles, drive shock waves, and stir up turbulence that keeps the gas too hot to collapse.8Monthly Notices of the Royal Astronomical Society. Mechanical AGN feedback: controlling the thermodynamical evolution of elliptical galaxies
Supermassive Black Holes and the Galaxies That Host Them
Elliptical galaxies sit at the center of one of modern astrophysics’ most striking correlations: the mass of a galaxy’s central black hole scales tightly with properties of the galaxy itself, such as the speed at which its stars move (the velocity dispersion). For most ellipticals, you can predict the black hole mass from how fast the stars are buzzing around, and vice versa.9Monthly Notices of the Royal Astronomical Society. Ultramassive black holes and the three M–sigma relations This tight coupling strongly suggests that black holes and their host galaxies grow together, regulating each other through feedback.
At the very highest masses, however, the relationship between black hole mass and stellar velocity dispersion starts to break down. In the biggest ellipticals, which have grown mainly through dry mergers, each merger adds the two progenitor black holes’ masses together but barely changes the overall velocity dispersion of the stellar system. As a result, the black hole mass grows faster than velocity dispersion would predict. For these galaxies, the total luminosity turns out to be a better proxy for black hole mass than the stars’ velocity dispersion.10The Astrophysical Journal Letters. THE L∝σ8 CORRELATION FOR ELLIPTICAL GALAXIES WITH CORES: RELATION WITH BLACK HOLE MASS
The aftermath of black hole mergers also leaves a visible imprint on the galaxy itself. When two supermassive black holes spiral together and eventually coalesce, they eject stars from the galaxy’s center through gravitational interactions, carving out a low-density “core.” This scouring process is now considered the leading explanation for why the most luminous ellipticals have shallow, flat-topped brightness profiles in their central regions rather than the steep cusps seen in less massive systems.11The Astrophysical Journal. Core Formation by Binary Scouring and Gravitational Wave Recoil in Massive Elliptical Galaxies
Dark Matter in Elliptical Galaxies
Like all large galaxies, ellipticals are embedded in halos of dark matter that extend far beyond the visible stars. Measuring the dark matter distribution is trickier in ellipticals than in spirals, because ellipticals lack the neat, rotating gas disks that make it easy to weigh a spiral galaxy from its rotation curve. Instead, astronomers rely on methods like strong gravitational lensing, where the gravity of a foreground elliptical bends and distorts light from a more distant background galaxy.
An analysis of 161 strong-lens systems found that the total mass profile of elliptical galaxies is well described by a power-law density that falls off roughly as the inverse square of the distance from the center. Disentangling the contributions of stars and dark matter is harder, because the answer depends on assumptions about how many low-mass stars the galaxy contains. Combining lensing with independent constraints from quasar microlensing observations helps break this ambiguity, and the best-fitting models point to a dark matter fraction that is somewhat smaller in the central regions than some theoretical models predict.12Monthly Notices of the Royal Astronomical Society. The stellar and dark matter distributions in elliptical galaxies from the ensemble of strong gravitational lenses Independent strong-lensing decompositions reinforce the value of separating stellar from dark matter mass, as both components carry clues about how the galaxy assembled: the stellar mass profile reflects the galaxy’s history of gas accretion and feedback, while the dark matter profile connects to the larger framework of cosmic structure formation.13Monthly Notices of the Royal Astronomical Society. Galaxy structure with strong gravitational lensing: decomposing the internal mass distribution of massive elliptical galaxies
Where Ellipticals Live
Elliptical galaxies are not distributed evenly across the cosmos. They crowd toward the densest environments: the centers of galaxy clusters and the cores of galaxy groups. This “morphology-density relation” has been confirmed across large samples: as local galaxy density increases, the fraction of early-type galaxies (ellipticals and lenticulars) climbs while late-type spirals become rarer.14Monthly Notices of the Royal Astronomical Society. The morphology—density relation in the Sloan Digital Sky Survey The correlation makes physical sense. Dense environments offer more opportunities for galaxy-galaxy mergers, more tidal interactions that strip gas and disrupt disks, and a hot intracluster medium that can ram-pressure strip gas from infalling galaxies, effectively killing their star formation.
That said, ellipticals are not exclusive to clusters. Isolated ellipticals exist, and they tend to have slightly younger stellar populations than their cluster counterparts, consistent with having had a less rushed evolutionary history.
The Giants at the Centers of Clusters
Sitting at or near the bottom of a galaxy cluster’s gravitational well, you often find the most massive galaxy in the entire cluster: a giant elliptical, sometimes classified as a cD galaxy because of an extended, diffuse stellar envelope. The traditional picture for how these monsters form is “galactic cannibalism,” in which the central galaxy swallows smaller galaxies that lose orbital energy through dynamical friction. While that process does occur, simulations suggest that the bulk of the central galaxy’s mass actually assembles through the merger of several massive galaxies along a cosmic filament early in the cluster’s history, rather than through the gradual accretion of many small systems over time.15The Astrophysical Journal. The Origin of the Brightest Cluster Galaxies
Observational examples of this process in its early stages have been elusive, but one study identified a galaxy cluster at a redshift of about 0.59 where two massive galaxies near the center appear to be in the initial phases of a cannibalizing merger.16Monthly Notices of the Royal Astronomical Society. Galactic cannibalism in the galaxy cluster C0337-2522 at z = 0.59 These brightest cluster galaxies can reach stellar masses more than ten times that of the Milky Way, and their extended halos blur the boundary between the galaxy and the diffuse intracluster light.
Not All Ellipticals Are Giants
At the opposite end of the mass scale sit dwarf elliptical galaxies and their even smaller cousins, the dwarf spheroidals. These are low-luminosity systems with old stellar populations and very little gas. Unlike the giant ellipticals that grew through mergers, many dwarf ellipticals are thought to have been small spiral or irregular galaxies that were transformed by their environment. In galaxy clusters, infalling dwarfs can have their gas stripped away by the pressure of the hot intracluster medium, shutting off star formation abruptly. IC 3418 in the Virgo Cluster appears to be caught in precisely this act: its gas is being ripped out, leaving fireballs of star-forming clumps trailing behind it in a clear case of a dwarf irregular turning into a dwarf elliptical.17The Astrophysical Journal. TRANSFORMATION OF A VIRGO CLUSTER DWARF IRREGULAR GALAXY BY RAM PRESSURE STRIPPING: IC3418 AND ITS FIREBALLS
Theoretical modeling supports this picture. Ram-pressure stripping alone can remove a small galaxy’s gas and quench its star formation, and the sudden drop in gas content is enough to reshape the galaxy’s structure from disk-like to something resembling a dwarf elliptical.18Astronomy & Astrophysics. MB and Kormendy relations in dwarf elliptical galaxies When tidal forces from the cluster’s gravitational field are combined with ram pressure, the result can be even more dramatic, producing gas-poor systems that resemble dwarf spheroidals.19Monthly Notices of the Royal Astronomical Society. Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas So while giant and dwarf ellipticals may look superficially similar in their smooth, featureless appearance, they arrived at that state by very different roads.
Compact Ellipticals and the Case of M32
One of the strangest members of the elliptical family is a class of tiny, dense galaxies called compact ellipticals. The prototype is M32, a small satellite of the Andromeda Galaxy. These objects pack a surprisingly large stellar mass into a very small volume, giving them surface brightnesses far higher than typical dwarf ellipticals of similar mass. For decades, M32 was essentially the only well-studied compact elliptical, which made it hard to say whether it represented a common type of galaxy or a one-off curiosity.
A Virtual Observatory search changed that picture by turning up 21 compact elliptical galaxies beyond the Local Group. All of them showed old, metal-rich stellar populations similar to those of much more massive ellipticals, not the younger, metal-poorer populations of ordinary dwarf ellipticals of comparable mass. This chemical fingerprint strongly supports the idea that compact ellipticals are the stripped remnants of once-larger galaxies, not small galaxies that formed in isolation.20PubMed. A population of compact elliptical galaxies detected with the Virtual Observatory Two additional compact ellipticals discovered later show clear tidal streams, the smoking-gun evidence that material is actively being pulled away from them by a larger neighbor.21Monthly Notices of the Royal Astronomical Society. Tidal streams in newly discovered M32 analogues: evidence for the stripping scenario
Even M32 itself, which sits suspiciously close to Andromeda’s disk, shows kinematic signatures in its outer stars consistent with ongoing tidal distortion.22The Astrophysical Journal. Kinematical Modeling of the Resolved Stellar Outskirts of M32: Constraints on Tidal Stripping Scenarios The emerging view is that compact ellipticals represent a formation channel distinct from both giant ellipticals (merger-built) and dwarf ellipticals (environment-quenched small galaxies). They are the dense cores left behind after a larger galaxy was stripped of most of its stars by a massive companion.
Globular Clusters as Fossils of Elliptical Galaxy Formation
Large elliptical galaxies are often surrounded by swarms of thousands of globular clusters, ancient, dense balls of stars. In many ellipticals, the color distribution of these clusters appears bimodal: one group looks red and the other blue. For years, this was interpreted as evidence that ellipticals formed in two distinct episodes, perhaps an early burst that produced the blue clusters and a later merger that added the red ones. However, a reexamination of the problem found that the color bimodality could be an illusion. Because the relationship between a star cluster’s metal content and its observed color is nonlinear, a single population of old clusters spread across a range of metal abundances can produce what looks like two separate color peaks when plotted. Horizontal-branch stars, which are old stars in a particular evolutionary phase, appear to drive much of this nonlinearity.23Science. Explaining the color distributions of globular cluster systems in elliptical galaxies If this interpretation holds up broadly, it would simplify the formation story for ellipticals by removing the need for two distinct cluster-forming episodes.
Gas and Dust in Supposedly “Dead” Galaxies
The “red and dead” label is a useful shorthand, but it oversimplifies. Many elliptical galaxies, particularly those with visible dust lanes, turn out to harbor substantial reservoirs of molecular and atomic gas. A survey of dust-lane ellipticals found molecular hydrogen masses ranging from several hundred million to about twenty billion times the mass of the Sun.24Monthly Notices of the Royal Astronomical Society. Molecular and atomic gas in dust lane early-type galaxies – I. Low star formation efficiencies in minor merger remnants Despite all that fuel, these galaxies form stars at rates far below what you would expect from their gas content. The star formation efficiency is low, possibly because the gas was acquired recently through minor mergers and has not yet settled into conditions favorable for collapse, or because AGN feedback and the hot X-ray-emitting halo keep the gas heated and turbulent.
Speaking of hot gas, massive ellipticals are bathed in diffuse X-ray-emitting halos of gas at temperatures of tens of millions of degrees. These halos are visible in X-ray observations and can extend out to a hundred kiloparsecs or more. In compact elliptical galaxies, X-ray luminosities have been measured at levels indicating substantial hot gas reservoirs, and the pressure balance of this gas offers a way to measure the galaxy’s total mass, including its dark matter.25The Astrophysical Journal. The Luminous X-Ray Halos of Two Compact Elliptical Galaxies The hot halo is not merely a bystander; it is intimately involved in the feedback loop with the central black hole, serving as both the fuel supply and the medium through which the black hole’s energy is deposited.
Red Nuggets and What They Tell Us About Early Galaxy Evolution
Some of the most intriguing ellipticals are the ones that appear to have barely changed since the early universe. At high redshifts, surveys have uncovered a population of extremely compact, massive galaxies sometimes called “red nuggets.” These are small, dense, and already finished forming stars despite existing when the universe was only a few billion years old. Most red nuggets are thought to have eventually puffed up through dry mergers, growing in size more than in mass. But a handful may have survived relatively intact to the present day. Local compact elliptical galaxies that resemble red nuggets in their size, mass, and stellar density are studied as potential descendants, offering a window into galaxy evolution at early cosmic times.26The Astrophysical Journal. The Luminous X-Ray Halos of Two Compact Elliptical Galaxies These relic galaxies allow astronomers to study conditions that would otherwise require looking billions of light-years away, essentially fossils from the early universe sitting in our cosmic backyard.
Spatially resolved studies of low-mass compact ellipticals also support the relic interpretation: they tend to be old and metal-rich in their centers, with both age and metal content declining toward their outskirts, a gradient consistent with having formed their cores early and added little new material since.27Monthly Notices of the Royal Astronomical Society. Low-mass compact elliptical galaxies: spatially resolved stellar populations and kinematics with the Keck Cosmic Web Imager