From the smallest dwarf galaxies to the enormous superclusters dominating the cosmic web, galaxies are among the most impressive and long-lived structures in the Universe. Yet what we see — the shining light of billions of stars — tells only part of the story: behind that light lie massive dark matter halos, complex gas flow systems, and black holes with masses exceeding billions of Suns. All these components work together, determining how galaxies form, grow, and change over billions of years.
In the third major topic — Galaxy formation and evolution — we will focus on how galaxies form and interact, shaping much of the visible Universe's structure. We will examine the balance between dark and baryonic matter, the intriguing diversity of galaxy types (spiral, elliptical, irregular), and the powerful internal and external forces governing galaxy life cycles — from passive phases to intense starburst periods. Here is a brief overview of each key topic we will cover in upcoming articles.
Dark matter halos: the foundation of galaxies
Galaxies form and evolve within dark matter halos — gigantic, invisible "frames" that make up the majority of the mass. These halos provide the gravitational "glue" that holds stars and gas together, influencing the galaxy's shape, rotation curve, and long-term stability. We will discuss why these halos are important, how they emerge from initial density fluctuations, and how they channel gas into galaxy centers, promoting star formation and affecting galactic dynamics. Understanding dark matter halos is essential to explain the motion of stars in galaxies (rotation curves) and why galaxies contain more mass than we can directly observe.
Hubble galaxy classification: spirals, ellipticals, irregulars
One of the most famous and longest-used galaxy classification systems is Hubble’s "tuning fork." It divides galaxies into spirals, ellipticals, and irregulars, each type having distinct structures and star formation properties:
- Spiral galaxies often have clearly visible disks, dust lanes, and star-forming regions in spiral arms.
- Elliptical galaxies feature older stellar populations, almost no gas, and have a more spheroidal shape.
- Irregular galaxies lack a clear shape, characterized by chaotic star-forming regions and disturbed gas flows.
We will discuss how the concept of Hubble’s classification evolved with improved observations and how different morphologies are influenced by galaxy history, environment, and evolution.
Collisions and mergers: the engine of galaxy growth
Galaxies are not static "islands" in space – they often collide and merge, especially in denser environments. These interactions can dramatically alter galaxy properties:
- Starburst events – when gas in merging galaxies collides and triggers intense star formation.
- Central black holes can suddenly attract more material and transform a passive galaxy nucleus into a bright quasar or active galactic nucleus (AGN).
- Morphological changes, e.g., the merger of two spirals resulting in the formation of an elliptical galaxy, demonstrate how collisions cause major structural changes on both small and large scales.
Mergers are inseparable from hierarchical cosmic growth models and show how galaxies continuously evolve by "devouring" smaller neighbors or merging with partners of similar size.
Galaxy clusters and superclusters
On scales larger than the galaxy itself are clusters containing hundreds or thousands of galaxies bound by common gravity, dominating the cosmic web. In clusters we find:
- Intracluster medium (ICM): Hot gas emitting strong X-rays.
- Dark matter halos: Even more massive than in individual galaxies, connecting the entire cluster.
- Dynamic interactions: Galaxies in clusters experience gas pressure stripping, harassment, and other rapid collisions.
On an even larger scale – superclusters, loosely bound chains of clusters connected by dark matter filaments. These structures reveal the hierarchical evolution of the Universe, linking galaxies in a vast network and influencing star systems over cosmic timescales.
Spiral arm structures and cross-sections in galaxies
Many spirals have ornate, clearly visible arm structures dotted with star-forming regions. Some galaxies show a bar—a stretched stellar feature crossing the center. We will discuss:
- Formation of spiral arms: From density wave models to swing amplification explaining how such structures can persist or change in disk systems, promoting new star formation.
- Bars: How they channel gas toward the galaxy center, feed central black holes, and can even trigger nuclear starburst.
These morphological features emphasize that not only external collisions but also internal dynamics strongly influence the long-term appearance and star formation rate of a galaxy.
Elliptical galaxies: formation and characteristics
Most commonly found in denser regions, such as clusters, elliptical galaxies are massive, mature stellar systems characterized by:
- Little cold gas or active star formation, but dominated by older, redder stars.
- Random distribution of stellar orbits rather than orderly rotating disks.
- Often formed through major mergers that destroy disk structures and funnel gas into the central region.
By studying ellipticals, we can understand the impact of major mergers, the role of feedback in quenching star formation, and the processes that allow the largest galaxies in the Universe to form. Dynamical relaxation and possible supermassive black holes continue shaping these grand, spherical structures.
Irregular galaxies: chaos and "starbursts"
Not all galaxies fit into clear categories. Some are distinctly irregular, featuring disrupted disk traits, displaced stellar clumps, or intense star formation arcs. They are caused by:
- Tidal interactions or partial collisions disrupting the internal structure of the galaxy.
- Low mass and shallow gravitational potential well where outflows or inflows from the cosmic web can distort the shape.
- Sudden star formation "bursts" triggered by gas compression; these can cause superwinds, blowing material out of the galaxy.
These galaxies demonstrate how gravitational interactions, environment, and internal feedback can unexpectedly create chaotic or "starburst" states both locally and in the distant Universe.
Evolutionary paths: secular or merger-driven
Galaxies evolve through different pathways determined by both internal processes (secular evolution) and external impulses:
- Secular evolution: Slowly redistributing mass through bars, spiral density waves, or stellar migration. Over billions of years, these factors can alter disks, form pseudobulges, and affect star formation without major collisions.
- Mergers: Sudden, often "violent" events that can radically change morphology, star formation intensity, and the accretion state of the central black hole.
We will compare these paths, showing how galaxy mass, environment, and dynamical history determine whether it remains a calm disk, transforms into a massive elliptical, or acquires hybrid features.
Active galactic nuclei and quasars
At the centers of some galaxies lie exceptionally bright nuclei (AGN or quasars), powered by supermassive black holes that can outshine the entire galaxy. These sources flare up when:
- A large gas inflow feeds the central black hole, causing intense radiation.
- Radiation and winds from AGN can suppress or regulate further star formation in the galaxy.
- Mergers or interactions trigger gas inflows, igniting quasar phases.
Thus, AGN demonstrate a crucial feedback loop — rapid black hole growth can alter a galaxy's fate by quenching star formation or driving powerful outflows that affect the local and wider environment.
The future of galaxies: "Milkomeda" and beyond
Cosmic evolution continues: even the Milky Way will eventually merge with Andromeda, forming one larger elliptical or lenticular galaxy, sometimes called "Milkomeda." Beyond local events, with galaxies existing in the expanding Universe, star formation rates decline as gas supplies dwindle. The accelerating effect of dark energy raises questions about how the future of clusters and superclusters will unfold over the coming billions of years:
- Will galaxy clusters remain gravitationally bound?
- How will star formation exhaust when gas is locked in long-lived stellar remnants or expelled into the intergalactic medium?
- Will the large-scale structure simply "freeze," as the Universe expands and systems separate?
These future visions are shaped by our models of dark matter dynamics, stellar evolution, and cosmic expansion, linking them to the overall theme of galaxy formation and evolution.
Final thoughts
Together, these topics reveal a broad picture of galaxy life — from invisible dark matter halos, to which stars and gas attach, to constant collisions and transformations, ultimately meeting us in future scenarios where galaxies merge into giants in the expanding Universe. By examining spirals, ellipticals, and irregular galaxies, studying star formation bursts, explaining AGN mechanisms, and predicting future mergers, we expand understanding of how from initial density fluctuations in the Universe we arrived at the rich and diverse galaxy population we observe.
In the upcoming series of articles, we will delve into each of these topics: reviewing the latest discoveries and theoretical models explaining the cosmic dance performed by galaxy formation and evolution. Along the journey, we will see how dark matter determines galaxy structure, how morphological types depend on evolutionary paths, and how both internal and external processes — from secular dynamics to intense mergers — shape the diversity of galaxies observed in our Universe.