Galaxy Formation Statistics And Facts (2025)
Updated · Sep 04, 2025
WHAT WE HAVE ON THIS PAGE
- Introduction
- Editor’s Choice
- Origin and Timeline
- How Many Galaxies Are There?
- The Cosmic Star Formation History
- The Galaxy Stellar Mass Function
- Morphology, Types, and Demographics
- Dark Matter Halos and Baryon Accounting
- Mergers and Assembly
- Role of Black Holes and Feedback
- Observational Data Sets and Surveys
- Simulations and Models
- Open Tensions and Statistical Puzzles Right Now
- Statistical Tools and Methods Used in Galaxy Formation Studies
- Conclusion
Introduction
Galaxy Formation Statistics: Galaxy formation simply means how our universe built its grandest structure. After the Big Bang, the cosmos was mostly just gas and dark matter. Over billions of years, gravity pulled this matter together, making the first stars and slowly creating the galaxies we see today.
When we say galaxy formation, we are really talking about numbers. How many galaxies exist, when stars formed the fastest, how dark matter shaped them, and how collisions changed them. Today, we estimate there may be around two trillion galaxies, with star formation peaking about 10 billion years ago.
In this article, I’ll completely walk you through galaxy formation statistics, showing not just how galaxies formed, but how scientists measure and understand it. Without further a due, let’s get started.
Editor’s Choice
- Galaxy formation began nearly 13 billion years ago, only a few hundred million years after the Big Bang.
- The universe may contain around 2 trillion galaxies, according to the latest Hubble and JWST estimates.
- Star formation peaked around 10 billion years ago, with galaxies producing stars at rates up to 100 times faster than today.
- Dark matter halos play a central role in pulling gas together, making galaxy formation possible.
- Collisions and mergers shaped galaxy structure, with large galaxies often built by consuming smaller ones.
- Our own Milky Way Galaxy formed roughly 6 billion years ago and still grows by merging with dwarf galaxies.
- About 70% of galaxies in the universe are spiral or irregular, while 30% are elliptical, showing how formation pathways differ.
- Supermassive black holes at galaxy centers grew alongside their host galaxies, linking galaxy formation with black hole evolution.
- On average, galaxies form stars at rates between 1 to 100 solar masses per year, but starburst galaxies can go much higher.
- By studying Galaxy formation statistics, scientists can trace the universe’s history and predict its future structure.
| Aspect | Insights |
| Age of first galaxies | Around 13 billion years ago |
Total galaxies in the universe | 2 trillion |
| Peak star formation era | 8 to 10 billion years ago |
Average star birth rate | 1 to 100 solar masses per year |
| Spiral vs Elliptical Galaxies | 70% spirals/irregulars, 30% ellipticals |
Milky Way age | 13.6 billion years |
| Role of dark matter | Backbone for galaxy growth |
Impact of mergers | Major driver of galaxy growth |
| Central black holes | Present in nearly every galaxy |
Ongoing growth | Galaxies are still evolving today |
Origin and Timeline
(Source: nature.com)
- The universe began nearly homogeneously, and tiny fluctuations grew under gravity into dark matter halos.
- The first stars (Pop III) likely formed inside small halos at redshifts roughly 20 to 30, a few hundred million years after the Big Bang.
- These first episodes seeded the earliest enrichment and set the stage for galaxy assembly.
- Between redshifts 6 and 15, the universe went through reionizations. The earliest galaxies contributed ionizing photons, but they were rare and faint.
- Observationally, reionization ended by 6 according to quasar absorption spectra and cosmic microwave background constraints.
- The densest buildup of stars across the universe happens around 1.5 to 2. This era, sometimes called “cosmic noon,” is where galaxy growth via cold gas accretion and mergers is most rapid.
- The cosmic star formation rate density (SFRD) peaks here, a core number for galaxy formation.
- After the peak, the global SFRD declines by roughly an order of magnitude to 0, leaving the present universe dominated by fewer actively star-forming systems and many quiescent galaxies.
| Epoch | Redshift | Age (approx) | Key process |
| First stars | 20 to 30 | 100 to 300 Myr | Pop III star formation in minihalos |
| Reionization end | 6 | 1 Gyr | The universe becomes ionized, first galaxies abundant |
| Cosmic noon | 1.5 to 2 | 3 to 4 Gyr | Peak Galaxy Formation: star formation |
| Decline to present | 0 | 13.8 Gyr | Quenching, mergers, and formation of passive galaxies |
How Many Galaxies Are There?
(Source: wikipedia.org)
- Work combining deep Hubble fields with extrapolations for faint, small galaxies estimated there are roughly 2 trillion galaxies in the observable universe, an order of magnitude above older 2000s estimates of a few hundred billion.
- That number comes from integrating observed stellar mass functions and accounting for populations below current detection thresholds.
- That 2 trillion is a model-dependent extrapolation. It depends on the assumed low-mass slope of the galaxy mass function and how many ultra-faint dwarfs exist at high redshift. Deeper surveys and JWST results are refining this number now.
- Deep Hubble images (HUDF, XDF) show thousands of galaxies in a tiny patch, and wide surveys like SDSS provide spectroscopic samples of up to a million galaxies for robust low-redshift statistics.
- Together, deep + wide is how we translate local snapshots into a global galaxy census.
| Quantity | Typical Value |
| Observable galaxy estimate (extrapolated) | 2 x 10 power 12 galaxies |
| Galaxies in the Hubble Ultra Deep Field | 10,000 in a tiny patch |
| Spectra in SDSS (approx) | 0.9 to 1 million galaxy spectra in older releases; DR16+ expands coverage |
The Cosmic Star Formation History
(Source: caltech.edu)
- The cosmic star formation rate density peaks around 1.9, roughly 3.5 Gyr after the Big Bang, and then declines.
- Typical SFRD values: order of 0.1 M at the peak, falling to around 0.01 to 0.02 M in the local universe, depending on calibration and IMF assumptions.
- These are the backbone numbers used to calibrate galaxy formation models.
- Roughly half of the stars that exist today formed before 1.3, and only a small fraction formed during reionization.
- These integrals of the SFRD give the stellar mass history used to check models of galaxy formation.
- New JWST data at very high redshifts are forcing re-evaluation of the earliest SFRD points because unexpectedly massive and luminous early galaxies have been found. They could increase estimates of high SFRD and shift some integrals.
| Redshift | SFRD |
| 0 (today) | 0.01 to 0.02 |
| 1.9 (peak) | 0.1 (order of magnitude) |
| 6 (reionization) | 0.01 |
The Galaxy Stellar Mass Function
(Source: researchgate.net)
- A Schechter function has a characteristic mass M and slope parameters. For the local Universe, the characteristic stellar mass log(M/M) is roughly 10.7 to 10.8, with a double Schechter shape often needed to capture the faint end and the bright exponential cutoff.
- The normalization phi is of order 10 power 3 to 10 power 2 per cubic Mpc, depending on the parametrization.
- The faint end slope measures how many low-mass galaxies exist and is sensitive to feedback processes, while the high-mass exponential cutoff tells you how common massive galaxies are.
- Integrating the SMF gives the stellar mass density in stars at a given epoch.
- The SMF evolves with redshift: massive end grows mostly by mergers and in situ star formation at early times, while the faint end evolves depending on feedback and gas accretion rates.
- Surveys like COSMOS2015 and GAMA map this evolution up to 4 to 6.
| Parameter | Typical value (local) |
| logM | 10.7 to 10.8 for characteristic mass. |
| phi (normalization) | few × 10 power -3 Mpc power -3 dex power -1 depending on fit. |
| faint-end slope | -1.1 to -1.6 (double Schechter components) |
Morphology, Types, and Demographics
(Source: researchgate.net)
- In the nearby Universe, a large fraction of stellar mass is in spheroid-dominated systems (ellipticals and S0), while disk galaxies hold a significant fraction of the number counts.
- Surveys that classify morphology statistically find roughly half the stellar mass in spheroids and the rest in disks when integrated over mass.
- At fixed stellar mass, the fraction of quiescent galaxies increases with mass and with cosmic time.
- At cosmic noon, most massive galaxies are still forming stars, whereas at 0, a substantial fraction of high-mass galaxies are quiescent.
- Galaxy demographics strongly depend on the environment. Dense environments like clusters have higher quiescent fractions and a larger fraction of early-type morphologies compared to the field.
| Category | Local fraction (by stellar mass or number) |
| Spheroid-dominated mass fraction | 50 to 70% of local stellar mass in spheroids, depending on the sample. |
| Disk galaxies (stellar mass fraction) | 30 to 50% |
| Quiescent fraction (high mass) | High increases with stellar mass and decreasing redshift. |
Dark Matter Halos and Baryon Accounting
(Source: mdpi.com)
- Planck cosmology yields a baryon fraction in the universe of about Ω_b / Ω_m 15 to 0.16. That sets a maximum baryon budget for galaxies inside halos; in reality, only a fraction of those baryons turn into stars. This fraction strongly depends on halo mass and redshift.
- Peak galaxy formation efficiency is around halo masses 10 power 11 to 10 power 12 M, where roughly 10% or a few tens of percent of baryons get converted into stars.
- In much smaller or much larger halos, the efficiency drops, partly because of stellar feedback at low mass and AGN feedback at high mass.
- Dark matter halo abundance is much steeper at low masses than the observed galaxy stellar mass function.
- That mismatch is a core statistical constraint forcing feedback processes in galaxy formation models.
| Quantity | Typical value |
| Cosmic baryon fraction Ω_b/Ω_m | 0.155. |
| Peak star formation efficiency | several to 20% baryons to stars at Mhalo 10 power 11 to 10 power 12 M |
| Why not all baryons? | Feedback, heating, stripping, and inefficiencies in cooling |
Mergers and Assembly
(Source: researchgate.net)
- Empirical and simulation-based studies find major merger rates (mass ratio roughly 1:4) that depend strongly on mass and redshift.
- Typical numbers: a few percent up to tens of percent per Gyr for massive galaxies around 1 to 3, falling to a few percent per Gyr at
- Integrated over cosmic time, a typical massive galaxy may experience order-unity numbers of major mergers.
- Minor mergers are more common than major ones and cumulatively can add a substantial fraction of a massive galaxy’s mass, especially at late times.
- Merger signatures vary in observability. Pair counts, disturbed morphologies, and kinematic signatures each have different observability windows and must be corrected with timescales to yield merger rates. That calibration is a statistical challenge in Galaxy Formation studies.
| Measure | Typical value |
| Major merger rate (per Gyr) for massive galaxies | 0.02 to 0.2 per Gyr, depending on redshift and mass. |
| Minor merger dominance | Minor mergers are 3 times more common than major mergers at some epochs. |
Role of Black Holes and Feedback
(Source: nature.com)
- Observed central black holes correlate with bulge mass and velocity dispersion. Typical black hole mass is around 0.1% to 0.5% of the bulge mass, depending on the sample and measurement method. That scaling is used in co-evolution models.
- To reproduce the observed high-mass cutoff of the SMF and the number of massive quiescent galaxies, AGN feedback, injecting a small fraction of the black hole’s rest-mass energy over time, is sufficient.
- This constraint is statistical: models tuned without AGN feedback overproduce massive galaxies.
| Parameter | Typical |
| BH mass fraction of the bulge | 0.1 to 0.5% (observationally). |
| Why important | Explains suppressed SFR and exponential cutoff in SMF |
Observational Data Sets and Surveys
(Source: mdpi.com)
- Hubble Deep Fields (HUDF, XDF) show thousands of galaxies in a tiny area, probing to very faint magnitudes and giving the high-redshift counts that inform the 2 trillion extrapolation.
- SDSS provided imaging for hundreds of millions of objects and spectra for up to about a million galaxies, which anchors local mass functions, environmental measures, and morphological statistics.
- JWST’s infrared sensitivity is revealing numerous bright, massive systems at very high redshift, which has forced rethinking of the earliest growth models. That is an active statistical tension area in Galaxy Formation literature.
| Survey | Role | Scale |
| Hubble Ultra Deep Field (HUDF) | Deep counts to faint magnitudes | 10,000 galaxies in a tiny patch. |
| Sloan Digital Sky Survey (SDSS) | Local universe, spectra | 0.9 to 1M galaxy spectra in older releases; imaging hundreds of millions. |
| JWST | Deep IR, early galaxies | Detects high luminous galaxies changing early statistics. |
Simulations and Models
(Source: simonsfoundation.org)
- Large cosmological hydrodynamical simulations like IllustrisTNG and EAGLE simulate volumes of tens to hundreds of Mpc with billions of resolution elements and include subgrid models for star formation, feedback, and black hole growth.
- They reproduce many statistical observables like the SMF and SFRD when calibrated properly.
- TNG50, TNG100, and TNG300 trade resolution for box size so that different scientific questions can be addressed statistically.
- TNG300 covers large volumes for rare objects, while TNG50 gives high resolution for galaxy internal structure.
- Simulations provide priors and forward models used in abundance matching, halo occupation distribution modeling, and other statistical techniques that connect observed galaxy counts to dark matter halos.
| Simulation | Box size (approx) | Note |
| TNG50 | 50 Mpc | High resolution for galaxy structure. |
| TNG100 | 110 Mpc | Balanced resolution and volume. |
| TNG300 | 300 Mpc | Large volume for rare objects. |
| EAGLE (Ref-L100N1504) | 100 Mpc | Public catalogs are used for statistical comparisons. |
Open Tensions and Statistical Puzzles Right Now
(Source: nature.com)
- Several JWST studies found apparently massive galaxies at 9 whose presence and inferred masses are higher than many models predicted.
- This is a current statistical puzzle that may reflect selection biases, modeling uncertainties, or genuinely surprising early efficiency.
- Cold dark matter predicts many more subhalos than observed luminous satellites, but improved completeness corrections and deeper imaging have narrowed the tension.
- There remain statistical questions about baryonic processes and detectability.
- Comparisons of the time-integral of SFRD and observed stellar mass density show discrepancies at some redshifts, leading to checks on IMF variations, dust corrections, and photometric uncertainties.
Statistical Tools and Methods Used in Galaxy Formation Studies
(Source: mdpi.com)
- Stellar mass and luminosity functions are most often fitted with Schechter or double-Schechter forms using 1/Vmax, maximum likelihood, or SWML estimators to correct for survey limits.
- These fits summarize the mass distribution with a few numbers.
- Abundance matching ties observed SMF to halo mass functions to infer how galaxies occupy halos. Halo occupation distributions (HOD) and conditional luminosity functions statistically describe galaxy populations in halos.
- Modern work increasingly uses forward modeling of entire surveys with synthetic catalogs and Bayesian parameter estimation to extract physical parameters from noisy data.
Conclusion
So overall, galaxy formation is really the story of how the universe built itself. What we understand is that, from clouds of gas and invisible dark matter, gravity sculpted the first stars, merged them into galaxies, and kept changing them through collisions and growth.
Every galaxy carries clues about the universe’s history, how fast stars were born, how black holes grew, and how cosmic structures evolved. With new telescopes like the James Webb Space Telescope, we are going deeper into the past, getting closer to seeing the first galaxies form.
In short, galaxy formation is an ongoing process, one that began billions of years ago and continues even now. And by studying it, we’re not just learning about galaxies, we’re learning about where we came from, and where the universe is heading. It is one of the most interesting articles I have ever written. I had fun, I hope you guys did too. If you have any questions, kindly let me know in the comments section. Thanks.
Sources
FAQ.
Galaxy formation is the process through which the universe’s matter, dark matter, gas, and dust, come together under gravity to form galaxies. It started shortly after the Big Bang and continues today as galaxies grow and merge.
Current estimates suggest there are around 2 trillion galaxies in the observable universe. This number comes from deep imaging and models that correct for galaxies too faint to see.
Star formation peaked around 8 to 10 billion years ago (redshift 1.5 to 2), a time known as “cosmic noon,” when galaxies formed stars at their highest rates.
Dark matter creates the gravitational backbone for structure formation. Its clumps, called halos, attract gas, which cools and forms stars, making galaxy formation possible.
Galaxies grow by forming new stars from gas and by merging with other galaxies. Major mergers (roughly equal-mass collisions) and minor mergers (accretion of smaller systems) both contribute to structural growth.
Yes. The Milky Way began forming around 13.6 billion years ago, shortly after the first galaxies appeared. It continues to evolve by merging with smaller galaxies.
Spiral galaxies have flat, rotating disks with ongoing star formation and younger stars. Elliptical galaxies are rounder, older, and have little new star formation. Today, about 70% of galaxies are spirals or irregulars, and 30% are ellipticals.
Nearly every large galaxy hosts a supermassive black hole at its center. These black holes grow along with their host galaxies and play a key role in regulating star formation.
Absolutely. Galaxies continue to form new stars, though at lower rates than in the past, and to merge with others. Galaxy formation is an ongoing process, visible throughout cosmic history.
Scientists use large surveys (like SDSS and COSMOS), deep fields (like Hubble’s Ultra Deep Field), and space telescopes (like JWST) to observe galaxies across time. They combine these observations with simulations (IllustrisTNG, EAGLE) and statistical methods to understand galaxy formation trends and test theories.
Jeeva Shanmugam is passionate about turning raw numbers into real stories. With a knack for breaking down complex stats into simple, engaging insights, he helps readers see the world through the lens of data—without ever feeling overwhelmed. From trends that shape industries to everyday patterns we overlook, Jeeva’s writing bridges the gap between data and people. His mission? To prove that statistics aren’t just about numbers, they’re about understanding life a little better, one data point at a time.
