The first galaxies formed when the universe was still young, but scientists cannot visit them directly.
Instead, they piece together their history using powerful telescopes, computer simulations, and clues left across cosmic time.
This article explains how do scientists study the first galaxies, what evidence they rely on, and why each new observation changes what we know about early galaxy formation.
What scientists mean by the first galaxies
The term “first galaxies” usually refers to the earliest bound systems of stars, gas, dust, and dark matter that formed after the Big Bang.
These objects likely appeared within the first few hundred million years of cosmic history, during the era of reionization.
Because light travels at a finite speed, observing distant galaxies is effectively looking back in time.
A galaxy seen 13 billion light-years away is being viewed as it existed billions of years ago, long before the Milky Way reached its current form.
- Lookback time: The time difference between the light’s emission and its detection on Earth.
- Redshift: The stretching of light to longer wavelengths as the universe expands.
- Cosmic dawn: The period when the first stars and galaxies began to shine.
How do scientists study the first galaxies with telescopes?
Telescopes are the main tool for detecting galaxies from the early universe.
Scientists use both space-based and ground-based observatories to collect faint light that has traveled for billions of years.
Why infrared observations matter
Many of the earliest galaxies are invisible or extremely faint in visible light because cosmic expansion shifts their light into the infrared.
This is why infrared astronomy is central to early-galaxy research.
The James Webb Space Telescope, or JWST, has transformed the field by observing deep infrared wavelengths with high sensitivity and resolution.
It can detect galaxies that formed only a few hundred million years after the Big Bang, and it can study their structure, brightness, and chemical signatures.
What Hubble, JWST, and large ground telescopes contribute
Each observatory fills a different role in the search for the first galaxies:
- Hubble Space Telescope: Helped identify very distant galaxy candidates using deep imaging and color selection.
- JWST: Measures infrared light from extremely early galaxies and reveals details about star formation and composition.
- Very Large Telescope, Keck Observatory, and other ground-based facilities: Provide spectroscopy and follow-up observations, often with adaptive optics.
Together, these instruments allow astronomers to move from a candidate detection to a more complete physical picture.
How do scientists determine distance and age?
Distance is one of the most important measurements in early-galaxy research.
Scientists use redshift, denoted by the letter z, to estimate how far back in time a galaxy lies.
When light from a distant galaxy is stretched, known atomic features move to longer wavelengths.
By identifying those shifted features, astronomers can calculate the galaxy’s redshift and infer its age in cosmic history.
Photometric redshifts versus spectroscopic redshifts
There are two main methods:
- Photometric redshift: Estimated from brightness measurements in multiple filters.
This method is fast and useful for identifying candidates, but it is less precise.
- Spectroscopic redshift: Measured from detailed spectra showing specific emission or absorption lines.
This method is more accurate and confirms the galaxy’s distance.
For the earliest galaxies, spectroscopic confirmation is especially valuable because many objects initially appear distant but later turn out to be closer, dusty galaxies or unusual sources.
What light tells us about early galaxies
Light from the first galaxies carries information about the stars inside them, the gas around them, and the processes powering them.
Astronomers analyze brightness, color, and spectral features to infer physical properties.
Star formation rates
Young galaxies often form stars rapidly.
Their ultraviolet light, once redshifted into the infrared, can indicate how many massive stars they contain and how quickly those stars are being born.
Chemical composition and metallicity
The first generations of stars formed from hydrogen and helium, with almost no heavier elements.
As stars lived and died, they enriched galaxies with carbon, oxygen, nitrogen, and other elements.
Measuring metallicity helps scientists estimate how much prior star formation has occurred.
Dust and gas content
Gas fuels star formation, while dust can absorb and re-emit starlight.
In the earliest galaxies, dust is expected to be sparse at first, so studying its presence or absence helps reveal how quickly galaxies evolved.
What role do computer simulations play?
Because direct observations reach only part of the story, scientists use computer simulations to model galaxy formation from the Big Bang onward.
These simulations follow dark matter clustering, gas cooling, star formation, feedback from supernovae, and black hole growth.
Simulations help researchers answer questions that telescopes alone cannot resolve:
- How quickly did small protogalaxies merge into larger systems?
- How much did supernova feedback suppress star formation?
- What types of galaxies produced enough radiation to reionize the universe?
By comparing simulated galaxies with real observations, astronomers test whether their models match the early universe.
If the predicted number of bright galaxies or the observed star formation rates do not agree, the models must be revised.
How do scientists use gravitational lensing?
Gravitational lensing occurs when a massive object, such as a galaxy cluster, bends and magnifies the light from a more distant galaxy behind it.
This natural cosmic telescope can make extremely faint early galaxies detectable.
Lensing is especially useful for studying objects near the limits of current instrumentation.
It can reveal smaller, dimmer galaxies that might otherwise remain invisible, although astronomers must carefully model the lens to correct for distortion and magnification.
Why is the era of reionization important?
The first galaxies likely played a major role in reionization, the process that transformed the intergalactic medium from opaque to transparent.
During this epoch, energetic radiation from early stars and galaxies ionized surrounding hydrogen.
Studying reionization helps scientists understand how early galaxies affected the wider universe.
Key questions include:
- Were small galaxies numerous enough to drive reionization?
- How much ultraviolet radiation escaped from early galaxies?
- How did the growing galaxy population change the intergalactic gas?
Answering these questions links galaxy formation to the evolution of cosmic structure on the largest scales.
What limits do astronomers face?
Studying the first galaxies is difficult because the signals are faint, distant, and often contaminated by closer objects.
Even with advanced telescopes, uncertainty remains high.
- Low brightness: The earliest galaxies are extremely dim.
- Source confusion: Nearby stars or dust-obscured galaxies can mimic distant targets.
- Instrument limits: Sensitivity and wavelength coverage still constrain detections.
- Cosmic variance: Small survey fields may not represent the universe as a whole.
These challenges mean that one discovery rarely settles a question by itself.
Researchers build confidence by combining multiple observations across different regions of the sky.
What will future observations reveal?
Upcoming telescopes and surveys will sharpen the picture of the first galaxies.
The Nancy Grace Roman Space Telescope, thirty-meter-class ground telescopes, and next-generation radio observatories will extend the reach of current studies.
Future research will likely improve measurements of galaxy masses, star formation histories, and the connection between early galaxies and the dark matter halos that hosted them.
As data improves, scientists will better understand how the universe’s first luminous structures emerged and evolved.
The question of how do scientists study the first galaxies continues to drive modern astronomy because every new method exposes a different layer of the early universe.
With each deeper image and sharper spectrum, the first pages of cosmic history become clearer.