28.1 Observations of Distant Galaxies

Observations of Distant Galaxies

Looking at distant galaxies means looking back in time. Light travels at a finite speed, so when we observe a galaxy billions of light-years away, we're seeing it as it existed billions of years ago. This gives astronomers a direct way to study how galaxies formed and changed across cosmic history.

Information from Distant Galaxies

The most fundamental tool here is light travel time. Light from the Andromeda galaxy, for example, takes about 2.5 million years to reach us, so we see Andromeda as it was 2.5 million years ago. For galaxies billions of light-years away, we're looking billions of years into the past.

• Redshift is how we measure those vast distances. As the universe expands, light from distant galaxies gets stretched to longer, redder wavelengths. The greater the redshift, the farther away the galaxy is and the further back in time we're observing. Quasars, among the most distant objects detected, have extremely high redshifts.
• Hubble's Law gives us the mathematical relationship between distance and redshift: $v = H_0 \times d$, where $v$ is the galaxy's recessional velocity (how fast it's moving away from us), $H_0$ is the Hubble constant, and $d$ is the galaxy's distance. A faster recession means a more distant galaxy.
• Lookback time is the amount of time that has passed since the light we're observing was originally emitted. It's calculated using the galaxy's redshift and the expansion rate of the universe. For instance, the cosmic microwave background has a lookback time of about 13.8 billion years, representing the oldest light we can detect.
• Spectroscopy breaks a galaxy's light into its component wavelengths, revealing details about its chemical composition, temperature, and motion. This is one of the most information-rich tools astronomers have for studying distant galaxies.

Evidence of Early Star Formation

Several types of observations tell us that galaxies in the early universe were forming stars at very high rates.

• Lyman-alpha emission is an ultraviolet spectral line produced by hydrogen atoms in regions of active star formation. Detecting this line in a distant galaxy signals the presence of young, massive stars. Starburst galaxies, which form stars at exceptionally high rates, are strong Lyman-alpha emitters.
• Infrared emission provides another clue. Dust in star-forming regions absorbs ultraviolet light from hot young stars and re-emits that energy in the infrared. NASA's Spitzer Space Telescope was instrumental in detecting this infrared glow from distant galaxies.
• The Hubble Ultra Deep Field is a long-exposure image that revealed thousands of galaxies in the early universe. Many of these galaxies appear blue, which indicates young stellar populations and active star formation. Some of the faintest, most distant galaxies in the image were only visible thanks to gravitational lensing, where the gravity of a foreground galaxy cluster magnifies background objects.
• Spectroscopic detections of heavy elements (elements heavier than hydrogen and helium) in distant galaxies confirm that stars had already formed and died, enriching the surrounding gas. Facilities like the Very Large Telescope have been key to these measurements.
• The James Webb Space Telescope (JWST) now provides unprecedented infrared views of the earliest galaxies, pushing observations to higher redshifts than ever before and revealing galaxy formation in greater detail.

Early vs. Modern Galaxy Characteristics

Comparing distant (early) galaxies with nearby (modern) ones reveals clear patterns of change over cosmic time.

• Size and shape: Early galaxies tend to be smaller and more irregular than modern galaxies. Many are classified as "proto-galaxies" or "galaxy fragments" that hadn't yet settled into the familiar spiral and elliptical forms of the Hubble sequence.
• Stellar populations: Stars in early galaxies are younger and more metal-poor (they contain fewer heavy elements). The most distant galaxies may even contain Population III stars, the theoretical first generation of stars formed from nearly pure hydrogen and helium. By contrast, nearby structures like globular clusters contain some of the oldest surviving stars, but even these are metal-poor compared to younger stars in the Milky Way's disk.
• Star formation rates: Early galaxies formed stars much faster than most modern galaxies. The cosmic star formation rate peaked roughly 10 billion years ago and has been declining since. The Milky Way today forms only a few new stars per year, a modest rate compared to early starburst galaxies.
• Chemical composition: Early galaxies have lower abundances of heavy elements. Over time, successive generations of stars produce and disperse heavier elements through supernovae and stellar winds, gradually increasing a galaxy's metallicity.

Large-Scale Structure and Galaxy Environment

Galaxies don't exist in isolation. Their distribution across the universe follows a distinct large-scale pattern.

• Galaxy clusters are the largest gravitationally bound structures, containing hundreds to thousands of galaxies held together by gravity (and large amounts of dark matter).
• The cosmic web describes the overall arrangement of matter in the universe. Galaxies are concentrated along filaments and sheets of matter, with enormous nearly empty voids between them. This web-like pattern was shaped by gravity acting on tiny density variations in the early universe.
• Active galactic nuclei (AGN) are extremely luminous galaxy centers powered by supermassive black holes actively consuming surrounding material. AGN were far more common in the early universe, suggesting that black hole growth and galaxy evolution are closely linked.
• Dark energy is the mysterious force driving the accelerating expansion of the universe. Over cosmic time, this acceleration affects how galaxies cluster together and how large-scale structure evolves, making dark energy a central factor in understanding galaxy distribution.