26.3 Properties of Galaxies

Galactic Properties and Dark Matter

Mass estimation of galaxies

The most powerful tool astronomers have for measuring a galaxy's mass is its rotation curve, a plot of how fast stars and gas orbit at different distances from the galactic center. These curves are built by measuring Doppler shifts of spectral lines from different regions of a galaxy.

Kepler's laws predict that orbital velocity should decrease with distance from the center, just like planets farther from the Sun orbit more slowly. The expected relationship is:

$v = \sqrt{\frac{GM}{r}}$

where $G$ is the gravitational constant, $M$ is the mass enclosed within the orbit, and $r$ is the orbital radius.

But here's what actually happens: observed rotation curves stay flat or even rise at large distances from the galactic center. Stars in the outer regions orbit far faster than the visible matter alone can explain. This means there must be additional, unseen mass pulling on those outer stars.

The gap between predicted and observed rotation curves lets astronomers estimate a galaxy's total mass, including both visible matter (stars, gas, dust) and dark matter. Much of this extra mass resides in enormous galactic halos that extend far beyond the visible disk.

Mass-to-light ratios across galaxy types

The mass-to-light ratio ($M/L$) compares a galaxy's total mass to its luminosity, expressed in solar units ($M_{\odot}/L_{\odot}$). If a galaxy were made entirely of Sun-like stars, you'd expect $M/L \approx 1$. Higher ratios mean the galaxy contains a lot of mass that isn't producing light, which points to dark matter.

Different galaxy types show very different ratios:

• Spiral galaxies (like the Milky Way and Andromeda) have $M/L$ ratios of about 10–20 $M_{\odot}/L_{\odot}$, indicating significant dark matter.
• Elliptical galaxies (like M87 and Centaurus A) have higher ratios, typically 50–100 $M_{\odot}/L_{\odot}$, suggesting an even greater proportion of dark matter relative to visible matter.
• Dwarf spheroidal galaxies (like Draco and Ursa Minor) have the highest ratios, reaching up to 1000 $M_{\odot}/L_{\odot}$. These small, faint galaxies are almost entirely dominated by dark matter.

This pattern tells you something important: dark matter plays a larger role in low-luminosity and low-surface-brightness galaxies (dwarf irregulars, ultra-diffuse galaxies). The stellar populations within a galaxy also affect its $M/L$ ratio, since older, redder stars have higher mass-to-light ratios than young, bright ones.

Dark matter in galactic evolution

Multiple independent lines of evidence point to dark matter:

• Flat rotation curves remain the strongest direct evidence. Extended dark matter halos explain why orbital velocities stay high well beyond the visible disk.
• Gravitational lensing reveals unseen mass in galaxies and galaxy clusters. The deflection of light from background sources is greater than visible matter alone can account for. The Bullet Cluster and Abell 1689 are classic examples.
• Velocity dispersions in galaxy clusters show that galaxies within clusters move too fast to be held together by visible matter alone. Without additional mass, these clusters would fly apart.
• Cosmological simulations require dark matter to reproduce the large-scale structure of the universe. Cold dark matter (CDM) models, such as the Millennium Simulation and IllustrisTNG, successfully match observed galaxy formation and clustering patterns.

Dark matter halos shape galaxies in two key ways:

1. They provide gravitational potential wells where ordinary (baryonic) matter can collapse and form stars.
2. They influence galaxy morphology and dynamics through gravitational interactions like tidal stripping and mergers.

The actual nature of dark matter remains one of the biggest unsolved problems in astrophysics. Leading candidates include weakly interacting massive particles (WIMPs) and axions. Some researchers have also proposed modified gravity theories (like MOND) as an alternative, though these struggle to explain all the observations that dark matter models handle well.

Galaxy Composition and Structure

• Galaxy morphology describes a galaxy's overall shape and structure, with the main categories being spiral, elliptical, and irregular.
• The interstellar medium (gas and dust between stars) fuels star formation and plays a central role in how galaxies evolve over time.
• Active galactic nuclei (AGN) are extremely energetic central regions found in some galaxies, powered by matter falling into supermassive black holes.
• Galaxy clusters are large-scale structures containing hundreds to thousands of galaxies bound together by gravity, and they serve as important laboratories for studying dark matter.