21 Galaxy Evolution and Cosmology Notes

Galaxies and Beyond

The Cosmic Distance Ladder

  • Methods to measure distances are useful for specific ranges.

  • Combining and calibrating methods allows reliable distance determination over a broad range.

  • Standard Candle:

    • Known luminosity without prior distance knowledge.

    • High luminosity for observation at great distances.

    • Common for frequent distance measurements.

  • 1AU=4.85×10−6pc1 AU = 4.85 \times 10^{-6} pc

  • 1pc=206,265AU=3.1×1013km=3.26LY1 pc = 206,265 AU = 3.1 \times 10^{13} km = 3.26 LY

  • Redshift: Furthest galaxies' distances measured from their redshifts.

Implication: The Age of the Universe

  • Hubble's discovery indicated the universe had a beginning.

  • Age Calculation:

    • H=67.4kms⋅MpcH = 67.4 \frac{km}{s \cdot Mpc}

    • H=2.18×10−18s−1H = 2.18 \times 10^{-18} s^{-1}

    • Age=1H=14.5×109yrAge = \frac{1}{H} = 14.5 \times 10^9 yr

Cosmological Principle

  • The universe appears the same on large scales, regardless of location.

  • Matter is evenly distributed without a center or edge.

Implication: Cosmological Redshift

  • Redshift: Stretching of photon wavelength by the expansion of space.

  • Doppler Effect: Wavelength changes when source and observer move relative to each other.

    • Blueshift: Wavelength reduced as source moves towards observer.

    • Redshift: Wavelength increased as source moves away.

Galaxy Evolution

  • Studying distant galaxies reveals their past appearances in a younger universe.

  • Comparing galaxies of the same type at different distances/times helps understand evolution.

The Earliest Galaxies

  • Rely on theoretical modeling.

  • Key Assumptions:

    1. He and H were nearly uniformly distributed.

    2. Matter distribution was not perfectly uniform.

  • Gravity in denser regions attracted matter, seeding early galaxy formation.

Hubble’s Classification of Galaxies

  • Tuning fork diagram classifies galaxies.

  • E = elliptical, S = spiral (ordinary and barred).

Galactic Type Differentiation Mechanisms

  • Initial Conditions (Rotation, Density)

  • Galaxy Collisions

  • Galactic cannibalism

Galactic Collisions

  • Collisions compress gas clouds, triggering star formation.

  • Collisions were more likely in the early universe due to closer galaxies.

Starbursts

  • Forming stars so quickly they will use gas in less than a billion years.

Quasars and Other Active Galactic Nuclei

  • Active Galactic Nuclei (AGN) are unusually luminous galactic centers; Quasars are the most luminous.

  • Quasars: quasi-stellar radio sources.

  • Found primarily at great distances/early times.

  • Luminosity L > 10^{12} L{Sun} > 10^2 L{Milky Way}

  • Redshift:

    • z=λ<em>shift−λ</em>restλrestz = \frac{\lambda<em>{shift} - \lambda</em>{rest}}{\lambda_{rest}}

*Recessional Speed:
* vrc=(z+1)2−1(z+1)2+1\frac{v_r}{c} = \frac{(z+1)^2 - 1}{(z+1)^2 + 1}

Characteristics of Active Galaxies

  • Enormous luminosity (>10^{12} L_{Sun}

  • Rapidly varying luminosity.

  • Energy emission across a wide range of wavelengths.

  • Jets of plasma at near light speed, producing giant radio lobes.

Power Source for Quasars and Active Galactic Nuclei

  • Accretion of gas onto a supermassive black hole.

  • Gravitational potential energy converted to kinetic energy.

  • Friction turns kinetic energy into thermal energy.

  • Heat produces thermal radiation (photons).

  • This process can convert 10–40% of energy into radiation.