This lecture provides a comprehensive introduction to galaxies, delving into their fundamental composition, structural classifications, and underlying dynamics. It details the various components of galaxies, including stars, gas (ionized, neutral, and molecular hydrogen), and dust, highlighting their differing distributions and properties in spiral versus elliptical galaxies. The talk covers essential astronomical concepts like distance scales (parsec, kiloparsec, megaparsec) and celestial coordinates (Right Ascension, Declination). A significant portion is dedicated to explaining the distinct dynamical behaviors of spiral galaxies (rotationally supported, differential rotation, density waves, dark matter halos) and elliptical galaxies (pressure-supported, random stellar motion, virial equilibrium). The lecture also touches upon the Hubble sequence for galaxy classification, different types of galaxies (irregular, lenticular, barred, ring, polar-ring), and the phenomenon of gravitational clustering. Finally, it briefly discusses the role of supermassive black holes and the importance of multi-wavelength observations in understanding galactic phenomena.
Summary of the Transcript:
0:06 Galaxy Composition: Galaxies are primarily composed of stars, gas, and dust. Gas is predominantly hydrogen, existing in three forms: hot ionized hydrogen (protons), neutral atomic hydrogen, and molecular hydrogen. Molecular hydrogen forms on dust grains, which are crystalline particles (silicon, germanium).
1:41 Gas Distribution in Galaxy Types: Spiral galaxies contain ionized, neutral, and molecular hydrogen. Elliptical galaxies, conversely, only possess ionized gas, appearing as a hot halo, and lack neutral or molecular hydrogen.
2:17 Astronomical Distance Scales: A parsec is defined by stellar parallax, equating to 3 x 10^18 cm. Galactic distances are measured in kiloparsecs (kpc) and megaparsecs (Mpc), with Andromeda being 750 kpc away and large elliptical galaxies several Mpc away.
4:14 Calculating Distances (Example): The simple R-theta formula (D = theta * d) is used to calculate distances, such as the diameter of a galaxy's nuclear region given its angular size and distance.
6:31 Celestial Coordinates: When viewing galaxies, the celestial sphere uses Right Ascension (RA) for the x-axis and Declination (Dec) for the y-axis, similar to longitude and latitude on Earth, helping to project distant objects onto our view.
9:11 Importance of Multi-Wavelength Observations: Different wavelengths reveal distinct features and physical processes within galaxies, as exemplified by molecular hydrogen observations and composite optical/gas images.
Spiral galaxies are discs of rotating stars embedded in massive, unseen dark matter halos (10x stellar mass).
They exhibit differential rotation, where layers move at different velocities, causing a shearing effect.
Spiral arms are now understood as density waves, slowly moving regions of higher density that trigger star formation through gas collision and cooling.
The Milky Way is a prime example, with our solar system near the Orion arm.
Many spiral galaxies have a central bulge, often spherical, containing older stars (Pop II) distinct from the younger disk stars (Pop I).
Prehistoric globular clusters (compact, spherical star clusters, 5 parsecs in radius, Pop II stars) are found in the dark matter halo.
Neutral hydrogen is the most massive ISM component in the Milky Way, extending far beyond the stellar disk (twice the stellar disk radius).
Molecular hydrogen is clumpy and associated with dust, forming stars when gas condenses.
Ionized hydrogen is totally distributed within the galaxy disk.
Dust particles are silica-based, micron-sized, and crucial for molecular hydrogen formation, as well as polarizing light and forming visible filaments (e.g., in JWST images).
19:41 Disc Thickness: Discs can be thin (hosting young stars, molecular hydrogen) or thick (hosting older stars). Older discs become thicker due to gravitational scattering from internal stellar interactions or external galaxy encounters.
22:01 Gas Cooling & Star Formation: Gas collision in density waves leads to shocks, causing radiation, which cools the gas. Cooled gas fragments and forms stars.
22:43 Globular Clusters: They are compact, spherical star clusters in virial equilibrium (2T + V = 0), with density peaking towards the center. They are old, metal-poor (Pop II stars), and a significant source of binaries and pulsars. Their number correlates with the dark matter halo mass.
Edwin Hubble classified galaxies based on a "tuning fork" diagram.
Elliptical Galaxies (E0-E7): Classified by flattening (E0 = spherical, E7 = most squashed). Beyond E7, they become unstable due to "firehose instability."
Spiral Galaxies (Barred and Unbarred):
Unbarred spirals (Sa, Sb, Sc): Classified by bulge size (smaller from Sa to Sc) and tightness of spiral arms (looser from Sa to Sc).
Barred spirals (SBa, SBb, SBc): Similar classification but with a central bar structure. Bars are ellipsoidal stellar structures that develop their own potential, causing stars to move in elliptical orbits within them, leading to dust lanes. Two-thirds of galaxies are barred.
The Hubble plot is not an evolutionary sequence; changes in galaxy morphology primarily occur through mergers or gravitational interactions.
Irregular Galaxies: Have irregular shapes and rotations (e.g., Magellanic Clouds).
Lenticular Galaxies (S0): Intermediate between ellipticals and spirals, having both a bulge and a disk but often lacking active star formation. They can form from mergers or stripping.
Ring Galaxies: Have a central nucleus surrounded by a ring of stars, often formed by gravitational resonances or face-on collisions (e.g., Cartwheel Galaxy).
Polar-Ring Galaxies: Galaxies with a ring of gas/stars orbiting perpendicularly to the main galactic disk, usually formed from mergers at a high angle.
Gravity causes galaxies to cluster: galaxy pairs (may merge), galaxy groups (3-50 galaxies, e.g., Stephan's Quintet), galaxy clusters (hundreds to thousands of galaxies, e.g., Coma Cluster), and superclusters (clusters of clusters).
Beyond superclusters, the universe exhibits vast empty regions called voids.
SMBHs reside at the center of most galaxies, forming from seed black holes (primordial or from early stars) and growing significantly through galaxy and stellar mergers.
Active Galactic Nuclei (AGN) occur when SMBHs accrete mass, releasing immense energy.
"Wandering black holes" are SMBHs from merged smaller galaxies that haven't yet spiraled to the center of the larger galaxy.
59:59 Multi-Wavelength Observations (Recap): Observing galaxies across different electromagnetic wavelengths (gamma-ray, X-ray, optical, radio) reveals diverse physical processes and structures, as shown for Crab Nebula and M31.