11.1 Galaxy classification and properties

Galaxies come in diverse shapes and sizes, from elegant spirals to smooth ellipticals. We classify them using Hubble's tuning fork diagram, which considers features like spiral arms and bulge size. This helps us understand their structure and evolution.

Beyond appearance, galaxies differ in properties like stellar populations, gas content, and star formation rates. We use spectroscopy to analyze their composition and motion, while luminosity functions help us study galaxy populations and evolution across cosmic time.

Galaxy Classification and Properties

Galaxy classification scheme

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• Hubble's tuning fork diagram organizes galaxies into ellipticals (E), spirals (S), barred spirals (SB), lenticulars (S0), and irregulars (Irr) based on visual appearance
• Morphological features considered include presence of spiral arms, bulge-to-disk ratio, and presence of a bar
• Elliptical galaxies classified E0 to E7 based on ellipticity (roundness to flattened shape)
• Spiral galaxies classified Sa, Sb, Sc based on tightness of spiral arms and bulge size (tightly wound to loosely wound)
• Limitations include not accounting for galaxy evolution and difficulty classifying distant or faint galaxies

Properties of galaxy types

• Elliptical galaxies exhibit smooth, featureless appearance with little gas or dust, contain older stellar populations, and supported by random stellar motions
• Spiral galaxies feature disk component with spiral arms, central bulge, younger stellar populations in disk, ongoing star formation in spiral arms (HII regions), and rotation-supported structure
• Irregular galaxies lack definite shape or structure, often result from galaxy interactions (mergers), typically smaller and less massive, and display active star formation regions

Spectroscopy for galaxy analysis

• Emission and absorption lines indicate presence of specific elements (hydrogen, helium, oxygen)
• Doppler shift measures galaxy's radial velocity (approaching or receding)
• Spectral energy distribution reveals galaxy's overall composition and age (young vs old stars)
• Redshift measurements determine galaxy distances and calculate Hubble constant
• Metallicity indicators trace chemical evolution of galaxies (iron, oxygen abundance)
• Star formation rate indicators include H-alpha emission line strength
• Active galactic nuclei identified by broad emission lines (quasars, Seyfert galaxies)

Galaxy luminosity functions

• Number of galaxies per unit volume as a function of luminosity
• Schechter function models luminosity function: $\phi(L) = \phi^* (L/L^*)^\alpha e^{-L/L^*}$
• Parameters include $\phi^*$ (normalization factor), $L^*$ (characteristic luminosity), and $\alpha$ (faint-end slope)
• Applications study galaxy evolution across cosmic time, constrain galaxy formation models, and estimate total luminosity density of the universe
• Variations exist with galaxy type and environment (ellipticals vs spirals, clusters vs field)