11.1 Galaxy classification and properties

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. Spectroscopy lets us analyze their composition and motion, while luminosity functions describe galaxy populations and how they evolve across cosmic time.

Galaxy Classification Scheme

Hubble's tuning fork diagram is the foundational system for organizing galaxies by visual morphology. It splits galaxies into several major types:

• Elliptical galaxies (E) are classified E0 through E7 based on their projected ellipticity, where E0 appears circular and E7 is highly elongated. The number $n$ in the designation comes from $n = 10(1 - b/a)$, where $b/a$ is the ratio of the semi-minor to semi-major axis.
• Lenticular galaxies (S0) sit at the fork's junction. They have a disk and bulge like spirals but lack prominent spiral arms, and they contain relatively little gas.
• Spiral galaxies (S) are classified Sa, Sb, Sc based on two correlated features: the tightness of the spiral arms and the size of the central bulge. Sa galaxies have tightly wound arms and a large bulge; Sc galaxies have loosely wound arms and a small bulge.
• Barred spirals (SB) follow the same Sa–Sc progression but have a prominent bar structure running through the center (SBa, SBb, SBc).
• Irregular galaxies (Irr) fall outside the tuning fork entirely, lacking the symmetry needed for classification in the other categories.

The main morphological features Hubble considered are the presence of spiral arms, the bulge-to-disk ratio, and whether a bar is present.

The tuning fork has real limitations. It's a classification of appearance, not a sequence of evolution (Hubble himself cautioned against reading it that way). It also struggles with distant or faint galaxies where fine morphological detail is lost, and it doesn't capture the full range of properties that modern surveys reveal.

Properties of Galaxy Types

Each morphological class corresponds to distinct physical properties. Understanding these connections is where classification becomes genuinely useful.

Elliptical galaxies have a smooth, featureless appearance. They contain very little cold gas or dust, so star formation has largely ceased. Their stellar populations are predominantly old, red stars. Dynamically, they're supported by the random (dispersion-dominated) motions of their stars rather than organized rotation.

Spiral galaxies have two main structural components: a central bulge (older stars, similar to a small elliptical) and a flat disk with spiral arms. The arms are sites of active star formation, traced by bright HII regions and young O/B stars. The disk is rotationally supported, meaning ordered circular motion dominates over random motion. Gas and dust are abundant in the disk, fueling ongoing star formation.

Irregular galaxies lack a well-defined shape or rotational symmetry. Many are the products of gravitational interactions or mergers. They tend to be smaller and less massive than spirals or ellipticals, and they often host vigorous star formation.

Spectroscopy for Galaxy Analysis

Spectroscopy is the primary tool for measuring physical properties of galaxies beyond what imaging alone can reveal.

• Emission and absorption lines identify specific elements (hydrogen, helium, oxygen, iron) and tell you about the physical conditions of the gas and stars.
• Doppler shifts of spectral lines measure a galaxy's radial velocity. Blueshifted lines mean the galaxy is approaching; redshifted lines mean it's receding.
• Redshift ($z$) is used to determine cosmological distances via Hubble's law ($v = H_0 d$), connecting observed recession velocity to distance.
• The spectral energy distribution (SED) across all wavelengths reveals the mix of stellar populations. A galaxy dominated by young, hot stars peaks in the blue/UV; one dominated by old, cool stars peaks in the red/infrared.
• Metallicity indicators, such as the relative strength of iron or oxygen lines, trace the chemical enrichment history of a galaxy over successive generations of stars.
• Star formation rate can be estimated from the strength of the H$\alpha$ emission line ($\lambda = 656.3$ nm), which traces ionized hydrogen around massive young stars.
• Active galactic nuclei (AGN) are identified by unusually broad emission lines, indicating gas moving at thousands of km/s near a supermassive black hole. Quasars and Seyfert galaxies are subcategories of AGN.

Galaxy Luminosity Functions

The luminosity function describes the number density of galaxies (number per unit volume) as a function of their luminosity. It's a statistical tool for characterizing entire galaxy populations rather than individual objects.

The standard model is the Schechter function:

$\phi(L)\,dL = \phi^* \left(\frac{L}{L^*}\right)^\alpha e^{-L/L^*} \frac{dL}{L^*}$

The three parameters each control a different aspect of the distribution:

• $\phi^*$ is the normalization, setting the overall number density of galaxies.
• $L^*$ is the characteristic luminosity, marking the "knee" of the function where it transitions from a power law to an exponential cutoff. Galaxies much brighter than $L^*$ are exponentially rare.
• $\alpha$ is the faint-end slope. A more negative $\alpha$ means a steeper rise in the number of faint galaxies. Observations typically give $\alpha \approx -1.0$ to $-1.3$.

The luminosity function is used to constrain galaxy formation models, estimate the total luminosity density of the universe, and track how galaxy populations evolve with redshift. It also varies with environment and galaxy type: cluster environments tend to have a different mix of galaxy luminosities than the field, and ellipticals and spirals have distinct luminosity functions.