Galaxy population: the observed properties of galaxies

In this module, we will discuss properties of the global population of galaxies, for example, the distributions of color and luminosities of galaxies, and relationships between global properties of galaxies. We will separately discuss additional properties of elliptical and spiral galaxies, and will discuss the distribution of galaxies in space.

Learning objectives:

• Explain what is meant by the bimodality in galaxy colors, including terms like “blue cloud”, “red sequence”, “green valley”.
• Sketch what the color-magnitude relation of galaxies looks like, with axis labels and characteristic numerical values.
• Describe scenarios that can cause galaxies can “move” in the color-magnitude diagram.
• Describe what is measured by a galaxy luminosity function (LF).
• Sketch what the local galaxy LF looks like, with axis labels and characteristic numerical values.
• Describe how the galaxy LF may be different for galaxies of different morphology, and the corresponding implication that the galaxy LF may vary with environment.
• Explain how the galaxy LF is often parameterized by a Schechter function, know the different parameters of this function, and how variations in these parameters affect the LF.
• Define terminology used in discussing LF evolution: no evolution, passive evolution, luminosity evolution and density evolution.
• Describe the characteristics of the nearby elliptical galaxy population: surface brightness profiles, non-axisymmetric structure, core properties, 3D shapes, kinematic properties, spectral properties, and trends that exist among these characteristics, e.g., as a function of galaxy luminosity.
• Describe the relationships between the global observables for elliptical galaxies: the fundamental plane.
• Explain the argument of how virial equilibrium might underly the fundamental plane, and the limitations/assumptions of this argument.
• Derive the rough scaling of the fundamental plane using virial equilibrium arguments.
• Describe how there may be several “families” of elliptical galaxies and plausible physical mechanisms that might be related to their formation.
• Sketch a typical elliptical galaxy spectrum, including labeling at least 2 key features with wavelengths.
• Describe what E+A (k+a) galaxies are, and be able to sketch a typical E+A spectrum.
• Describe the interstellar medium (ISM) in elliptical galaxies.
• Describe the characteristics of the nearby spiral galaxy population: typical disk and bulge scale lengths and radial surface brightness profiles, and disk vertical surface brightness profiles.
• Describe examples of non-axisymmetric structure in disk galaxies: bars, spiral arms, warps, and stellar streams in the halo.
• Describe the types of spiral arms observed in galaxies, and the mechanism(s) behind spiral arms.
• Identify and be able to interpret different ways of measuring and presenting spiral galaxy kinematics.
• Describe the basic principle of using rotation curves for mass modeling, being able to describe and apply the basic equation relating rotational velocity, mass, and distance from the galaxy center.
• Describe the variety of rotation curve shapes and amplitudes and how they roughly correspond to galaxies of different absolute magnitude.
• Explain the concept of the velocity ellipsoid on top of pure rotation, and how its amplitude varies with stellar population, e.g., asymmetric drift, radial blurring, radial migration.
• Describe the evidence that stars and gas move through the spiral arm pattern.
• Explain how we can determine whether a galaxy’s arms are leading or trailing.
• Describe what the Tully-Fisher relation is, and why it is important.
• Derive the rough scaling from Virial equilibrium arguments.
• Sketch a typical spiral galaxy spectrum, including labeling at least 2 key features with wavelengths.
• Explain what the galaxy “main sequence” is, and what “specific star formation rate” means.
• Describe the interstellar medium (ISM) in spiral galaxies, and how gas fractions vary systematically among spiral galaxies.
• Describe the mass-metallicity relation, and what factors may contribute to it.
• Describe additional global parameter relations that galaxies obey, i.e., the Fundamental Manifold, the Fundamental Metallicity Relation, and the M-sigma relation.
• Describe how galaxies are qualitatively distributed in space.
• Sketch and explain what a correlation function is and how it is measured.
• Describe ways in which the galaxy population in clusters differs from that in the field.
• Describe the intracluster medium (ICM) and its typical properties.
• List the typical masses of clusters and how those can be measured.