Galaxies, a graduate level course

Building Blocks of Galaxies

In this module, we will delve deeper into what we can learn about individual components of galaxies: stars, gas, central black holes, and dark matter.

Lecture Slides

Learning Objectives

Stars

Describe the basic stages of stellar evolution, and where stars fall on an HR diagram during different phases.

Hydrogen core burning - Main sequence
Hydrogen shell burning – Giant branch
Helium core burning – Horizontal branch
Helium shell burning – Asymptotic Giant branch
End stages
- Low mass – white dwarf
- High mass – supernova

Explain the distinction between stellar evolutionary tracks and stellar isochrones, and how stellar isochrones change as a function of age and metallicity.

Stellar evolutionary tracks predict the evolution for stars of a given mass.
Stellar isochrones are a cross section of properties for a range of stars at a given time.
With time, stars of decreasing masses evolve off the main sequence. This is reflected in the isochrone as the main sequence grows shorter with time.
Higher metallicity increases opacity in the interior of a star, making them cooler and thus redder. Higher metallicity also leads to atmospheric absorption, especially in the blue leading to line blanketing. This also makes stars appear redder regardless of temperature.

Identify key features on Color-Magnitude Diagrams (CMD), describe how they connect to specific phases of stellar evolution, and sketch how they may be affected by properties such as metallicity. Describe how binaries can alter stellar evolution and what changes are observed in the CMD of the stellar population. Explain the concept of a single stellar population (SSP) and how well it relates to actual globular clusters. Describe what a Hess diagram is and how it differs from a normal CMD or HR diagram. Describe what an initial mass function (IMF) is and what typical observed IMFs look like. Sketch the approximate SFH (SFR vs. time) for a system given an observed CMD, describing relevant CMD features. Explain how star formation histories (SFH) can be derived quantitatively from observations of resolved stellar populations, and describe some results from SFH studies of local galaxies. Explain which stars contribute the most light vs. mass to a stellar population, and how that changes with age. Describe how the color and luminosity of unresolved stellar populations change with age and metallicity. Explain the concept and goals of using stellar population synthesis to model composite galaxy spectra. Sketch the spectral energy distribution (SED) of both simple and composite stellar populations as a function of age, and identify which features are sensitive to star formation rate (SFR) or stellar mass. Describe what a stellar M/L ratio is and how it varies with wavelength. Explain the problem posed by the age-metallicity degeneracy in the context of fitting galaxy SEDs, and what strategies are typically used to overcome it. Describe potential uncertainties when using stellar population synthesis models to derive star formation histories for unresolved stellar populations (e.g., from stellar evolution, binaries, IMF).

Gas and dust

Describe the different phases of gas in galaxies, the interstellar medium (ISM), and how different phases can be observed. Describe the basic processes of heating and cooling in the ISM and the concept of a cooling curve, including an understanding of the units. Explain how the ISM can be studied either by absorption or emission, including the distinction between emission measure vs. column density. Describe the concepts of an HII region and a Strömgren Sphere, and what variables affect the Strömgren Radius. Explain how emission lines from HII regions (e.g., Hα) can be used to estimate star formation rates, and the assumptions that go into this. Describe why certain emission lines can be used to estimate gas temperatures and densities. Describe how emission line diagnostic diagrams can be used to probe different ionization sources for dense ionized gas, and the physical picture for why AGN have relatively higher fluxes in lines of [OI], [SII], [NII], and [OIII] vs. in Balmer lines. Describe the main ways we derive star formation rates (SFR) in galaxies, and some pros/ cons of each. Explain the importance of parameterizing star formation, and what the Schmidt and Kennicutt- Schmidt Laws refer to. Describe the concepts of stellar feedback, superbubbles, and the galactic fountain. Sketch and describe an extinction curve, how it is used, and how it is affected by dust properties. Sketch the SED of a galaxy including dust emission, labeling key features and what they are coming from.

Central black holes and AGN

Describe the observational characteristics of different types of AGN and the arguments suggesting AGN are powered by central black holes. Sketch the unified AGN model, including the basic nomenclature of different regions of an AGN, and explain how it accounts for different types of AGN. Describe some of the ways that black hole masses can be measured, and the relation between central black hole masses and spheroid luminosity / velocity dispersion.

Dark matter

Describe how rotation curves and mass modeling are used to determine the distribution of dark matter, what the maximum disk hypothesis is, and what kinds of galaxies are more dark matter dominated. Describe the dark matter profiles predicted theoretically and the core vs. cusp debate. Describe some of the methods and challenges associated with determining the distribution of dark matter in ellipticals. Explain the basic idea of gravitational lensing, the differences between strong and weak lensing, and how these techniques are used to measure masses of galaxies or clusters.

\( \Sigma(r) = \Sigma_s e^{-r/r_s} \)