Observing galaxies

In this module, we will discuss basic observables and observations of galaxies. These will include understanding the basic imaging observations made of galaxies, namely surface brightness and surface brightness distributions and the basic spectroscopic observations that provide information on motions within galaxies and on the nature of the components producing the light. We will also discuss methods for determining distances to galaxies and issues involving observational biases and selection functions.

Learning objectives. After completing this module, you should be able to:

• Describe (in a few sentences) the main historical developments in extragalactic astronomy.
• Describe what surface brightness is and how it varies with distance/redshift.
• Explain why galaxies are often characterized by 1D surface brightness profiles, and how this is done in principle.
• Identify the different profiles that are typical of disk and elliptical galaxies.
• Describe the Sersic profile, what parameters it has, and typical values.
• Describe how galaxies can have multiple components, and how these might be separated using bulge-disk decomposition.
• Describe how sizes and integrated brightnesses of galaxies can be characterized, e.g. using isophotal quantities or Petrosian measurements, and some of the issues with isophotal quantities.
• Explain why corrections for foreground and internal extinction are necessary, and how foreground extinction is determined.
• Describe what a K-correction is, under what circumstances it is used, and qualitatively how one would compute one.
• Describe how spectra of galaxies are composite, i.e., arising from multiple components, each with its own spectrum and its own velocity.
• Sketch the basic features of the stellar and gas components of galaxy spectra, including labeling wavelenghts (!), for different type galaxies.
• Describe the two kinematic extremes of purely organized and purely random velocities, and how the two are typically compared.
• Explain how mean velocities and velocity dispersions are measured. Understand how the ability to measure velocity dispersion depends on spectral resolution. Understand how to determine the redshift given a wavelength of a known spectral line.
• Describe, for the random component of velocities, the concept of the line-of-sight velocity distribution.
• Describe how velocities are driven by the mass distribution, how this is recovered in the case of pure rotation, and why it is more challenging to recover in a kinematically “hot” system
• Describe multiple methods for getting distances to galaxies in some detail.
• Define the terms “standard candle” and “standard ruler”.
• Describe the Hubble flow, peculiar velocities, and their impact on derived distances.
• Explain how redshifts are generally measured observationally, including spectroscopic and photometric redshifts
• Describe the basics of the classical morphological classification systems.
• Explain why morphology is a function of wavelength and the resulting effect on observing galaxies at higher redshift.
• Describe a range quantitative morphological statistics in some detail (e.g., B/D, global Sersic index, CAS, Gini, M20).
• Define the difference between a parametric and non-parametric statistic is, and why one might choose one approach vs. the other
• Describe some examples of large surveys from which galaxy catalogs are constructed.
• Define what is meant by a “selection function” and a “complete” catalog.
• Explain key observational biases such as Malmquist Bias, Eddington Bias, and surface brightness bias, and what effect they will have on a galaxy catalog.

  Date	Topic	Assignments	Supplemental reading
  8/18	Class introduction and history of developments in extragalactic astronomyDownload Class introduction and history of developments in extragalactic astronomy
  8/23	surface brightness and surface brightness profilesDownload surface brightness and surface brightness profiles	Reading: Galaxies: an overview
  8/25	sizes, extinctions, k-correctionDownload sizes, extinctions, k-correction
  8/30	spectra, velocitiesDownload spectra, velocities
  9/1	distances and redshiftsDownload distances and redshifts	Paper Summary 1
  9/6	distances and redshifts (cont)
  9/8	galaxy morphologyDownload galaxy morphology galaxy surveys and selection functions Problems 1
  9/13	galaxy surveys and selection functionsDownload galaxy surveys and selection functions