Galaxies, a graduate level course

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.

Lecture Slides

Learning Objectives

Describe (in a few sentences) the main historical developments in extragalactic astronomy. Describe what surface brightness is and how does it vary with wavelength?

Basic measured property for the imaging of a resolved or extended object. It measures the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy.
Units: mag/arcsec2 or W/m2arcsec2
Often given in terms of light in a bandpass.
In a simple static universe, SB is independent of distance but in an expanding universe, SB decreases as (1+z)4.

Explain why galaxies are characterized by 1D SB profiles and how this is done in principle.

Most galaxies are axisymmetric at a significant level and can be fit well with elliptical isophotes.
The SB profiles are fit using parametrized profiles.
Spirals can be fit using the exponential profile, taking the scale radius (r_s) (radius where the surface brightness drops by 1/e) and SB at the scale radius (sigma_s) as parameters

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

Elliptical galaxies can be fit using the de Vaucouleurs profile, taking the effective radius r e (half-light radius, radius that encloses half of the total light if the model is extrapolated to infinity) and the SB at the effective radius Σ e as the parameters.

\( \Sigma(r) = \Sigma_e e^{-7.67 (-r/r_s)^{1/4} - 1 } \)

These models can be generalise using the Sersic profile, taking three parameters: the half light radius, the effective SB and the Sersic Index (n = 1 for spirals, n=4 for ellipticals) 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. Using isophotes, the size can be measured up to a minimum SB. For e.g., Holmberg Radius at SB = 26.5 mag/sq.arcsec or half light radius, which is the radius that encloses half the brightness of the galaxy if the SB model was extrapolated to infinity. Isophotal quantities are dependent on distance. Galaxies that are farther away might appear smaller since they are fainter.• Similarly, the integrated brightness can be measured within an aperture of fixed angular size (metric magnitude), within a specified SB contour (isophotal magnitude) or by summing over the entire model (model magnitude).

The Petrosian Radius measures the radius at which the SB drops to some fraction of the average surface brightness within that radius. This quantity is significantly less dependent on distance.

The Petrosian Magnitude is the magnitude within a fixed number of Petrosian radii. 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