This project aims to provide a first hands-on experience at doing actual research in observational cosmology. The goal of this project is to carry out a comparative study of the physical properties of two distinct galaxy populations in the early universe. The data you will be using in this project consist of the two deepest images of the universe ever taken: the Hubble Deep Fields (North and South) obtained by the Hubble Space Telescope. During this investigative study you will learn to derive the physical properties of galaxies from the actual observations, and to develop a critical intuition of the role selection effects, both experimental and cosmological, may play in your conclusions.

The objects to be studied in the HDF for this project are the populations of red and blue galaxies. Below you will find detailed instructions to carry out this comparative analysis. The results will be presented in a written report as described below. The deadline for submitting this report is April 24.

• Title Page (1 page):
  • Title

  • Name

  • Affiliation (including email)

  • Abstract (20 lines maximum)

• Observations (3 pages)
  • Data Selection (1page)
    • Your galaxy sample will consist of 20 blue galaxies and 20 red galaxies selected from the HDFN and HDFS fields. Your selection should be based not only on the color but also on another galaxy property (e.g, elliptical-type, irregular-type, similar magnitude, similar size...). You are free to define your samples as you wish.
    • The description of your selection criteria and stamp-size images of all objects in the blue and red samples should be presented in one page.

  • Overview of Observed Galaxy Properties (3 pages)
    • By clicking in each galaxy from the HDF N/S you will access some basic information about the object, including Galaxy ID, UBRI magnitudes, and photo-redshifts. In addition, you will be able to measure approximately the angular galaxy size from the stamp-size color image. Assume the stamp image is 10''x10'' in size. Using the observed angular radius (r) and B-band apparent magnitude you will be able to derive the observed B-band surface brightness, defined as mu_obs = m_B + 2.5 log(pi r²).

    • A table listing Galaxy ID, UBRI magnitudes, photo-redshifts angular sizes, and B-band surface brightnesses should be presented for each galaxy in the blue and red samples (two tables in one page).

    • Histograms showing the distribution of values for the photo-redshifts, B-band magnitudes, B-R colors, angular sizes, and B-band surface brigthnesses for the two galaxy samples should be presented in two pages.

• Measurements of Physical Galaxy Properties (3 pages)
  • Description of the formulas to be used (1 page).
    • Assume the universe is well represented by a RWF cosmological model with zero curvature and Ho=70 km/s/Mpc. For this model universe, the definition of the distance modulus can be rewritten as follows:
      m(z) = M + 5 x log d(z) - 5 x log(Ho/100) + 42.38 + K(z) + E(z) + A(z)

      where:

      • m = apparent magnitude in a given passband (or filter)
      • M = absolute magnitude in the same passband
      • d(z) = 2 x (1+z) x (1 - 1/sqrt(1+z))
      • K(z) = K-correction
      • E(z) = Evolutionary correction
      • A(z) = Extinction correction

      For simplicity, let's assume also that no corrections for evolutionary effects or IGM extinction are needed, and that the K-correction in the B-band can be approximated by the following expressions:

      • K(z) = 4 x z (for red galaxies at z<=1)
      • K(z) = 4 + 0.1 x z (for red galaxies at z>1)
      • K(z) = z (for blue galaxies at z<=1)
      • K(z) = 1 - 0.5 x z (for blue galaxies at z>1)

    • Describe the formulas you will need to calculate: (i) the luminosity distance (d_L); (ii) the angular distance (d_A); the absolute B-band magnitude; (iii) the intrinsic B-band surface brightness; and (iv) the linear size.

    • A table listing Galaxy ID, absolute B-band magnitudes, linear sizes, and B-band intrinsic surface brightnesses should be presented for each galaxy in the blue and red samples (two tables in one page).

    • Histograms showing the distribution of values for the absolute B-band magnitudes, linear sizes, and B-band intrinsic surface brightnesses for the two galaxy samples should be presented in one page.

• Discussion (1 page)
  • How do the physical properties of each galaxy sample compare to each other?
  • What do you think is the role cosmological effects and your selection criteria may play in your results?
  • Assume your galaxy samples are representative of the blue and red populations of galaxies in the HDF N/S. Considering the number density of red galaxies is comparable to that of blue galaxies in today's universe, what can you conclude about the early universe?