The student will analyze galaxies in and the structure of the Hubble Deep Field.
Background and Theory
As part of this exercise, you need to take a look at the
Hubble Deep Field (215K), an image that takes us
far out into space and far back in time. There are thousands of galaxies of
many shapes and colors. By deep, astronomers mean dim and distant.
This is an image of the faintest objects ever detected. It reaches 30th
magnitude, or about 4 billion times fainter than the naked human eye can see.
To create it, the Hubble Space Telescope was programmed to expose its
electronic detectors for about 100 hours over the course of 10 days,pointed
at the tiny region of space near the constellation Ursae Majoris.
This image covers an area about 1/100 that of the full Moon. After this image
was obtained, the 10-m Earth-based Keck telescope was used to observe the
faint blue galaxies in the image. Astronomers have concluded that the small
blue shards are among the most distant objects ever seen. These objects may
represent galaxies caught in the act of formation. In all, the number of
galaxies in the image implies that there are about 40 billion galaxies in the
Take a closer look at the reproduction of the Hubble
Deep Field (215K). Next to many of the galaxies is the redshift,
z, for that galaxy (except for a few cases, the corresponding galaxy
is usually the galaxy located to the upper left of the redshift
number). For nearby galaxies, where z is much less than 1, the
redshift is defined as:
It is humbling to realize that as we look at galaxies at distances
of 12 or 15 billion light years, we are viewing the Universe as it was 12 or
15 billion years ago. The concept of the finite speed of light and
look-back time means we actually can see our Universe as it was a
few billion years after its creation.
Where ' is the measured wavelength, is the rest wavelength, and v is the recessional
velocity. If you look closely at a few of the galaxies in
the Deep Field, you will note that there are redshifts of 1.36, 2.80, 3.23,
even 4.02. Does this mean v/c > 1 and these galaxies are traveling
faster than the speed of light?
Well, no. Once the redshift of a galaxy approaches 1, we must take special
relativity into account. We use a modified formula of:
As the recessional velocity of the galaxy approaches the speed of light, the
denominator becomes very small, so z approaches infinity.
Print out the worksheet
Most of the calculations have been done for you for this part of the exercise.
In addition to the Hubble Deep Field (215K) image
you will find a histogram of the cosmic redshifts
for a selection of galaxies in this field. That is, the redshifts have been
binned from z = 0.0 to z = 4.0 in 0.1 increments. Review
this histogram of the cosmic redshifts and the image
of the Hubble Deep Field (215K) and answer the
- Assume the Deep Field represents the actual distribution of galaxies in
this portion of the sky. (Note: This is an extremely small slice of the
cosmos. As more redshifts are observed, our conclusions here most certainly
will need to be modified.) Look at the histogram for
- Can you see possible large scale structure such as clustering? If so,
at what redshifts? (Look for redshifts where there are lots of galaxies.)
- Are there voids in this field? If so, at what redshifts?
(Look for redshifts where there are few or no galaxies.)
- There are a number of clearly identifiable galaxy types included in the
Deep Field image. Your instructor identified at least 5 spirals, 1 barred
spiral, and more than a dozen ellipticals. There are also 4 (and only 4,
look for diffraction spikes) dim stars from our Galaxy in the image. Locate
at least 10 galaxies and 2 of the stars and sketch them on grid 1.
- Label each galaxy sketched with the Hubble type of that galaxy.
- This is an extremely small slice of the Universe. We could identify a
cluster if we noted a number of galaxies close together in this 2-D image,
each of those galaxies having similar cosmological redshifts. Look for a
group of galaxies that you would classify as a cluster, and sketch 4 or more
galaxies belonging to that "cluster" in grid 2. Write their redshifts in the
- The galaxies have noticeably different colors. Do you see any overall
pattern between the color of the galaxy and its Hubble type? Based on what
you know about the colors of stars, give a brief description of the types of
stars that make up the different types of galaxies.
- Speculate on what astronomers would see if we had even bigger, more
powerful telescopes and detectors and looked even deeper into space. Explain
your answer based upon the history of our observations of the structure of the
Universe and what we have discovered as our instruments and technology have
© 2003 Weber State University
Revised: 24 April, 2003