Homework #7: Properties of Galaxies in the Hubble Deep Field Name:_____________________
Due: Friday, October 31
30 points
Profs. Rieke
You are going to work with some famous astronomical data in this homework. The image data is
available at http://ircamera.as.arizona.edu/Astr170B1/Exercise_page/exercise.html. Follow the narrative
below, and go to the web site as needed to examine the images. Print out this pdf and turn it in with your
answers. You must analyze the image sections that were assigned to you on D2L to receive full
credit.
Introduction to the Hubble Deep Field
In 1996 an experiment was initiated by former UA professor Bob Williams in how to take pictures of the
sky was executed using the Hubble Space Telescope. This was both an experiment in the sense of whether
images that collected photons over a period of several days would reveal anything fainter than was
already know and an experiment in the sense of a single dataset being made available to astronomers
worldwide for them to analyze as they wished. The Hubble Deep Field is described at the end of the Oct
15 lecture on “Types of Galaxy”.
To get oriented, look at the images below. The notches in the images are due to the fact that one of the
camera’s light sensors was designed to take a more magnified views of the sky than the other three
sensors. The higher magnification means that less of the sky is seen. We won’t be using any of the
imagery from this section of the camera. The two images cover the same area of the sky, but the galaxy in
the right hand image is only 56 million light years away, so close that it overfills the camera’s view.
Left hand image: Hubble Deep Field
Right hand image: Spiral Galaxy M100
1) Given how large M100 appears compared to the galaxies in the Hubble Deep Field image, what can
you conclude about the distance to the largest galaxies in the Hubble Deep Field as compared to the
Name:_____________________
distance to M100? Make a quantitative estimate by assuming that M100 and the yellowish spiral near the
star in the lower left (
) are physically the same size and only appear different in size due to distance.
2) The total exposure time for the Hubble Deep Field was about 240 hours. The total exposure time for
the M100 image was 3.3 hours. Review the end of the lecture on Light which discusses how the apparent
brightness of an object changes with its distance from us. Explain whether this exposure time difference
is in agreement with your answer to question 1.
Part 1: How many objects are visible?
When the Hubble Deep Field data were first shown to astronomers, the images raised a number of
questions in the astronomers’ minds. One obvious question was “How many objects are present in the
image?”. You are going to measure how many galaxies are in the image, and use that to estimate how
many galaxies there across the entire sky.
At the web site mentioned above, follow the Part 1 link which will take you to a copy of the Hubble Deep
Field data. If we wanted to keep you busy for hours, we could just have you count the objects that you
see in this image. Rather than have you count everything in the image, we will have you use a procedure
that scientists often employ when faced with a large data sample. When we ask “how many objects are
there?”, we can use sampling to save work. We will assume that objects are distributed uniformly across
the image. You can tell by eye that this is not quite strictly correct, and sampling techniques can be used
to check the assumption of uniformity.
3) Click on your assigned square and check the box for the camera that you were assigned:
A:

B:

C:

4) Click on your assigned rectangle in the new image and enter its number:_______________
5) Count how many objects you see in this image section:___________________________
6) Select a second area: Camera:
A:

B:

C:

Section Number:__________
7) Count how many objects you see in this image section:___________________________
8) Compute the average of your two counts and enter here:__________________________
This represents the average number of objects in a section based on studying two sample sections.
Name:_____________________
The power of the sampling technique lies in taking subset of the data and using it to give an estimate for
what’s present in the entire data set. Here’s how to do this in this case to derive an estimate of the
number of galaxies in the Hubble Deep Field and ultimately in the entire sky:
9) How many sections are there in the entire deep field (ignoring the camera with the different
magnification, ie. for three cameras) ?
10) Use your average count for a section from 8) and the fraction of the deep field represented by one
section to compute the total number of objects:
Average count per section x
________________
Number of sections = Total number of objects in
the Deep Field
x ________________ = __________________
11) Knowing that it would take about 30 million images of the area represented in the Hubble Deep Field
image to cover the entire sky, compute the number of objects in the entire universe that would be seen if
we could take images of the entire Universe as deep as the Hubble Deep Field. Setup the calculation in a
manner similar to 10).
12) Review your counts for your two sections from 5) and 7).
your average value?
By what percentage do they differ from
13) Give two reasons why the two counts might differ even if you did everything correctly (Hint:
Review the lecture on Distribution of Galaxies in Space and the discussion of counting statistics from Oct
17 discussion sections).
Name:_____________________
Part 2: Classifying the Deep Field Objects
Knowing how many objects are visible in the Hubble Deep Field is just the start of using such data.
Ideally we would like to know what the objects are (foreground stars in the Milky Way or small, dim
nearby galaxies or large, bright extremely distant
Object Shape
galaxies). Three key factors that influence a galaxy’s
1
2
3
4
5
color are 1) the types of stars that dominate the light
output; 2) how much interstellar gas and dust are
present in the galaxy; and 3) the galaxy’s redshift (or
Color
equivalently, its distance). The light output from
Blue
elliptical galaxies is dominated by red giant stars while
White
spiral galaxies have significant contributions from
young and blue stars. Irregular galaxies tend to be
Yellow
dominated by young, blue stars. Ellipticals have very
Red
little interstellar material while spirals and especially
irregulars have a lot. As described in the lecture “Other
Stars”, any star seen in the deep field might have any color in the visible spectrum but recall that red stars
are the most common type of star in the Milky Way (but don’t assume that this means that galaxies like
the Milky Way look red because these common stars are also very dim so they do not dominate the light
output). Click on Part 2 from the exercise start page. Click on your assigned camera section. A labelled
image will appear. Examine the image carefully and classify each numbered object using the scheme in
the table. Enter an object’s number into the correct bin in the table.
14) Objects in the column 1 are stars (the spikes are diffraction spikes cause by structures inside the
Hubble Space Telescope) while the other objects are galaxies. Did you find more stars than galaxies?
Give a reason why this is the case.
15) Identify columns 2-4 with the galaxy types discussed in lecture. A galaxy type might contribute to
more than one column and indicate whether you found any such examples.
16) What type of galaxy (elliptical, spiral, irregular) is most common in your sample?
Name:_____________________
Part 3: Distances to the Deep Field Objects
Return to the exercise main page and click on Part 3. A section of the Hubble Deep Field will pop up with
some galaxies labelled from A to F. In the table below, enter the letters in what you think is in order of
distance:
Nearest -------------------------------------------------------------------------------------Most Distant
17) Explain what criteria you used to put the galaxies in distance order.
18) By taking spectra and measuring redshifts, we have determined that galaxy ‘F’ is actually closer than
galaxy ‘C’. Explain how this can be true.
Part 4: What is this?
Click on Part 4 from the exercise main page. An image of a single object will appear.
19) From what you have learned in this exercise and in lecture, what type of object do you think this is?
Give at least two reasons for your choice or why you eliminated possibilities.