Homework on Properties of Galaxies in the Hubble Deep Field Name: Due: Friday, April 8 30 points Prof. Rieke & TA Melissa Halford

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1 Homework on Properties of Galaxies in the Hubble Deep Field Name: Due: Friday, April 8 30 points Prof. Rieke & TA Melissa Halford You are going to work with some famous astronomical data in this homework. The image data is available at 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. Introduction to the Hubble Deep Field In 1996 an experiment 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 Mar 30 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 configured 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 Hubble Deep Field as compared to the distance to the Hubble Deep Field? Make a quantitative estimate by assuming that M100 and the spiral in the yellow oval in the lower left are physically the same size and only appear different in size due to distance. An object s apparent size decreases the further it is away so you can assume that the diameters of objects that you measure from pictures like these can be directly ratioed to indicate relative distances.

2 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 one of three squares completely filled with an image. From the new image that appears, check the box for the camera that you have selected: A: B: C: 4) Click on one of the rectangles 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. 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)?

3 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). By what percentage do they differ from your average value? 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 Feb 29 available from the class home page). 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. Object Shape 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 galaxies). Three key factors that influence a galaxy s Color color are 1) the types of stars that dominate the light output; 2) how much interstellar gas and dust are Blue present in the galaxy; and 3) the galaxy s redshift (or White Yellow Red equivalently, its distance). The light output from elliptical galaxies is dominated by red giant stars while spiral galaxies have significant contributions from young and blue stars. Irregular galaxies tend to be dominated by young, blue stars. Ellipticals have very

4 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 one of the three camera sections. 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? 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.

5 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.