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To Start Data Analysis You Must Log Into Overview Your task is to use Spitzer Space Telescope observations to determine how much infrared energy is being emitted from the center of certain galaxies. You will use images of galaxies in 4 different wavelengths from the Infrared Array Camera (IRAC); 3.6, 4.5, 5.8 and 8.0 microns. The galaxies we have targeted are called Active Galactic Nuclei, or AGN. They are called "active" because they emit large amounts of energy at many wavelengths above and beyond what we would calculate based on the total numbers of stars at their center. We believe the excess energy is generated by material flowing into a huge black hole at their centers. Your measurements of the infrared emission from these objects will be combined with your (or other students’) measurements of the radio emission from these same objects, as well as estimates made by professional astronomers of the masses of the black holes at their centers. We hope that we will learn more about how AGNs work by determining if and how their infrared and radio emissions depend on the mass of the black hole. To determine the infrared energy emitted from the surrounding region of the black hole, you will do these things: 1) Log onto a Lewis Center computer, and run two computer programs (called IDL and ATV) that are used for visualizing and manipulating data. 2) Load your Spitzer data into ATV and adjust the image of the AGN so you can more easily see it. 3) Determine how much energy was received from the center of the AGN. This is a two step process. First, you measure how much total energy is coming from the central region. Second, you must subtract off any background emission that is present. (This background is emission from many other things that are in the area, in addition to the nucleus of the galaxy.) As you will see, the ATV program is set-up to make this very easy for you to do! 4) Answer some questions about what you have learned and relate them to your radio work. At the end of this document we list the masses of the black holes you have observed. You will use this information in the analysis of the data. 1. Start the Data Analysis Software To start data analysis you must log into: http://idl2.gavrt.org/ You will get a page with a password requirement. The password is: g4vrt4u Then you will get the following screen that may also require a password. Put in the same password. You will have an image of another desktop. On that desktop is an icon called “IDL.” Double click on it to open This will start two screens, one is running a program called IDL: and the other has a program called ATV which is generated by IDL. It is the tool you will use to display and analyze the data from the Spitzer Space Telescope. Right now it should be displaying a cross, which is the default image: IMPORTANT NOTE: At times the cursor can leave a trail that can cover the text (see figure on left). If you need to refresh the screen DO NOT use the “Page Reload” option of the browser (the one with the circular arrow on top)!!! Either use the “Refresh” button at the top center, or minimize and then maximize the screen. If you use the “Page Reload” button you will be taken to the login screen again, though your work will not be destroyed. 2. Loading Your Data and Adjusting the Image Spitzer files are saved in FITS format which is the most common format that astronomical images are saved. It stands for Flexible Image Transport System. Go to the “File” menu and open the image that you wish to analyze using “Readfits.” These images should have been uploaded to the Lewis Center and placed in a directory where you should be able to open them. If the images you are looking for do not show up when you use the “Readfits” function, contact the Lewis Center. As an example you can load in the image n4945-24um.fits. It should look like this: Once you have opened an image you can change the display in two forms to get a better look at the data. This is called changing the “stretch” of the image. First you can press the left mouse button and hold it down and move the mouse. This will change the relative brightness on the screen. If you only have one mouse button, which is typical on Apple computers, don't worry! You can jump straight to the second way of stretching the image. You also could try pressing either the "option" key, or the down- arrow key (if you have one) at the same time as pressing your mouse button. Second you can apply a mathematical function to the image. This is the most useful method since often the main problem is that there are very bright and very faint things in an image so trying to display both simultaneously is difficult. But if you apply a logarithm function to the images where each pixel value is replaced by the log of that value then it becomes easier to see bright things next to faint things. For example if one pixel has a value of 10 and the pixel next to it is 10 times brighter and has a value of 100, it is difficult to analyze. But if you took the log of both pixel values then their pixel values change from 10 and 100 to 1 and 2 (log(10)=1, log(100)=2) so it become easier to see the brighter value next to the fainter one. Go to the “Scaling” menu and select “log” for the image you have loaded. You can still use the left mouse button to change the relative brightness on the screen. A final way to change the display is to change the minimum (Min) and maximum (Max) values displayed. If the image has a value higher than the maximum you specify, all pixels higher than that maximum value are displayed as if they had that maximum value. Similarly, if you specify a minimum value to plot, image values lower than that are treated as if they have the minimum value. You specify the minimum and maximum values in the upper middle of the ATV window, just to the left of the two small images. Change those numbers to see how the image changes. Now you are ready to do photometry. 3. Determining the Light From the AGN Photometry is the measurement of the brightness of an object. The brightness that is recorded on an electronic detector (or any kind of detector) is a combination of the brightness of the source plus the brightness of the background that the source is on. In the case of the center of the galaxy above, a great deal of the "background" is in fact from the surrounding host galaxy. But what we are interested in is only the light from the center and not from the rest of the galaxy. To account for this, we will determine how much light is coming from where the center of the AGN is, and then compare it to how much light is coming from near the center of the AGN. We assume that the "background" near the AGN is the same as the background right on the AGN, so by subtracting the two, we are left with only the brightness of the center of the AGN. To start the photometry process, place the cursor on the center of the galaxy (or in general on whatever you want to do photometry on) and press the “P” key. Notice that on the upper right of ATV, there is a window that magnifies the region your cursor is on, or you can simply zoom in using the “ZoomIn” function just above the image. Once you press “P,” then a new window opens with a magnified region around where your cursor was with 3 circles drawn around your photometry target. The green circle is around the target whose light you want to measure and it is called your “photometry aperture.” The blue and purple circles define a ring in which the sky measurement will be made which will give the average background value per pixel that will be subtracted from each pixel within the aperture value. Then after all the pixels within the photometry aperture have this average per pixel sky value subtracted from them, they are totaled up and are displayed on the right as “Object Counts: 53770.5” The next step revolves around the radius of the photometry aperture. How do you know if it is big enough or maybe if it is too big? To see if you have included the light mainly from the center and not mostly from the background, press the “Show Radial Profile” key: This is a one dimensional representation of the image. The brightest part of the image is at zero radius on the x-axis (the numbers are partially hidden at the bottom of the screen) and has the most counts (2300 counts on the y-axis). But as you go away from the center of the image and to the right on the plot you see that the counts come down and then they are almost flat but they don’t actually reach zero counts. This is because there is background or sky contribution which in this case comes from the rest of the host galaxy and is noted in red as “sky level.” The sky level is measured within a ring between the inner sky measurement radius (insky) and the outer sky measurment radius (outsky).
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