Education Activities To Accompany Chandra Data Analysis Software M31 & Coma
Big, Bigger, Biggest
Activity 6:get a radial profile of the surface brightness of the Coma cluster
Just like we did for Cas-A in another set of pages, do the following:
A) Select an annulus for your region shape, and center the cursor over the brightest part of the cluster. Click once to create the region, and again to select it. Grab a corner of the region and drag it outward until you are covering most the x-ray cluster gas with the outer annulus ring.
B) Go to the Region > Get info... dialog box and enter an inner radius of "0", and 10 for the number of annuli. (Note: you MUST select the region by clicking once anywhere inside it before you go to the "get info" dialog box).
C) Click on "generate" and then "apply". You will notice that 10 concentric rings appear like a bulls-eye surrounding the cluster. What we will find out is how bright the gas is as a function of how far you are away from the center.
D) Under analysis, click on "radial profile".
Your graph shows you that the intensity of the cluster steadily decreases from the center.(How is this different from the Cas-A result?) This is indicative of a mass of X-ray gas filling a region with the brightest area at the gravitational center of the cluster. Using this brightness profile, coupled with the result for the temperature you saw earlier, scientists can model the cluster to determine how much mass must be present in order to have the X-ray gas "float" in the gravitational field set up by all the galaxies.
What we find is astonishing. In order for the gas to be at the brightness and temperature observed, we need about 100 times more mass in the cluster than that which is seen in the star-filled galaxies. This is the famous "missing-mass" problem, which has been dogging astronomers for well over 50 years. What this is apparently telling us is that there is unseen matter in the universe, amounting to 100 times what we can see with all our telescopes. This result is consistent no matter which cluster we look at. All clusters appear this way.
This "missing matter" is inferred by observations of individual galaxies as well. Here, the stars in these galaxies appear to be moving in such a way as to require, again,about 100 times more mass in any galaxy than we can see in gas and stars.
More recently, though, this extraordinary result is being called into question. For if we just assume that this missing matter exists, we are forced to predict other consequences in the appearances and motions of stars and galaxies that don’t really seem to fit the observed data. This has led some scientists to question our assumptions about the nature of the law of gravity itself. Our understanding of this law is definitely incomplete (since, for example, we have no quantum theory of gravity at all, and therefore have no understanding about the way masses behave gravitationally on the scale of atoms), so a new form of this law may allow us to explain this puzzle without the need for "missing" mass. In any event, we are left with an amazing legacy: either 99% of the universe is in the form of some exotic type of unseen matter, or else the known forces in the Universe are going to be radically changed. Stay tuned for the latest in this intensively studied area of research.
