Education Activities To Accompany Chandra Data Analysis Software M31 & Coma
Big, Bigger, Biggest
Activity 5: How bright is the gas?
A) In the text box you generated above, you will find another very useful bit of information. Look for the line that gives you the "flux". This should be a number approximately equal to 10-10 ergs/sec/cm2. What this means is that through each cm2 of area (about the size of your little finger's nail) at the satellite, about 10-10 ergs pass each second from the direction of the Coma cluster. This is a tiny amount of energy, but since the cluster emits x-rays in all directions, if we take this number and multiply it by the number of square centimeters in all space at the distance of the Earth from the cluster, we will get the entire energy output of the gas. To visualize this more clearly, imagine a basketball, with the cluster gas at the center. The Chandra satellite (and the Earth) is on the surface of this ball, and through each little 1 cm2 box on this surface about 10-10 ergs pass each second. Therefore, if we multiply this flux by 4*pi*r2 (where r is the ball's radius), we will get the total energy passing each second through the ball. This must be the total output of the gas. Do this! You should get an answer of about 1044 ergs/sec. This is about the same as the output of 100 billion Suns! (Since we are dealing with only the central part of the cluster, the actually result will be greater still).
B) Now, just for fun, suppose we call up our friends on another galaxy that is twice as far away from Coma as we are. They, too, have just launched a duplicate Chandra satellite, and are measuring the energy from the Coma cluster also. What will they get for the flux? What about the total energy output?
Further research showed that the mass of this X-ray emitting gas was roughly comparable to the total mass of the (optically visible) member galaxies. Where could this gas come from?
At first, it was thought that the clusters captured this gas from intergalactic space, but soon, it was found that a spectral line of highly ionized iron appears in all the x-ray spectra at an energy of about 6 keV. (Note that in the spectrum that you examined in Activity 4, this feature was not prominent. This is due to specific characteristics of the Coma cluster. For an advanced exercise, experiment with subtracting the background and rebinning the photons. You will be able
to pop out the iron line as a somewhat broadened, low peak. It is not a very strong feature in the Coma cluster. Also, bombardment by cosmic rays early in the Chandra mission degraded the ACIS instrument somewhat, and the emission is spread out over a greater range of energy. Astronomers must refine their analysis of each object to best bring out the features that they are looking for). Iron can only be formed within the interiors of stars, it is later released into the galactic environment through supernova explosions. Thus, the gas must be the product of the galaxies themselves.But how does it get distributed between the galaxies?
Several mechanisms have been proposed. For example, if two galaxies collide, the stars, being widely separated, will pass by each other like ships in the night, virtually unaware of each other's existence. But the gas in the interstellar medium will interact, and be left behind.In this picture, as the galaxies recede from each other, the gas will remain between the galaxies. However, further calculations seemed to indicate that there wouldn't be enough collisions between galaxies to account for all the X-ray emitting gas that is observed.
More likely it is a process we call "ram pressure stripping".In this scenario, the gas is lost by having the galaxies just move through intergalactic space. This process is similar to the wind knocking the hat off a rapidly pedaling cyclist (ALWAYS wear a helmet!). Calculations show that gas already expelled from stars via supernova explosions would have sufficient energy to blow enough interstellar gas out of even large spiral galaxies to account for the quantity of observed X-ray gas.
So the gas eventually fills an enormous region from one end of the cluster to the other. Because this gas responds and orients itself (as does all mass in the Universe) to the gravitational fields present in the cluster of which it is a part, it acts like a "map" of this field and the material therein. Thus, by examining the distribution of X-ray gas, we can deduce the amount of mass in the entire cluster.Let's see how this is done.
