question, starting with how to quantify the trend and then determine the procedure themselves for making the calculations below. Using a calculator, find the percentages of each galaxy type in the cluster versus the field( Ignore IRs and INTs).
Use your numbers from the table above to calculate the percentages and fill in each of these blanks below: In the Cluster:% of Ellipticals( e / h) = % % of Lenticulars( f / h) = % % of Spirals( g / h) = % In the Field:% of Ellipticals( i / m) = % % of Lenticulars( j / m) = _ % % of Spirals( k / m) = %
Question: Where did you find a higher percentage of spirals- in the Cluster or in the Field? Answer: _ Tell students: The percentages that you just found tell us which types of galaxies are common in the Coma Cluster versus which types are common in the field.
Astronomers have done this same experiment on hundreds of thousands of galaxies in the nearby universe, and discovered that the following percentages are pretty typical:
ˆ In dense clusters, 40 % of the galaxies are ellipticals, 50 % are lenticulars, and 10 % are
spirals. ˆ In the field, 10 % of the galaxies are ellipticals, 10 % are lenticulars, and 80 % are spirals.
When galaxies are found very close together there are more ellipticals and lenticulars. When galaxies are far apart there are more spirals. Astronomers call this the " morphology-density effect." This term basically means that in crowded galaxy neighbourhoods, like clusters, there are different types of galaxies than are found in open areas, like the field.
Step 9
Students should by now( from Step 7) have asked the question, " Why do we see more elliptical and lenticular galaxies in clusters and more spirals in the field?"( This question can also be phrased, " Why do we observe the morphology-density effect?") They should also have had the idea that interactions could be involved, and maybe even the idea that more interactions take place in denser environments, like the center of a cluster. Below is information that can be used to answer this question. You can give students this text to read, then ask them to discuss and write down an explanation for this effect; or you can continue to prompt students to brainstorm and discuss ideas for possible explanations, then potentially have them do research in textbooks / on the internet on their own or in groups, and then have them share their explanations with each other.
Explanation: Many galaxies contain what astronomers call " gas," which generally means hydrogen gas, sometimes mixed with the gases of other elements, and sometimes mixed also with dust. Gas clouds can collapse by gravity, which leads to the formation of stars. Astronomers have observed many spiral galaxies( S and SB) and find that most of these galaxies contain a lot of gas, and are currently forming lots of new stars. Elliptical and lenticular galaxies( E, S0, and SB0) are gas-poor and are not making many new stars. Galaxies that are very close to each other, such as those in clusters, often undergo many violent interactions with each other. When a gas-rich spiral galaxy interacts with another galaxy, it tends to quickly use up most of its gas to make new stars, leaving little gas behind. Galaxy-galaxy interactions often change gas-rich galaxies into gas-poor galaxies. Many lenticular galaxies are the remains of old spirals that have lost their gas, and many elliptical galaxies are the remains of several spiral galaxies that have collided. Galaxy clusters are usually filled with a lot of extremely hot gas that is spread between galaxies throughout the cluster. However, there is no hot gas like this out in the field. When the radiation from this hot gas hits a spiral galaxy, it strips the spiral galaxy of its much cooler gas
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