Quiet Galaxies, 22 November 2013!Opening questions: !1.What is dark/unseen matter and gravitational lensing? !A.The unseen matter is undetectable and probably transparent, but it does a.have mass. The mass of dark matter is enough to bend the path of light from something behind it, according to the rules of gravitation. Lenses also bend the path of light, so this effect is called "gravitational lensing."!Why do spiral galaxies have halos? !B.No one can answer that. !a.Quiet galaxies, as opposed to active galaxies, do not interact.!2.You can see galaxies in all directions in the sky.!3.Galaxies were spotted as fuzzy blotches in the 1700s, and called nebulae. It A.wasn't until the 1900s that astronomers knew that galaxies were different from nebulae. !Hundreds of millions of galaxies can be seen with current telescopes, and B.there is no reason to think there aren't hundred's of million elsewhere as well. !Each other galaxy is equivalent to our own spiral disc galaxy. !C.Some galaxies settled into stars earlier or later that our 10 billion year old D.galaxy. !The oldest galaxies formed about 13.5-6 years ago, after the big bang a.13.8 billion years ago cooled enough. !Galaxies look different as they age. !b.While you will not need to know Edwin Hubble's tuning fork diagram, you will 4.need to know another diagram from the supplement (p 154): !Popular NamePhysical structureGalaxy type Star Populations 1 2elliptical ellipticalnoneallspiral discdiscbulge + haloirregularelliptical withoddityelliptical withodditysomtimes inthe odditymostlydisc withodditydiscdisc +sometimes inthe odditybulge + halounsymmetricalirregularallnoneAstronomers have learned a lot of information since the 1930s that A.antiquates the tuning fork diagram. It doesn't say anything useful any more, so there is no reason to use it. It is still in textbooks because there is nothing better to replace it.!Population 2 areas often look blue, and population 1 areas look yellow. !B."Irregular" refers to two types of galaxies: !C.Elliptical galaxies with oddities turn out to elliptical galaxies. Disc galaxies a.with oddities also turn out to be disc galaxies.!Disc galaxies are also called spiral galaxies, but the spiral is mostly an 1.illusion. !Unsymmetrical galaxies are truly irregular. !b.Elliptical galaxies: !5.This name is very right because these galaxies have an elliptical profile from A.any angle. !Elliptical galaxies are population 2 areas–"they have old stars with a little bit B.of gas and dust. !In elliptical galaxies, no star formation has occurred in the last 2 billion a.years. !Population 2 areas look as they do because they stopped forming 1.stars 2 billion years ago. The boundary between new stars and old stars is 2 billion years. !This measurement was made about 20 years ago, but it is not A.widely known. This is probably because astronomers mostly ignore elliptical galaxies. !Most elliptical galaxies are dwarf ellipticals. !C.For example, the Andromeda galaxy has two dwarf elliptical satellites a.about 2 million light years away (close by galaxy standards). !Dwarf ellipticals have a few billion stars in them, rather than the usual b.100 billion stars in large disc galaxies. !There do not seem to be any dwarf spiral galaxies, and there are only a c.few dwarf irregular galaxies. !One dwarf irregular galaxy is Sextans Dwarf. It looks like a dwarf 1.elliptical galaxy, except it is square. There is no known way to make a celestial object square according to gravity, but it happened anyway. !Any student who solves this problem to the satisfaction of experts A.in the specialty, gets an instant A for the entire course, regardless of anything else. !There are some medium sized elliptical galaxies. !D.Giant elliptical galaxies get all the attention: !E.No one talks about anything but regular sized spiral galaxies and giant a.elliptical galaxies. !Virtually all giant galaxies are ellipticals:!b.There are no giant irregular galaxies. !1.Giant elliptical galaxies are bigger than large spiral galaxies. !2.In any galaxy cluster, there are either 0, 1, or 2 giant elliptical galaxies. !c.Giant elliptical galaxies only occur near the middles of galaxy clusters, d.never in the outskirts. !The leading explanation for this is that giant elliptical galaxies are the 1.results of a bunch of galaxies merging together. !One giant elliptical galaxy is at the center of the Great Virgo Cluster, the e.largest cluster of galaxies close to earth. !The Virgo cluster is also notable in that it has several hundred 1.globular clusters around it. There are few other galaxies which have this many. ! The reason for this is not known, though it has attracted attention A.for 70-80 years. !Elliptical galaxies are the most abundant type of galaxy, but they rarely make F.it into slide sets.!They are faint, and not much activity is happening in them (star a.formation is already over). !Only giant elliptical galaxies are ever included, and then only rarely. !b.Spiral galaxies sell better. !c.Elliptical galaxies, like red dwarfs, are caught in a self reinforcing cycle of d.dullness. !No one learns anything new about them, so no one approves 1.telescope time to observe them, so no one learns anything new about them, etc.!You can get around this cycle by using full sky surveys like Sloan 2.Digital Sky Survey which incidentally photograph elliptical galaxies. !These can be found online. !A.This bias has lasted for 70-80 years, and is out of date. !3.Less than 3% of galaxies are truly irregular galaxies: !6.Most irregular galaxies are spiral or ellipticals with an odd growth. !A.It is possible to get time on telescopes for these galaxies because there is a B.lot of action in them. However, there are so few of these that there are still not many images. !The two nearest irregular galaxies are the Large Magellanic Cloud and the C.Small Magellanic Cloud.!They are satellites of the milky way galaxy: !a.They are called Magellanic clouds because Ferdinand Magellan's crew b.were the first to circumnavigate the globe and therefore the first to see them, and report them to Europe. !The clouds can only be seen in the southern hemisphere. !1.The Large Magellanic Cloud is blue, meaning that it contains a lot of c.recently formed blue giants.!In a long exposure of the Magellanic Clouds, you can see hydrogen pink d.nebulae around them and some non-blue areas where star formation happened more than 2 million years ago. !They are known as