1Dark Matter•Dark matter may be:•MACHOS (MassiveCompact Halo Objects),dwarf stars, planets•WIMPS (WeaklyInteracting MassiveParticles), or other newsub-atomic particles•Primordial black holes•Non-luminous gas32-Mpc simulation cold-dark matter Universe. Courtesy R. Cen, Princeton University.A History of Galactic Discovery• In the early 20th century, the existenceof other galaxies was unknown– The Milky way was the Universe!– Other galaxies were called nebulae• Light from galaxies always appearsfuzzy and diffuse, due to the vastseparation between the Sun and theobserved galaxy, as well as theseparation between the stars of thatgalaxy!– The paleness of visible light from distantgalaxies is called the surface brightness.• Galaxies are therefore difficult toobserve, even with good telescopes.2• In the 1700’s, Charles Messier wasobserving comets, and kept finding objectsthat while fuzzy, were not comets– He made a list (or catalog) of these undesiredobjects, so he could avoid seeing them– They became known as Messier Objects, anumber preceded by an M.– M31 (the Andromeda galaxy) is one suchobject• William and Caroline Herschel (1800’s)developed a catalog of faint objects in theheavens– Now known as the New General Catalog– Objects are known by a number preceded bythe letters NGC– Objects can appear in both the Messier andNGC catalogs!M31A History of Galactic DiscoveryA Sky Full of Galaxies• Technology hasadvanced to the pointwhere we have foundas many galaxies asthere are stars in theMilky Way!3A Sky Full of Galaxies•Diameter of the known universe ~ 8 x 10^10 LY•Avg distance between galaxies ~ 1 million LY(10 to 30 big galaxy diameters)•If lake Mendota was the observable universethen each galaxy would be about 25 mm acrossand separated by about 1/2 meter away.•In contrast if the galaxy was the size of Mendotathe solar system would be 25 microns indiameter and the nearest star would be 1/2 meterOur Galactic Neighborhood• The smallest organization ofgalaxies are called galaxy groups• Our local group is called the LocalGroup• The Local Group contains 40 knownmembers, including the AndromedaGalaxy and the Large and SmallMagellanic clouds, dwarf satellitegalaxies of the Milky Way4Distances to other galaxies• We can use Cepheid variablestars to measure the distanceto other galaxies• A Cepheid’s luminosity isproportional to its period, so ifwe know how rapidly itbrightens and dims, we knowmuch energy it emits• If we see a Cepheid in anothergalaxy, we measure its period,determine its luminosity, andcalculate its distance!• Distance between galaxies ishuge!– M31 is 2 million Lightyearsaway– M100 is 55 million Light yearsaway.The Redshift and Expansion of the Universe• Early 20th centuryastronomers noted that thespectra from most galaxieswas shifted towards redwavelengths• Edwin Hubble (and others)discovered that galaxiesthat were farther away(dimmer) had even morepronounced redshifts!• This redshift wasinterpreted as a measure ofradial velocity, and itbecame clear that the moredistant a galaxy is, thefaster it is receding!5The Hubble Law• In 1920, Edwin Hubbledeveloped a simpleexpression relating thedistance of a galaxy to itsrecessional speed.• V = H × d– V is the recessionalvelocity– D is the distance to thegalaxy– H is the Hubble Constant(70 km/sec per Mpc)• This was our first clue thatthe universe is expanding!Spiral Galaxies• Spiral arms and a central bulge• Type S6Elliptical Galaxies• No spiral arms• Ellipsoidal shape• Smooth, featureless appearance• Type EIrregular Galaxies• Stars and gas cloudsscattered in random patches• No particular shape• Type Irr7Barred Spirals• Related to regular spiralgalaxies, barred spirals arenoted for their large bar ofstars across the central bulge• Type SB• The arms of the spiral begin atthe end of the bars• Recently discovered that theMilky Way is a barred spiral!S-zero galaxies• S0 galaxies are in theshape of a disk, buthave no spiral arms• It is likely that the gasand dust have beenblown out of suchgalaxies• The lack of gas anddust means that no newstars can form, so thereare no spiral arms8The Tuning Fork• Edwin Hubble (busy guy!)organized these differentgalaxy types into a tuning forkshaped diagram• Ellipticals are labeled E0-E7– E0 is almost perfectly spherical,E7 is quite flattened• Spirals are labeled Sa – Sd– Sa galaxies have tightly woundarms and a large central bulge– Sd galaxies are loosely woundand have a small central bulge• Barred Spirals are labeled SBa– SBd– Same flow as the SpiralsAdditions to the list…• Dwarf galaxies (left) are difficult todetect, and may be the building blocksof larger galaxies• Low Surface Brightness galaxies (aboveleft) are very large, yet very faintgalaxies that have very little new starformation occurring9Differences in Star andGas Content• Ellipticals:– Low in gas and dust, so containsmostly older Pop II stars– Contain very high temperature,very low density clouds of gasthat cannot condense into stars.• Spirals:– Lots of gas and dust, so haveactive regions of star formation– Have both Pop II and youngerPop I stars• Irregulars:– Many hot, young stars– Large amounts of interstellarmatter– Might be young galaxiesThe nuclei of most spiral galaxies appear redder than their spiralarms because of a) young blue stars in the arms, and old red ones in the nuclei b) emission nebulae and dust in the nuclei c) receding nuclei and advancing spiral arms (Doppler shifts) d) nuclear reactions10A look back in time• The Hubble Space Telescopewas pointed at a part of the skythat looked empty, taking a 100-hour exposure• Very distant galaxies weredetected, some closer than others• This technique allows us to seegalaxies at various stages offormation• These early galaxies tend to besmaller than the Milky Way, andto not fall into Hubble’sclassification schemeGalactic Collisions• Galaxies can collide, though not in the sense of a car accident!• The galaxies pass through one another, and their immensegravitational pull tears both galaxies apart!• Eventually, a new elliptical galaxy will form…11Galaxy collision and mergerCollision movies12The Mice• These two interacting galaxies are tidallydistorting each other.Which of the following is least easily explainable as a result of interaction between galaxies? a) Some galaxies have long "tails" of