Interacting Massive. Particles), or other new sub-atomic particles. •Primordial
black holes. •Non-luminous gas. 32-Mpc simulation cold-dark matter Universe.
Dark Matter •Dark matter may be: •MACHOS (Massive Compact Halo Objects), dwarf stars, planets •WIMPS (Weakly Interacting Massive Particles), or other new sub-atomic particles •Primordial black holes •Non-luminous gas 32-Mpc simulation cold-dark matter Universe. Courtesy R. Cen, Princeton University.
A History of Galactic Discovery
• In the early 20th century, the existence of other galaxies was unknown – The Milky way was the Universe! – Other galaxies were called nebulae
• Light from galaxies always appears fuzzy and diffuse, due to the vast separation between the Sun and the observed galaxy, as well as the separation between the stars of that galaxy! – The paleness of visible light from distant galaxies is called the surface brightness.
• Galaxies are therefore difficult to observe, even with good telescopes.
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A History of Galactic Discovery • In the 1700’s, Charles Messier was observing comets, and kept finding objects that while fuzzy, were not comets – He made a list (or catalog) of these undesired objects, so he could avoid seeing them – They became known as Messier Objects, a number preceded by an M. – M31 (the Andromeda galaxy) is one such object
• William and Caroline Herschel (1800’s) developed a catalog of faint objects in the heavens – Now known as the New General Catalog – Objects are known by a number preceded by the letters NGC – Objects can appear in both the Messier and NGC catalogs!
M31
A Sky Full of Galaxies
• Technology has advanced to the point where we have found as many galaxies as there are stars in the Milky Way!
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A 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 universe then each galaxy would be about 25 mm across and separated by about 1/2 meter away. •In contrast if the galaxy was the size of Mendota the solar system would be 25 microns in diameter and the nearest star would be 1/2 meter
Our Galactic Neighborhood
• • •
The smallest organization of galaxies are called galaxy groups Our local group is called the Local Group The Local Group contains 40 known members, including the Andromeda Galaxy and the Large and Small Magellanic clouds, dwarf satellite galaxies of the Milky Way
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Distances to other galaxies • We can use Cepheid variable stars to measure the distance to other galaxies • A Cepheid’s luminosity is proportional to its period, so if we know how rapidly it brightens and dims, we know much energy it emits • If we see a Cepheid in another galaxy, we measure its period, determine its luminosity, and calculate its distance! • Distance between galaxies is huge! – M31 is 2 million Lightyears away – M100 is 55 million Light years away.
The Redshift and Expansion of the Universe • Early century astronomers noted that the spectra from most galaxies was shifted towards red wavelengths • Edwin Hubble (and others) discovered that galaxies that were farther away (dimmer) had even more pronounced redshifts! • This redshift was interpreted as a measure of radial velocity, and it became clear that the more distant a galaxy is, the faster it is receding! 20th
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The Hubble Law
• In 1920, Edwin Hubble developed a simple expression relating the distance of a galaxy to its recessional speed. • V=H×d – V is the recessional velocity – D is the distance to the galaxy – H is the Hubble Constant (70 km/sec per Mpc)
• This was our first clue that the universe is expanding!
Spiral Galaxies
• Spiral arms and a central bulge • Type S
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Elliptical Galaxies
• • • •
No spiral arms Ellipsoidal shape Smooth, featureless appearance Type E
Irregular Galaxies
• Stars and gas clouds scattered in random patches • No particular shape • Type Irr
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Barred Spirals
• Related to regular spiral galaxies, barred spirals are noted for their large bar of stars across the central bulge • Type SB
S-zero galaxies
• The arms of the spiral begin at the end of the bars • Recently discovered that the Milky Way is a barred spiral!
• S0 galaxies are in the shape of a disk, but have no spiral arms • It is likely that the gas and dust have been blown out of such galaxies • The lack of gas and dust means that no new stars can form, so there are no spiral arms
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The Tuning Fork
• Edwin Hubble (busy guy!) organized these different galaxy types into a tuning fork shaped diagram • Ellipticals are labeled E0-E7 – E0 is almost perfectly spherical, E7 is quite flattened
• Spirals are labeled Sa – Sd – Sa galaxies have tightly wound arms and a large central bulge – Sd galaxies are loosely wound and have a small central bulge
• Barred Spirals are labeled SBa – SBd – Same flow as the Spirals
Additions to the list…
• Dwarf galaxies (left) are difficult to detect, and may be the building blocks of larger galaxies • Low Surface Brightness galaxies (above left) are very large, yet very faint galaxies that have very little new star formation occurring
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Differences in Star and Gas Content
• Ellipticals: – Low in gas and dust, so contains mostly older Pop II stars – Contain very high temperature, very low density clouds of gas that cannot condense into stars.
• Spirals: – Lots of gas and dust, so have active regions of star formation – Have both Pop II and younger Pop I stars
• Irregulars: – Many hot, young stars – Large amounts of interstellar matter – Might be young galaxies
The nuclei of most spiral galaxies appear redder than their spiral arms 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 reactions
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A look back in time
• The Hubble Space Telescope was pointed at a part of the sky that looked empty, taking a 100hour exposure • Very distant galaxies were detected, some closer than others • This technique allows us to see galaxies at various stages of formation • These early galaxies tend to be smaller than the Milky Way, and to not fall into Hubble’s classification scheme
Galactic Collisions
• Galaxies can collide, though not in the sense of a car accident! • The galaxies pass through one another, and their immense gravitational pull tears both galaxies apart! • Eventually, a new elliptical galaxy will form…
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Galaxy collision and merger
Collision movies
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The Mice
• These two interacting galaxies are tidally distorting each other.
Which of the following is least easily explainable as a result of interaction between galaxies? a) Some galaxies have long "tails" of stars. b) Rich, regular clusters are dominated by central giant ellipticals. c) Both spiral and elliptical galaxies are seen at very high redshift. d) Some galaxies seems to be undergoing bursts of star formation.
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Rich and Poor Galaxy Clusters
• Rich clusters: – Contain hundreds to thousands of member galaxies – Are roughly spherical, with the largest galaxies near the center – Contain mostly elliptical and type S0 galaxies – Lots of hot gas and dust
• Poor clusters – Contain only tens of galaxies – Have a ragged, irregular appearance – More spiral and irregular galaxies
Superclusters
• Clusters of clusters are called superclusters – Contain a few to many dozen clusters of galaxies – Can be Mpc across! – The Local Group is part of the Local Supercluster, shown at left.
• The Local Supercluster is heading toward a region of space known as the Great Attractor, where there are a large number of massive superclusters • There may be super-superclusters!
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Large Scale Structure in the Universe
• Using modern technology, astronomers have mapped the location of galaxies and clusters of galaxies in three dimensions • Redshift is used to determine distance to these galaxies • Galaxies tend to form long chains or shells in space, surrounded by voids containing small or dim galaxies • This is as far as we can see!
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