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Galaxies Astronomy 1-2 Lecture 24-1 Is Our Galaxy All There Is Until the early twentieth century, the Milky Way was thought to be the extent of the universe All objects that were seen, such as nebulae, were thought to be contained within the Milky Way There were those who disagreed with this conclusion It was not until 1924, that the true location of many of these objects was determined Astronomy 1-2 Lecture 24-2 Its Bigger Than we Thought Edwin Hubble noticed that there were Cepheid variable stars in the Andromeda “nebula” Using the luminosity, period relation for Cepheids and the measured luminosity, he showed that the Andromeda system was far outside of the Milky Way Hubble did similar studies with respect to many of the other “nebulae” and found that they also were outside of the Milky Way Thus there are many systems like the Milky Way spread throughout the universe Astronomy 1-2 Lecture 24-3 Hubble’s Galaxy Classification This pair of images shows the Coma cluster of galaxies Almost every object visible is a galaxy Astronomy 1-2 Lecture 24-4 Galaxy Classification Hubble first classified the various types of galaxies He classified the galaxies into four basic groups: Astronomy 1-2 Elliptical Galaxies Spiral Galaxies Barred Spiral Galaxies Irregular Galaxies Lecture 24-5 Tuning Fork Description As part of his classifying the galaxies, Hubble produced a diagram that shows that transition from elliptical galaxies to spiral galaxies This diagram does not imply any evolutionary progression in galaxy formation Astronomy 1-2 Lecture 24-6 Spiral Galaxies These are galaxies with lanes of stars and nebula emanating from a central nucleus Spiral galaxies range from Tightly wound arms and Large central bulge Loosely wound arms and Small central bulge Direct relationship between the tightness of the arms and size of the central bulge Allows classification of the galaxy even when viewed from on edge Astronomy 1-2 Lecture 24-7 Spiral Galaxies Spiral galaxies have Flat, rotating disks Central bulges Gas and dusk in the disk Star formation in the spiral arms Spiral galaxies can range in size from 80,000 to 250,000 light years across They can range in mass from 109 to 1012 solar masses Astronomy 1-2 Lecture 24-8 Spiral Galaxies Classified according to the size of their central bulge: Astronomy 1-2 Lecture 24-9 Spiral Galaxies Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest Type Sa tends to have the most tightly bound spiral arms with Types Sb and Sc progressively less tight, although the correlation is not perfect The components of spiral galaxies are the same as in our own galaxy disk, core, halo, bulge, and spiral arms Astronomy 1-2 Lecture 24-10 Spiral Galaxies The Sombrero galaxy, with its large central bulge, is a type Sa We cannot see the spiral arms, as they are edge-on Infrared Astronomy 1-2 Lecture 24-11 Barred Spiral Galaxies Spiral arms emanate from the ends of a bar Classified in exactly the same way as regular spiral galaxies By the size of the central bulge and the tightness of the arms In both regular and barred spiral galaxies, the more tightly wound the arms - the less gas and dust There is mounting evidence for the Milky Way being a Barred Spiral Galaxy with a short bar Astronomy 1-2 Lecture 24-12 Barred Spiral Galaxies Astronomy 1-2 Lecture 24-13 Barred Spiral Galaxies Astronomy 1-2 Lecture 24-14 Elliptical Galaxies Elliptical galaxies have no spiral arms They range from being spherical to elliptical and are classified according to the Eccentricity Eccentricity = 10 (1 - b/a) Sphere has eccentricity equal to 0 Remember the eccentricity we measure is as seen from Earth Astronomy 1-2 Lecture 24-15 Elliptical Galaxies Elliptical galaxies come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars Ellipticals also contain very little, if any, cool gas and dust, and they show no evidence of ongoing star formation Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy Astronomy 1-2 Lecture 24-16 Elliptical Galaxies Classification according to their shape From E0 (almost spherical) to E7 (the most elongated) Astronomy 1-2 Lecture 24-17 Elliptical Galaxies Astronomy 1-2 Lecture 24-18 Lenticular Galaxies This is a subspecies of galaxy that is related to spiral galaxies Lenticular galaxies have Flat rotating disks Central bulges Very little gas and dust No spiral arms It may be that it is necessary a galaxy to have interstellar gas for spiral arms to form Astronomy 1-2 Lecture 24-19 Lenticular Galaxies S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas Astronomy 1-2 Lecture 24-20 Irregular Galaxies Irregular galaxies tend to contain lots of gas and interstellar dust Contain lots of star formation Star formation is not uniformly spread over the galaxy Gas and interstellar dust not uniformly spread through galaxy Astronomy 1-2 Lecture 24-21 Irregular Galaxies Astronomy 1-2 Lecture 24-22 Irregular Galaxies The small and large Magellanic Clouds are close neighbors to our own Milky Way Astronomy 1-2 Lecture 24-23 Irregular Galaxies NGC 4485 and NGC 4490 on the left, and M82 on the right Astronomy 1-2 Lecture 24-24 Summary of Galaxy Properties Astronomy 1-2 Lecture 24-25 Question Is the central bulge of a spiral galaxy in any way related to elliptical galaxies? Probably not Spiral galaxies rotate, whereas elliptical galaxies rotate very little, if at all Astronomy 1-2 Lecture 24-26 Two Questions 1) Is there a connection between the types of galaxies? 2) Does one type of galaxy evolve from the other? All galaxies are of about the same age Spiral galaxies have stars just as old as the stars in elliptical galaxies Hubble tried to devise an evolutionary scheme for galaxies There is no evolutionary staging of galaxies Astronomy 1-2 Lecture 24-27 Clusters of Clusters Clusters of galaxies also seem to be clustered into Superclusters Clusters of galaxies are not uniformly spread throughout the universe Astronomy 1-2 Lecture 24-28 Bound Together As galaxies are bound systems of stars, clusters are also bound systems of galaxies Measured masses within the clusters insufficient to generate the necessary gravitational forces to hold the cluster together The missing mass problem again Dark Matter Astronomy 1-2 Lecture 24-29 Collision Course Galaxies can and do pass through each other There are gravitational effects, tidal effects, between the galaxies Gases and dusts create shock waves Gases are stopped even though galaxies continue to move Gases are heated to high temperatures When gases cool, protostar formation begins and star formation can be prolific - Star-Burst Galaxies Astronomy 1-2 Lecture 24-30 Collision Course Gravitational forces seriously can distort the shape of both the interacting galaxies Galaxies can "combine", with the resulting galaxy being larger still Merger of spiral galaxies produces elliptical galaxies If the resultant galaxy is in a rich cluster, this larger galaxy can continue to gobble up other galaxies - Galactic Cannibalism Astronomy 1-2 Lecture 24-31 Collisions Whirlpool Galaxy Astronomy 1-2 Lecture 24-32 Collisions The “Mice” For a simulation of colliding galaxies look here Astronomy 1-2 Lecture 24-33 Collisions Cartwheel Galaxy Astronomy 1-2 Lecture 24-34 Mergers Antennae Galaxies Astronomy 1-2 Lecture 24-35 A Collision Coming Soon The Milky Way Galaxy and the Andromeda Galaxy are falling towards each other Tidal effects should start in about 4 to 5 billion years The two spiral galaxies will become a combined elliptical galaxy Astronomy 1-2 Lecture 24-36 A Collision Coming Soon M81 versus M82 Astronomy 1-2 Lecture 24-37 Distribution of Galaxies How are galaxies distributed throughout the Universe? Cepheid variables allow distance measurements of galaxies Astronomy 1-2 Lecture 24-38 Distribution of Galaxies However, some galaxies have no Cepheids, and most are farther away then 25 Mpc New distance measures are needed Tully–Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity Astronomy 1-2 Lecture 24-39 Distribution of Galaxies Type I supernovae can also be used They all have about the same luminosity, as the process by which they happen doesn’t allow for much variation They can be used as “standard candles” Objects whose absolute magnitude is known, and which can therefore be used to determine distance using their apparent magnitude Astronomy 1-2 Lecture 24-40 Distribution of Galaxies With these additions, the cosmic distance ladder has been extended to about 1 Gpc Astronomy 1-2 Lecture 24-41 Clustering of Galaxies Galaxies are not uniformly distributed throughout the universe Galaxies are clustered Two types of clusters: Rich Clusters - These contain many galaxies Poor Clusters - These contain few galaxies Astronomy 1-2 Lecture 24-42 Local Group The Milky Way is part of a cluster of about 45 galaxies within about 1 Mpc of it This is known as the Local Group Astronomy 1-2 Lecture 24-43 Local Group There are three spirals in this group—the Milky Way, Andromeda, and M33 These and their satellites—about 45 galaxies in all—form the Local Group Such a group of galaxies, held together by its own gravity, is called a galaxy cluster Astronomy 1-2 Lecture 24-44 Clustering of Galaxies A nearby galaxy cluster is the Virgo cluster It is much larger than the Local Group, containing about 3500 galaxies Astronomy 1-2 Lecture 24-45 Clustering of Galaxies This image shows the Abell 1689 cluster of galaxies, a very large cluster almost 1 billion parsecs away Astronomy 1-2 Lecture 24-46 Cluster of Galaxies Perseus Cluster > 1000 Galaxies Astronomy 1-2 Lecture 24-47 Characterization of Clusters Clusters of galaxies are also characterized by shape Regular Clusters Many elliptical galaxies, but few spiral galaxies Distinctly spherical shape Marked concentration of galaxies at the center Acting in concert for a long time Dominated by at least one giant elliptical galaxy near the center of cluster Irregular Clusters Marked by an asymmetrical distribution of the galaxies Astronomy 1-2 Lecture 24-48 Regular Cluster Coma Cluster ~ 10,000 Galaxies Astronomy 1-2 Lecture 24-49 Correlations in Clusters There is a strong correlation between Rich and Poor clusters and Regular and Irregular clusters Rich ==> Regular Poor ==> Irregular Astronomy 1-2 Lecture 24-50 Hubble’s Law Universal recession All galaxies (with a couple of nearby exceptions) seem to be moving away from us With the redshift of their motion correlated with their distance Astronomy 1-2 Lecture 24-51 Motion of Galaxies An examination of the spectra of other galaxies has shown that the spectra are red shifted A red shift in a spectrum tells us that the object that is emitting the spectra is moving away from us Therefore the other galaxies are moving away from us Astronomy 1-2 Lecture 24-52 Galactic Motion Hubble and his colleagues found a correlation between a galaxy's velocity and its distance from the Milky Way v = H0 r with H0 being Hubble's constant, the expansion rate of the universe The more distant the galaxy, the faster it was moving! Astronomy 1-2 Lecture 24-53 Hubble’s Law The relationship (slope of the line) is characterized by Hubble’s constant H0 recessional velocity = H0 × distance The currently accepted value for Hubble’s constant: H0 = 70 km/s/Mpc Measuring distances using Hubble’s law actually works better on farther away objects Random motions are overwhelmed by the recessional velocity Astronomy 1-2 Lecture 24-54 Distance Scales This puts the final step on our distance ladder Astronomy 1-2 Lecture 24-55 Distribution of Galaxies Astronomy 1-2 Lecture 24-56 Active Galactic Nuclei About 20–25% of galaxies don’t fit well into the Hubble scheme — they are far too luminous Such galaxies are called active galaxies They differ from normal galaxies in both the luminosity and type of radiation they emit Astronomy 1-2 Lecture 24-57 Active Galactic Nuclei The radiation from these galaxies is called nonstellar radiation Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor These galaxies are called starburst galaxies, and we will discuss them later The galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center Astronomy 1-2 Lecture 24-58 Active Galactic Nuclei This active galaxy has star-formation rings surrounding a very luminous core Astronomy 1-2 Lecture 24-59 Active Galactic Nuclei Active galaxies are classified into three types Seyfert galaxies, radio galaxies, and quasars Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more luminous: Astronomy 1-2 Lecture 24-60 Active Galactic Nuclei The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact Astronomy 1-2 Lecture 24-61 Active Galactic Nuclei Radio galaxies emit very strongly in the radio portion of the spectrum They may have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy Astronomy 1-2 Lecture 24-62 Active Galactic Nuclei Radio galaxies may also be core dominated Astronomy 1-2 Lecture 24-63 Active Galactic Nuclei Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles Astronomy 1-2 Lecture 24-64 Active Galactic Nuclei Many active galaxies have jets Most show signs of interactions with other galaxies Astronomy 1-2 Lecture 24-65 Active Galactic Nuclei Quasars — quasi-stellar objects Starlike in appearance Have very unusual spectral lines Astronomy 1-2 Lecture 24-66 Active Galactic Nuclei Eventually it was realized that quasar spectra were normal, but enormously redshifted Astronomy 1-2 Lecture 24-67 Active Galactic Nuclei Solving the spectral problem introduces a new problem Quasars must be among the most luminous objects in the galaxy, to be visible over such enormous distances. Astronomy 1-2 Lecture 24-68 Relativistic Redshifts and Look-Back Time The redshift of a beam of light is its fractional increase in wavelength Redshifts are measured directly Distances are calculated from them using Hubble’s constant, which is uncertain Astronomers therefore prefer to quote redshifts rather than distances The look-back time is the time when light was emitted from a distant object For very distant objects it is less than the redshift would indicate, as the object has receded in the meantime Astronomy 1-2 Lecture 24-69 The Central Engine of an Active Galaxy Active galactic nuclei have some or all of the following properties high luminosity nonstellar energy emission variable energy output, indicating small nucleus jets and other signs of explosive activity broad emission lines, indicating rapid rotation Astronomy 1-2 Lecture 24-70 The Central Engine of an Active Galaxy This is the leading theory for the energy source in an active galactic nucleus A black hole is surrounded by an accretion disk The strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis Astronomy 1-2 Lecture 24-71 The Central Engine of an Active Galaxy In an active galaxy, the central black hole may be billions of solar masses The accretion disk is whole clouds of interstellar gas and dust They may radiate away as much as 10–20% of their mass before disappearing Astronomy 1-2 Lecture 24-72 The Central Engine of an Active Galaxy This pair of images shows evidence for a black hole at the center of NGC 4261 Astronomy 1-2 Lecture 24-73 The Central Engine of an Active Galaxy The central portion of M87 shows rapid motion and jets characteristic of material surrounding a black hole Astronomy 1-2 Lecture 24-74 The Central Engine of an Active Galaxy One might expect the radiation to be mostly XRays and gamma-rays But apparently this radiation is often “reprocessed” in the dense clouds around the black hole and re-emitted at longer wavelengths Astronomy 1-2 Lecture 24-75 The Central Engine of an Active Galaxy Particles will emit synchrotron radiation as they spiral along the magnetic field lines This radiation is decidedly nonstellar. Astronomy 1-2 Lecture 24-76