Chapter 15 Normal and Active Galaxies

M51 - Whirlpool Galaxy Galaxies are among the grandest, most beautiful objects in the universe. They are colossal collections of typically a hundred billion stars, held together loosely by gravity. Despite their widespread existence in space, astronomers do not yet fully understand how galaxies originated. Here the graceful, winding arms of the majestic spiral galaxy, M51, also known as the Whirlpool Galaxy, resemble a curving staircase sweeping through space. This view, taken by the Hubble Space Telescope, shows the galaxy's yellowish central core, its reddish nebulae in the arms, and myriad stars throughout. (STScl)

Edwin Hubble Mt. Wilson observatory 100” (2.5 m) telescope Categorized galaxies in 1924

Figure 15.1 Coma Cluster (a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies more than 100 million pc from Earth. (The blue spiked object at top right is a nearby star; virtually every other object in this image is a galaxy.) (b) A recent Hubble Space Telescope image of part of the cluster. (AURA; NASA)

Hubble classification scheme Spirals Barred spirals Ellipticals Irregulars

Spiral galaxies Example: Milky Way

Spiral scheme Based on size of central bulge Sa - largest bulge, tightly wrapped arms, least gas and dust Sb - more open arms Sc - smallest bulge, loose spiral structure, most gas and dust

Figure 15.2 Spiral Galaxy Shapes Variation in shape among spiral galaxies. As we progress from type Sa to Sb to Sc, the bulges become smaller while the spiral arms tend to become less tightly wound. (R. Gendler; NOAO; D. Malin/AAT)

Tilted view Spiral structure can be hard to see on edge Presence of disk with dust, gas and newborn stars signifies spiral

Figure 15.3 Sombrero Galaxy The Sombrero Galaxy, a spiral system seen edge-on. Officially cataloged as M104, this galaxy has a dark band composed of interstellar gas and dust. The large size of this galaxy’s central bulge marks it as type Sa, even though its spiral arms cannot be seen from our perspective. The inset shows this galaxy in the infrared part of the spectrum, highlighting its dust content in false-colored pink. (NASA)

Barred spirals Elongated bar extending into disk Spiral arms project from ends of bar SBa, SBb, SBc Milky Way intermediate between spiral and barred spiral

Figure 15.4 Barred-Spiral Galaxy Shapes Variation in shape among barred-spiral galaxies. The variation from SBa to SBc is similar to that for the spirals in Figure 15.2, except that now the spiral arms begin at either end of a bar through the galactic center. In frame (c), the bright star is a foreground object in our own Galaxy; the object at top center is another galaxy that is probably interacting with NGC 6872. (NASA; AAT; ESO)

Elliptical galaxies No spiral arms No obvious disk Dense central nucleus E0 - most circular to E7 - most elliptical Classification depends on actual shape and orientation to earth

Figure 15.5 Elliptical Galaxy Shapes Variation in shape among elliptical galaxies. (a) The E1 galaxy M49 is nearly circular in appearance. (b) M84 is a slightly more elongated elliptical galaxy, classified as E3. Both galaxies lack spiral structure, and neither shows evidence of cool interstellar dust or gas, although each has an extensive X-ray halo of hot gas that extends far beyond the visible portion of the galaxy. (c) M110 is a dwarf elliptical companion to the much larger Andromeda Galaxy. (AURA; SAO; R. Gendler)

Elliptical galaxy sizes Sizes range from Dwarf ellipticals - 1 kpc diameter, million stars Giant ellipticals - few Mpc across, trillions of stars Dwarfs more common by 10 to 1 Most of mass in ellipticals is in giants

Other elliptical properties Little or no cool dust and gas No young stars or star formation Old reddish low mass stars Large amounts of very hot interstellar gas

Intermediate between E7 & Sa Thin disk and flattened bulge No spiral structure No gas and dust S0 - no bar SB0 - if bar present

Figure 15.6 S0 Galaxies (a) S0 (or lenticular) galaxies contain a disk and a bulge, but no interstellar gas and no spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties. (b) SB0 galaxies are similar to S0 galaxies, except for a bar of stellar material extending beyond the central bulge. (Palomar/Caltech)

Irregular galaxies Rich in interstellar matter and young stars Lack regular structure Irr I - look like misshapen spirals Irr II - often explosive or filamentary appearance Usually smaller than spirals, larger than dwarf ellipticals Between 108 and 1010 stars

Magellanic Clouds Pair of Irr I galaxies Orbit our galaxy 50 kpc from center of our galaxy Visible from southern hemisphere Lots of dust, gas and blue stars Also old stars and globulars

Figure 15.7 Magellanic Clouds The Magellanic Clouds are prominent features of the night sky in the Southern Hemisphere. Named for the sixteenth-century Portuguese explorer Ferdinand Magellan, whose around-the-world expedition first brought word of these fuzzy patches of light to Europe, they are dwarf irregular (Irr I) galaxies, gravitationally bound to our own Milky Way Galaxy. They orbit our Galaxy and accompany it on its trek through the cosmos. (a) The Clouds’ relationship to one another in the southern sky. Both the Small (b) and the Large Magellanic Cloud (c) have distorted, irregular shapes, although some observers claim they can discern a single spiral arm in the Large Cloud. (F. Espenak; Harvard Observatory)

Figure 15.8 Irregular Galaxy Shapes Some irregular (Irr II) galaxies. (a) The strangely shaped galaxy NGC 1427A is probably plunging headlong into a group of several other galaxies (not shown), causing huge rearrangements of its stars, gas and dust. (b) The galaxy M82 seems to show an explosive appearance, probably the result of a recent galaxy-wide burst of star formation. (NASA; Subaru)

Dwarf irregulars Most common irregular Dwarf ellipticals and irregulars most common galaxies in universe Often found close to a larger “parent” galaxy

Figure 15.9 Galactic “Tuning Fork” Hubble’s tuning fork diagram, showing his basic galaxy classification scheme. The placement of the four basic types of galaxies—ellipticals, spirals, barred spirals, and irregulars—in the diagram is suggestive, but the tuning fork has no physical meaning.

Table 15.1 Basic galaxy Properties by Type

Measuring galactic distance Cepheid variables to 25 Mpc Need “standard candles” - astronomical objects of known luminosity Luminosity + apparent brightness  distance

Tully-Fisher relation Correlation between rotation speed and luminosity of spiral galaxies Can be used out to about 200 Mpc Other related correlations for ellipticals

Figure 15.10 Galaxy Rotation A galaxy’s rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from an unmoving source). From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of the galaxy’s spectral lines. The amount of broadening is a direct measure of the rotation speed of the galaxy, such as the one at the right, NGC 4603, about 100 million light-years away. (NASA)

Type I supernovae Peak luminosity can be used as standard candle Can be used out to 1 Gpc

Figure 15.11 Extragalactic Distance Ladder An inverted pyramid summarizes the distance techniques used to study different realms of the universe. The techniques shown in the bottom four layers—radar ranging, stellar parallax, spectroscopic parallax, and variable stars—take us as far as the nearest galaxies. To go farther, we must use new techniques—the Tully–Fisher relation and the use of standard candles—based on distances determined by the four lowest techniques.

Local Group 45 galaxies in a local cluster Gravitationally attracted 3 spirals - Milky Way, Andromeda, M33 Remainder dwarf irregulars and ellipticals

The Local Group is made up of some 45 galaxies within approximately 1 Mpc of our Milky Way Galaxy. Only a few are spirals; most of the rest are dwarf-elliptical or irregular galaxies, only some of which are shown here. Spirals are colored blue, ellipticals pink, and irregulars white, all of them depicted approximately to scale. The inset map (top right) shows the Milky Way in relation to some of its satellite galaxies. The photographic insets (top left) show two well-known neighbors of the Andromeda Galaxy (M31): the spiral galaxy M33 and the dwarf elliptical galaxy M32 (also visible in Figure 14.2a, a larger-scale view of the Andromeda system). (M. BenDaniel; NASA) Figure 15.12 Local Group

Virgo cluster 17-18 Mpc from Milky Way 2500 galaxies 3 Mpc across

Figure 15.13 Virgo Cluster The central region of the Virgo Cluster of galaxies, about 17 Mpc from Earth. Many large spiral and elliptical galaxies can be seen. The inset shows several galaxies surrounding the giant elliptical known as M86. An even bigger elliptical galaxy, M87 noted at bottom, will be discussed later in this chapter. (M. BenDaniel; AURA)

Figure 15.14 Distant Galaxy Cluster - 2 Gpc distant The galaxy cluster Abell 1689 contains huge numbers of galaxies and resides roughly 2 billion parsecs from Earth. Virtually every patch of light in this photograph is a separate galaxy. Thanks to the high resolution of the optics onboard the Hubble Space Telescope, we can now discern, even at this great distance, spiral structure in some of the galaxies. We also see many galaxies in collision—some tearing matter from one another, others merging into single systems. Despite the frenzied activity of individual galaxies, though, all off them recede from us—like buzzing fireflies in a moving jar. (NASA)

Universal Recession 1912 - Slipher found almost all spiral galaxies are redshifted All galaxies, except some local ones, are receding Motion of galaxies in a cluster is random Clusters are receding, as are galaxies not part of a cluster

Analogy 15-1 Like fireflies in a moving jar

Figure 15.15 Galaxy Spectra Optical spectra, shown at left, of several galaxies named on the right. Both the extent of the redshift (denoted by the horizontal red arrows) and the distance from the Milky Way Galaxy to each galaxy (numbers in center column) increase from top to bottom. The vertical yellow arrow in each spectrum highlights a particular spectral feature (a pair of dark absorption lines). The horizontal red arrows indicate how this feature shifts to longer wavelengths in the spectra of more distant galaxies. The many white vertical lines at the top and bottom of each spectrum are laboratory references. (Palomar/Caltech)

Hubble’s Law 1920’s - Hubble plotted recessional velocity versus distance for galaxies Hubble diagrams Discovered rate at which a galaxy recedes is directly proportional to its distance from us - Hubble’s Law

Figure 15.16 Hubble’s Law Plots of recessional velocity against distance (a) for the galaxies shown in Figure 15.15 and (b) for numerous other galaxies within about 1 billion pc of Earth.

Hubble’s Law Universal recession known as Hubble flow Distances separating clusters and superclusters is expanding Universe (space itself) is expanding Individual objects are not expanding

Hubble’s Constant - H0 Recessional velocity = H0 X distance Some measurements give H0 between 70 - 80 km/s/Mpc Other types give 50 - 65 km/s/Mpc We will use H0 = 70 km/s/Mpc

Top of distance ladder Use Hubble’s law to find distances Redshift  recessional velocity  distance

Figure 15.17 Cosmic Distance Ladder Hubble’s law tops the hierarchy of distance-measurement techniques. It is used to find the distances to astronomical objects all the way out to the limits of the observable universe.

Redshifts Largest redshifts greater than 6 Means wavelengths shifted 7X UV spectral lines shifted to infrared Such objects 9000 Mpc away

Redshifts and look-back time Redshift of 6.0 means galaxy is receding at 96% of the speed of light It is now 8420 Mpc = 27.5 billion light-years away It was 12.7 billion light-years away when it emitted the light we see today Its look-back time is 12.7 billion years Light traveled 12.7 billion years to us

Table 15-2 Redshift, Distance, and Look-Back Time

Active galaxies More than 90% of all galaxies are normal Few percent of all bright galaxies are active galaxies Overall active are brighter than normal, and at more wavelengths Normal galaxy - accumulated light of stars Active galaxy - most of radiation nonstellar Violent events in galactic nucleus

Figure 15.18 Galaxy Energy Distribution The energy emitted by a normal galaxy differs significantly from that emitted by an active galaxy. This plot illustrates the general run of intensity for all galaxies of a particular type and does not represent any one individual galaxy.

Seyfert galaxies Discovered in 1943 by Carl Seyfert Resemble normal spiral galaxies except Seyfert nucleus emits the most energy Brightest Seyfert nuclei 10X brighter than entire Milky Way

Figure 15.19 Seyfert Galaxy The Circinus galaxy, a Seyfert with a bright compact core, lies some 4 Mpc away—it is one of the closest active galaxies. (NASA)

Seyfert spectrum Some produce from infrared to X-ray 75% emit most of their energy in infrared Broad spectral lines - rapid internal motion Varies in time within fraction of year - small energy producing region

Figure 15.20 Seyfert Time Variability This graph illustrates the irregular variations of a Seyfert galaxy’s luminosity over two decades. Because this Seyfert, called 3C 84, emits strongly in the radio part of the electromagnetic spectrum, these observations were made with large radio telescopes. The optical and X-ray luminosities vary as well. (NRAO)

Radio galaxies Active galaxies emitting large amounts of energy at radio wavelengths Radio energy from huge radio lobes

Centaurus A radio galaxy Visible light E2 galaxy with band of dust Perhaps merger of spiral and elliptical 4 Mpc from earth Radio lobes span half a Mpc Lobes 10X size of Milky Way Two lobed - roughly symmetrical

Figure 15.21 Centaurus A Radio Lobes Radio galaxies, such as Centaurus A shown here optically in (a), often have giant radio-emitting lobes (b) extending a million parsecs or more beyond the central galaxy. The lobes cannot be imaged in visible light and are observable only with radio telescopes. The lobes are shown here in false color, with decreasing intensity from red to yellow to green to blue. (ESO; NRAO)

Figure 15.22 Centaurus A, Close Up The main image (a) shows an optical photograph of Centaurus A, one of the most massive and peculiar galaxies known. The pastel false colors mark the radio emission shown in Figure 15.21; the data in this case were more recently acquired and have higher resolution. (b) Although the radio jets emit no visible light, they do emit X rays, as shown in this Chandra image. (c) Increasingly high-resolution optical views of the galaxy’s core region, taken by the Hubble Space Telescope. (NASA; SAO; J. Burns)

Figure 15.23 Cygnus A (a) Cygnus A also appears to be two galaxies in collision. (b) On a much larger scale, it displays radio-emitting lobes on either side of the optical image. The optical galaxy in (a) is about the size of the small dot at the center of (b). Note the thin line of radio-emitting material joining the right lobe to the central galaxy. The distance from one lobe to the other is approximately a million light-years. (NOAO; NRAO)

Radio galaxies Radio lobes emit roughly 10X energy of entire Milky Way Nucleus emits up to 100X energy of radio emission Total emission up to 1000X of Milky Way Not all have radio lobes - depends on view

Figure 15.24 Core-Dominated Radio Galaxy On this radio contour map of the radio galaxy M86, we can see that the radio emission comes from a bright central nucleus, which is surrounded by an extended, less-intense radio halo. The radio map is superimposed on an optical image of the galaxy and some of its neighbors, a wider-field version of which was shown previously in Figure 15.13. (Harvard-Smithsonian Center for Astrophysics)

Figure 15.25 Radio Galaxy - view determines lobe visibility A central energy source produces high-speed jets of matter that interact with intergalactic gas to form radio lobes. The system may appear to us as either radio lobes or a core–dominated radio galaxy, depending on our location with respect to the jets and lobes.

Common active features Huge energy generation in compact nucleus Evidence of interacting galaxies Many contain jets If view jets end-on, see intense doppler shifted radiation This is a blazar - X or gamma rays

Figure 15.26 M87 Jet The giant elliptical galaxy M87 (also called Virgo A) is displayed here in a series of zooms. (a) A long optical exposure of the galaxy’s halo and embedded central region. (b) A short optical exposure of its core and an intriguing jet of matter, on a somewhat smaller scale. (c) An infrared image of M87’s jet, examined more closely compared with (b). The bright point at left in (c) marks the bright nucleus of the galaxy; the bright blob near the center of the image corresponds to the bright “knot” visible in the jet in (b). (NOAO; NASA)

Quasars In 1960, faint blue star like object located with radio source 3C48 1963 - 3C 273 - spectral lines found to be redshifted 16% - moving at 48,000 km/s 3C 48 redshifted 37%

Figure 15.27 Quasar 3C 273 a) The bright quasar 3C 273 displays a luminous jet of matter, but the main body of the quasar is starlike in appearance. (b) The jet extends for about 30 kpc and can be seen better in this high-resolution image. (AURA)

Quasars Large redshift - enormous distance 3C 48 is 1400 Mpc away 3C 273 has luminosity of 1040 W Comparable to 20 trillion suns or 1000 Milky Way galaxies Quasars range from 1038 W to 1042 W

Figure 15.28 Quasar Spectrum Optical spectrum of the distant quasar 3C 273. Notice both the redshift and the widths of the three hydrogen spectral lines marked as Hβ, Hγ, and Hδ. •(Sec. 2.6) The redshift indicates the quasar’s enormous distance. The width of the lines implies rapid internal motion within the quasar. (Adapted from Palomar/Caltech)

Quasars Quasars look star-like because of great distance Quasi-stellar radio sources, shortened to quasars Quasi-stellar object or QSO is more common today More than 30,000 quasars known 250 Mpc to 9000 Mpc away

Quasars Only seen at great distances Means long ago in time Perhaps early phase of galaxy formation Variable over short periods (days or hours) Jets Bright cores of distant galaxies too faint to see

Figure 15.29 Quasar Jets Radio image of the quasar 3C 175, showing radio jets feeding faint radio lobes. The bright (white) central object is the quasar, some 3000 Mpc away. The lobes themselves span approximately a million light-years. (NRAO)

Active galactic nuclei features High luminosities, mostly nonstellar Highly variable (small region) Jets and explosive activity Optical spectra with broad emission lines

Active galaxy energy production Supermassive black hole - 106 - 109 M Accretion disk - infalling matter heated by friction Emits large amounts of radiation Jets - ejected material in magnetic fields along axis of accretion disk

Figure 15.30 Active Galactic Nucleus The leading theory for the energy source in active galactic nuclei holds that these objects are powered by material accreting onto a supermassive black hole. As matter spirals toward the hole, it heats up, producing large amounts of energy. At the same time, high-speed jets of gas may be ejected perpendicular to the accretion disk, forming the jets and lobes seen in many active objects. Magnetic fields generated in the disk are carried by the jets out to the radio lobes, where they play a crucial role in producing the observed radiation.

Active galaxy energy production 10% to 20% of infalling mass-energy converts to radiation One M consumed per decade can power 1038 W active galaxy

Figure 15.31 Giant Elliptical Galaxy (a) A combined optical/radio image of the giant elliptical galaxy NGC 4261, in the Virgo Cluster, shows a white visible galaxy at center, from which blue-orange (false color) radio lobes extend for about 60 kpc. (b) A close-up photograph of the galaxy’s nucleus reveals a 100-pc-diameter disk surrounding a bright hub thought to harbor a black hole. (NRAO; NASA)

Figure 15.32 M87 Disk Recent imaging and spectroscopic observations of M87 support the idea of a rapidly whirling accretion disk at its heart. (a) An image of the central region of M87, similar to that shown in Figure 15.26c, shows its bright nucleus and jet. (b) A magnified view of the nucleus suggests a spiral swarm of stars, gas, and dust. (c) Spectral-line features observed on opposite sides of the nucleus show opposite Doppler shifts, implying that material on one side of the nucleus is coming toward us and material on the other side is moving away from us. The strong implication is that an accretion disk spins perpendicular to the jet and that at its center is a black hole having some three billion times the mass of the Sun. (NASA)

Energy Emission Emitted energy at broad range of wavelengths Some radiation absorbed and reemitted by dust in surrounding disk

Figure 15.33 Dusty Donut The accretion disk surrounding a massive black hole, drawn here with some artistic license, consists of hot gas at many different temperatures (hottest nearest the center). When viewed from above or below, the disk radiates a broad spectrum of electromagnetic energy extending into the X ray band. However, the dusty infalling gas that ultimately powers the system is thought to form a rather fat, donut-shaped region outside the accretion disk (shown here in dull red), which effectively absorbs much of the high-energy radiation reaching it, reemitting it mainly in the form of cooler, infrared radiation. Thus, when viewed from the side, strong infrared emission is observed. The appearance of the jets, radiating mostly in radio and X-rays, also depends on the viewing angle (see Figure 15.25). (Adapted from D. Berry)

Synchrotron radiation Ejected charged particles in jet spiral around magnetic field at high speeds This is nonthermal radiation Jet slowed by intergalactic medium, magnetic field tangled, forms radio lobes

Figure 15.34 Nonthermal Radiation (a) Charged particles, especially fast-moving electrons (red), emit synchrotron radiation (blue) while spiraling in a magnetic field (black). This process is not confined to active galaxies. It occurs, on smaller scales, when charged particles interact with magnetism in Earth’s Van Allen belts •(Sec. 5.6), when charged matter arches above sunspots on the Sun •(Sec. 9.4), in the vicinity of neutron stars •(Sec. 13.2), and at the center of our own Galaxy. •(Sec. 14.7) (b) Variation of the intensity of thermal and synchrotron (nonthermal) radiation with frequency. Thermal radiation, described by a blackbody curve, peaks at some frequency that depends on the temperature of the source. Nonthermal synchrotron radiation, by contrast, is more intense at low frequencies and is independent of the temperature of the emitting object. Compare this figure with Figure 15.18.