Spiral Galaxies, the Most Familiar One Being the Galaxy Closest to Us

GALAXIES

© 2017 Pearson Education, Inc. Chapter 24 • Morphology – Hubble’s Galaxy Classification • The Distribution of Galaxies in Space – Extending the Distance Ladder • Hubble’s Law • Active Galactic Nuclei • The Central Engine of an Active Galaxy • Summary

Bode’s Galaxy Whirlpool Galaxy Pinwheel Galaxy (M81) (M51a) (M101) © 2017 Pearson Education, Inc. Hubble’s Galaxy Classification: Spirals • All spiral galaxies are denoted by the letter “S” • They are further classified by the letters “a”, “b”, “c”, etc., according to the size of the bulge, so that: – Type Sa has the largest central bulge, – Type Sb has a smaller bulge, and – Type Sc has the smallest bulge. • The tightness of the spirals is strongly correlated with the size of the bulge, – Type Sa tends to have the most tightly bound spiral arms, with types Sb and Sc progressively less tight. • The correlation is not perfect. Andromeda is classified as an Sb galaxy.

© 2017 Pearson Education, Inc. Hubble’s Galaxy Classification: Spirals • The components of spiral galaxies are the same as in our own Galaxy: halo, bulge, disk, spiral arms. – The bulge contains older stars on the outside and younger stars inside, – The halos contain older stars and globular clusters, – The galactic disks are rich in gas and dust and contains most of the star forming regions in the spiral arms. Most of the Luminosity of these galaxies comes from A – G type stars in the disk. • It turns out that the larger the bulge the less gas and dust contained by the disk: thus Sa galaxies have the smallest amount of gas and dust in the disk and Sc galaxies have the most

© 2017 Pearson Education, Inc. Hubble’s Galaxy Classification: Spirals • Most spirals will not be seen face to face. However, we do not need to see spiral arms to classify a galaxy as a spiral galaxy. A disk, dust and star formation is good enough.

The Spiral galaxy pair The Sombrero Galaxy as NGC 4302 and NGC 4298. seen by Hubble.

• The spirals project from the ends of the bar. • Barred spirals are designated the letters “SB” followed by the letters “a”, “b”, “c”, etc. depending on the size of the bulge. • In the case of SBc the spirals merge with the bar.

© 2017 Pearson Education, Inc. Galaxy Classification: Ellipticals • Elliptical galaxies have no spiral arms and no disk. They come in many sizes, – Giant Ellipticals contain trillions of stars – Dwarf Ellipticals contain fewer than a million stars. • Ellipticals contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation. They contain mostly old stars. • Many Ellipticals, however, have large clouds of very hot gas (millions of K), extending far beyond the visible boundaries of the galaxy. • The presence of this hot gas is known by X-ray observations.

• The Dwarf Elliptical M110 is a satellite of Andromeda

© 2017 Pearson Education, Inc. Galaxy Classification: Ellipticals • Some Giant Ellipticals are known to contain disks of gas and dust in which stars are forming. • Dwarf Ellipticals are the most common type of Elliptical galaxy. • Stars in ellipticals have more or less random motions. • Ellipticals are assigned the letter “E”. • They are classified according to their shape, from – E0 (almost spherical) to – E7 (the most elongated).

© 2017 Pearson Education, Inc. Galaxy Classification: Lenticulars • Lenticular galaxies have a thin disk and a flattened bulge, but no gas and dust. • They are known as Lenticular because their shape resembles a lens • They are known as “S0” galaxies or “SB0” galaxies, if they have a bar. • S0 and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas.

• They typically contain between 100 million and 10 billion stars.

© 2017 Pearson Education, Inc. Galaxy Classification: Irregulars • The stars in irregular galaxies have random motions, like their Elliptical counterparts. • These galaxies have vigorous star formation. • Irr I galaxies are by far the most common. They have hardly any nucleus and are rich in gas and dust, so they have active star formation. • Irr I are bluish in appearance. • Irr II galaxies redder than Irr I galaxies and rare. They appear more filamentary. • This type of galaxy is thought to result from normal galaxies that have undergone strong interactions with other galaxies.

• Eg. Spirals are not Ellipticals that grew arms!

• Cepheid variables allow measurement of galaxies to about 25 Mpc away. • However, most galaxies are farther away than 25 Mpc. New distance measures are needed. • There are two ways to do this: – Tully-Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its mass, and so to its Luminosity. – Standard Candles: these are intrinsically bright, easily recognizable objects whose Luminosity is known. RR Lyrae variables and Cepheid variables are examples, but they are generally not bright enough at very large distances. On the other hand, Type I supernovae all have about the same luminosity, as the process by which they happen doesn’t allow for much variation. The same can be said for Planetary Nebulae.

• The faster the rotation the more the broadening. • By measuring the broadening we can deduce the rotation rate.

© 2017 Pearson Education, Inc. The Tully Fisher Relation • The Tully-Fisher relation (TFR) is an empirical relation between the rotational velocity and the intrinsic luminosity of a galaxy. The relation is tighter when the rotation rate is compared with the baryonic mass. This latter form is known as the Baryonic Tully-Fisher (BTR) relation. By comparing the intrinsic luminosity to the apparent luminosity we determine the distance.

© 2017 Pearson Education, Inc. Distribution of Galaxies: Local Group • There are three spirals in the Local Group—the Milky Way, Andromeda, and M33 (the Triangulum Galaxy). There are more than 54 galaxies in all most of them dwarfs.

• A collection of galaxies between 2 and 10 Mpc across is called a Cluster. Clusters are large, eg. the Virgo Cluster is a cluster of about 2500 galaxies, whose center is roughly 16 Mpc away, in the constellation of Virgo, with a diameter of about 3 Mpc.

© 2017 Pearson Education, Inc. Distribution of Galaxies: Superclusters • Clusters form Superclusters. The Laniakeia super- cluster contains the local group and about 100,000 other galaxies; it has a diameter of about 77 Mpc. [Video]

© 2017 Pearson Education, Inc. 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.

Hubble’s constant H0:

recessional velocity = H0 ×distance • The value of Hubble’s constant today is approximately 73.4 km/s/Mpc. • Over the past year, there’s been some confusion about its value because recent observations from ESA’s Planck satellite indicates that its it more like 67.4 km/s/Mpc. • Measuring distances using Hubble’s law works better the farther away the object is; random motions are overwhelmed by the recessional velocity.

© 2017 Pearson Education, Inc. Active Galactic Nuclei • About 40 percent of galaxies don’t fit well into the “normal” category — they are far too luminous and the radiation does not look thermal. Such galaxies are called active galaxies. They differ from normal galaxies in both the luminosity and type of radiation they emit.

© 2017 Pearson Education, Inc. Active Galaxies • Most of a normal galaxy’s radiation is emitted in and around the visible portion of the spectrum. This is because most of the radiation is from stars. • Radiation from stars obeys the black body curve. • By contrast, the radiation from an Active Galaxy does not obey the black body curve. • On the contrary, it covers both longer and shorter wavelengths. • This is why the radiation from these galaxies is called nonstellar radiation.

© 2017 Pearson Education, Inc. Starburst Galaxies • Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor. These galaxies are called starburst galaxies. • The radiation from a starburst galaxy will be spread throughout the galaxy and distinctly stellar in nature. • The active galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center. The centers are called Active Galactic Nuclei or AGN for short: – Seyfert Galaxies – Radio Galaxies – Quasars

© 2017 Pearson Education, Inc. Active Galactic Nuclei • We can tell when the radiation is primarily coming from a small region by studying the variability of the light emitted in different wavelengths. • If the period of the variability, 푇, is small, then we expect that the emitting region is also small. Strictly, the size of a coherently emitting region cannot be larger than the light travel time: 푆푖푧푒 = 푐 푇 • For example, if the variability is 1 day, then the size will be 2.6 × 1010 km. This is only about 2000 times the horizon size of the black hole at the center of our galaxy.

© 2017 Pearson Education, Inc. AGN: Seyfert Galaxies • Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more luminous. • Seyferts have large radio and infrared luminosities but look ordinary in visible. • A Seyfert’s luminosity can double or halve in a fraction of a year. NGC 1566 lies about 13 Mpc away

© 2017 Pearson Education, Inc. AGN: Seyfert Galaxies • The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact. The cores of Seyfert Galaxies have luminosities roughly equal to the entire Milky Way, or 5 × 1036 W

© 2017 Pearson Education, Inc. AGN: Radio Galaxies • 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.

© 2017 Pearson Education, Inc. AGN: Radio Galaxies • The lobes may extend far out of the galaxy. • They are made up of multiple jets. • A typical radio galaxy has a luminosity that is about 100 times the Milky Way: 5 × 1038 W • The galaxy Centaurus A appears to be the result of a collision.

© 2017 Pearson Education, Inc. AGN: Blazars • According to the theory of relativity, if a jet is approaching the observer at speeds close to the speed of light, the radiation emitted will be concentrated in a cone (whose opening angle depends on the speed) in the direction of the motion. • To such an observer, this jet will appear both very intense (because the radiation is concentrated in a cone) as well as greatly blue-shifted (by the Doppler effect) • If we observe them radiating gamma rays, they are known as Blazars.

© 2017 Pearson Education, Inc. AGN: Quasars • Quasars: Quasi-Stellar Radio Sources are star-like in appearance, but have very unusual spectral lines. • For a long time the spectral lines could not be mapped to any element. • The first quasars to be detected were labeled 3C 48 and 3C 273

© 2017 Pearson Education, Inc. AGN: Quasars • This means that 3C 48 is receding from us at about 1/3 the speed of light and 3C 273 at 1/7 the speed of light! • According to Hubble’s law this puts 3C 48 and 3C 273 very far away! • Using Hubble’s constant of 70 km/s/Mpc we find

푣 푑 = Mpc 퐻 or – 3C 48 is 1.3 Gpc away, and – 3C 273 is 620 Mpc away!

• The farthest quasar observed is 13.1 b.ly away. • These distant quasars are very old, dating back to an earlier period of galactic evolution. • They must be among the most luminous objects in the galaxy to be visible over such enormous distances: 퐿 = 5 × 1040 W. • They are probably Radio galaxies, in which a lobe is viewed head on.

© 2017 Pearson Education, Inc. AGN: The Central Engine • Active galactic nuclei have some or all of the following properties: – High luminosity – Non-stellar energy emission – Variable energy output, indicating small nucleus – Jets and other signs of explosive activity – Broad emission lines, indicating rapid rotation

© 2017 Pearson Education, Inc. AGN: The Central Engine • This is the leading theory for the energy source in an active galactic nucleus: a black hole, surrounded by an accretion disk. The strong magnetic field lines around the black hole channel particles into jets perpendicular to the plane of the accretion disk.

© 2017 Pearson Education, Inc. AGN: The Central Engine • In an active galaxy, the central black hole may be billions of solar masses. Black holes have a small horizon radius. A billion solar mass black hole has a radius of only 3 × 109 km, which is only 20 AU. • The black hole is probably spinning very fast. • The accretion disk is whole clouds of interstellar gas and dust; • The rotational velocity of the accretion disk is very high. We can say this because of the spectral line broadening. • Fast spinning, hot matter will generate powerful magnetic fields.

© 2017 Pearson Education, Inc. AGN: The Central Engine • As the matter swirls around the central black hole, it heats up by friction. The temperatures can rise to tens of millions of degrees Kelvin. • Accretion disks are efficient at releasing energy in the form of radiation; they may radiate away as much as 10–20 percent of their mass energy before disappearing into the black hole. • Quick calculation: 20% of the mass energy of a 1 solar mass star is 3.6 × 1046 Joules. To radiate at a rate of about 100 times our galaxy an AGN would have to consume about 1 solar mass every 2.3 years! At 10,000 times our galaxy, a quasar would have to consume 1 solar mass every 8.3 days!

© 2017 Pearson Education, Inc. AGN: The Central Engine • The jets emerging from an active galaxy can be quite spectacular. • They consist of material (mostly charged) blasted out from the magnetic poles. • They are a result of matter being accelerated by the magnetic field produced by the accretion disk.

© 2017 Pearson Education, Inc. AGN: The Central Engine • One might expect the radiation to be mostly X-rays and gamma-rays, but apparently it is often “reprocessed” in the dense clouds around the black hole and reemitted at longer wavelengths.

© 2017 Pearson Education, Inc. Summary • Hubble classification organizes galaxies according to shape. • Galaxy types include spiral, barred spiral, elliptical, irregular. • Objects of relatively uniform luminosities are called “standard candles”; examples include RR Lyrae stars, Cepheids and type I supernovae. • The Milky Way lies within a small cluster of galaxies called the Local Group. • Other galaxy clusters may contain thousands of galaxies.

© 2017 Pearson Education, Inc. Summary • Hubble’s law: Galaxies recede from us faster the farther away they are. • Active galaxies are far more luminous than normal galaxies, and their radiation is nonstellar. • Seyfert galaxies, radio galaxies, and quasars all have very small cores; many emit high-speed jets. • Active galaxies are thought to contain supermassive black holes in their centers; infalling matter converts to energy, powering the galaxy.