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

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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 © 2017 Pearson Education, Inc. Hubble’s Galaxy Classification: Spirals • We’re acquainted with Spiral Galaxies, the most familiar one being the galaxy closest to us. © 2017 Pearson Education, Inc. Hubble’s Galaxy Classification: Spirals • Spiral galaxies do not all look the same. Hubble classified them by the size of the central bulge. 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. © 2017 Pearson Education, Inc. Galaxy Classification: Barred Spirals • Similar to the spiral galaxies are the barred spirals. • They contain a pronounced “Bar” through the center. • 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. © 2017 Pearson Education, Inc. Galaxy Classification: Ellipticals • The Giant Elliptical NGC1316 in the Formax Cluster. • 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. © 2017 Pearson Education, Inc. Galaxy Classification: Irregulars • The irregular galaxies have a wide variety of shapes. They are classified as “Irr I” and “Irr II”. • 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. © 2017 Pearson Education, Inc. Galaxy Classification: Irregulars • Here are the Large and Small Magellanic Clouds, which are two irregulars belonging to our local group. © 2017 Pearson Education, Inc. Hubble’s Galaxy Classification • A summary of galaxy properties by type © 2017 Pearson Education, Inc. Hubble’s “Tuning Fork” • Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning. • Eg. Spirals are not Ellipticals that grew arms! © 2017 Pearson Education, Inc. The Distance Ladder • 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. © 2017 Pearson Education, Inc. Measuring the Rotation Rate • The rotation of a galaxy results in Doppler broadening of its spectral lines. • 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. The Distance Ladder • With these additions, the cosmic distance ladder has been extended to about 1 Gpc. © 2017 Pearson Education, Inc. Distribution of Galaxies: Local Group • Here is the distribution of galaxies within about 1 Mpc of the Milky Way. We call it our local group. © 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. © 2017 Pearson Education, Inc. Distribution of Galaxies: Clusters • 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. © 2017 Pearson Education, Inc. Hubble’s Law • These plots show the relation between distance and recessional velocity for the five galaxies in the previous figure, and then for a larger sample. © 2017 Pearson Education, Inc. Hubble’s Law • The relationship (slope of the line) is characterized by 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. The Distance Ladder: Hubble’s Law • This puts the final step on our distance ladder. © 2017 Pearson Education, Inc. Large Scale Structure of the Universe © 2017 Pearson Education, Inc. Large Scale Structure of the Universe © 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.
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