Chapter 21
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Chapter 21

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Chapter 21

Chapter 21

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Galaxies

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Chapter 21

The components of spiral galaxies are the same as in our own Galaxy: disk, core, halo, bulge, spiral arms. 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.

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Similar to the spiral galaxies are the barred spirals. Historically they were recognized later but were found to be equally numerous. Like the spirals the are designated by how tightly wound they are.

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Elliptical galaxies have no spiral arms and no disk. They 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 show no evidence of ongoing star formation.

Many do, however, have large clouds of high temperature gas, extending far beyond the visible boundaries of the galaxy.

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Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated).

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S0 (Lenticular) and SB0 galaxies have a flattened disk and bulge, but no spiral arms and, like the Ellipticals, have little interstellar gas and dust

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The irregular galaxies have a wide variety of shapes, but no fixed shapes. The Small and Large Magellanic Clouds are close neighbors to our own Milky Way

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Here are three other irregular galaxies: NGC 4485 and NGC 4490 on the left, and M82 on the right

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Here is a summary of galaxy properties by type

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When Edwin Hubble announced that many of the strange nebulae in the sky were really Galaxies, he was tasked with sorting and classifying them. Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning

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  • Cepheid variables allow measurement of distance to galaxies to about 25 Mpc away. However, some galaxies have no Cepheids, and most are farther away than 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.

  • Type I supernovae all have about the same luminosity, since the process by which they happen doesn’t allow for much variation, so the inverse square can be applied

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With these additions then, the cosmic distance ladder has been extended to about 1 Gpc

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The Local Group

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Here is the dis-tribution of galaxies within about 1 Mpc of the Milky Way.

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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

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A nearby galaxy cluster is the Virgo cluster; it is much larger than the Local Group, containing about 3500 galaxies.

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All galaxies (with a few nearby exceptions) show a Doppler Red Shift that suggest they are moving away from us.

This chart shows red shift correlated with their recession velocity and distance:

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These plots show the relation between distance and recession velocity for the five galaxies in the previous figure, and then for a larger sample

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The relationship (slope of the line) is characterized by Hubble’s constant H0

The currently most accepted value for Hubble’s constant

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Measuring distances using Hubble’s law actually is better the farther away the object is; random motions are overwhelmed by the recessional velocity.

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The Doppler Effect relates the change in wavelength to the recession velocity

Δλ / λ0 = Vr / C

The Hubble Law relates recession velocity with distance

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This puts the final step on our distance ladder. However the Hubble Law does need some final tweaking when the recession velocities approach reletavistic values

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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

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The radiation from active galaxies is called non-stellar radiation.

Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor. These galaxies are called starburst galaxies

The galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center.

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This active galaxy has star-formation rings surrounding a very luminous core called an Active Galactic Core

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Active galaxies are classified into three types: Seyfert galaxies, radio galaxies, and quasars. Seyfert galaxies, such as this, resemble normal spiral galaxies, but their cores are thousands of times more luminous

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The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact

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Radio galaxies emit very strongly in the radio and X-ray portions of the spectrum. Many have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy.

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Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles.

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Quasars – quasi-stellar objects – are star-like in appearance, but have very unusual spectral lines. Eventually it was realized that quasar spectra were normal, but enormously redshifted:

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Solving the spectral problem introduces a new problem – quasars must be among the most luminous objects in the universe, to be visible over such enormous distances.

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  • 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

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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 magnetic axis.

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One might expect the radiation from such a powerful source to be mostly X rays and gamma rays and we often see quasars that way, but apparently it is often “reprocessed” in the dense clouds around the black hole and we see it reemitted at longer wavelengths

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Particles in the jet will emit synchrotron radiation as they spiral along the magnetic field lines; this radiation is decidedly non-stellar.

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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.

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Chapter 21

End of Chapter 21

End of Unit 5

End of Astro1040 and Phys 1040

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