Lecture 7: The Cosmic Tapestry of Galaxies

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Lecture 7 The Cosmic Tapestry of Galaxies

• Astronomy 5 The Formation and Evolution of the
  Universe
• Sandra M. Faber
• Spring Quarter 2007
• UC Santa Cruz

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Spiral galaxies are flattened, rotating disks
seen at various inclinations
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Spheroidal galaxies are roundish systems with
low net rotation and high internal velocity
dispersion
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Spheroidal components inside disk galaxies are
called bulges
Bulge
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The Hubble Sequence, schematically
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Another illustration of the scheme using
different galaxies
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Elliptical galaxies from the Sloan Digital Sky
Survey
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E/S0
Moving from an E/S0 through S0s to an Sa
SB0
S0
Sa
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Sas with weak spiral arms to an Sb
Sa
Sab
SBab
Sa
Sb
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Sb
Sc?
Sb
Sbc
Sc?
Sbs through Scs. Note both bluer and clumpier.
Sc
SBc
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Scs through Irrs. All very blue and
star-forming.
Sc
Sc
Sd
Irr
Irr
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How a spiral galaxy rotates differential rotation
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Peculiars objects that dont fit.
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How a spiral galaxy rotates differential rotation
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How a spiral galaxy rotates differential rotation
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Spheroidal systems with peculiar dust.
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An extreme peculiar The Antennae
Hubble image
Ground-based image
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How a spiral galaxy rotates differential rotation
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The future collision between Andromeda and the
Milky Way 5 billion years from now
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The Messier 81 group, where outer neutral
hydrogen gas disks are already starting to collide
Optical image
Radio image
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So-called cartwheel galaxies
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A giant cluster Abell 1689
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Swarm by John Dubinski
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1 Collisions easily transform disks into
spheroids. Disks are fragile they are easily
disrupted.
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The Haj in Mecca
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2 Gas clouds are large and fluffy. Hence, they
cannot have scrambled orbits, as in a spheroid.
Gas soon settles into a circularly rotating disk.
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3 Whenever you see a system of stars (or
planets) rotating in circles in a thin disk,
these stars must have formed from a previous gas
disk.
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Collisions were more frequent in the early
Universe, when galaxies were closer together.A
two-step process for galaxy formation1) Early
star formation in disks that were disrupted by
early collisions to make the spheroid
component.2) Later gas falls in via gravity and
is not disturbed by collisions. Makes younger
generation of disk stars.
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How stellar ages reveal the collision history of
the Milky Way
Oldest spheroid stars are nearly as old as
Universe, e.g., globular clusters.
Number of stars
Last big collision 10 Byr ago. Disk is
undisturbed after this.
Today
Spheroid stars
Disk stars
Years after Big Bang
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The last major collision in the Milky Way
happened about 10 Byr ago based on the ages of
the oldest disk stars
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The collision picture explains why

• Spheroid stars
• Are on scrambled orbits.
• Are very old.
• Are on average metal-poor.

Disk stars Are on circular orbits in a thin
disk. Have a wide range of ages. Are on
average metal-rich.
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The simplest nuclear reaction that makes stars
shine