Spiral Galaxies

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Spiral Galaxies

• Ron Buta
• University of Alabama

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What are spiral galaxies?

• Flattened systems of stars, gas, and dust
• stars seem to concentrate in spiral arms
• all bound by gravity into a single unit
• typical size50,000-100,000 light years
• typical mass 10billion-500billion suns
• often there is also a prominent bulge

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Face-on spiral galaxy
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Highly inclined spiral galaxy
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Edge-on spiral galaxy
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Edge-on Spiral Galaxy
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Why are spirals interesting?

• An intriguing morphology
• difficult to explain
• common and widespread thoughout the Universe
• new stars tend to form in spirals but not in
  other types of galaxies
• part of process of galaxy evolution and
  interaction
• our Galaxy is a spiral

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Discovery of Spirals

• 1845 William Parsons, Third Earl of Rosse,
  Ireland
• 72-inch reflector
• visually detected spiral arms in the Whirlpool
  Nebula M51
• many other nebulae found to be spirals

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William Parsons, Lord Rosse 1800-1867
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Discovery of spiral structure in M51
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What causes spiral structure?

• Natural phenomenon in flat, rotating stellar
  disks
• possibly triggered by an interaction with another
  galaxy
• or generated by bars
• density waves

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Spiral galaxy research at UA

• morphology
• sense of winding of spiral arms
• gravitational torques due to bars and spiral arms
• star formation in galactic rings

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The de Vaucouleurs Atlas of GalaxiesGerard de
Vaucouleurs 1918-1995
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The de Vaucouleurs Atlas what is it?

• Authors R. J. Buta (U. Alabama)
• H. G. Corwin, Jr. (Caltech)
• S. C. Odewahn (U. Arizona)
• publisher Cambridge Univ. Press
• Purpose to illustrate the de Vaucouleurs revised
  Hubble classification system with modern digital
  images

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NGC 7457 SA(rs)0-
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NGC 1553 SA( r )0
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NGC 1291 (R)SB(s)0/a
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NGC 2713 (R)SB(rs)ab
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NGC 4736 (R)SA( r )ab
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NGC 5850 SB( r )b
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NGC 4501 (M88) SA(rs)b
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NGC 4321 (M100) SAB(s)bc
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NGC 1566 SAB(s)bc
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NGC 6643 SA(rs)c
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NGC 7479 SB(s)c
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NGC 6946 SAB(rs)cd
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NGC 5585 SAB(s)d
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NGC 4618 SB(rs)m
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Sense of winding of spiral arms

• Trailing arms follow direction of rotation
• Leading arms oppose direction of rotation
• most spiral arms trail
• but one galaxy has leading arms!

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Trailing arm spiral M81
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Leading arm spiral galaxy NGC 4622
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Barred spiral galaxies

• A bar-like pattern of old stars crosses the
  center
• arms break from the ends of this bar
• 70 of spirals have a bar, including the Milky Way

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Barred spiral galaxy NGC 1365
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Barred spiral NGC 1300
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How stars move in presence of a bar

• In solar system, planets follow elliptical paths
  with Sun at one focus
• no torque on planet as it orbits
• in presence of a bar, star may follow a centered
  elliptical orbit
• there is a torque on star as it orbits
• amount of torque is a measure of bar strength

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Rings of Star Formation

• Real bar orbit
• lit up by new stars
• often not circular

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Gallery of spirals from the Hubble Space Telescope
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Typical spiral galaxy NGC 4414
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Starburst spiral galaxy NGC 3310
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Ringed spiral galaxy NGC 7742
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Superposed spiral galaxies NGC 3314
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Edge-on spiral galaxy ESO 510-13
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Edge-on spiral galaxy NGC 4013
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Peculiar ring galaxy Hoags Object
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Barred spiral galaxy NGC 4319 with quasar
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Polar ring galaxy NGC 4650A
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Compact galaxy group
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Colliding spiral galaxies NGC 6745
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The Spiral Menagerie Visual Observations of
Spirals from McDonald and Siding Spring
Observatories

• 1977-1984
• photoelectric galaxy photometry runs
• McDonald 30, 36, 82, and 107-inch telescopes
• Siding Spring 40-inch telescope

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NGC 5194-5 (M51) - McDonald 30-inch
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NGC 1566, SSO 40-inch
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NGC 2403, McD 30-inch
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NGC 3034 (M82), McD 36-inch
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NGC 3893(36McD), 4449(36McD), 4676(107McD),
3646(36McD), and 5921(36McD)
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NGC 4631 (left) and NGC 4565(right) (both 30McD)
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M99, M100, M88, and M61 (all 30McD)
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M77(36McD), NGC4535(30McD), NGC5394-5(36McD),
NGC3395-6(36McD), NGC5426-7(36McD), NGC5247(36McD)
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Thank you!