The Milky Way Galaxy and the Formation of

Spiral Galaxies

Prof. Ata SarajediniUniversity of Florida

Department of Astronomy

The Milky Way Galaxy - Our HomePole-On View

Edge-On View

The Andromeda Galaxy - Our Nearest Large Neighbor

M32

NGC 205

• What is the significance of these satellite galaxies?

• Since the late 1970s, theories of galaxy formation have suggested that disrupted satellite galaxies formed the Milky Way’s halo.

They are remnants from the formation of the Milky Way’s halo!

Satellite Galaxies of the Milky Way and Andromeda

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Fg

How do dwarf galaxies get disrupted?

Tidal Effects

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gravityF ∝1

2

Distance

The problem was that there was no concrete evidence for the buildup of spiral galaxies via the disruption and accretion of dwarf galaxies until 1994!

The “Smoking Gun” - the Sagittarius Dwarf Galaxy (Discovered in 1994)

Blue Red

Velocity along the line-of-sight for

stars toward

the Galaxy’s

Bulge

Bulge stars with an average velocity of 0 km/s

Sagittarius dwarf galaxy

M55 turns out to be a M55 turns out to be a foreground cluster!foreground cluster!

Observations of stars known to be a part of the Sagittarius dwarf galaxy and its tidal tails.

Theoretical model of how the Sagittarius dwarf galaxy should be disrupted by the Milky Way.

Canis Major Dwarf Galaxy

• In 2003, another disrupted and digested dwarf galaxy was found…

• Globular clusters are also disrupted and accreted to form the Galaxy’s halo.

• We have even detected tidal streams and tidal tails composed of stars in the halo of the Andromeda galaxy.

• The halos of the Milky Way and Andromeda do indeed show evidence of having been formed via the disruption and accretion of dwarf satellite galaxies.

Conclusion

The halo of a spiral galaxy is like a bowl of spaghetti!

Today, NASA and the Space Telescope Science Institute released images of the Hubble Ultra Deep Field - the result of 11.3 days of total exposure time with the Advanced Camera for Surveys instrument.

Satellite Galaxies of the Milky Way and Andromeda

• Similar searches for tidally disrupted dwarf galaxies around the spiral galaxy M33 have turned up nothing significant.