Galaxy Formation Scenarios

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Spirals vs Ellipticals
Morphology	smooth ellipsoids	disk + spiral arms; smaller bulge
Kinematics	random motions	rotational motion
Gas Content	hot X-ray emitting halos	disk of cold atomic/molecular gas
Star Formation History	mostly old stars; little ongoing star formation	young + old stars; active star formation
Environment	dense clusters, some field	field + clusters, but not cluster cores

Early speculation (1960s, 1970s) focused on primordial collapse models, where the galaxy type was determined by initial conditions (angular momentum content, density distribution, local environment) in collapsing clouds of gas. Once made, galaxies lived their live in relative isolation as "island universes."

More recently, hierarchical merger models have been developed which describe elliptical galaxies as forming through mergers of smaller galaxies. What motivated this change?
 
 
 

• Observations and modeling of colliding galaxies (Schweizer 1977, 1982; Toomre 1977)

 

 
 






NGC 7252 (Hibbard etal 1994)

• Theoretical models of structure formation in the universe

 

Structure Formation	Cluster Collapse
Paul Bode (UPenn)	John Dubinski (CITA)

• Observations of the high redshift universe

The Hubble Deep Field

So what does this mean?

Under hierarchical structure models, structure forms continuously over time (depending on cosmology). The collapse time of a volume of space depends on the mean density:

Early in the universe collapse and elliptical galaxy formation took place in the proto-cluster environment, while now it takes place in the field. Perhaps we should see differences in cluster and field ellipticals...