The Formation of Massive Ellipticals



First, remember basic properties of elliptical galaxies, from ASTR 222:
What can we say about when, where, and how massive E's form?



Colors are red, which by themselves are only a weak constraint on age. But the low scatter in color along the red sequence provides an important constraint.



Imagine that a galaxy forms in a single burst of star formation, and then ages passively from that point on. Also imagine that galaxies form at a time tform and that you observe them at a time tobs. Finally, imagine that galaxies can form all at the same time, or randomly over time. Then the scatter in the red sequence depends on:
  • spread in formation age (synchonization)
  • rate at which galaxies redden with time
  • when you observe the galaxies

write this scatter as


where beta is a synchronization parameter:
  • beta=0: pure synchronization
  • beta=1: pure random

Bower+ 92: Virgo and Coma clusters

observed scatter is ~ 0.04 magnitudes in color at z=0.

If no synchronization (beta=1), tform>13 Gyr ago.

If synchronized (beta=0.1),
tform>7 Gyr ago.



Virgo and Coma red sequences from Bower+ 1992

The scatter at higher redshift gives better constraints, since you are looking back closer to the formation era, when colors are changing more rapidly.

Ellis+ 97: clusters at z=0.5




But are we looking at early star formation or early galaxy formation? from De Lucia and Blaizot (2007) -- stars may be old, but galaxies formed later.

Merger tree:







Assembly (blue) and star formation (green) history:




But massive ellipticals do exist at high redshift. Why are they hard to find?

Cimatti+ 2004: sample of ellipticals at z ~ 1.6-1.9, stellar masses ~ 1011 Msun:





Spectra suggest ages of ~ 1-2 Gyr, so formation redshifts of z ~ 3-4:



These are estimated to be comparable in number to massive star forming galaxies at z~2 and could account for ~ 10% of the massive galaxies at z~0.

But most galaxy formation models find it hard to make this many massive galaxies this fast.





Massive ellipticals much more compact at high redshift. From Conselice 2014 ARAA:




So ellipticals seem to be growing "inside out" -- compact inner regions form first, they then accrete material at larger radius, growing in size. Two phase assembly:



Field ellipticals at z=0

Field ellipticals often show morphological peculiarities and (sometimes) bluer colors and evidence for gas accretion:

NGC 474:



Centaurus A (nearest E):
Optical/HI
(stars, neutral gas)

Spitzer
(warm dust, tracing gas)





NGC 7252 (merger remnant -- E to be?)




Summary galaxy formation scenario:



This is a schematic "starting point" -- contradictory in places, exceptions exist, and details are complicated and uncertain!