Which came first: black holes or galaxies?

Question

In other terms, did galaxies grow around black holes at their center?

Answer 1

_{Note: I am not an astrophysicist. If someone with the relevant expertise finds my analysis of the source paper to be invalid, I would appreciate your help with correcting my answer here.}

This is a somewhat complicated question to answer. Black holes may precede galaxies in the form of primordial black holes which according to some models arose in the early Universe from quantum fluctuations. Primordial black holes are currently hypothetical. Supermassive black holes (that you find at the center of galaxies) may not come from these, however. There are a few possible ways that supermassive black holes may have formed:

• Molecular cloud / dwarf galaxies collapsing to form supermassive stars (SMS) which later further collapsed into supermassive black holes (SMBH)
• Dwarf galaxies forming massive stars which collapse into massive black holes, which later coalesce into SMBHs
• Primordial black holes accreting sufficient matter to become massive black holes, which later coalesce into SMBHs

Depending on how strictly you define "galaxy" it does seem most likely that the supermassive black holes formed inside early galaxies, but a type of black hole (primordial black holes) may have preceded galaxies.

Source:

• Formation of the First Supermassive Black Holes - Bromm and Loeb

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UPDATE 10/21/2013:

This just in, recent observations have ruled out the possibility that SMBHs gain mass only through merging with other black holes. For more, see this Astronomy.com article.