ブラックホールと銀河のどちらが先か？

言い換えれば、銀河は中心にあるブラックホールの周りに成長しましたか？

_{注：私は天体物理学者ではありません。関連する専門知識を持つ人が、ソースペーパーの分析が無効であると判断した場合は、ここで私の回答を修正してください。}

This is a somewhat complicated question to answer. Black holes do precede galaxies in the form of primordial black holes which arose in the early Universe from quantum fluctuations. 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) did precede galaxies.

Source:

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

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.