Title: Is Hierarchical Merging Broken?
Charles Steinhardt (Caltech, DARK)
I will describe two sharp puzzles suggesting a possible mismatch
between the current hierarchical merging paradigm and observations of
evolving galaxies. Current models predict that the z ~ 4-8 universe
should be a time in which the most massive galaxies are transitioning from
their initial halo assembly to the later baryonic evolution seen in
star-forming galaxies and quasars. Instead, massive galaxies appear to
exist impossibly early, before their halos should even have been able to
assemble. Further, small halos should have generally formed earlier than
larger halos, yet the observed hierarchy in galaxy formation is inverted,
with more massive galaxies completing their growth earlier. After
describing these puzzles, we will consider hierarchical merging is truly
inconsistent with observation or whether other explanations might be more
A .pdf of the talk can be found here.