redshift 5, accretion

On day two of The Fabulous, Ando showed an absolutely incredible mean Subaru spectrum of about 10 redshift 5 (!) galaxies. It shows strong, very well-detected interstellar absorption lines, so presumably the ISM is metal-enriched. Good question: Is the enrichment one infers consistent with the hypothesis that earlier generations of stars in these objects were responsible for re-ionizing the Universe at some earlier redshift?

Romeel Davé gave a very nice overview of the status of simulations/theory in galaxy evolution, including not just the successes, but a thoughtful list of the big, generic conflicts/issues. He also emphasized the importance of accretion, noting that in the simulations, the bulk of galaxy growth is through slow, steady (not major-merger) accretion, and that such accretion is substantial. He distinguished two kinds of accretion, a "hot" mode in which accreted gas hits a shock at the virial radius and is heated to near the virial temperature as it falls in, and a "cold" mode in which no shock occurs. The latter is more common in lower-mass halos and at earlier times, and it can lead to very efficient building of stellar (or black hole) mass. Romeel suggested that the dichotomy of these modes might be related to the "bimodality" in galaxy spectral energy distributions.


  1. Remember:
    Cold mode dominates in low mass halos,
    Hot mode dominates in massive halos.

    Call if you need help.

  2. i thought in the simulations the as galaxy mass is built through merger? i'm a bit lost! so the DM halo mass is built through merger events but the galaxies grow by accretion of gas?

  3. I think the issue is that there are huge - even Hubble-time-length delays in the simulations for many of the galaxy pairs. But I don't think this can be correct for the central galaxies. Unfortunately, every paper and "punchline" on the subject uses different conditions for calling something a merger.