General Discussion (day 2)

Farhad discussion points:
  • Fraction of dense molecular gas in CMZ?
  • Volume filling factor of dense gas?
  • Abundances in CMZ?
  • Lada 2012 plot: SFR vs Molecular Gas Mass. Density is crucial
    • Steve: Not volume, column density. Assumptions about 3rd dimension. All gas above Lada's column density threshold.
    • Gallagher: exgal community correlates surface density with SFR. Variations on local scales
  • Aalto: HCN as dense gas tracer in ULIRGs - problem.
  • Steve: comparison to centers of other galaxies... rings have low volume filling factor, not pancakes but rings
    • unlikely that column density uniformly distributed along line of sight; likely to be clumped
  • Bally: comment on face-on views. Mostly filaments. Filaments tend to be end-on on positive side, face-on on negative side. Could Brick be an end-on filament? + Gallagher: +1
  • Mills: ALMA data can answer that with excitation. NH3 thought of as high-density tracer, but not in CMZ. + plug by me
  • Juergen Ott: Interferometer and single-dish, 10% filling factor
  • Gallagher: Density limit on SF from star clusters. 10^6-ish, high SFE. Star clusters may imply upper bound
  • Farhad: Mass of molecular clouds from dust emission. Mass of low and high density gas + Rathborne: Opacity, chemistry very tough
  • Aalto: "Medusa merger" large-scale, very high efficiency. Timescales are a problem: no high-density gas tracers seen, but plenty of star formation is seen
  • Mills: How do you trace this highest density gas, then? + Aalto: H2CO, Ao et al. Weak chemical variation. + Rathborne: "Million dollar question". H2CS follows continuum better than other tracers
  • Dust normally considered better tracer of mass. Do we really know dust to gas ratio? Factor of 2 at best
  • Farhad: Question raised yesterday... asymmetry of SFR/mass How do we explain distribution
    • Steve: How measure SFR?
    • Farhad: YSOs counting
    • Bally: You sure they're YSOs?
    • Farhad: Yes, best we can do, age ~few 10^5 yrs. Maybe YSOs obscured? If not, interesting issues
    • Steve: Could YSOs have moved after they formed? Dynamical time...
  • What's the extinciton at 24um? Selection effect? + Farhad: Could be selection. Sgr B2 makes things worst. + Not clear 24um sources come from same volume
  • Dust is the best measure for mass. Only free parameter is temperature. KS not very fundamental.

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