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.