ohp_meeting_september_2018 [mist]

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EDITH FALGARONE

Reservoir of energy : gravity and nuclear processes.
Interact at the scale of SF and starburst
Explain the low-rate of SF observed (low compared to what is observed in numerical simulations)
Feedback is invoked to explain why the many very low-mass and very massive galaxies formed in simulations are not observed. What is it ?
Peak of SF around z=2
Many knobs to turn in simulations (positive and neative feedbacks at stellar and AGN ends) to meet the observations…
Interaction of cold streams accreting onto pro-galactic disk shoudl be soruce of turbulence and shocks. Maby too tenuous gas for observations ?
Absorption spectroscopy of CH+ line in M82 against continuum of local ULIRGs
Strongly lensed submm high-z galaxies can be used to serve as lighthouses to probe the turbulent gas in absorption spectroscopy.
CH+ formation endothermic, very easily destroyed, so lifetime is 1 year / fH2 / n50 : it shines where it forms
ISM is turbulent : very large Reynolds numbers
Energy injection at large scales, destabilisation over a dynamical timescale L/v, cascade in k space to dissipation when advection and diffusion become similar in magnitude.
Dissipation scale propto n^(-3/4) while mean free path is propto 1/n and the two are about the same for the diffuse ISM (100 cm-3)
Bulk of the motion is in solenoidal modes, not compressive. More compressive in SF regions. Observations of Pety et al. and also simulations by PL.
Simulations of PL : half of dissipation is concentrated in 10% of the volume. Sign of intermittency.
Expriments of Douady 1991 : filamentary structures of dissipation, living for a turnover time scale of the large scales (rotating disks at top and bottom of the cylinder) so very long if compared to what is expected at their size.
Observations of CH+ absorption in high-z lensed starburst galaxies, but also very broad emission lines. Much broader than the CO line emission in the same objects.
Description of tasks : CH+ at high-z, turbulence in the local ISM, turbulent magnetic field using the Planck data, the LOFAR conection (how thermal electrons and dust are entwined)
Starburst galaxies lie above the main sequence of galaxies SFR vs stellar mass. About one order of magnitude above the MS.
14 observations, 3 have no CH+ line. Those three are known to host AGNs.

BENJAMIN GODARD

Critical density at which emission starts dominating absorption in CH+ is about 1e4 cm-3. Critical density C21/A21 of CH+ is 1e7
Broad emission must come from the centre of the source. Absorption is much narrower.
Suprise is that the emission is much broader than in CO and H2O.
Other hydrides have similar cirticial densities (such as OH)
CH+ in emission requires nH>1e4 and T>1000 K
Eyelash : Source 1kpc with SFR 1000 Msun/yr so 1e4 to 1e5 times the UV flux of the MW. Heats the gas through photoelectric effect (1% of UV energy transferred).
PDRs would be everywhere and following the kinematics of the source : but why would you observe CH+ and not CO or H2O ?
Shocks fed by Galactic outflows ? Outflow interacting with environment, creates turbulent cascade, very many low velocity structures. OK to form molecules (not possible in high velocity shocks). Could be galacitc-scale wind related to high-pressure from combination of SN remnants.
Does not explain why we do not observe CO and H2O.
Irradiated shocks ? That would explain why CO and H2O are not observed. Models show that CH+ is on the contrary enhanced by an increase in G0. Also SH+, but for a different reason.
Observational diagnostics.
PDR physics : H2 and CO are dissociated only through lines, while C/C+ is in the continuum. H2 therefore self-shields.
Formation of a shock front. Distribution of the energy of a particle going through the discontinuity. Conversion from kinetic to thermal (e.g. viscosity) to internal (rovibration levels) to radiation
In the presence of a B field (depending on its strength and ionization fraction), some information is sent in advance because magneto-sonic speed in ions is larger than in neutrals, which irself is larger than the sonic speed in neutrals. If shock speed in between the two magneto-sonic speeds, decoupling the two species with a magnetic precursor.
Goes naturally from J shock to CJ shock to C shock at steady state. Ions are slowed down ahead of the shock, transferring some of this information to the neutrals through drag.
Increase of B → increase of width of magnetic precursor → neutrals slowed down more → ends in a C-type shock.
C-type shocks reach much lower temperatures than J-type shocks, so tracers may be different
Model of irradiated shocks : shock + PDR, in steady state, with B perpendicular to shock propagation.
Paris-Durham shock code version Lesaffre et al. (2013) includes irradiation with low G0.
Some reactions have a strong influence on the dynamics of the shock (e.g. dissociative recombination). Dynamical reduction of the network ? What is the effect of uncertainties in parameters (rates) ? Bistability ? Chaos ? Our experience is that it is quite robust.
Added radiative transfer in the UV, H2 self-shielding, dust ionization, possibility to treat C* and CJ shocks.
Chernoff (1987), Roberge & Draine (1989) description of trajectories of particles (ions and neutrals) across a shock. Forbidden regions of phase space in v_i and v_n. Trajectory can remain supersonic region (C-type shock). Other trajectories require a jump and just one can meet the condition to arrive at the trajectory going continuously through C’ (CJ shock). C* shocks are continuously going through C and C’ points. Do we actually necessarily reach a steady state ? Also there are steady state shock solutions that are actually unstable. Many trajectories go through C, but only one then goes through C’. Shooting technique varying slightly the position around C, exhibits the one trajectory going to C’.
End up with a phase space diagram of the different types of shocks as a function of nH, B0, G0, Av (« initial conditions »)
Increase of G0 increases ionization fraction and couplign of neutrals and ions. Reduce the size of the shock, increases the temperature, so does the sound speed, and some parts of the trajectory becomes subsonic. Numerical integration DVODE.
Drastic increase of CH+ abundance for irradiated shocks.
Should be able to observe CH+ ladder, with a ratio of 2-1 to 1-0 and 3-2 to 1-0 depending on shock velocity.
Excitation ladder of H2 has signatures of both C and J type shocks. In irradiated models, H2 intensity approximately constant whatever G0. The shock adapts to the type that allows for evacuating the kinetic energy through H2 radiation, because H2 is main coolant. But excitation diagram changes dramatically. Observing H2 will give some clues on shocks, but quite degenerate.

ANDREW LEHMANN

Self-irradiated shocks : shocks hot enough to self-irradiate.
Could be the population of shocks out of the cascade from a high-velocity outflow
AGN-type of outflow vs Galactic wind from more diffuse sources (combined SN explosions).
M82 : IR emission from dust at the edge of the outflow seen in X-ray (hot gas)
500-1000 km/s shock at large scale is in part raiated in the X-ray, but also in part must cascade down to smaller scale lower velocity shocks, where energy radiates in H2 cooling lines.
Odd presence of H2 gas colocated with X-ray : we do not expect H2 to survive in such hot environments.
Distribution of shock velocities to model the cascade.
Cooling time overall several 100 Myr, much longer than the cycle WIM → WNM → Warm H2 → Cold H2
Mass flow rate from WIM to WNM much smaller (100 Msun/yr) than warm H2 → cold H2 (2e4 Msun/yr) so need for a process to replenish Warm H2 from cold H2. MHD shocks.
Fitting H2 rotational data has quite a strong degeneracy (2 gaussians at 5 and 20 km/s or a piecewise exponential both fit correctly)
« Slow shocks » are the case of B inclined with respect to the shock interface. « Fast shocks » have B parallel to interface. Lehmann et al. (2016) searches for distribution of shocks in a turbulent MC simulation. Velocities range to 5 km/s for slow shocks, 12 km/s for fast shocks.
Need to go to to high-J CO lines to disentangle fast and slow shocks.
At these speeds, temperatures reached are of 2e4 to 1.2e5 K, excitation of electronic levels of H
At 30 km/s Ly alpha radiation from these is comparable to ISRF. Then you have to take this into account (« like a PDR in the middle of your shock »)
UV photons can travel ahead of the shock, so quite complicated, needs to be solved iteratively. UV heats dust and gas ahead, and so changes « initial » state…. But not yet done.
LVG approximation not valid because the postshock region is not showing large gradients.
Implemented ALI+Ng for radiative transfer.