Creasey, Peter; Theuns,Tom; Bower, Richard G.; How supernova explosions power galactic winds

How supernova explosions power galactic winds

by: Creasey, Peter; Theuns,Tom; Bower, Richard G.

[ADS:2013MNRAS.429.1922C; pdf; First author; second; third]
I thought that this was an interesting paper. Because it is very similar to what I am doing it gave me a number of ideas about how to set up specific tests and a way of organizing my data to pick out the important details. This paper is a subset of Peter Creasey's PhD work so the general approach and specific questions that he is exploring are different from mine, but on some level we are doing similar work.

He is using the MHD code FLASH, but only in the hydro configuration (like I am doing with Athena). Even though FLASH has the great advantage of being a AMR code they ran into some problems where the AMR was trying to refine the simulation beyond what was practical so they turned off the AMR component of FLASH and used it as a fixed grid solver (which automatically removed the major advantage of using FLASH, and basically turned FLASH into a FORTRAN version of Athena).

When I read that in the paper I instantly knew why they had done that. Because of the increased resolution from the AMR the time step in the simulation would drop to something incredibly small. Thus to do a 5 Myr simulation like they are doing, it would take a VERY LONG TIME to do a single simulation. I would estimate that a single run may take several months on 128+ processors. By turning off the AMR they could force a coarser resolution and thus take larger time steps, and thus run a number of simulations in less time (I think they did 61 simulations if I remember correctly).

Their resolution for their simulations ranged from 32x32x160 to 256x256x1280 over a spacial range of 200x200x1000 pcs. Thus their highest resolution runs had about 50% fewer cells than my highest resolution runs, but my simulations are spread out over a box of 1000 pc³, so I have slightly lower spacial resolution.

They are looking at how supernova feedback affects mass loading which they define as β≡M˙_wind/M˙_⋆ (those are supposed to be M dots, as in change of Mass). This is a measure of how much mass is flowing out of the grid over how much gets converted into new stars (oh and as a note, for this major important equation they reference Stringer et al. (2011), but there is no reference to Stringer et al. (2011) in the bibliography. There is a Stringer et al. (2012), but even though that paper is the intended paper it does not have the β in it as advertised. Stringer may use another symbol but they don't use β in any of their equations. Anyway minor thing.) Basically they are looking at how much gas escapes the galaxy based on global properties of the galaxy which can then be matched to larger simulations where galactic outflow of individual galaxies is important.

Below is Figure 8 from the paper with the original caption:

Figure 8. Matrix view of simulations varying gas surface density (Σ_g) and gas fraction (f_g), each panel showing a time-averaged vertical velocity for the upper half plane of each simulation (i.e. the disc is at the base of each panel). Gas surface density increases from left to right, gas fraction increases from bottom to top. There appears to be a strong trend in wind velocity towards the lower right-hand panels, i.e. a disc with low gas fraction but high gas surface density tends to generate a faster wind.

I have two comments. First: Cool. That is interesting. Second: AAAAGGGHHHH!!!!!! Rainbow color map!! Why?!?!? AND they used it with a diverging scale, but they cut off the bottom half of each grid so they didn't even need half of their color map. Use something else! Not the rainbow color map. I had to stare at this plot for several minutes to even figure out what they were showing. If they had used something else like a diverging scale, or an incremental luminous scale then it would have been much easier. Anyway, they had other things to worry about.

Wagner, A. Y.; Bicknell, G. V.; Umemura, M.; Driving Outflows with Relativistic Jets and the Dependence of Active Galactic Nucleus Feedback Efficiency on Interstellar Medium Inhomogeneity

Driving Outflows with Relativistic Jets and the Dependence of Active Galactic Nucleus Feedback Efficiency on Interstellar Medium Inhomogeneity

by: Wagner, A. Y.; Bicknell, G. V.; Umemura, M.

[arXiv:1205.0542, pdf, First author, second, third]
I noticed this paper because the second author on it has also written several papers with Jackie Cooper, whose research forms the jumping off point for my own work. There are many things about the set up to this problem that mirror what was already done by Cooper and Bicknell in previous papers. So they drew upon years of previous work to create this set of simulations that form the foundation of this paper. Having taken all the tools and mechanics that were developed previously all they had to do was modify it for this specific problem and let it run. No need to reinvent the wheel. If you take a glance at this paper you may not realize the large amount of work that went into setting up this problem by many other people before this problem was solved. This is the way almost all science works and this is how I have done a lot of my work. As one of my former professors would say, "A lazy physicist is a good physicist!"

They use the code FLASH in its relativistic hydrodynamics configuration to do a series of simulations of relativistic jets streaming off of an AGN and into the local ISM. They use a fractal distribution to create the initial density of the warm phase ISM (~10⁴K). The rest is hot gas (~10⁷K). Their study consists of 29 different simulations, where 15 of those were done for this paper and the rest were included in a previous paper. In their different simulations they tested different parameters such as the jet pressure, the density of the hot phase medium, average density of warm phase, volume filling factor of warm phase, maximum cloud size, total mass in warm phase, and other parameters that depend of the sampling wavenumber (effectively a measure of the fractal scale relative to the overall size of the fractal cube).

In this paper they are looking at the potential for galactic feedback, effectively a measure of how much of the energy in the AGN gets transferred into the warm ISM, thereby dispersing the clouds. They find that the feedback is sensitive to the maximum cloud size but not so much the filling factor. In order to determine the veracity of their findings they compare the velocity dispersion of the ablated clouds with observations of galaxies with radio lobes (i.e. those that have powerful AGN jets).

I thought that this paper was interesting since it deals with something close to what I am doing, with similar problems, methods and solutions. I will definitely use it as a reference for references, ideas and ways of organizing my own simulations. There are some questions that they explored here and things that they calculated or measured that I will definitely consider including in my own work.