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.

Melioli,C. et al.; Evolution of M82-like starburst winds revisited: 3D radiative cooling hydrodynamical simulations

Evolution of M82-like starburst winds revisited: 3D radiative cooling hydrodynamical simulations

by: C. Melioli, E. M. de Gouveia Dal Pino, F. Geraissate

[arXiv:1301.5005, pdf, first author, second, third]

This is an interesting paper because it is closely related to work done by Cooper et al. (first paper 2008, second paper 2009) that I have been looking at for some time. The paper deals with simulations done using a hydro AMR that has radiative cooling and some species tracking. It is more work on superbubbles and AGNs. They specifically use M82 as a test case.

The authors are from Brazil (Sao Paulo), and the code is named YGUAZU, which is a Paraguayan spelling of Iguazú (sort of appropriate for a hydro code since it means "Big Water"). Other than some basics (they use a Van Leer integrator) they only provide references and no explanation. Also interesting is the fact that they cite Strickland & Stevens (2000) in their explanation of how they set their initial conditions, but they don't use the notation of Strickland and Stevens. They use the notation of Jackie Cooper (2009) (she did work with Strickland and Stevens and used their code and set up). But these guys don't cite here even though they have copied her equations exactly.

Their energy injection centers around super stellar clusters (SSCs) "with an average size of ∼ 5.7 pc and mass (of stars) between 10⁴ and 10⁶ M_⊙(Melo et al. 2005)." They look at metals and how much gas escapes the galaxy and how much metals produced by supernovas escapes the galaxies. They conclude that most of the gas mass stays in the galaxy even with a superbubble blow out. Also most of the metals stay in the galaxy but some get transported out in the galactic winds that form due to the supernovas (the SN's pump out metal rich winds).

Mac Low, M.-M.; McCray, R.; Superbubbles in disk galaxies

Superbubbles in disk galaxies

by: Mac Low, M.-M.; McCray, R.

[ADS: 1988ApJ...324..776M, pdf, first author: personal site, second author]
This paper is part of a series of papers done by Mac Low and McCray on this subject. This paper contains the theory and analytic backing on the subject, another paper in 1989 contains more of the simulations on the same subject. This paper and others formed the basis of Mac Low's PhD dissertation, "Interactions of Massive Stars with the Interstellar Medium: Bow Shocks and Superbubbles". McCray was his advisor. Mac Low would later go on to advise my advisor Fabian Heitsch for his PhD.

This particular paper is heavy on the theory and equations. Basically they are looking at how supernovas (SNs) interact with the ISM. As supernovas explode they release significant energy into the ISM and create a bubble of hot gas. If the bubble is large enough it gets classified as a superbubble, which has the possibility of blowing out of the galactic disk, which then affects the halo and galactic accretion.

This paper represents a significant step forward in our understanding of the structure of the ISM. The models produced here are idealized and smooth, meaning everything looks nice, flat and symmetric. 25 years later the technology and experience available to researchers has improved and thus we have moved on to solving this exact same problem, except in 3D and with a much more complex setup.

There are two important conclusions that I wanted to mention. They provide a parameter that determines whether or not a superbubble will blow out of a stratified galactic disk. They give it as:

D = L_SN ρ₀^1/2 P_e^-3/2 H^-2

where L_SN is the luminosity from the SNs, ρ₀ is the density of the galactic disk (or ISM), P_e is the external pressure from the ISM, and H is the scale height of the disk. If D > 100 then there will be blowout even if the center of the superbubble begins to collapse. It will be interesting to find a corresponding parameter for a more complex set of simulations.

The second important point is that if there is a dense cloud in the ISM then when the edge of the superbubble over runs it will not "puncture" the bubble leading to a release of pressure. The bubble will instead travel around it and continue expanding. This is something that has become a very important consideration since it is the thing that allows molecular clouds to survive strong shocks like this. This was essentially a hint at the beginning of the study of the survivability of cold molecular clouds when they have been strongly shocked. The problem is that if molecular clouds are strongly shocked then they will heat up, expand and will not collapse gravitationally to form stars. So there has to be some way for them to survive long enough to form stars. Many people will look into this problem later, and research is still going on.

As a note, they used the 2D hydro code Zeus, which was very influential back in the day. The creators of Zeus rewrote the code for MHD and 3D and named it Athena (original site). Athena is the code that I use for my research.

Tenorio-Tagle, G.; Rozyczka, M.; Bodenheimer, P. ; The hydrodynamics of superstructures produced by multi-supernova explosions

The hydrodynamics of superstructures produced by multi-supernova explosions

by: Tenorio-Tagle, G.; Rozyczka, M.; Bodenheimer, P.

[ADS: 1990A&A...237..207T, pdf, first author: personal site, second, third]
This is an important historical paper from 1990. The motivation behind this paper goes back several years before this when astronomers were considering the effect that supernovas in OB regions (regions with type O and B stars) would have on the ISM and the general shape and structure of the galactic disk (see references in the introduction for history, VERY important!!! as in I will use these references in my dissertation).

A single supernova will ionize a section of the ISM and will create a small bubble with a well defined boundary and interior and exterior properties. If we consider multiple supernovas then our region begins to become much bigger. At some point the radius of the bubble exceeds the scale height of the galactic disk, thus we are no longer considering a series of blasts in a uniform medium. We now have a stratified medium with a gravitational potential. This changes the properties of the blast region and greatly affects the shape, internal structure and characteristics of the superbubble. There is still a sharp boundary for the region and this transition is termed a "supershell" (Heiles 1979, 1984). The formation of this structure is very important as it is linked to the formation of molecular clouds, which in turn collapse and form stars, thus feeding star formation in a galaxy.

In this paper the authors do 2D simulations of a stratified disk and vary a number of parameters to see how much energy is needed to achieve blow out (i.e. at what point does a bubble become a superbubble). The different parameters tested are summed up in their first table. All units are cgs.

For the density distribution the tried an exponential fall off (exp. 1 and 2), uniform, Gaussian distribution and a composite. Each one is defined in the paper. They also looked at the effect of a hot halo placed on top of the disk and how that changed the blow out.

What is interesting is that the basic structure of the ISM determines the shape and strength of the blow out. Also the velocity of the escaping gas is strongly constrained by the ISM. In the end these superstructures can readily be created by OB complexes and it is assumed that they can persist for many millions of years. The blow out can create a metal rich fountain that when it rains back down on the galaxy will fuel metal rich star formation.

They reference two papers by Mac Low (and others) who were working on this problem at the same time. I may review those papers next. They are: Mac Low and McCray 1988, and Mac Low, McCray and Norman 1989.

Papers Cited:
Heiles, C.; 1979, ApJ, 229, 533-537, 539-544.
Heiles, C.; 1984, ApJS, 55, 585-595.
Mac Low, M.-M. & McCray, R.; 1988 ApJ, 324, 776-785.
Mac Low, M.-M., McCray, R. & Norman, M. L.; 1989, ApJ, 337, 141-154.