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).

J. R. Dawson ; The Supershell-Molecular Cloud Connection: Large-Scale Stellar Feedback and the Formation of the Molecular ISM

The Supershell-Molecular Cloud Connection:

Large-Scale Stellar Feedback and the Formation of the Molecular ISM

by: J. R. Dawson

[arXiv:1301.1419 [astro-ph.GA], pdf, first author: personal site]
This is a review paper on most of the current major papers dealing with how molecular clouds form. It is a good reference paper for me to use. A number of the major papers that he cites are ones that I have seen before. She cites five of Fabian Heitsch's papers (my advisor) and a few from Mordecai-Mark Mac Low (Fabian's advisor). So this is a review of many of the papers that are similar to things that I am doing.

The basic purpose of the paper is to review the current state of simulations and observations of molecular clouds in their natural habitat, the ISM. It tries to address the fundamental question, "How, when and where do we form molecular clouds." The problem with molecular clouds is that in order for them to form is that you need sufficient density and column density to shield the cloud from UV radiation that will ionize the cloud and destroy any molecules that form. Any significant heating will also destroy the cloud and prevent it from achieving densities high enough to allow for gravitational collapse.

The current review is focused on how large scale stellar feedback can contribute to the formation and evolution of molecular clouds. In other words how can O and B type stars contribute to an environment that is conducive for the formation and survival of molecular clouds. The idea is that the OB regions will form a super bubble in the galaxy which will form shell walls of gas that has been swept up and compressed by the strong stellar winds. These shells will rapidly cool, and along with hydrodynamics (and magnetohydrodynamics!) and thermal instabilities will cause the shell walls to collapse and form molecular clouds. The interest is in modeling these effects, and also trying to model the interacting flows at the shell boundaries where the molecular clouds actually form.

There are two possible processes for the formation of molecular clouds. The first is through global gravitational collapse of the the galactic disk. This model requires significant inflows of matter from outside the galaxy, usually from the halo or galactic neighborhood. This is where high velocity clouds come in and play a role. Also galactic fountains can be placed in this process, but galactic fountains can also be part of the second process. The second process is from the shocks and turbulence inherent in the galactic disk from gas flows, and star formation. This paper focuses on the second process.

The general idea can be summed up from the first figure in the paper:

In order to have the blow out as shown in figure 1 there needs to be enough energy from the OB region to push its way out of the galactic disk. There region will sweep up gas from the ISM and will form a shell wall. The question is, what are the characteristics of this wall? And do molecular clouds form there (Section 3), or are they formed elsewhere (Section 4) and are caught up in the wall?

The question of how much energy is needed to cause a blow out depends heavily on the ISM and where the OB region is located and how strong it is. He cites Mac Low et al. 1989 and Tenorio-Tagle et al. 1990 (also see Tenorio-Tagle et al. 1990) on this one. These supershells are defined to have formation energies of E ≥ 10⁵² erg. If there is a blow out then the ejected material will supply the halo with metal rich material and more energy.

Here are the section headings and subsection headings for sections 3 and 4, with a brief explanation of what is in the sections. These are the sections that cover modeling and are the sections that I am currently most interested in.

3 Molecular Cloud Formation in Supershells: Theory & Modelling
3.1 Molecule Formation & Destruction
This section covers the conditions that are needed for molecules to survive. We need to know under what conditions molecules can survive in order to constrain our models. This deals with the strength of the UV field that will cause dissociation and ionization.

3.2 Gravitational Instability of Expanding Shells
This section covers the question of at what point will the shell begin to collapse gravitationally. There are conditions that affect whether or not a shell can collapse, such as the strength of the supernovas or OB stars that create the shell. If there is too much energy then the shell will be too hot for collapse. Farther away the shell may begin to collapse, but this is dependent on the ISM and the speed of the shell. Even still there may not be enough time for the shell to collapse before the proto molecular clouds fragment (the fragmentation timescale is shorter than the collapse timescale).

3.3 Molecular Cloud Formation in Colliding Flows
If you have two interacting flows (i.e. two shells raming into each other) then there may be favorable conditions to form molecular clouds. This model relies on thermal instabilities and turbulence to form molecular clouds. The thermal instabilities rely on an interesting property of the pressure density relation of cooling gas. The role of magnetic fields is still being investigated in all of this.

3.4 Whole-Disk Models of the Feedback Structured ISM
These models look at how certain gas structures are formed by having large scale simulations of the entire disk. Dawson includes a figure from Hill et al. (2012). The third author on the paper is Mordecai-Mark Mac Low. I spoke to Mordecai-Mark last year about this paper when he visited UNC last year. He was able to give me some pointers about how to fix my own problems because they had run into the exact same problems in their simulations. As in I was talking to him and I said, "This is what I am working on, but I am having some problems." and he said, "And you are getting negative temperatures under these conditions. We had the same problem so we hired a math PhD to fix the problem. This is what we did ..."

4 Pre-Existing Molecular Clouds
4.1 Cloud Disruption
This section looks into the conditions associated with the shells and how they interact with a non-homogeneous ISM. This is critical for the survival, growth collapse of pre-existing molecular clouds.

Section 5 looks into observations being made of the Milky Way and of near neighbors (Large and Small Magellanic clouds). By mapping the CO and H₂ in reference to star forming regions we can get a sense of whether or not molecular clouds form inside the shells or form outside the shells and are caught up in them as they sweep by. Also we can look at column densities, structure and expected life span of the clouds. Dawson has done a number of observations of these supershells and her work will be interesting to look into. I may review some of her papers in the future.

Papers Cited:

Hill, A. S., Joung, M. R., Mac Low, M.-M., Benjamin, R. A., Ha ner, L. M., Klingenberg, C., & Waagan, K. 2012, ApJ, 750, 104

Mac Low, M., McCray, R., & Norman, M. L. 1989, ApJ, 337, 141

Tenorio-Tagle, G., Rozyczka, M., & Bodenheimer, P. 1990, A&A, 237, 207