Thursday, August 15, 2013

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 (Σg) and gas fraction (fg), 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.

Tuesday, July 2, 2013

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.

Wednesday, May 29, 2013

Barger, K.A.; Haffner, L.M.; Bland-Hawthorn, J.; Warm Ionized Gas Revealed in the Magellanic Bridge Tidal Remnant: Constraining the Baryon Content and the Escaping Ionizing Photons around Dwarf Galaxies

Warm Ionized Gas Revealed in the Magellanic Bridge Tidal Remnant: Constraining the Baryon Content and the Escaping Ionizing Photons around Dwarf Galaxies
This paper deals with the "bridge" (a remnant of tidal interaction) between the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The bridge in this case is a region of ionized gas that stretches between the two clouds. In the paper they present the results of an Hα survey which they did using the Wisconsin Hα Mapper (WHAM) observatory. Some of the interesting results that they got were that the ionization fraction was higher in the bridge than in the SMC tail (36 − 52% compared to 5 − 24%).

They also found that the amount of ionizing radiation from the Milky Way and from extragalactic sources is insufficient to ionize the bridge. Thus they conclude that there needs to be a small amount of ionizing radiation that leaks out of the SMC and the LMC (about 4-5%). This has implications about how dwarf galaxies affect their surroundings and how much they affect their surroundings.
Figure 1 from Barger et al. The contours show column density of H I.
The contours are at 10, 20, 35, and 50 x 1019 cm−2.
I thought that this was a good paper that included a good discussion about how they made their observations and how they had to work with their data in order to subtract off atmospheric and other interference. This is relevant to me because in my simulations I am trying to reproduce some of these features observed here so I need to know what people are observing, how they are observing it and what is possible to observe so that I can fit that with my models.

Tuesday, May 14, 2013

McClure-Griffiths, N. M.; et al.;Atomic Hydrogen in a Galactic Center Outflow

Atomic Hydrogen in a Galactic Center Outflow
[arXiv:1304.7538v1, pdf, First author, second, third, fourth]
This is part of the results from a survey where the authors found several cool, T < 4000 K, H I clouds in a definite pattern around the Galactic Center. The distribution and kinematics of the cloud implied that they were created from a starburst located at the center of the galaxy that has been affecting the galaxy for the past ∼ 2 × 106 years.

They did this by calculating the local standard of rest (LSR) velocities for the cloud centers and then using a simulation that calculated the effect on clouds with a random distribution of velocities and comparing to their observations they determined that the observed clouds did not have kinematics similar to galactic rotation but have a distribution that implies ballistic motion away from the galactic center (see figure below).

They also give a bound on the velocity of the stellar winds that can create this type of distribution  "The [Kolmogorov-Smirnov] test implies that wind velocities greater than 270 km s−1 and less than 150 km s−1 are not consistent with the observed cloud velocities."

They conclude that these clouds are the remnants of a superbubble which was created by a starburst at the center of the Milky Way. They also mention survivability and life spans of the clouds. They provide some bounds to the life spans. They also rely heavily on the simulations of Jackie Cooper to reach some conclusions about the environment of the wind and clouds.

Sunday, April 14, 2013

Dobbs, C. L.; Pringle, J. E.; The exciting lives of giant molecular clouds

The exciting lives of giant molecular clouds
by: Dobbs, C. L.; Pringle, J. E.
[arXiv:1303.4995v1, pdf, first author, second]
This paper does a particle simulation with 8 million particles with each particle having a mass of ~300 Msun. They include heating and cooling following the method of Glover and Mac Low (2007), with cooling being switched off at 50K. Each particle contains a fraction of H2 which they recalculate each timestep based on expected formation and destruction events. They also include stellar feedback with the feedback relation given in equation (1) of their paper. It depends on an arbitrarily chosen parameter (ε), the  H2 mass, and a factor that comes from their chosen IMF. This is multiplied by 10^51 ergs, which comes from the average energy of a supernova events. The energy is deposited half in thermal energy, half in kinetic. No explanation on how they choose the deposit of kinetic energy, but that may be covered in a previous paper (actually a lot of what they are doing is covered in previous papers, they are always referring to previous papers for their set up). They also had a background stellar potential, with perturbations for the spiral arms.

They ran their simulations for ~300 Myr, and found that giant molecular clouds form in the spiral arms through the conglomeration of smaller clouds. The GMCs are then disrupted mostly through sheer effects, but in the smaller clouds there is more disruption through stellar feedback. The larger clouds can also stretch out into a spur in the spiral arm which in turn can form its own GMCs, from the remnants of the original cloud. Below is figure 1 from the paper which shows column density at 250 Myr. One particular GMC that they studied in detail is marked with a red square.
Below is figure 3 from the paper which shows the evolution of the cloud marked in figure 1.

Thursday, February 21, 2013

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

[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).

Friday, February 1, 2013

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 = LSN ρ01/2 Pe-3/2 H-2
where LSN is the luminosity from the SNs, ρ0 is the density of the galactic disk (or ISM), Pe 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.

Monday, January 14, 2013

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.

Saturday, January 12, 2013

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

Wednesday, January 9, 2013

The Purpose of This Blog

I decided to start this blog as a way to force myself to cover the published (or soon to be published) literature of my field of study. I have my personal blog that I update from time to time that I use to post mostly about stuff that is not related to my research. Because I know that I need to keep up on recent literature I plan on trying to pick one paper a week and read/skim and give a brief summary of the points that stuck out to me.

This is not intended to be a comprehensive review of the literature, nor is it supposed to cover the best or most important papers. I select my papers by looking at the title/abstract and saying, "Hey that looks interesting!" or, "I should look at that paper and find out what they are doing." I may also review a paper that was discussed in my research group meeting, or in some other random meeting. In other words, there is no real method or intent other than I need to keep up on recent goings on in my field. I will try to do one paper a week, but if that doesn't happen...meh.

My sources will generally come from arXiv (or from ADS for older papers), usually from recent posts on the following categories/subcategories:
The name of the blog comes from my online name Quantumleap42 which, with a little bit of imagination, can be thought of a first name, "Quantum", a middle name, "leap" and a last name "42". Thus the name of the blog is Quantum Musing, as in, Quantum is musing.