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

by: K. A. Barger, L. M. Haffner, J. Bland-Hawthorn

[arXiv:1305.4932v2, pdf, First author, second, third]

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.

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 M_sun. 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.

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).

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.