The Impact of Starbursts on the CGM

I have read https://arxiv.org/abs/1707.05933v1 by Heckman et al.

They discuss about the impact of starbursts on the circumgalactic medium. Their method is to compare the properties of the absorption lines(Lya, CIV, SiII, OIV… ) around starbursts galaxies within about the viral radius of galaxies with ones around normal star-forming galaxies. The sample of starburst galaxies they used is from Comic Origins Spectrograph on the Hubble Space Telescope. The result is

1. The equivalent width of absorption lines around starburst galaxies are systematically higher than star-forming galaxies. (We can’t make strong statement for OIV and close regions to galactic center since their sample is few for them.) For Lya (saturated), the equivalent width traces the spread in velocity of the absorbing gas along the line of sight, while for the other(unsaturated and optical depth ~ 1), does the ionic column densities.

2. The FWHM and velocity offsets of Lya around starburst galaxies are wider than star-forming. These imply that the cloud traced by Lya is super-viral.

3. The ratios of equivalent width of Si III and CIV (unsaturated) lines to Lya (saturated) are larger too.

4.The amount of metals is enhanced around starburst galaxies and the dynamical state of the outer CGM is significantly different.

They discuss some models to explain these results. The main point is whether the unusual CGM cause starbursts or starbursts cause unusual CGM. They conclude the latter is reliable and explain how starburst driven galactic winds affect the CGM assuming the simple model. To understand the mechanism, I need to learn the galactic winds more.

Core-cusp problem

Learned a core-cusp problem.

The core-cusp problem is one of the problem of Lambda CDM model in small scales. The simulation suggests that dark matter density profile follows models which density is proportional to r^(-1), meaning that it diverges at r = 0, e.g. NFW, Harnquist etc… Such small scale diverging structure is called ‘cusp’. However, the observation shows that galaxies have a core structure which has constant density at small scales. This mismatch between observation and simulation is called ‘Core-cusp problem’.

Yesterday, the paper about this problem was published(https://arxiv.org/abs/1707.06303). This paper discusses whether dwarf galaxies, which is galaxies strongly dominated by dark matter, have cusp structures or not, I do not still finish reading though(7/22 14:10).

To understand the background of this paper, I learned the situation of the core-cusp problem. There are the solutions by considering the different dark matter models from CDM, e.g. WDM or SIDM. But here I focus on the solutions with baryon physics in the LCDM framework. There seems to be 4 solutions. The key point is to inject energy or angular momentum to dark matter.

1. Let the gas spread from the center by supernovae. They would come back to the center after being pressure equilibrium. If repeat this process adiabatically, dark matter (+ baryon) potential would get large and form the core.(arXiv:1106.0499v2)

2. Let galaxies form disks and expel the disks by supernovae (only once). Then, dark matter gets left angular momenta, which they got when disk was formed.(J F.Navarro et al. MNRAS 283, L72-78, 1996)

3. Consider dynamical friction of the central gas. When the gas loses the energy by the friction, dark matter gets the energy due to the energy conservation. (AMR EL ZANT et al. Apj 560:636-643, 2001)

4. Inject the angular momentum to dark matter through the bar structure of galaxies. (M D.Weingberg and N. Katz Apj 580:627-633, 2002)

I don’t fully understand the mechanism of 3 and 4. Therefore, further study is recquired. Also, I’m interested in the solutions by SIDM.

 

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