Determining the Dark Matter distribution of barred galaxies

Based on Fragkoudi et al., (2017), MNRAS 466, 474

The nature of dark matter, which makes up 85% of the mass of the Universe, is still unknown. One of the major roadblocks is that while we can measure the total mass of a galaxy -- due to its gravitational effects -- we do not know how much of it is dark, and how much of it is ``normal'' baryonic matter.

 

The density distribution of the dark matter halo in the central regions of disc galaxies has proven hard to determine due to uncertainties in the Mass-to-Light ratio (M/L) of the stellar disc. This gives rise to the disc-halo degeneracy (see Figure 1) when trying to fit the various components that make up the rotation curve of the galaxy.


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Figure 1: The disc-halo degeneracy (van Albada & Sancisi 1986)

Figure 2: The galaxy NGC 4303. The white arrows indicate the dust lanes on the leading edges of the bar. Right: An example of one of my gas dynamical models of NGC 4303.

A way of breaking this degeneracy is to model, via hydrodynamic simulations, the gaseous response to non-axisymmetries in the baryonic component of galactic potentials, such as bars and spirals. Gas placed in an axisymmetric potential will follow circular orbits; however, when strong non-axisymmetries are present, the gas is forced to shock, thus creating dust lanes, as observed in many barred galaxies (see left panel of Figure 2). The strength of these shocks depends, among other things, on the ratio between the amount of baryonic to DM. Therefore, by tuning the M/L of the disc, such that the simulations (see right panel of Figure 2 and Video 1) reproduce the morphology and kinematics of the gas in observations, we can also constrain the amount of DM.

 

I'm currently working on a sample of disc galaxies taken from the S4G survey (Sheth et al. 2010) for which we have the stellar light distribution in the NIR, as well as their gas kinematics and morphology. The results from the first galaxy we modelled, NGC 1291, have been published in Fragkoudi et al., (2017). I found that the M/L of NGC 1291 agrees with that predicted by stellar population synthesis models in the near-infrared (≈0.6\,M⊙/L⊙) and that the disc of NGC 1291 is maximal, which means that the DM in the central regions is sub-dominant. Furthermore, the bar rotates fast, with a corotation radius which is ≤ 1.4 times the bar length.

This technique will greatly benefit from upcoming IFU and sub-millimeter observations, which will provide full 2D spatial coverage of the kinematics as well as spatial variations of the stellar M/L ratio.

 

Video 1: Simulation of the gas flows in a barred galaxy model. Produced using glnemo2. Copyright F.Fragkoudi 2015.