The effects of boxy/peanut bulges on galaxy models and gas inflow

Based on Fragkoudi et al., (2015), MNRAS 450, 229; Fragkoudi et al., (2016), MNRAS 462, L41

Non-axisymmetries, such as bars are the main drivers of the late time secular evolution of disc galaxies, responsible for rearranging their stellar and gaseous content and driving gas to the central regions where it can form discy (-pseudo) bulges and create a fuel reservoir for AGN activity.


In order to study the dynamics of galaxy discs one can create models from observations by deriving the potential of these galaxies directly from images. As it is common to use galaxies which are almost face-on, one of the assumptions which has to be made in order to obtain their 3D density distribution, is the shape of the vertical density distribution, i.e. height function. Previous works have all assumed a constant height function. However, bars can "puff up" due to vertical orbital resonances and the buckling instability (Combes et al. 1990, Raha et al. 1991) which leads to the formation of boxy/peanut (b/p) bulges (see Figure 1). Therefore by using a constant height function we may be significantly overestimating the torques in disc galaxies, which ultimately dictate the gas inflow to the central regions.

I explored the impact of realistic boxy/peanut height functions (see Figure 2) on the derived potentials of galaxies, on their orbital structure and bar strength. I found that boxy/peanut bulges reduce the forces in the plane of the galaxy by up to 40%. Additionally, they reduce the bar strength (see Figure 3) and significantly change the orbital structure of the galaxy. Therefore, non-uniform height functions are necessary to create accurate models of real galaxies (Fragkoudi et al., 2015) if a galaxy contains a b/p bulge.

Figure 1: A striking example of a galaxy with a boxy/peanut bulge (ESO597). Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA).

Figure 2: a flat (left) and a boxy/peanut (right) height function. Credit F. Fragkoudi.

Figure 3: (Left:) Some characteristic periodic orbits in the bar region for our model with a boxy/peanut bulge. (Right:) The bar strength as a function of radius for models with (black line) and without (red line) a boxy/peanut height function. We see that the bar strength is reduced in the model with a boxy/peanut bulge. Credit: Fragkoudi et al. 2015

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

In Fragkoudi et al., (2016) I showed that the inward flow of gas towards the central regions of galaxies is reduced in the presence of boxy/peanut bulges by up to an order of magnitude (see Figure 4 below), which could have significant consequences for the formation of discy bulges and on the build up of a fuel reservoir for nuclear star formation and AGN activity. 


To investigate the effect of boxy/peanut bulges on gas inflow to the central regions, I used 2D hydrodynamical simulations of the gas response in models with and without boxy/peanut bulges, using a modified version of the AMR code RAMSES (see Video 1). 

I made a catalogue of gas flow simulations in different analytic potentials of barred galaxies. You can find videos of the gas morphology, as well as plots of the jump in gas density and velocity along the shocks created at the leading edge of the bar. The parameters of the analytic models are slowly varied, so one can see the effect of each parameter on the gas flow.

Figure 4: Left: Gas inflow rate as a function of time for three models, one without a b/p, one with a weak and one with a strong b/p bulge. Right: Mass buildup in the central kiloparsec for the three models. We see that the gas inflow rate is reduced in the model with a prominent b/p (by up to an order of magnitude) and the gas mass buildup is also reduced. From Fragkoudi et al. 2016