Measuring the evolution of stellar bars with the host galaxy's spin

Abstract

We examine to what extent the galaxy spin parameter proxy ($λ_R$) is affected by bar formation and how it is related to the strong and weak classifications of stellar bars. By creating mock observations of a simulated galaxy, we show that the emergence of a stellar bar can cause mass-weighted $λ_R$ to decrease by up to 16%, depending on the bar's orientation. This decrease can be exaggerated if there is a burst of star formation due to the bar driving gas to the center of the galaxy. We use the SAMI galaxy survey to show that weakly barred galaxies have statistically significant younger average stellar populations, higher galaxy spin proxy and higher specific star formation rates compared to strongly barred galaxies within one effective radius. If we consider galaxies with average light-weighted stellar population age less than 3 Gyr within one effective radius, we still find weakly barred galaxies to have a higher galaxy spin proxy than strongly barred galaxies. Based on these trends found from the SAMI galaxy survey, we suggest weakly barred galaxies are rapidly forming, similar to the bar formation process seen in simulations, while strongly barred galaxies are undergoing slower (secular) evolution.

Figures (15)

• Figure 1: Face-on surface density maps of our simulation depicting different stages in the evolution of the bar. Panel A: the disc before bar formation. Panel B: when $A_2/A_0$ first exceeds 0.2. Panel C: when the exponential growth of $A_2/A_0$ ends. Panel D-E: evolution of the disc after bar formation. The black circle shows where we define the bar radius.
• Figure 2: Top panel: Bar strength on a natural log scale. The dotted blue line shows the fit while the bar forms undergoing exponential growth. Bottom panel: Bar strength on a linear scale. The dotted green line shows when the bar strength peaks, or when the bar matures, while the dashed red line indicates $A_2/A_0 = 0.2$.
• Figure 3: Evolution of the kinematic properties in our simulation. The dotted, vertical green line shows the formation time of the bar. Solid lines show the evolution of the indicated quantity within the bar radius and dashed lines outside the bar radius. Top panel - the evolution of the change in velocity dispersions ($\sigma_i(t) - \sigma_i(100)$) in each region. Bottom panel - evolution of change in mean azimuthal velocity in each region ($v_\phi(t) - v_\phi(100)$).
• Figure 4: Star formation rate inside the bar region is shown by the solid black line while outside the bar region is shown by the dashed red line. The dotted, vertical green line shows the bar formation time.
• Figure 5: The ratio of tangential to radial force before bar formation (left), during bar formation (middle) and after bar formation (right). The black contours outline the mass distribution at each timestep.