Projection Effects in Barred Galaxies Causing Wrong Interpretation of Radial Flows

TL;DR

This study investigates how projection effects in barred galaxies can masquerade as radial gas flows when modeling kinematics with axisymmetric tilted-ring methods. Using a GalMer-based, strongly barred galaxy, the authors generate mock cubes and apply 3D-Barolo across bar orientations, including bars along the kinematic axes and at ±45°, while varying inclination and position angle. They find that bar-driven elliptical orbits produce large, projection-dependent radial velocities up to about $|V_{ m rad}|\, ext{~} obreak oughly 150$ km s$^{-1}$ inside the bar, and that the inferred rotation curves inside the bar are substantially biased; these spurious flows can mimic inflows or outflows depending on geometry. The results underscore the necessity of incorporating non-axisymmetric diagnostics, such as gravitational torque analyses and multi-tracer data, to distinguish true radial transport from projection artifacts in barred galaxies. Overall, the work cautions against over-interpreting axisymmetric kinematic models in barred systems and provides a benchmark for interpreting observations with 3D-Barolo.

Abstract

Galaxy disks in rotation are sometimes the site of radial flows, especially in their gas component. It is important to estimate the outflows, due to AGN or supernovae feedback, or inflows due to bar gravity torques. However, these radial flows may be confused with non-circular motions, which are quite frequent in the center of galaxy disks. We use a simulated giant, barred spiral galaxy from the GalMer database to study the non-circular motions induced by the bar. Our goal is to identify the spurious radial flows that kinematics modeling algorithms can detect, assuming circular orbits for the gas. Using mock data of a strongly barred galaxy, we quantify the radial velocities computed by the 3D-Barolo algorithm for different disk inclinations and several bar orientations in the plane of the sky: along the major and minor kinematic axes and at 45° from them. Our results show that projection effects cause kinematics modeling algorithms to confuse the radial component of velocity due to elliptical orbits with significant radial flows with mean values up to 84 km.s$^{-1}$, within the bar region. The computed rotation curve is also wrongly estimated inside the bar region, by as much as 150 km.s$^{-1}$ for the highest inclination.

Figures (10)

• Figure 1: The data moment maps of the gas in the central 20 kpc produced with the symmetrized gSb at the selected snapshot ($T = 500$ Myr). The pixel scale is 0.2 kpc. The point (0, 0) corresponds to the center of the bar. The gas component has been inclined by $i_{\rm{true}} = 30^\circ$ and rotated so the bar is aligned with the kinematic axes: the galaxy minor axis (left column) and major axis (right column). First row: integrated intensity maps (moment 0) in logarithmic norm. Second row: mean velocity field (moment 1) maps. The black curves show the isovelocity contours of levels [-150, -100, -50, 0, 50, 100, 150] km s$^{-1}$.
• Figure 2: Each column corresponds to one bar orientation of Fig. \ref{['fig:bar_symaxes_moms']}. Left column: bar along the minor axis and right column: bar along the major axis. Parameters fitted by 3D-Barolo with a free PA and a free inclination angle. Each marker correspond to one 'true' inclination value $i_{\rm{true}}\in\pm[30^\circ,80^\circ]$, the one we input to create the cube. The different parameters are plotted as function of the galactocentric radius. First row: the PA obtained at the end of step 1. Second row: the inclination angles obtained at the end of step 1. Third row: rotation curves obtained after step 2. The black dashed curve represents the expected circular velocity (Eq. \ref{['eq:Vcirc']}) computed from the gravitational potential (Eq. \ref{['eq:miyamoto']}). Fourth row: Radial velocity with the sign convention: negative towards the center.
• Figure 3: Each row corresponds to one configuration. Left column: moment 0 maps. Middle column: moment 1 maps. The gas component has been inclined by $i_{\rm{true}} = 40^\circ$ and rotated so the bar could form a 45$^\circ$ angle with respect to the galaxy kinematic axes. The image parameters and the isovelocity contours are the same as in Fig. \ref{['fig:bar_symaxes_moms']}. Right column: the corresponding bar diagram with the radial motion interpretation according to the gas velocity vector projected on the minor axis in red and the major axis is in green.
• Figure 4: Same as Fig. \ref{['fig:bar_symaxes_params']} but each column corresponds to one configuration presented in the rows of Fig \ref{['fig:configs']}.
• Figure 5: Rows correspond to the configurations presented in Fig. \ref{['fig:configs']} with a disk inclination of $i_{\rm true}=30^\circ$. Left column: modeled moment 1 maps with the corresponding parameters plotted in Fig. \ref{['fig:fits_PA_free']} with a free PA and a free inclination angle. Right column: residuals maps ($V_{\rm LoS}-V_{\rm mod}$).