Projection effects in barred galaxies cause the wrong interpretation of radial flows

¹ Observatoire de Paris, LUX, CNRS, PSL University, Sorbonne University, 75014 Paris, France
² Collège de France, 11 Pl. Marcelin Berthelot, 75005 Paris, France
³ Centre national d’études spatiales (CNES), 2, Pl. Maurice Quentin, 75039 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 November 2025
Accepted: 5 February 2026

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 active galactic nucleus or supernovae feedback, or the inflows, due to bar gravity torques. However, these radial flows may be confused with noncircular motions, which are quite frequent in the center of galaxy disks. We used a simulated giant, barred spiral galaxy from the GalMer database to study the noncircular 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 quantified the radial velocities computed by the 3D-Barolo algorithm for different disk inclinations and several bar orientations on 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 92 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.