Integral Field Spectroscopy of Collisional Ring Galaxies I: Stellar Populations Analysis

Abstract

Collisional ring galaxies are produced by the collision of a disk galaxy with a compact galaxy plunging through the disk, forming a ring-shaped expanding density wave, triggering star formation at its wake. The wave expansion is expected to produce negative stellar age gradients in radial profiles of post-collision stellar populations. Integral field spectroscopy combined with stellar population synthesis allows us to spatially resolve the stellar populations, to separate the post-collision and pre-collision components, and to produce the radial profiles. We analyse three candidate galaxies: Arp~143, NGC~2793, and VII~Zw~466. Observations were performed with the Calar Alto 3.5~m~telescope using the PMAS/PPak spectrophotometer. NGC 2793 presents a positive stellar age gradient, dismissing the collision hypothesis. For Arp~143 and VII~Zw~466, we found negative stellar age gradients for the youngest stellar populations, up to the ring radii, consistent with the collision hypothesis. We estimated that the collisions occurred $\sim$300~Myr and $\sim$100~Myr (expansion velocities of 33~$\pm$~10 km s$^{-1}$ and 108~$\pm$~26 km s$^{-1}$), respectively, before the density waves reached the observed ring radii. A spatially resolved analysis of the specific star formation histories (sSFH), reveals an expected star formation enhancement following the collision. The sSFH also allowed to identify the most probable intruder galaxy for VII~Zw~466. We report new redshifts for its group members. Finally, radial profiles of light contributions from pre-collisional and post-collisional stars show that the density wave dragged old pre-collisional stars along, as predicted by simulations.