Large scale nested stellar discs in NGC 7217

TL;DR

This study analyzes the stellar populations and kinematics of NGC 7217, revealing a multi-component structure likely shaped by past minor mergers, including a thin inner disc and a thick, young outer disc with star-forming rings.

Contribution

It provides a detailed dynamical and stellar population analysis of NGC 7217, highlighting the galaxy's multi-disc structure and its likely formation through minor merger events.

Findings

Inner disc is thin with intermediate age stars (~5 Gyr)

Outer disc is thick, metal-poor, and young (~2 Gyr)

Minor mergers likely formed the galaxy's rings and polar disc

Abstract

NGC7217 is an unbarred early-type spiral galaxy having a multi-segment exponential light profile and a system of starforming rings of the unknown origin; it also possesses a circumnuclear gaseous polar disc. We analysed new long slit spectroscopic data for NGC7217 and derived the radial distributions of its stellar population parameters and stellar and gaseous kinematics up to the radius of r~100 arcsec (~8 kpc). We performed the dynamical analysis of the galaxy by recovering its velocity ellipsoid at different radii, and estimated the scaleheights of its two exponential discs. The inner exponential stellar disc of NGC7217 appears to be thin and harbours intermediate age stars (t(SSP) ~ 5 Gyr). The outer stellar disc seen between the radii of 4 and 7 kpc is very thick (z0 = 1...3 kpc), metal-poor, [Fe/H]<-0.4 dex, and has predominantly young stars, t(SSP) = 2 Gyr. The remnants of minor mergers of gas-rich satellites with an early-type giant disc galaxy available in the GalMer database well resemble different structural components of NGC7217, suggesting two minor merger events in the past responsible for the formation of the inner polar gaseous disc and large outer starforming ring. Another possibility to form the outer ring is the re-accretion of the tidal streams created by the first minor merger.