The difference in age of the two counter-rotating stellar disks of the spiral galaxy NGC 4138

TL;DR

This study investigates the age, composition, and origin of two counter-rotating stellar disks in galaxy NGC 4138, revealing that the younger disk likely formed from retrograde gas accretion or merging events.

Contribution

It provides detailed spectroscopic analysis of the two stellar components, offering new insights into their formation history and ruling out bar dissolution as a cause.

Findings

The younger stellar component is marginally more metal-poor and more alpha-enhanced.

The younger component is associated with a star-forming ring.

Properties support formation via gas accretion or retrograde merging.

Abstract

The Sa spiral NGC 4138 is known to host two counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured the kinematics and properties of the two counter-rotating stellar populations to constrain their formation scenario. A spectroscopic decomposition of the observed major-axis spectrum was performed to disentangle the relative contribution of the two counter-rotating stellar and one ionized-gas components. The line-strength indices of the two counter-rotating stellar components were measured and modeled with single stellar population models that account for the alpha/Fe overabundance. The counter-rotating stellar population is younger, marginally more metal poor, and more alpha-enhanced than the main stellar component. The younger stellar component is also associated with a star-forming ring. The different properties of the counter-rotating stellar components of NGC~4138 rule out the idea that they formed due to bar dissolution. Our findings support the results of numerical simulations in which the counter-rotating component assembled from gas accreted on retrograde orbits from the environment or from the retrograde merging with a gas-rich dwarf galaxy.