The vertical structure of the spiral galaxy NGC 3501: first stages of the formation of a thin metal-rich disc

TL;DR

This study uses deep integral-field spectroscopy to analyze the vertical structure and stellar populations of NGC 3501, revealing early stages of thin disc formation within a thicker, metal-poor component.

Contribution

It provides the first detailed stellar population and kinematic maps of NGC 3501, identifying distinct inner and outer disc components and proposing an early phase of thin disc formation.

Findings

NGC 3501 is a young galaxy with stars aged 2-8 Gyr.

The galaxy has a thicker, metal-poor, alpha-rich outer structure.

An inner, metal-rich, alpha-poor thin disc is in early formation stage.

Abstract

We trace the evolution of the edge-on spiral galaxy NGC 3501, making use of its stellar populations extracted from deep integral-field spectroscopy MUSE observations. We present stellar kinematic and population maps, as well as the star formation history, of the south-western half of the galaxy. The derived maps of the stellar line-of-sight velocity and velocity dispersion are quite regular, show disc-like rotation, and no other structural component of the galaxy. However, maps of the stellar populations exhibit structures in the mass-weighted and light-weighted age, total metallicity and [Mg/Fe] abundance. These maps indicate that NGC 3501 is a young galaxy, consisting mostly of stars with ages between 2 to 8 Gyr. Also, they show a thicker more extended structure that is metal-poor and $\alpha$-rich, and another inner metal-rich and $\alpha$-poor one with smaller radial extension. While previous studies revealed that NGC 3501 shows only one morphological disc component in its vertical structure, we divided the galaxy into two regions: an inner metal-rich midplane and a metal-poor thicker envelope. Comparing the star formation history of the inner thinner metal-rich disc and the thicker metal-poor disc, we see that the metal-rich component evolved more steadily, while the metal-poor one experienced several bursts of star formation. We propose this spiral galaxy is being observed in an early evolutionary phase, with a thicker disc already in place and an inner thin disc in an early formation stage. So we are probably witnessing the birth of a future massive thin disc, continuously growing embedded in a preexisting thicker disc.