Kinematics of the lens host S0 galaxy NGC 1553: role of secular processes

TL;DR

This study uses high-resolution MUSE integral-field spectroscopy to dissect the central region of the S0 galaxy NGC 1553. The authors identify a rapidly rotating nuclear disc–lens embedded in the main disc, with a hot inner lens and a central classical bulge, evidenced by a strong central $oldsymbol{\sigma}$ peak and distinctive Gauss-Hermite signatures ($h_3$, $h_4$). Stellar population analyses show an old, metal-rich central component with an inside-out age gradient, alongside a younger, moderately metal-poor disc, suggesting in-situ disc-lens formation with later rejuvenation likely from minor interactions. The results support an early formation of the disc-lens through secular processes, with subsequent evolution driven by both internal bar-driven dynamics and external processes in the Dorado group, offering insights into the origin of kinematically distinct substructures in S0 galaxies.

Abstract

We present an investigation of the central structure of the S0 galaxy NGC 1553, to understand its origin and the underlying dynamical processes that shape it. The high-resolution integral field spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE) reveal a well-ordered rotation pattern, consisting of a rapidly rotating nuclear disc that is somewhat decoupled from the main disc, together with an inner lens; we collectively refer to these structures as the "disc-lens". The central peak in the velocity dispersion indicates the presence of a classical bulge. The nuclear disc is dynamically colder than the surrounding disc, while the lens is dynamically hotter. The higher-order Gauss-Hermite moments, $h_{3}$ and $h_{4}$, further characterise the stellar kinematics. An anti-correlation between the line-of-sight velocity and skewness ($h_{3}$) is consistent with regular rotation. In contrast, the ring-like enhancement in kurtosis ($h_{4}$) confirms the presence of the nuclear disc component. Unsharp masking of HST images (Erwin et al. 2015) reveals a nuclear bar and faint spiral structures within the central 10 arcsec, supporting the role of secular evolution. The mass-weighted stellar age map shows an old stellar population in the central regions, with high metallicity that suggests the in-situ formation of the disc-lens from disc material. We discuss possible formation scenarios for the disc-lens, including both minor mergers and secular processes, and examine the influence of the Dorado group environment on NGC 1553. Our findings suggest that the disc-lens in NGC 1553 formed during the early stages of the galaxy's evolution. However, its subsequent development has been shaped by internal and external processes. These results provide new insights into the origin and evolution of kinematically distinct substructures in S0 galaxies.

Figures (6)

• Figure 1: DECaLS color composite image (left) and residual image (right) of NGC 1553 and nearby galaxy NGC 1549. The white arrow in both images represents the faint shell feature in NGC 1553. North is oriented upward, and east is to the left. The size of the bow is 22 arcmin in each panel.
• Figure 2: Stellar kinematic maps of NGC 1553 derived from the MUSE data cube using the GIST pipeline. The line-of-sight (LOS) stellar velocity ($V$) distribution is presented in the top-left panel, while the corresponding stellar velocity dispersion ($\sigma$) map is shown in the top-right panel. The nuclear disc is outlined by the solid black ellipse, while the dashed ellipse indicates the hot inner lens. The third ($h_{3}$) and fourth ($h_{4}$) Gauss-Hermite velocity moments are displayed in the bottom-left and bottom-right panels, respectively. The overlaid contours, extracted from the MUSE intensity maps, are spaced at uniform intervals of 0.2 mag. The orientation is such that the North is up, and the East is to the left.
• Figure 3: 1D radial profiles from the corresponding 2D maps. Left: The intensity map of NGC 1553 is constructed using a MUSE datacube over the wavelength range of 480–580 nm. To extract the 1D profiles, we employed a rectangular pseudo-slit of width 8 arcsec and a step size of 1 arcsec along its length. Right: 1D radial profile of (a) $V$, (b) $\sigma$, (c) $h_{3}$ and (d) $h_{4}$ along the white rectangular pseudo-slit as shown on the left panel. The vertical dashed line in each profile signifies the position of the central slit. Typical median errors are $V_{err} \sim 6.1$$km$$s^{-1}$, $\sigma_{err} \sim 8.1$$km$$s^{-1}$, $h_{3,err} \sim 0.028$ and $h_{4,err} \sim 0.033$.
• Figure 4: Deprojected radial profiles of $V/\sigma$ at each Voronoi bin, with velocity ($V$) corrected for inclination. The peak value $V_{dp}/\sigma$ and the radius at which this peak is located ($r_{k}$) are shown with the horizontal and vertical dashed lines, respectively, and the corresponding values are also given.
• Figure 5: Mass-weighted stellar age map (left) and stellar metallicty map (middle) of NGC 1553 derived from the Voronoi-binned MUSE data cube. Overlaid black contours represent isophotes from the MUSE white-light image, spaced at intervals of 0.2 mag. The dashed black ellipse marks the central 13 arcsec region corresponding to the nuclear disc-lens. North is up and east is to the left. The star formation history (SFH) is shown on the right, where each profile represents the SFH of an individual bin.