Disc growth and vertical heating of lenticular galaxies in the Fornax cluster

TL;DR

This work analyzes the vertical and radial structure of mono-age stellar populations in three edge-on Fornax lenticular galaxies using deep MUSE data. By deriving $R_{50}$ and $z_{50}$ across 1 Gyr age bins, the authors find a largely constant disc thickness for ages up to ~6 Gyr, indicating minimal secular heating and limited impact from cluster environment on the inner discs. Older populations thicken, likely reflecting ancient mergers and accreted stars, which are typically metal-poor and distributed in thicker configurations. The radial structure shows limited inside-out growth, suggesting strangulation as the dominant quenching mechanism over the past 8 Gyr, while the cluster environment primarily suppresses gas supply rather than driving strong inner-disc heating. The results demonstrate the value of mono-age population analyses for constraining galaxy evolution mechanisms and foreshadow broader insights from the GECKOS survey of 36 nearby edge-on discs.

Abstract

We present a detailed analysis of the vertical and radial structure of mono-age stellar populations in three edge-on lenticular galaxies (FCC 153, FCC 170, and FCC 177) in the Fornax cluster, using deep MUSE observations. By measuring the half-mass radius (R$_{50}$) and half-mass height (z$_{50}$) across 1 Gyr-wide age bins, we trace the spatial evolution of stellar populations over cosmic time. All galaxies exhibit a remarkably constant disc thickness for all stars younger than ~6 Gyr, suggesting minimal secular heating and limited impact from environmental processes such as tidal shocking or harassment. Evidence of past mergers (8-10 Gyr ago) is found in the increase of z$_{50}$ for older populations. We find that accreted (metal-poor) stars have been deposited in quite thick configurations, but that the interactions only moderately thickened pre-existing stars in the galaxies, and only caused mild flaring in the outer regions of the discs. The radial structure of the discs varies across galaxies, but in all cases we find that the radial extent of mono-age populations remains constant or grows over the past 8 Gyr. This leads us to argue that within the radial range we consider, strangulation, rather than ram-pressure stripping, is the dominant quenching mechanism in those galaxies. Our results highlight the usefulness of analysing the structure of mono-age population to uncover the mechanisms driving galaxy evolution, and we anticipate broader insights from the GECKOS survey, studying 36 nearby edge-on disc galaxies.

Figures (7)

• Figure 1: The top row shows the average mass-weighted age distributions for the three galaxies studied here, while the rest of the figure shows the surface density for four different mono-age populations in each galaxy (12--13 Gyr old, 8--9 Gyr old, 4--5 Gyr old, and finally 0--1 Gyr old). In all panels, the vertical lines enclose the central bulge or bar-dominated region, while the horizontal lines delimitate the thin- and thick disc-dominated regions Pinna2019aPinna2019b. In all galaxies, we find strong differences in the spatial distribution of stars of different ages: one striking aspect is the much thinner distribution of young stars compared to older stars.
• Figure 2: The first and second columns show R$_\mathrm{50}$ and z$_\mathrm{50}$ as a function of age for FCC 153, FCC 170 and FCC 177. The black dots represent the values obtained from our best fit to the spectra, while the blue violin plots represent the distribution of values obtained from the 50 Monte Carlo iterations (the upper and lower limits of the violin plots correspond to the full range of values from the 50 iterations, while the shape of the blue region represents the distribution of the values). The grey dots represent the values from the best fits, but excluding the very central regions of the galaxies, where nuclear star clusters are found. The third column represents z$_\mathrm{50}$ as a function of R$_\mathrm{50}$, color-coded by age, for each galaxy. Here, the error bars represent the 16$^\mathrm{th}$-84$^\mathrm{th}$ percentile range.
• Figure 3: Structure of FCC 153 (top row), FCC 170 (middle) and FCC 177 (bottom). The first column shows radial profiles of z$_\mathrm{50}$ for mono-age populations, with each line color-coded by the age of the corresponding population. The thick black line represents the radial evolution of the global z$_\mathrm{50}$ for each galaxy. The vertical dashed lines delimit the regions of the galaxies we use to compute the mean z$_\mathrm{50}$ values shown in Fig. \ref{['fig:r50_z50_all']}. The second column shows the mass fraction in each mono-age population, at each radius. In this column as in the first one, we only plot populations containing at least 1 per cent of the total mass of each galaxy. The last column shows radial profiles of the average mass-weighted age along the mid-plane (blue solid line) and the thick disc (violet dashed line). The shaded area represents the 16-84th percentile range, and the line the median.
• Figure 4: R$_\mathrm{50}$ (top row) and z$_\mathrm{50}$ (bottom row) as a function of age for FCC 153, FCC 170 and FCC 177. The grey dots represent the values obtained for the global population (they are the same as the black dots in Fig. \ref{['fig:r50_z50_all']}), while the blue and red dots correspond to metal-poor and metal-rich stars, respectively. The surface of each blue and red dot is proportional to the fraction of the mass in this metallicity range at a given age. Pinna2019aPinna2019b proposed that the 8--11 Gyr old metal-poor stars could be accreted from small galaxies: we indeed find that these stars have a different spatial distribution compared to the metal-rich stars (possibly formed in-situ).
• Figure 5: This figure shows the surface density for all mono-age populations in FCC 153 (from 0--1 Gyr old in the top left panel, to 13--14 Gyr old in the bottom right panel). In all panels, the vertical lines enclose the central bulge or bar-dominated region, while the horizontal lines delimitate the thin- and thick disc-dominated regions Pinna2019aPinna2019b.