Tracing the relic nature of compact galaxies through their globular cluster systems

Abstract

We investigate the synthetic model of globular cluster (GC) systems of 17 compact massive galaxies (CMGs) from the Illustris TNG100 simulation to explore their connection with massive relic galaxies, systems that have undergone little structural evolution across cosmic time. The co-evolution of the GC systems and their host galaxies is based on a GC formation and evolution model that assigns clusters to stellar particles according to age and local conditions, providing positional, kinematic, and chemical information for individual GCs. By combining stellar assembly histories, effective radius evolution, and GC properties such as in-situ vs. ex-situ origin, metallicity, and spatial distribution, we identify consistent signatures of early formation and late-time accretion. We find that the GC mass fraction traces the host assembly history more robustly than the GC number fraction, as massive clusters better preserve the imprint of the early accretion history. Three CMGs from TNG100 emerge as strong massive relic analogs, exhibiting high in-situ GC fractions, narrow metallicity distributions, and compact spatial distributions. A tight correlation between the host stripped fraction and the extent of the ex-situ GC population further reveals the possibility to consider GC spatial profiles as a signature to identify tidal stripping processes. These results indicate that the combined analysis of GC populations and host stellar assembly offers a robust diagnostic for identifying massive relic galaxies and constraining their evolutionary histories.

Figures (12)

• Figure 1: Stellar mass--size relation for the selected sample of CMGs at $z=0$. A sample of TNG100 subhalos are shown as light gray circles for reference, while selected compact subhalos are represented by open circles. Dotted black line indicates the threshold of 2 kpc, while the dashed red line follows the surface density parameter for the selected sample ($\rm{R}_e<2\,\rm{kpc}, log\,\Sigma_{1.5}>10.0\,dex$).
• Figure 2: Stellar mass assembly histories of the compact massive host galaxies as a function of cosmic time. The stellar mass enclosed within the effective radius, $R_{e,\star}$, is normalized to its value at $z=0$. Each host galaxy is shown in colored lines, where solid lines correspond to systems with satellite accretion fractions below $20\%$, while dashed lines indicate galaxies with more extended accretion histories ($>20\%$). The vertical line marks $z = 2$, commonly adopted as the onset of the second phase of massive galaxy evolution. Overall, $58.8\%$ of the hosts have accretion fractions below $10\%$, while $88.2\%$ have ex-situ stellar mass fractions below $20\%$.
• Figure 3: GC in-situ fraction vs. host in-situ mass fraction. The diagonal dashed line indicates the 1:1 relation. The left panel shows the GC in-situ ($N_{gc}$) number fraction as a function of host in-situ mass fraction, the center panel shows the GC in-situ mass fraction as a function of host in-situ mass fraction, and the right panel shows the GC number vs. the GC mass fraction. We are considering just GCs with masses higher than $10^5\,\rm{M_\odot}$ to compute the fractions. The shaded region indicates where the in-situ fractions are $>80\%$ for x and y-axis. Similarly, the '*' symbol in the legend indicates the host IDs where the in-situ fraction (for hosts and GCs) is $>90\%$ among the three panels.
• Figure 4: [Fe/H] distribution over ages for all GCs in each host galaxy. GCs in-situ are displayed in red, and ex-situ GCs are displayed in blue in all the frames. The horizontal gray indicates the [Fe/H]=$-1$, for reference. The most GC in-situ -dominated hosts are highlighted with '*' symbol. The last row and column display the dispersion $\sigma$[Fe/H] for each host galaxy. The IDs 69530, 60753, 69512 are highlighted with red boxes in the legend frames
• Figure 5: Histograms of GC metallicity ([Fe/H]) for each host galaxy in the sample, showing the number of GCs (N$_{\text{GC}}$) as a function of [Fe/H]. The distributions are color-coded in red and blue to represent in-situ and ex-situ populations, respectively. All the frames are ordered to follow the same sequence as in Fig. \ref{['fig:age-met_compacts']}.