SIRIUS: The relation between the diversity of dwarf galaxies and their formation histories

Abstract

Low-mass dwarf galaxies ($M_{\rm vir} \lesssim 10^9\rm\ M_\odot$) are fundamental cosmological building blocks, yet the physical processes driving their structural diversity remain poorly understood. Recent numerical simulations have suggested a diversity in the stellar-to-halo mass ratio in this halo mass range, but either the number of samples obtained from the same simulation setup or the numerical resolution was limited. We performed high-resolution cosmological zoom-in simulations for eight galaxies with a dark matter halo mass of $\sim 10^9\rm\ M_{\odot}$ up to $t=1.2$ Gyr at which most gas in the galaxies has been expelled. Our samples have a scatter of an order of magnitude in the halo mass at the reionization epoch. The stellar-to-halo mass ratio expected at $z=0$ scatters nearly two orders of magnitude with $5\times10^{-5}$ to $2\times10^{-3}$. We also observed variation in the compactness of their stellar distributions. Some of our simulated galaxies exhibit a stellar half-mass radius of $\sim30$ pc, which is as small as that of ultra-compact dwarfs. The formation condition for such a compact stellar distribution is understood as an analog of the condition for the formation of dense, massive star clusters. We found that when the central gas surface density exceeds a critical threshold ($Σ_{\rm gas} \gtrsim 30\rm\ M_\odot \rm\ {pc}^{-2}$), the star formation becomes highly efficient and results in dense stellar systems. These results suggest that UCDs can form in situ even in isolated dark matter halos.

Figures (14)

• Figure 1: Snapshots of Halos 198, 215, 219, and 230 (rows from top to bottom). The left and middle panels display the properties at the EoR: DM surface density (grayscale) with stellar distribution (yellow dots), and cold gas surface density ($T<1000$ K), respectively. The right panels show the DM and stellar distribution at the end of the simulation ($t=1.2$ Gyr). The red circles and inscribed numbers indicate the virial radius and virial mass, while yellow crosses mark the halo centers. Alt text: Four rows of halo snapshots (Halos 198, 215, 219, 230). Each row shows three panels: left = dark-matter surface density with stellar positions dots; middle = cold gas surface density (T<1000 K) colormap; right = dark-matter and stellar distribution at simulation end (t = 1.2 Gyr). Red dashed circle and number annotate the virial radius and virial mass; yellow cross marks the halo center.
• Figure 2: Same as Fig. \ref{['fig:snapshots1']}, but for Halos 236, 281, 284, and 299. Alt text: Four rows of halo snapshots (Halos 236, 281, 284, 299). Each row shows three panels: left = dark-matter surface density with stellar positions dots; middle = cold gas surface density (T<1000 K) colormap; right = dark-matter and stellar distribution at simulation end (t = 1.2 Gyr). Red dashed circle and number annotate the virial radius and virial mass; yellow cross marks the halo center.
• Figure 3: Dark matter halo mass evolution. The lower and upper horizontal axes indicate cosmic age in Gyr and redshift ($z$), respectively, while the vertical axis displays halo mass in solar masses ($\rm\ M_{\odot}$). Each colored curve represents the mass assembly history of an individual halo; the color coding is consistent across all figures to facilitate comparison in this paper. Star markers denote the onset of star formation in each halo. The vertical dashed line marks the reference epoch of reionization ($z=8.5$) Alt text: Line plot of dark-matter halo mass evolution for several halos from cosmic age 0 to about 1.2 Gyr. Each colored curve is one halo’s mass assembly history in $\rm log_10$ solar masses (vertical axis $10^4 \rm\ to\ 10^9$). Star symbols mark the onset of star formation in each halo; a vertical dashed line labeled REIONIZATION indicates the reference epoch ($z = 8.5$). An inset zoom at early times highlights rapid early growth differences. The legend identifies halo IDs (e.g., H198, H215, H219, H230).
• Figure 4: Star formation histories and mass evolution for four High-Mass Halos (Halos 198, 236, 284, and 299, arranged from top-left to bottom-right). The left vertical axis displays cumulative mass ($\rm M_\odot$): black dotted lines represent dark matter, dashed lines indicate cold dense gas ($T<1000$ K, $n>100\,\mathrm{cm}^{-3}$), and solid lines show stellar mass within the virial radius. The right vertical axis corresponds to the star formation rate (SFR) averaged over 6.9 Myr, shown by the colored curves. The vertical gray dashed line marks the epoch of reionization ($z=8.5$). Alt text: Star-formation histories and cumulative mass evolution for four high-mass halos (H198, H236, H284, H299): the left vertical axis shows cumulative mass (dark matter, cold dense gas, and stellar mass within the virial radius, plotted as dotted/dashed/solid lines), the right vertical axis shows the time-varying star-formation rate (colored trace) with bursts as spikes, and a vertical dashed line marks the epoch of reionization ($z = 8.5$).
• Figure 5: Same as Fig. \ref{['fig:H_SFH_cont']} but for the four Low‑Mass Halos, Halos 215, 219, 230, and 281, from top-left to bottom-right. Alt text: Star-formation histories and cumulative mass evolution for four high-mass halos (H215, H219, H230, H281): the left vertical axis shows cumulative mass (dark matter, cold dense gas, and stellar mass within the virial radius, plotted as dotted/dashed/solid lines), the right vertical axis shows the time-varying star-formation rate (colored trace) with bursts as spikes, and a vertical dashed line marks the epoch of reionization ($z = 8.5$).