Co-evolution of baryons and dark matter halos of LYRA dwarf galaxies

TL;DR

This work probes how baryons sculpt the co-evolution of dwarf galaxies and their dark matter halos using ultra-high-resolution LYRA zoom-in simulations of six halos near 10^9 M_sun. By tracking merger histories and star-formation episodes, the authors identify a dichotomy between rejuvenators (post-reionization star formation) and reionization relics (quenched after reionization), linking their SFHs to metallicity distributions and orbital morphologies. They find that baryons drive inner halo shapes toward rounder configurations, but do not generally form DM cores in these dwarfs, with Halo A showing a contracted core-like feature due to a central baryon-dominated phase during a starburst. Post-reionization mergers contribute 15–40% of the final stellar mass, and the scatter in the low-mass mass–metallicity relation correlates with SF history and galaxy shape, offering observable predictions for upcoming surveys such as Rubin and Euclid.

Abstract

We use the extremely high-resolution ($m_{\rm bary}=4\rm{M}_\odot$) LYRA cosmological galaxy formation simulations of six dwarf galaxies with $M_{\rm 200c}\sim10^9\rm{M}_\odot$ at $z=0$ to investigate their stellar assembly histories. Based on the age of stars in these galaxies at $z=0$, $40-100\%$ of their stellar mass was formed by the time of reionization, when star formation (SF) abruptly shuts down. Depending on their halo mass evolution, some of the dwarfs reignite SF post-reionization (rejuvenators), while others remain quenched for the rest of cosmic time (reionization relics). However, the stellar mass of relics can still grow by more than $50\%$ through mergers post-reionisation. We find clear correlations between metallicity distributions of the galaxies and the fraction of stars formed post-reionization ($f_{\rm post-reio}^\star$) such that relics have lower median $\rm [Fe/H]$ with a more prominent low metallicity tail. Moreover, the shape of the galaxies at $z=0$ correlates with their $f_{\rm post-reio}^\star$, with rejuvenators showing more spherical stellar distribution than relics. This difference arises only post-reionization when rejuvenators become rounder with more SF activity. Similarly, the shape of dark matter (DM) halos in the inner regions display more spherical distributions in rejuvenators than in relics. The shape evolution shows that DM haloes in all galaxy formation simulations become rounder in comparison to their collision-less, DM-only counterparts. However, DM haloes of rejuvenators evolve more significantly. We do not find any correlation between SF activity and formation of shallow DM density cores in these galaxies. These predictions can be tested using upcoming observational data. In particular, our results indicate that the scatter in the mass-metallicity relation in the low mass regime is correlated with SF histories and the shape of galaxies.

Figures (15)

• Figure 1: Galaxy stellar mass vs dark matter halo mass relation. The diamonds show the six LYRA galaxies in this relation. Other symbols show isolated galaxies from different simulations (data compiled in Sales:2022). In addition, the solid lines illustrate abundance matching results with the dashed and dotted lines corresponding to extrapolations on the low mass regime.
• Figure 2: Stellar (top) and cold gas (mid) mass growth vs z at different radius, where cold gas refers to gas cells with temperature $T_\mathrm{gas}<10^3 \,\mathrm{K}$. The bottom panel shows the SFH for each halo in 50 Myr bins. The left panels correspond to rejuvenated galaxies whereas the right ones are reionization relics. The grey vertical band illustrates reionization while the red translucent vertical lines indicate the corresponding significant mergers for each halo. Fainter merger lines correspond to smaller merger mass ratios. We see overall significant differences between the relics and rejuvenated dwarf galaxies, in terms of their SFHs and their cold gas content, illustrating our categorization of each galaxy in the sample.
• Figure 3: Top: Stellar metallicity distribution for each LYRA halo. The dot highlights the median of the distribution with the thicker line showing the interquartile range. Bottom: The median stellar metallicity against the mass fraction of stars formed post-reionization, $f^{\star}_\mathrm{post-reio}$ (left), and against the total stellar mass (right). On the right panel we include a mass-metallicity relation for dwarf galaxies from Kirby:2013. Relic galaxies show in general more extended metallicity distributions with smaller median values at smaller post-reionization mass fractions.
• Figure 4: Ellipsoidal axis ratios for the stellar component of the LYRA dwarf galaxies at $z=0$, measured enclosed to the stellar half mass radius, $r_{1/2}$ (see Sec. \ref{['sec: shape-algorithm']} for the details). The colour of the symbols represents their SFH category, rejuvenator (blue) or relic (red), and the size of each symbol is proportional to the fraction of stellar mass formed after reionization. This figure illustrates that SFH activity of LYRA dwarf galaxies are correlated with their shape.
• Figure 5: Evolution of the ellipsoidal axis ratios and triaxiality parameter for the stellar component of each simulated galaxy. The shapes are measured at fixed physical radius, corresponding to the $z=0$ stellar half mass radius, $r_{1/2}$. The top and bottom sets of panels correspond to the rejuvenated and relics galaxies, respectively. The orange and dark blue curves correspond to the intermediate-to-major (q) and minor-to-major (s) axis ratios, respectively. Below each panel we show the evolution of the triaxiality parameter.