Two-phase formation of galaxies: the coevolution between galaxies and dark matter halos

Abstract

We use FIRE-2 cosmological zoom-in hydrodynamic simulations to investigate the co-evolution between Milky Way-size galaxies and their host dark matter halos. We find that the formation of these galaxies follows a two-phase pattern, with an early phase featured by hot dynamics, bulge-dominated structure and bursty star formation, and a later phase featured by cold dynamics, disk-dominated structure and steady star formation. The transition times of these galaxy properties are correlated with the time when the host halo transits from fast to slow accretion, indicating the two-phase assembly of halos as a potential mechanism that drives the two-phase formation of galaxies. The physical origin of dynamical hotness can be summarized into two modes of star formation: a scattered mode in which stars form at large radii within cold gas streams associated with fast assembly of halos, and a concentrated mode in which stars form at small radii through violent fragmentation from globally self-gravitated gas when halo assembly is about to slow down. Cold gaseous and stellar disks can form when the conditions of the two modes are removed by the stall of fast halo assembly and the reduction of gas by feedback processes. The two modes of star formation leave distinct imprints on the structural properties of high-redshift galaxies, providing implications to be tested by JWST and future observations.

Figures (12)

• Figure 1: Visualization and profiles of the galaxy m12f at $z=0$. (a)-- (d), 2-D stellar mass surface density, viewed edge-on (row 1) and face-on (row 2). The four columns show the results for all stellar particles, and those belonging thin disk ($\epsilon\geq0.8$), thick disk ($0.2\leq\epsilon<0.8$) and bulge ($\epsilon<0.2$), respectively. (e), mass-weighted distribution of circularity ($\epsilon$) of star particles. Colored solid curve shows the distribution of all star particles within $R_{\rm 90,*}$, with three colors indicating the three kinematic components, respectively. Black solid curve shows the distribution of only young star particles (age $<100\,{\rm Myr}$) within $R_{\rm 90,*}$. The mass fraction of each component in indicated in the legend for all/young stars. (f), mass surface density profiles, obtained using all (solid curves) and young (dashed curves) stars, showing in total (black) and separately for the three kinematic components (colored). (g), hotness as a function of radius, obtained by using all (solid curve) and young (dashed curve) stars enclosed within the radius. We mark the galaxy sizes, $R_{\rm 50,*}$ (blue) and $R_{\rm 90,*}$ (red), by circles in (a2) and vertical lines in (f) and (g).
• Figure 2: Properties of the galaxy m12f and its host halo as functions of the cosmic time ($t$) or redshift ($z$). Each panel shows a different property. (a), stellar mass in total (gray) and separately for the three kinematic components (colored), and $M_{\rm vir}$ of the host halo (black). Three vertical lines indicate the epochs at which the bulge mass reaches 20%, 50% and 80%, respectively, of its final value at $z=0$, with that corresponding to 50% denoted as $t_{\rm bulge, 1/2}$. (b), the growth rate of the three kinematic components (colored). For reference, we show the accretion rate of baryons, defined as the growth rate of $M_{\rm vir}$ times the cosmic baryon fraction (black). (c), mass fractions of the three kinematic components, using young stars only. Grey vertical band indicates the transitional interval, $t_{\rm thin}^{\rm (start)}$--$t_{\rm thin}^{\rm (end)}$, of the thin-disk fraction. (d), dynamical hotness of the galaxy ($\mathcal{H}$), calculated using young stars only. Grey vertical band indicates the transitional interval, $t_{\mathcal{H}}^{\rm (start)}$--$t_{\mathcal{H}}^{\rm (end)}$ of $\mathcal{H}$. (e), burstiness of star formation ($\mathcal{B}$). Vertical line indicates $t_{\mathcal{B}}$, the epoch when the star formation transits from bursty to steady (defined by $\mathcal{B} = 0.2$, indicated by the dashed horizontal line). (f), cold-gas spin $\lambda_\text{cold-gas}$, and fractions of cold ($f_\text{cold-gas}$), warm and hot gas. (g), (h), virial velocity ($V_{\rm vir}$) and specific growth rate of the host halo ($\gamma$; see Eq. \ref{['eq:halo-gamma']} for definition). Grey vertical bands indicate the transitional interval, $t_{\gamma}^{\rm (start)}$--$t_{\gamma}^{\rm (end)}$, of halo assembly, defined by $\gamma = 3/8$--$0$ (horizontal lines in h) In (c), (d), (g) and (h), each solid curve is obtained from the simulation, while the associated dashed curve is obtained by a parametric fitting (see §\ref{['ssec:galaxy-properties']} and §\ref{['ssec:halos']} for details). This figure shows that the galaxy evolves from an early phase featured by hot dynamics, bulge-dominated structure and bursty star formation, to a later phase featured by cold dynamics, disk-dominated structure and steady star formation, and that the assembly of the host halo decelerates with time. See §\ref{['sec:co-evolution']} for details.
• Figure 3: Correlation between halo transition time ($t_\gamma$) and galaxy transition times. Each panel uses one definition of galaxy transition time: (A), $t_{\rm thin}$; (B), $t_{\rm \mathcal{H}}$; (C), $t_{\rm \mathcal{B}}$; (D), $t_{\rm bulge,1/2}$. Each marker represents one galaxy in our sample, with error bar spanning from the start to the end of the transition interval (for $t_\gamma$, $t_{\rm thin}$ and $t_{\rm \mathcal{H}}$) or from 20% to 80% formation time of bulge mass (for $t_{\rm bulge,1/2}$). The ending epochs of the transitions of $f_{\rm thin}$ and $\mathcal{H}$ are undefined for m12z, for which we only show the starting epochs of the transitions with arrows pointing upward. For each pair of variables, the Spearman correlation coefficient ($\rho$) is indicated in the corresponding panel (see also Table \ref{['tab:corr']} for a full list of $\rho$ and $p$-values), and black line shows a linear fitting. Markers with grey edges represent the exceptional galaxies (m12w, m12r and m12z) excluded from correlation analysis and linear fitting. This figure demonstrates a co-evolution pattern between halos and galaxies. See §\ref{['sec:co-evolution']} for details.
• Figure 4: Durations of transitional phases for individual galaxies. Here the the duration, $\Delta T_{\rm dyn}$, is normalized by the dynamical timescale of the host halo (see Eq. \ref{['eq:duration']}), and the transitional phase is defined by the evolution of (a), $f_{\rm thin}$; (b), $\mathcal{H}$. Markers are the same as in Fig. \ref{['fig:Correlation']}. Grey shading indicates the range $1 \leqslant \Delta T_{\rm dyn} \leqslant 3$. See §\ref{['sec:co-evolution']} for details.
• Figure 5: The quadrant diagram showing the evolution of galaxies in the $f_{\lambda}$-$\gamma$ plane. In each panel, dots show the snapshots of all galaxies in our sample, color-coded by a property labeled in the color bar. $f_\text{gas+star}$ in (b) are evaluated within the inner $1\, {\rm {kpc}}$ of galaxies. The growth rate of stellar bulge ($\dot{M}_{\rm bulge}$, in $\rm M_{\odot}yr^{-1}$) shown in (h) is smoothed by the LOESS method Cleveland01091988, using the Python package Cappellari2013. White path linking the dots with black edges shows the evolution of m12f. Two vertical lines show $\gamma = 3/8$ and $\gamma = 0$, respectively, which define the transitional regime of halo assembly. One horizontal line shows $f_{\lambda} = 1$, above which self-gravity of gas is expected to prevent the formation of a dynamically cold disk. The $f_{\lambda}$-$\gamma$ plane is partitioned into four quadrants, Q1--Q4, by these lines, within which galaxies are expected to have distinct dynamical states. The FIRE-2 galaxies follow a general evolutionary trajectory in this plane, from Q1 at high $z$ to Q3 at low $z$. See §\ref{['ssec:quadrant']} for details.