Stellar Mass Growth in the First Galaxies: Theory and Observation

TL;DR

This work tests whether the stellar-mass growth of the first galaxies, as measured by JWST in the redshift window $6<z<12$, can be reproduced by the Galacticus semi-analytic model. By leveraging JWST-derived SFHs and a flexible, outflow-based feedback framework, the authors find that the shape of the stellar-mass growth is matched without tuning, and the amplitude can be reconciled when feedback is weakened by a factor of about three. They further show that a substantial fraction of high-redshift galaxies experience rapid, burst-like growth linked to mergers or gas inflows, but these bursts account for only a minority of the total stellar mass formed. Extending the model to $z\sim0$ reveals the necessity of a redshift-dependent feedback prescription to simultaneously fit early and present-day stellar-mass statistics, highlighting the importance of mass growth as a stringent test for galaxy formation theories.

Abstract

We compare the growth in stellar mass of galaxies in the $6<z<12$ epoch with predictions of a semi-analytic galaxy formation model - Galacticus. In contrast to diverse and controversial results that compare models and data for the \emph{luminosity} evolution of galaxies -- reported in an abundance of recent papers, we find very good, unambiguous agreement in the more fundamental quantity of stellar mass - measured from JWST observations - and Galacticus predictions. Specifically, we find good agreement for the shape of the integrated stellar mass as a function of redshift without any adjustment of parameters, and in \emph{amplitude} as well, when 'feedback' is lowered by a factor of 3 compared to that required to match later-universe models and data. This result emerged from detailed investigation of the claim by Dressler et al. that bursts of star formation dominated the growth in stellar mass, specifically, that half of the galaxies with stellar mass growth of $M_* > 2\times10^8 \mathrm{M}_\odot$ in the epoch $8<z<6$ had less than $M_*<\times10^8 \mathrm{M}_\odot$ prior to $z = 8$. Here too we find agreement between models and data, namely that these ~100 Myr 'bursts' had strong in situ growth at $z\le8$, or showed (in Galacticus) substantial stellar and/or gas-rich mergers, and 30-40 Myr 'starbursts' as are common in $z<3$ galaxies. We note that, if a theoretical simulation is unable to pass the test of matching the growth of stellar mass, any success in reproducing the luminosity function is meaningless.

Figures (6)

• Figure 1: The demographics of star formation histories represented as the growth of stellar mass in the first billion years of cosmic time. Circles show individual galaxies at their observed epochs from 2024ApJ...964..150D. The color of each point indicates its classification based on its star formation history as defined by 2024ApJ...964..150D and as indicated in the panel. Grey vertical numbers along the bottom axis show the age of the universe in Myr, while gray horizontal numbers show the duration in Myr between each identified epoch. The seven epochs shown cover $\sim$1 Gyr of cosmic time, in intervals that grow from $\sim$40 Myr at $z=12$ to almost $\sim$200 Myr at $z=6$. A steep decrease in detection sensitivity occurs at redshift $z>10$, but is is relatively constant at $10^8$$\,$M$_{\odot}$ for $z<10$, so we adopt constant detectivity in mass. (The gap at $z\sim6.5$ is an artifact of the way redshifts are determined by interpolation in a scheme that is working near its resolution limit---see 2024ApJ...964..150D for details.) This figure is reproduced with permission from 2024ApJ...964..150D.
• Figure 2: The growth of stellar mass (shown as the total stellar mass in galaxies per square degree per unit redshift) from $z=12$ to $z=6$. Colors indicate the stellar mass selection: blue shows the total mass in galaxies with stellar masses above $10^{7.8}\mathrm{M}_\odot$ while purple shows the total mass in galaxies with stellar masses above $10^{8.6}\mathrm{M}_\odot$. Circles indicate observed galaxies from 2024ApJ...964..150D. Lines show results from our Galacticus model, with dashed lines showing our default feedback model ($V_\mathrm{outflow}=150$ km/s; $\alpha_\mathrm{outflow,(disk|spheroid)}=(3.8,2.7)$), and solid lines indicating our reduced feedback model ($V_\mathrm{outflow}=120$ km/s; $\alpha_\mathrm{outflow,(disk|spheroid)}=(3.0,3.0)$). For comparison, we also show results from a very weak feedback model ($V_\mathrm{outflow}=20$ km/s; $\alpha_\mathrm{outflow,(disk|spheroid)}=(2.0,2.0)$) as dot-dashed lines. Black, dotted lines show the total baryonic mass (assuming the universal baryon fraction) in halos with masses above $10^{10.7} \mathrm{M}_\odot$ and $10^{11.5} \mathrm{M}_\odot$ (upper and lower lines, respectively).
• Figure 3: Examples of star formation histories of Galacticus central galaxies classified into the 'burst' category. The total stellar mass at $z=6$ is shown above each panel, along with the fraction of stellar mass formed in 'starbursts' (indicated by the vertical dotted lines, and identified as times at which the star formation rate in a peak exceeds twice the mean star formation rate) in the $z=7$ and $z=6$ bins. Solid blue lines show the total star formation rate (left axis), while dotted blue lines indicate the in situ star formation rate. Green circles show the integrated stellar mass (right axis) formed in each unit redshift interval. The horizontal dotted line indicates a stellar mass of $10^8\mathrm{M}_\odot$ as used in the classification of galaxies into 'bursts' and 'non-bursts.' Open black circles/stars (shown at arbitrary position on the $y$-axis) indicate the stellar mass ratio (defined as the stellar mass of the satellite galaxy divided by the sum of the stellar masses of the satellite and central galaxies) in merger events, while filled red circles/stars show the gas mass ratio in the same merger events---larger circles indicate larger mass ratio. Black/red circles indicate minor mergers, while black/red stars indicate major mergers.
• Figure 4: Examples of star formation histories of Galacticus central galaxies classified into the 'non-burst' category. The total stellar mass at $z=6$ is shown above each panel, along with the fraction of stellar mass formed in bursts (indicated by the vertical dotted lines, and identified as times at which the star formation rate in a peak exceeds twice the mean star formation rate) in the $z=7$ and $z=6$ bins. Solid blue lines show the total star formation rate (left axis), while dotted blue lines indicate the in situ star formation rate. Green circles show the integrated stellar mass (right axis) formed in each unit redshift interval. The horizontal dotted line indicates a stellar mass of $10^8\mathrm{M}_\odot$ as used in the classification of galaxies into 'bursts' and 'non-bursts.' Open black circles/stars (shown at arbitrary position on the $y$-axis) indicate the stellar mass ratio (defined as the stellar mass of the satellite galaxy divide by the sum of the stellar masses of the satellite and central galaxies) in merger events, while filled red circles/stars show the gas mass ratio in the same merger events---larger circles indicate larger mass ratio. Black/red circles indicate minor mergers, while black/red stars indicate major mergers.
• Figure 5: Left panel: The stellar mass--halo mass relation at three different redshift ranges: $z=0.2$--0.5 (blue), $z=2.5$--3.0 (purple), and $z=5.5$--6.0 (orange). Solid lines indicate the median relation inferred from observed galaxies by 2025AA...695A..20S, with the shaded regions indicating the $1\sigma$ confidence interval on this median. Filled points show the results from our Galacticus model that matches the assembly of stellar mass at $z > 6$. Open circles show a variant of this model in which we set $V_\mathrm{outflow} = 150 (1+z)^{-0.115}$ km/s. Right panel: The stellar mass function at the same three redshift ranges. Points with error bars show observational results from 2025AA...695A..20S, while lines show results from our variant Galacticus model (i.e., that shown by open symbols in the left panel).