---
title: 'FirstLight: Simulating Cosmic Dawn Galaxies'
url: https://www.emergentmind.com/topics/firstlight
type: topic
---

# FirstLight: Simulating Cosmic Dawn Galaxies

FirstLight is a suite of cosmological zoom-in hydrodynamical simulations constructed to follow the formation and evolution of primeval galaxies during cosmic dawn and the epoch of reionization, typically from $z \gtrsim 15$ down to $z \simeq 5$–6. Across the FirstLight series and its later extensions, the project couples high-resolution adaptive-mesh simulations to post-processing for stellar populations, nebular emission, dust radiative transfer, and synthetic observables, with the explicit aim of building statistically meaningful, mass-limited samples of early galaxies and predicting their UV luminosities, star-formation histories, metallicities, line emission, dust attenuation, morphologies, and sizes [1703.02913, 2102.12343].

## 1. Project scope and development

FirstLight was introduced as a large database of high-resolution zoom-in simulations of galaxy formation around the epoch of reionization. The initial project design selected halos from parent boxes of 10, 20, and 40 $h^{-1}\,\mathrm{Mpc}$, with a main halo catalog of 978 objects across all boxes and cosmologies; the first published tests analyzed 15 zoom-in halos with $M_h \approx 10^9$–$10^{11}\,M_\odot$ at $z=5$ [1703.02913]. Subsequent analyses focused on a mass-selected sample of 290 halos at $z=5$, yielding roughly 300 distinct galaxy histories over $z=5$–15, and later extensions expanded the database to 377 and 430 zoom-in simulations for studies of efficiency and synthetic size measurements [1801.10382, 2404.02537, 2603.05045].

The simulated galaxy population spans broad ranges in intrinsic properties. The database described in the early series covers rest-frame UV magnitudes $M_{\rm UV} \simeq -12$ to $-22$ and stellar masses $M_\star \simeq 10^6$–$10^{9.5}\,M_\odot$, while specific studies of burst histories quote $M_\star \approx 10^6$–$3\times10^9\,M_\odot$ [2102.12343, 1801.10382]. Later analyses of bright and massive systems extend to $M_\star \sim 10^{10}$–$10^{11}\,M_\odot$ in selected subsamples [2402.08911, 2603.05045].

The internal structure of the series is cumulative. FirstLight I established the UV luminosity function, stellar mass function, and basic halo–galaxy scaling relations at $z \gtrsim 6$; FirstLight II quantified the highly bursty star-formation histories from $z=5$ to 15; FirstLight III modeled rest-frame UV–optical spectral energy distributions; FirstLight IV added nebular-line physics and spatially resolved emission in sub-$L_\star$ systems at $z \simeq 6$ [1703.02913, 1801.10382, 1810.09754, 2102.12343]. Later papers extended the framework to the mass–metallicity relation, [O III]-bright emitters, dust attenuation and infrared emission, clump formation, galaxy compactness, and accelerated size evolution [1910.11729, 2301.02416, 2304.10150, 2402.08911, 2510.05299, 2603.05045].

| Series component | Primary focus |
|---|---|
| FirstLight I | UV luminosity function, SMF, scaling relations |
| FirstLight II | Bursty SFR histories and SF main sequence |
| FirstLight III | Rest-frame UV–optical SEDs |
| FirstLight IV | [O III], H$\alpha$, and spatially resolved nebular structure |

This progression made FirstLight not merely a galaxy catalog, but an interpretive framework for JWST-era studies of reionization galaxies.

## 2. Numerical framework and subgrid physics

Most FirstLight analyses use the ART code, an adaptive mesh refinement Eulerian gravity-plus-hydrodynamics solver. In the standard database, the simulations adopt WMAP5 cosmology with $\Omega_m=0.27$, $\Omega_b=0.045$, $h=0.7$, and $\sigma_8=0.82$; some later 40 $h^{-1}\,\mathrm{Mpc}$ extensions use Planck13 or Planck 2014 cosmologies [1801.10382, 1910.11729, 2402.08911, 2602.07347]. Target halos are selected by $V_{\max}$ at $z=5$, with thresholds that depend on box size, and are re-simulated at high resolution.

The canonical FirstLight resolution is $m_{\rm DM} \approx 10^4\,M_\odot$ for dark matter, minimum star-particle mass $\approx 100\,M_\odot$, and maximum spatial resolution 8.7–17 proper pc; the more massive 40 $h^{-1}\,\mathrm{Mpc}$ samples typically use $m_{\rm DM}=8\times10^4\,M_\odot$, minimum stellar particle mass $10^3\,M_\odot$, and 17–32 pc proper resolution [1801.10382, 2102.12343, 2402.08911]. Snapshot cadence is typically 7–10 Myr in the high-cadence outputs, while the dynamical integration reaches adaptive timesteps down to $\sim 1$ kyr in the sub-$L_\star$ nebular analyses [2102.12343, 2510.05299].

The physics model includes atomic H/He cooling, metal-line cooling, and in several descriptions molecular $\mathrm{H}_2$ cooling; a cosmological UV background with partial self-shielding; stochastic star formation in gas with $n>1\,\mathrm{cm}^{-3}$ and $T<10^4\,\mathrm{K}$; and multi-channel stellar feedback combining thermal, kinetic, and radiative terms [1703.02913, 1801.10382, 2510.05299]. In the FirstLight I description, radiative feedback is implemented through a non-thermal pressure term in cells containing stars younger than 5 Myr if the gas column density exceeds $10^{21}\,\mathrm{cm}^{-2}$, with moderate IR photon trapping above $300\,\mathrm{cm}^{-3}$; momentum injection from supernovae and winds is applied over 40 Myr with IMF-integrated specific momentum $3.75\times10^3\,\mathrm{km\,s}^{-1}$ [1703.02913]. Metal enrichment from SNe II and Ia is tracked with yields approximating Woosley & Weaver (1995) [1801.10382, 2102.12343].

The project’s post-processing stack varies by application. Stellar continua are commonly generated with BPASS; nebular emission is computed either from CLOUDY-based grids or from pixel-by-pixel H II-region models; dust radiative transfer is performed with POLARIS or SKIRT in the dust-focused studies [1810.09754, 2102.12343, 2208.08658, 2304.10150, 2602.07347]. This layered design allows FirstLight to separate hydrodynamical evolution from observable synthesis while retaining the simulated spatial and temporal structure of the ISM.

A recurring limitation is methodological rather than conceptual. H II regions are unresolved in several line-emission papers and are treated through subgrid models with fixed or grid-derived densities; dust studies often adopt a constant dust-to-metal ratio $=0.4$ and fixed MW- or SMC-type grain models; AGN feedback is not included in the standard sub-$L_\star$ analyses and is explicitly stated to be absent in the compactness work [2102.12343, 2301.02416, 2304.10150, 2510.05299]. These choices delimit the regime in which FirstLight predictions should be interpreted.

## 3. Bursty star formation and redshift-dependent efficiency

A central FirstLight result is that star formation at cosmic dawn is strongly time-variable. In FirstLight II, galaxies spend about 70% of their time in star-formation bursts across $z>5$ [1801.10382]. At $z \approx 6$, a typical burst has $\mathrm{sSFR}_{\max}\approx 5$–$15\,\mathrm{Gyr}^{-1}$ and an effective width of about 100 Myr, while roughly one quarter of bursts populate a high-intensity tail with $\mathrm{sSFR}_{\max}\approx 20$–$30\,\mathrm{Gyr}^{-1}$ and widths of 40–80 Myr [1801.10382]. The mean time between consecutive peaks is about $200\pm100$ Myr, with only mild mass dependence.

These burst cycles imprint the star-forming main sequence. At $z\approx 6$, the overall scatter is $\sigma\approx0.3$ dex, rising to $\sigma\approx0.6$ dex at $M_\star\approx10^7\,M_\odot$ because of a low-sSFR tail of quiescent dwarfs with $\mathrm{sSFR}\lesssim0.3\,\mathrm{Gyr}^{-1}$ [1801.10382]. The mean sSFR grows with redshift approximately as $\mathrm{sSFR}_{\rm mean}(z)\propto(1+z)^{5/2}$, and the typical burst height remains a factor $2\pm0.5$ above the mean, implying $\mathrm{sSFR}_{\max}\approx20$–$30\,\mathrm{Gyr}^{-1}$ at $z\approx9$–10, with a tail reaching $\approx60\,\mathrm{Gyr}^{-1}$ [1801.10382].

Later work reframed this burstiness in terms of galaxy formation efficiency. FirstLight defines an instantaneous efficiency
$$
\epsilon=\dot M_\star/(\dot M_{\rm vir} f_B)
$$
and an integrated efficiency
$$
\epsilon_\star=M_\star/(M_{\rm vir} f_B).
$$
At fixed $M_{\rm vir}\approx10^{11}\,M_\odot$, the instantaneous efficiency rises from about 10% at $z\approx6$ to about 30% at $z\approx12$, while the integrated efficiency increases from about 0.1 at $z\approx6$ to about 0.2 at $z\approx10$ [2404.02537]. The same study argues that this redshift dependence is driven by high gas densities in compact early galaxies: efficient systems at $z\approx9$ reach $n\ge3000\,\mathrm{cm}^{-3}$, free-fall times $<1$ Myr, and $\Sigma_{\rm SFR}\ge10^3\,M_\odot\,\mathrm{yr}^{-1}\,\mathrm{kpc}^{-2}$ in central regions and off-center clumps [2404.02537].

This combination of high duty cycle, short depletion times, and rising efficiency provides the project’s physical explanation for why galaxies at fixed halo mass are brighter and more rapidly growing at higher redshift. A plausible implication is that FirstLight’s burst statistics are not an incidental by-product of zoom-in numerics, but one of the main mechanisms by which the simulations reproduce the slow evolution of the bright-end UV luminosity function.

## 4. Spectral energy distributions, metallicity, and nebular diagnostics

FirstLight III established the UV–optical spectral diversity of early galaxies by coupling simulated star-formation histories to BPASS-based stellar populations and CLOUDY nebular grids [1810.09754]. At $z\approx6$, across $-22\le M_{1500}\le -14$, most systems have steep intrinsic UV slopes $\beta\approx-2.5$ to $-2.2$ and high ionizing-photon production efficiencies $\zeta\approx25.0$–25.5, corresponding to $\xi_{\rm ion}\approx10^{25.0}$–$10^{25.5}\,\mathrm{s}^{-1}\,(\mathrm{erg\,s}^{-1}\,\mathrm{Hz}^{-1})^{-1}$ [1810.09754]. Optical lines are correspondingly strong: EW(H$\alpha$+[N II]) commonly lies between 300 and 3000 Å, and broad-band colors in rest-frame $B$, $V$, and $R$ are significantly modified by [O III]+H$\beta$ and H$\alpha$+[N II] contamination [1810.09754].

The chemical-evolution work predicts that a tight mass–metallicity relation is already present by $z=5$–8 [1910.11729]. At fixed $M_\star=10^8\,M_\odot$, the metallicity declines by $\le0.2$ dex from $z=8$ to 5, with a mean decline of about 0.15 dex, similar to the intrinsic scatter [1910.11729]. The scatter correlates with gas fraction: in the $M_\star=10^8\,M_\odot$ bin, systems with $F_{\rm gas}\approx0.7$ are systematically metal-poor, whereas those with $F_{\rm gas}\approx0.3$ are richer by about 0.2 dex [1910.11729]. The same paper finds that the strong-line index $R3=([\mathrm{O\,III}]\,\lambda5007+[\mathrm{O\,III}]\,\lambda4959)/\mathrm{H}\beta$ is informative for $12+\log(\mathrm{O/H})<8$ but saturates at $R3\approx3$ for higher metallicities, and that higher sSFR raises $R3$ at fixed metallicity through a higher ionization parameter [1910.11729].

FirstLight IV specialized this picture to sub-$L_\star$ galaxies with $M_{\rm UV}=-19$ to $-19.5$ at $z=5.5$–6.5 [2102.12343]. In that sample, a factor $\sim40$ variation in sSFR, from about 1.7 to $30\,\mathrm{Gyr}^{-1}$, drives a $\sim1$ dex spread in EW([O III] $\lambda5007$): extreme bursts reach EW $\simeq1000$ Å, main-sequence systems have EW $\simeq300$ Å, and the most massive, lower-SFR sub-$L_\star$ galaxies show EW $\le100$ Å [2102.12343]. Even at fixed sSFR, EW([O III]) retains an intrinsic dispersion $\sigma\simeq0.1$ dex whose residuals correlate with both $R_{\rm OIII/H\alpha}$ and SFR-weighted $\log U$ [2102.12343].

The [O III]-to-H$\alpha$ ratio is used as a morphological and physical classifier:
$$
R_{\rm OIII/H\alpha}\equiv F_{[\mathrm{O\,III}]\,\lambda5007}/F_{\mathrm{H}\alpha}.
$$
By convention, [O III]-bright emitters have $R_{\rm OIII/H\alpha}>1$, while H$\alpha$-bright emitters have $R_{\rm OIII/H\alpha}<1$ [2102.12343]. In the sub-$L_\star$ sample, [O III]-bright systems tend to have higher ionization parameters and/or higher nebular metallicities, with [O III] up to a factor of $\sim2$ brighter than H$\alpha$; H$\alpha$-bright galaxies dominate the sample and can have H$\alpha$ up to $\sim3\times$ brighter than [O III] [2102.12343].

The more massive [O III] studies extend the same logic to multi-line diagnostics with ALMA-accessible far-infrared lines [2301.02416]. They find that some $M_\star>10^9\,M_\odot$ galaxies reach $12+\log(\mathrm{O/H})\sim8.5$ already by $z=9$, and that high-redshift systems lie above the local $L_{\rm [OIII],88}$–SFR relation because very young stellar populations boost ionizing-photon production [2301.02416]. The line-ratio plane combining [O III] $\lambda5007$, [O III] 88 $\mu$m, and [O III] 52 $\mu$m is then used to infer typical ISM conditions of $n_e\sim50$–$300\,\mathrm{cm}^{-3}$, $\log(Z/Z_\odot)\sim-1.0$ to $-0.4$, and $\log U\sim-3$ to $-2$ [2301.02416].

Together, these results make FirstLight a line-physics framework as much as a galaxy-formation suite: stellar age, burst phase, metallicity, and local H II-region conditions are all required to interpret the nebular output of reionization galaxies.

## 5. Clumps, compactness, and size evolution

The spatial structure of FirstLight galaxies is neither smooth nor static. A merger-driven clump analysis identifies [O III]-bright clumps in 1828 snapshots from 62 galaxies over $z=9.5$–5.5, with clumps found via a threshold $\Sigma_{\rm SFR}>10^{1.5}\,M_\odot\,\mathrm{yr}^{-1}\,\mathrm{kpc}^{-2}$ and minimum radius $R_{\min}=113$ pc [2402.08911]. About one tenth of snapshots are clumpy systems with two or more components [2402.08911]. Young off-centered clumps have mass-weighted ages of about 35 Myr, radii $R_c\approx142$ pc, gas fractions $\approx0.95$, and sSFR peaks around $55\,\mathrm{Gyr}^{-1}$, while older clumps in clumpy systems peak around $10\,\mathrm{Gyr}^{-1}$ and single-component central clumps around $6\,\mathrm{Gyr}^{-1}$ [2402.08911]. The inferred baryonic merger mass ratios are major, $1\le q<4$, and the clumpy configurations are short-lived, with off-centered clumps merging into the center on timescales of several tens of Myr, consistent with $t_{\rm cross}\approx50$ Myr and $t_{\rm dyn}\approx25$ Myr [2402.08911].

A distinct but related line of work studies global compactness. In the compactness analysis, the stellar half-mass radius $r_{\star{\rm hm}}$ is normalized by the halo radius $r_{200}$ through
$$
\xi_\star \equiv r_{\star{\rm hm}}/r_{200}.
$$
The distribution of $\xi_\star$ is bimodal over $5.25<z<9$, with median $\langle\xi_\star\rangle\approx0.07$ and a compact population defined by $\log_{10}\xi_\star\lesssim-1.6$ [2510.05299]. Galaxies are found to undergo an expansion–compaction–re-expansion cycle. Compaction begins at $\log_{10}(M^\star_{\rm on}/M_\odot)=8.53$ and ends at $\log_{10}(M^\star_{\rm off}/M_\odot)=9.57$, over which the size–mass relation becomes inverted, with a negative slope [2510.05299]. The proposed mechanism is a self-reinforced, approximately spherical gas inflow driven by the transition of the central potential from dark-matter dominated to baryon dominated, triggering a strong localized starburst within $\lesssim1$ kpc [2510.05299]. Unlike low-redshift wet-compaction scenarios, this process ends without central gas depletion or star-formation quenching because the replenishment time becomes comparable to the depletion time [2510.05299].

The later synthetic-observation study extends size evolution to $z\approx14$–5 [2603.05045]. In JWST-like rest-frame UV measurements, the size–mass relation is already in place by $z\simeq14$ and is fit as
$$
\log(R_e/{\rm kpc})=\alpha \log(M_\star/10^9\,M_\odot)+\beta.
$$
For rest-UV sizes, the slope is $\alpha=0.214\pm0.013$ at $z=5$–6, $\alpha=0.21\pm0.03$ at $z=7$–9, and the normalization evolves rapidly, increasing by about 0.5 dex between $z\simeq14$ and $z\simeq6$ in 600 Myr [2603.05045]. At $M_\star=10^9\,M_\odot$, the fitted rest-UV size grows from about 0.17 kpc at $z\approx13$–15 to about 0.60 kpc at $z\approx5$–6 [2603.05045]. The same paper finds large diversity at fixed mass: extended galaxies tend to have higher sSFRs and gas fractions and sit above the main sequence, while compact galaxies tend to have lower sSFRs and can occupy “mini-quenching” phases [2603.05045].

The size analysis also shows that dust is dynamically unimportant but observationally decisive. Differential attenuation dims galaxy centers more strongly than outer regions, increasing apparent half-light radii and modifying the size–mass slope even in the rest-frame optical [2603.05045]. In the $z=5$–6 rest-optical band, the fitted slope changes from $\alpha=-0.016\pm0.003$ without dust to $\alpha=0.122\pm0.008$ with dust [2603.05045]. This suggests that some of the apparent disagreement between simulations and observed size–mass relations can arise from radiative transfer rather than from dynamics alone.

## 6. Dust, observability, and the role of FirstLight in JWST-era interpretation

Dust was not included in the intrinsic SED predictions of FirstLight III, but later studies turned it into a major observable channel [1810.09754, 2304.10150]. In the POLARIS-based dust-emission study at $z=6$ and 8, the CMB sets a floor for diffuse dust temperatures: $T_{\rm CMB}(z=6)=19.11$ K and $T_{\rm CMB}(z=8)=24.57$ K [2208.08658]. The resulting equilibrium temperatures exhibit strong spatial gradients, with hot central regions near young stars and outskirts driven toward the CMB floor; including the CMB boosts mid- and far-infrared emission and weakens simple $L_{\rm IR}$–$T$ correlations [2208.08658].

The attenuation-curve study for $z=6$–8 galaxies finds a broad diversity of attenuation laws arising from the clumpy ISM and starbursting geometry [2304.10150]. Low-mass systems with $M_\star\approx10^7$–$10^8\,M_\odot$ typically show steep, SMC-like attenuation, whereas more massive galaxies with $M_\star\gtrsim10^9\,M_\odot$ approach Calzetti-like behavior [2304.10150]. At $z=6$, the IRX–$\beta_{\rm UV}$ relation follows the Calzetti model with a shift to slightly lower $\beta_{\rm UV}$ at low attenuation due to low metallicity; at $z=8$, the locus shifts to higher IRX because of stronger CMB heating [2304.10150].

A clump-scale extension then decomposes galaxies into clumps, diffuse regions, and system-integrated emission [2602.07347]. System-integrated attenuation curves are grayer than Calzetti even for fixed MW- or SMC-type dust, with median slopes $S_{\rm sys}=1.84$ for MW dust and $S_{\rm sys}=1.99$ for SMC dust [2602.07347]. Individual clumps are even grayer and substantially more obscured: for MW dust, median clump values are $S_{\rm cl}=1.60$, $A_{V,\rm cl}=2.50$ mag, $A_{\rm UV,cl}=4.13$ mag, and $\log_{10}\mathrm{IRX}_{\rm cl}=1.75$ [2602.07347]. Diffuse regions are steeper and optically thinner, with $S_{\rm diff}=2.90$, $A_{V,\rm diff}=0.34$ mag, and $\log_{10}\mathrm{IRX}_{\rm diff}=0.30$ [2602.07347]. The paper interprets this through a toy model on the IRX–$\Delta\beta$ plane, parameterized by the dust-to-star scale-height ratio $R\equiv H_d/H_\star$ and a fiducial optical depth $\tau_{\rm fid}$ [2602.07347].

These radiative-transfer outputs feed directly into instrumental predictions. For the sub-$L_\star$ sample at $z\simeq6$, galaxies with $M_{\rm UV}\simeq-19$ have observed $H$-band magnitudes 27–28 and typical optical-line fluxes of a few $\times10^{-18}\,\mathrm{erg\,s}^{-1}\,\mathrm{cm}^{-2}$, making them accessible to JWST/NIRSpec in 0.5–2 hour exposures [2102.12343]. FirstLight IV predicts that [O III]-bright emitters will often be barely resolved at JWST’s nominal FWHM $\simeq0.12$ arcsec, whereas H$\alpha$ emission should be better resolved and more extended where it is observable [2102.12343].

The MIRI-oriented spectroscopy study uses the FirstLight database to forecast detectability at $6.5\le z\le10.5$ [1907.06962]. In 40 ks with MIRI/MRS, only the luminous tail is detectable at $\mathrm{S/N}\ge5$: 6.2%, 1.1%, and 0.4% of the simulated population in the redshift bins 6.5–7.5, 7.5–8.5, and $z>8.5$, corresponding to intrinsic limits of $\mathrm{SFR}\gtrsim1.6$, 1.9, and $3.9\,M_\odot\,\mathrm{yr}^{-1}$ and stellar masses $M_\star\gtrsim4$, 9, and $14\times10^7\,M_\odot$, respectively [1907.06962]. For bright or lensed sources, the same study finds that 10–40 ks spectra can recover H$\beta$, [O III] $\lambda\lambda4959,5007$, H$\alpha$, He I 1.083 $\mu$m, and, in deeper integrations, [N II], [S II], and [S III], enabling extinction, excitation, hardness, and metallicity diagnostics [1907.06962].

The broader significance of FirstLight lies in this observational closure. The suite was built to reproduce the UV luminosity function and scaling relations of early galaxies, but it now functions as a multi-layer inference engine linking bursty accretion, feedback-regulated star formation, chemical enrichment, nebular line physics, dust geometry, compactness, and synthetic observables. This suggests that its enduring value is not any single prediction, but the internally connected description it provides of how reionization-era galaxies grow, radiate, and appear in data.

Source: https://www.emergentmind.com/topics/firstlight