---
title: 'Sunrise Galaxy: Lensed Arc & Young Clusters'
url: https://www.emergentmind.com/topics/sunrise-galaxy
type: topic
---

# Sunrise Galaxy: Lensed Arc & Young Clusters

Searching arXiv for the specified Sunrise Galaxy papers and closely related work.
arxiv_search(query="2507.05483 OR \"Is Earendel a Star Cluster?\" OR \"Sunrise Arc\" OR 2211.09839", max_results=10)
Searching for exact titles individually to ensure precise citation targets.
arxiv_search(query="ti:\"Is Earendel a Star Cluster?: Metal Poor Globular Cluster Progenitors at z~6\"", max_results=5)
arxiv_search(query="ti:\"JWST/NIRCam Probes Young Star Clusters in the Reionization Era Sunrise Arc\"", max_results=5)
The **Sunrise galaxy**, commonly termed the **Sunrise arc**, is a strongly lensed galaxy at $z \approx 5.93$ observed behind the massive foreground cluster **WHL0137** at $z_{\rm cluster}=0.566$. In imaging it appears as a highly stretched, $\sim 17''$-long tangential arc with four multiple images (“a, b, c, d”), while source-plane reconstructions resolve it into compact knots down to $\sim 1$ pc scales. JWST/NIRCam imaging and JWST/NIRSpec PRISM spectroscopy have made Sunrise a reference system for parsec-scale star formation in the first Gyr of cosmic history, for the study of young massive star clusters (YMCs), and for the reassessment of the highly magnified feature **Earendel**, previously identified as a candidate star or binary but now also analyzed as a possible star cluster [2211.09839], [2507.05483].

## 1. Lensed morphology and source-plane geometry

Sunrise is lensed by WHL0137 and reconstructed with lens-modeling frameworks including **WSLAP+**, **Lenstool**, and **GLAFIC**. The full arc spans $\sim 17''$ in the image plane, corresponding to $\sim 95$ kpc tangentially, and to $\sim 1.6$ kpc length in the source plane. The principal star-forming complex (**SFC**) has size $\sim 200$ pc, and the entire analyzed host region (“TOTd”) extends $\sim 1000$ pc [2211.09839].

The global magnification is reported as $\mu_{\rm tot} \simeq 300$, while individual “b” knots have $\mu_{\rm tot}\gtrsim 30$ and knot **1b** has $\mu_{\rm tot}\gtrsim 66$. Lens-model reconstructions place critical curves through the arc and yield tangential stretches $\mu_t=\mu_{\rm tot}/\mu_{\rm rad}\simeq 30$–$70\times$, with $\mu_{\rm rad}\approx 1.1$ for the “b” knots. This geometry resolves the galaxy into compact components on source-plane scales that are otherwise inaccessible at $z\sim 6$ [2211.09839].

For **Earendel**, the magnification is model-dependent. Macro models discussed by Welch et al. (2022b) predict tangential magnifications of order $\mu_t\sim 10^3$–$10^4$, implying an intrinsic FUV half-light radius $r\lesssim 0.02$ pc. Ji & Dai (2024) showed that including dark subhalos can relax the limit to $r\lesssim 3$ pc without violating observed flux ratios or astrometric constraints. Scofield et al. (2025) instead find $\mu\sim 50$–$70$ from a joint strong+weak lens model, still sufficient for Earendel to remain unresolved in the F090W, F115W, and F200W NIRCam images, whose native pixel scale is $0.031''\simeq 200$ pc at $z=5.93$ [2507.05483].

## 2. Redshift determination and spectroscopic basis

Deep archival **JWST/NIRSpec PRISM** spectroscopy established a spectroscopic redshift for Sunrise by extracting the NIRSpec MSA slit **2282_12001**, which covers the SFC, and simultaneously fitting Gaussian profiles to the nebular lines [O II] $\lambda\lambda3727,3729$, [Ne III] $\lambda3869$, H$\beta$ $\lambda4861$, [O III] $\lambda\lambda4959,5007$, and H$\alpha$ $\lambda6563$. The formal fit yields

$$
z_{\rm spec}=5.926^{+0.013}_{-0.012},
$$

in excellent agreement with the Lyman-continuum break seen in the continua of Earendel and 1b [2507.05483].

This spectroscopic redshift supersedes earlier photometric estimates of $z\approx 6.2$ from Welch et al. (2022b). The same analysis places Sunrise at a lookback time of $\sim 12.7$ Gyr. The earlier JWST/NIRCam study had already inferred $z=6.0\pm 0.2$ at 95% confidence from SED fitting, consistent with HST estimates; the PRISM result converts that approximate placement into a line-anchored systemic redshift suitable for continuum modeling and lensing interpretation [2211.09839], [2507.05483].

The spectroscopic confirmation is methodologically important because the stellar-population analysis of Earendel and 1b uses observed-frame $1.0$–$5.0\,\mu$m spectroscopy, corresponding to $\sim 1400$–$8300$ Å in the rest frame. At this redshift, the Balmer-break region and the rest-UV continuum both fall within the PRISM coverage, which is central to the subsequent age–metallicity analysis [2507.05483].

## 3. Young massive star clusters in the Sunrise arc

JWST/NIRCam imaging identified **six young massive star clusters** in Sunrise with measured radii spanning $\sim 20$ pc down to $\sim 1$ pc, estimated stellar masses of $\sim 10^{(6-7)}\,M_\odot$, and ages $1$–$30$ Myr based on SED fitting to photometry measured in 8 filters extending to rest-frame $7000$ Å [2211.09839].

The size-measurement procedure uses F150W for UV-continuum knots and F277W for nebular-line knots. Each candidate is modeled as a 2D Gaussian with $(\sigma_x,\sigma_y)$, ellipticity $\epsilon$, and position angle $\theta$, convolved with the filter PSF and fit in a $9\times 9$ pixel box. The Gaussian is extrapolated to infinite radius to obtain total flux, and the intrinsic source-plane half-light radius is computed as

$$
r_h=\frac{\mathrm{FWHM}}{2}=0.5\times 2\sqrt{2\ln 2}\,\sigma_{\rm eff},
$$

with $\sigma_{\rm eff}$ converted from image-plane $\sigma$ by dividing angular size by $\mu_t$ and multiplying by $5.61$ kpc/$''$ at $z=6.0$ [2211.09839].

The six clusters span the following measured values: **1b** has $r_h=1.4^{+0.3}_{-0.7}$ pc, $M_*=7.1^{+1.6}_{-4.6}\times 10^6\,M_\odot$, and age $30^{+0}_{-22}$ Myr; **2b** has $r_h=6.3^{+1.1}_{-1.3}$ pc, $M_*=3.9^{+0.2}_{-1.3}\times 10^6\,M_\odot$, and age $10^{+0}_{-1}$ Myr; **3b** has $r_h=6.1^{+12.5}_{-3.6}$ pc, $M_*=1.1^{+8.7}_{-0.5}\times 10^6\,M_\odot$, and age $4^{+36}_{-3}$ Myr; **4b** has $r_h=24.8^{+62.6}_{-12.3}$ pc, $M_*=10.1^{+11.0}_{-0.2}\times 10^6\,M_\odot$, and age $1^{+3}_{-0}$ Myr; **5b** has $r_h=4.9^{+10.6}_{-1.7}$ pc, $M_*=3.1^{+10.2}_{-2.0}\times 10^6\,M_\odot$, and age $6^{+74}_{-5}$ Myr; **6b** has $r_h=8.5^{+2.1}_{-3.0}$ pc, $M_*=3.3^{+3.2}_{-0.8}\times 10^6\,M_\odot$, and age $4^{+2}_{-3}$ Myr [2211.09839].

The stellar surface density is defined as

$$
\Sigma_*=\frac{M_*}{2\pi r_h^2},
$$

and exceeds $1000\,M_\odot\,{\rm pc}^{-2}$ for the sample, reaching up to a few $10^5\,M_\odot\,{\rm pc}^{-2}$. For 1b specifically, the tabulated value is $311^{+446}_{-158}\times 10^3\,M_\odot\,{\rm pc}^{-2}$ [2211.09839].

A dynamical classification was carried out through the crossing time and the dynamical age,

$$
T_{\rm CR}=10\left[\frac{r_h^3}{G\,M_*}\right]^{1/2}, \qquad \Pi=\frac{\mathrm{Age}}{T_{\rm CR}},
$$

with $G\approx 0.0045\,{\rm pc}^3\,M_\odot^{-1}\,{\rm Myr}^{-2}$. The resulting values indicate that five of the six candidates, all except **4b**, satisfy $\Pi\gtrsim 1$ and therefore qualify as gravitationally bound; for **1b**, $\Pi=314^{+298}_{-156}\gg 1$ [2211.09839].

## 4. Continuum fitting of Earendel and 1b

The 2025 spectroscopic analysis tested whether Earendel could be explained by a compact stellar population rather than an individual star or binary. Over the observed-frame range $1.0$–$5.0\,\mu$m, the rest-UV through optical continua of **Earendel** and **1b** were fit with instantaneous-burst **simple stellar population** (SSP) models from three libraries: **BPASS v2.3** with binaries and a broken power-law IMF ($\alpha_1=-1.3$ below $0.5\,M_\odot$, $\alpha_2=-2.35$ above), **BC03 (2016)** with a Kroupa IMF ($0.1$–$100\,M_\odot$), and **FSPS/MIST** with MILES empirical stellar spectra and a Kroupa IMF ($0.1$–$120\,M_\odot$) [2507.05483].

The spectral-energy distributions were generated on a fine grid of $\log t_{\rm age}$ from $1$ Myr to $1$ Gyr and $\log (Z/Z_\odot)$, guided by the native grid points of each library and linearly interpolated in log–log space. A nebular component computed with **CLOUDY 17** tied $Z_{\rm gas}=Z_*$ and allowed $\log U\in[-4,-1]$ and covering factor $x\in[0,1]$ to vary. Dust attenuation followed the **Salim et al. (2018)** prescription with slope $\delta\in[-2,0.75]$ and 2175 Å bump strength $B\in[0,3]$. The fit adopted a Gaussian prior on $z$ centered at $5.926\pm 0.013$, convolved each model to the NIRSpec PRISM resolution, and minimized

$$
\chi^2=\sum_{i=1}^N \frac{[F_{\rm obs}(\lambda_i)-F_{\rm mod}(\lambda_i)]^2}{\sigma^2(\lambda_i)},
$$

including an extra free white-noise term $\alpha$ to scale the pipeline uncertainties upward as needed. Parameter inference used **Nautilus** nested sampling with 3,000 live points across a 10–12-parameter space [2507.05483].

All three SSP libraries produce formally acceptable fits, with reduced $\chi^2_\nu\sim 1.7$ before noise scaling, and the BC03 and FSPS results agree with BPASS within $1$–$2\sigma$. The paper reports that the continuum of Earendel is well described by an SSP nearly equivalently to 1b, which is confidently a star cluster [2507.05483].

| Property | Earendel | 1b |
|---|---:|---:|
| Age | $\log(t_{\rm age}/{\rm yr})=7.79^{+0.08}_{-0.27}$ | $\log(t_{\rm age}/{\rm yr})=7.64^{+0.06}_{-0.08}$ |
| Interpreted age | $\sim 60^{+30}_{-30}$ Myr | $\sim 45\pm 10$ Myr |
| Metallicity | $\log(Z/Z_\odot)=-2.28^{+0.90}_{-0.38}$ | $\log(Z/Z_\odot)=-2.17^{+0.18}_{-0.21}$ |
| Metallicity limit | $Z_*\lesssim 10^{-1} Z_\odot$ (95% C.L.) | $Z_*\lesssim 5\times 10^{-2} Z_\odot$ (99% C.L.) |
| Stellar mass | $\log(\mu M_*/M_\odot)=9.06^{+0.07}_{-0.23}$ | $\log(\mu M_*/M_\odot)=9.33^{+0.04}_{-0.06}$ |
| Interpreted lensed mass | $\mu M_*\approx 1.1\times 10^9\,M_\odot$ | $\mu M_*\approx 2.1\times 10^9\,M_\odot$ |
| Dust | $A_V\approx 0.03$ mag, $\delta\approx -1.2$, $B\approx 0.8$ | $A_V\approx 0.02$ mag |

A central result is that the pronounced Balmer break at $\sim 4000$ Å rest, together with the UV and optical continuum slopes, provides leverage to disentangle age, metallicity, and $A_V$ even without strong absorption features at PRISM resolution. In the authors’ formulation, this is what enables spectroscopic characterization of intermediate-age clusters that are seldom probed at high redshift [2507.05483].

## 5. Earendel, 1b, and the proto-globular-cluster interpretation

The status of **Earendel** is the main interpretive controversy associated with Sunrise. It was previously identified as a candidate star or binary because extreme lensing magnification appeared to require a sub-parsec source size. The subsequent relaxation of the size constraint to $r\lesssim 3$ pc opened the possibility that Earendel could instead be a compact stellar cluster. The 2025 analysis explicitly explored that hypothesis and found that Earendel’s continuum is compatible with an SSP solution similar to that of 1b [2507.05483].

The neighboring knot **1b** provides an anchor for that interpretation. In the NIRCam study, 1b was already one of the most compact and dense YMCs, with $r_h=1.4^{+0.3}_{-0.7}$ pc, $M_*=7.1^{+1.6}_{-4.6}\times 10^6\,M_\odot$, age $30^{+0}_{-22}$ Myr, and $\Pi\gg 1$, which qualified it as gravitationally bound. In the later spectroscopic analysis, adopted magnifications $\mu\approx 70$–$80$ together with photometrically inferred sizes $r_{\rm FUV}\sim 5$–$10$ pc imply stellar surface densities $\Sigma_*\approx 10^6\,M_\odot\,{\rm pc}^{-2}$ [2211.09839], [2507.05483].

Both Earendel and 1b fall in the intermediate-age ($30$–$150$ Myr), metal-poor ($Z_*\lesssim 0.1\,Z_\odot$) regime and are reported to be consistent with the formation age–metallicity trend seen in local globular clusters. In the comparison presented in Fig. 4 of the 2025 study, their positions on the age–metallicity plane align most closely with the simulated relation for **SMC/LMC-mass halos** in the **E-MOSAICS** cosmological runs, although the total stellar mass of Sunrise, $10^8$–$10^9\,M_\odot$, also leaves a Milky Way-like host as a plausible alternative. The same paper states that their inferred $Z_*$ and $t_{\rm age}$ align with in-situ or ex-situ formation epochs envisaged for metal-poor GCs [2507.05483].

The earlier NIRCam analysis had already advanced a related argument for 1b and 2b: with $M_*\gtrsim 4\times 10^6\,M_\odot$, $r_h<5$ pc, $\Sigma_*>10^3\,M_\odot\,{\rm pc}^{-2}$, $\Pi\gg 1$, ages $10$–$30$ Myr, and inferred $[{\rm Fe/H}]\lesssim -1$, they were described as direct analogues of proto-globular clusters. Even with $\sim 75\%$ mass loss over a Hubble time, they would retain $M_f\gtrsim 1\times 10^6\,M_\odot$, comparable to present-day metal-poor GCs [2211.09839].

## 6. Reionization-era star formation and observational significance

The Sunrise arc is also used to investigate the relation between clustered star formation and ionizing output during the reionization era. The ages of the six YMCs map a progression of star formation along the arc, with evolved systems older than $\sim 10$ Myr followed by very young clusters. The youngest stellar clusters, younger than $5$ Myr, show photometrically inferred rest-frame equivalent widths $\mathrm{EW}_{\rm rest}([\mathrm{O\,III}]4959,5007+\mathrm{H}\beta)\gtrsim 1000$ Å, and they are hosted in a $200$ pc-sized SFC [2211.09839].

For the SFC region, the reported photometric excesses correspond to $\mathrm{EW}_0(\mathrm{[O\,III]}+\mathrm{H}\beta)\simeq 1300$ Å in F356W and $\mathrm{EW}_0(\mathrm{H}\alpha+\mathrm{[N\,II]})\simeq 800$ Å in F444W. The ionizing photon production efficiency is defined as

$$
\xi_{\rm ion}\equiv \frac{N_{\rm ion}}{L_{\rm UV}}\;[{\rm Hz\,erg}^{-1}],
$$

and the inferred value is $\log \xi_{\rm ion}\simeq 25.7$, assuming at least 50% conversion to nebular lines. The SFC is described as dominating the ionizing photon production [2211.09839].

The integrated star-formation accounting is likewise cluster-centric. SED fits with **Prospector** non-parametric and **BAGPIPES/BPASS** constant-SFH models yield a host stellar mass $M_*\sim (0.3$–$2.2)\times 10^9\,M_\odot$ formed over $\sim 200$ Myr. The six YMCs together contain $\sim 3\times 10^7\,M_\odot$ formed over $\sim 30$ Myr, implying $\mathrm{SFR}_{\rm cl}\sim 1\,M_\odot\,{\rm yr}^{-1}$, while the host has $\mathrm{SFR}\sim 3$–$10\,M_\odot\,{\rm yr}^{-1}$. The resulting cluster formation efficiency is

$$
\mathrm{CFE}\equiv \frac{\mathrm{SFR}_{\rm cl}}{\mathrm{SFR}_{\rm host}}\simeq \frac{1}{3\text{--}10}\simeq 10\text{--}30\%.
$$

The 2022 study speculated that YMC-driven feedback may carve low-density channels and that progressively older clusters could facilitate Lyman-continuum escape during bursty episodes of star formation [2211.09839].

The 2025 work extends the significance of Sunrise from photometric identification of very young clusters to **continuum-only spectroscopy** of intermediate-age systems. It argues that the ability to measure ages and metallicities of $\sim 10^6$–$10^7\,M_\odot$ star clusters at $z>5$ from continuum-only spectroscopy constitutes a critical new frontier. At NIRSpec PRISM resolution and for magnitudes $m_{\rm F200W}\sim 27$–$29$ AB, broad continuum features can break the classical age–dust–metallicity degeneracies when the signal-to-noise per resolution element is $\gtrsim 5$. Strong-lensing caustics with $\mu$ boosted by factors of tens to hundreds are therefore essential, and the same paper anticipates that similar continuum-fitting techniques will become applicable to statistical samples of proto-globular clusters as lensed arcs accumulate in the JWST archives [2507.05483].

Source: https://www.emergentmind.com/topics/sunrise-galaxy