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We present JWST/NIRSpec prism spectroscopy of MACS0647−JD, a triply lensed z ∼ 11 candidate discovered in Hubble Space Telescope imaging and spatially resolved by JWST imaging into two components, A and B. Spectroscopy of component A yields a spectroscopic redshift z = 10.17 based on seven detected emission lines: C iii ] λ λ 1907, 1909, [O ii ] λ 3727, [Ne iii ] λ 3869, [Ne iii ] λ 3968, H δ λ 4101, H γ λ 4340, and [O iii ] λ 4363. These are the second-most distant detections of these emission lines to date, in a galaxy observed just 460 million years after the Big Bang. Based on observed and extrapolated line flux ratios we derive a gas-phase metallicity 12 + log(O/H) ∼ 7.5–8.0, or Z ∼ (0.06–0.2) Z _⊙ , ionization parameter $\mathrm{log}(U)$ = −1.9 ± 0.2, and an ionizing photon production efficiency $\mathrm{log}({\xi }_{\mathrm{ion}})=25.2\pm 0.2\,$ erg ^−1 Hz. The spectrum has a softened Ly α break, evidence for a strong Ly α damping wing. The Ly α damping wing also suppresses the F150W photometry, explaining the slightly overestimated photometric redshift z = 10.6 ± 0.3. MACS0647−JD has a stellar mass log( M / M _⊙ ) = 8.1 ± 0.3, including ∼6 × 10 ^7 M _⊙ in component A, most of which formed recently (within ∼20 Myr) with a star formation rate ∼ 2 ± 1 M _⊙ yr ^−1 , all within an effective radius 70 ± 24 pc. Spectroscopy of a fainter companion galaxy C separated by a distance of ∼ 3 kpc reveals a Lyman break consistent with z ∼ 10.17. MACS0647−JD is likely the most distant galaxy merger known.

MACS0647–JD is a triply lensed z ∼ 11 galaxy originally discovered with the Hubble Space Telescope. The three lensed images are magnified by factors of ∼8, 5, and 2 to AB mag 25.1, 25.6, and 26.6 at 3.5 μ m. The brightest is over a magnitude brighter than other galaxies recently discovered at similar redshifts z > 10 with JWST. Here, we report new JWST imaging that clearly resolves MACS0647–JD as having two components that are either merging galaxies or stellar complexes within a single galaxy. The brighter larger component “A” is intrinsically very blue ( β ∼ −2.6 ± 0.1), likely due to very recent star formation and no dust, and is spatially extended with an effective radius ∼70 ± 24 pc. The smaller component “B” ( r ∼ 20 ${}_{-5}^{+8}\,$ pc) appears redder ( β ∼ −2 ± 0.2), likely because it is older (100–200 Myr) with mild dust extinction ( A _V ∼ 0.1 mag). With an estimated stellar mass ratio of roughly 2:1 and physical projected separation ∼400 pc, we may be witnessing a galaxy merger 430 million years after the Big Bang. We identify galaxies with similar colors in a high-redshift simulation, finding their star formation histories to be dissimilar, which is also suggested by the spectral energy distribution fitting, suggesting they formed further apart. We also identify a candidate companion galaxy “C” ∼3 kpc away, likely destined to merge with A and B. Upcoming JWST Near Infrared Spectrograph observations planned for 2023 January will deliver spectroscopic redshifts and more physical properties for these tiny magnified distant galaxies observed in the early universe.

The study of galaxy evolution hinges on our ability to interpret multiwavelength galaxy observations in terms of their physical properties. To do this, we rely on spectral energy distribution (SED) models, which allow us to infer physical parameters from spectrophotometric data. In recent years, thanks to wide and deep multiwave band galaxy surveys, the volume of high-quality data have significantly increased. Alongside the increased data, algorithms performing SED fitting have improved, including better modeling prescriptions, newer templates, and more extensive sampling in wavelength space. We present a comprehensive analysis of different SED-fitting codes including their methods and output with the aim of measuring the uncertainties caused by the modeling assumptions. We apply 14 of the most commonly used SED-fitting codes on samples from the CANDELS photometric catalogs at z ∼ 1 and z ∼ 3. We find agreement on the stellar mass, while we observe some discrepancies in the star formation rate (SFR) and dust-attenuation results. To explore the differences and biases among the codes, we explore the impact of the various modeling assumptions as they are set in the codes (e.g., star formation histories, nebular, dust and active galactic nucleus models) on the derived stellar masses, SFRs, and A _V values. We then assess the difference among the codes on the SFR–stellar mass relation and we measure the contribution to the uncertainties by the modeling choices (i.e., the modeling uncertainties) in stellar mass (∼0.1 dex), SFR (∼0.3 dex), and dust attenuation (∼0.3 mag). Finally, we present some resources summarizing best practices in SED fitting.

We report on the Hubble Space Telescope (HST) detection of the Lyman-continuum (LyC) radiation emitted by a galaxy at redshift z = 3.794 dubbed Ion1. The LyC from Ion1 is detected at 820−890 Å with HST WFC3/UVIS in the F410M band (m410 = 27.60 ± 0.36 mAB, peak signal-to-noise ratio (S/N) = 4.17 in an r = 0.″12 aperture) and 700−830 Å with the Very Large Telescope (VLT)/VIMOS in the U band (mU = 27.84 ± 0.19 mAB, peak S/N = 6.7 with an r = 0.″6 aperture). A 20 hr VLT/VIMOS spectrum shows low- and high-ionization interstellar metal absorption lines and the P Cygni profile of C iv and Lyα in absorption. The latter spectral feature differs from what observed in known LyC emitters, which show strong Lyα emission. An HST far-UV color map reveals that the LyC emission escapes from a region of the galaxy that is bluer than the rest. The F410M image shows that the centroid of the LyC emission is offset from the centroid of the nonionizing UV emission by 0.″12 ± 0.″03, corresponding to 0.85 ± 0.21 kpc, and that its morphology is likely moderately resolved. These morphological characteristics favor a scenario where the LyC photons produced by massive stars escape from low H i column density “cavities” in the interstellar medium. We also collect the VIMOS U-band images of 107 Lyman-break galaxies at 3.40 < zspec < 3.95, i.e., sampling the LyC, and stack them with inverse-variance weights. No LyC emission is detected in the stacked image, resulting in a 32.5 mAB flux limit (1σ) and an upper limit of absolute LyC escape fraction fescabs ≤ 0.63%. [ABSTRACT FROM AUTHOR]

Nonparametric star formation histories (SFHs) have long promised to be the “gold standard” for galaxy spectral energy distribution (SED) modeling as they are flexible enough to describe the full diversity of SFH shapes, whereas parametric models rule out a significant fraction of these shapes a priori. However, this flexibility is not fully constrained even with high-quality observations, making it critical to choose a well-motivated prior. Here, we use the SED-fitting code Prospector to explore the effect of different nonparametric priors by fitting SFHs to mock UV–IR photometry generated from a diverse set of input SFHs. First, we confirm that nonparametric SFHs recover input SFHs with less bias and return more accurate errors than do parametric SFHs. We further find that, while nonparametric SFHs robustly recover the overall shape of the input SFH, the primary determinant of the size and shape of the posterior star formation rate as a function of time (SFR(t)) is the choice of prior, rather than the photometric noise. As a practical demonstration, we fit the UV–IR photometry of ∼6000 galaxies from the Galaxy and Mass Assembly survey and measure scatters between priors to be 0.1 dex in mass, 0.8 dex in SFR100 Myr, and 0.2 dex in mass-weighted ages, with the bluest star-forming galaxies showing the most sensitivity. An important distinguishing characteristic for nonparametric models is the characteristic timescale for changes in SFR(t). This difference controls whether galaxies are assembled in bursts or in steady-state star formation, corresponding respectively to (feedback-dominated/accretion-dominated) models of galaxy formation and to (larger/smaller) confidence intervals derived from SED fitting. High-quality spectroscopy has the potential to further distinguish between these proposed models of SFR(t). [ABSTRACT FROM AUTHOR]

Parametric models for galaxy star formation histories (SFHs) are widely used, though they are known to impose strong priors on physical parameters. This has consequences for measurements of the galaxy stellar-mass function, star formation rate density (SFRD), and star-forming main sequence (SFMS). We investigate the effects of the exponentially declining, delayed exponentially declining, lognormal, and double power-law SFH models using Bagpipes. We demonstrate that each of these models imposes strong priors on specific star formation rates (SFRs), potentially biasing the SFMS, and also imposes a strong prior preference for young stellar populations. We show that stellar mass, SFR, and mass-weighted age inferences from high-quality mock photometry vary with the choice of SFH model by at least 0.1, 0.3, and 0.2 dex, respectively. However, the biases with respect to the true values depend more on the true SFH shape than the choice of model. We also demonstrate that photometric data cannot discriminate between SFH models, meaning that it is important to perform independent tests to find well-motivated priors. We finally fit a low-redshift, volume-complete sample of galaxies from the Galaxy and Mass Assembly (GAMA) Survey with each model. We demonstrate that our stellar masses and SFRs at redshift z ∼ 0.05 are consistent with other analyses. However, our inferred cosmic SFRDs peak at z ∼ 0.4, approximately 6 Gyr later than direct observations suggest, meaning that our mass-weighted ages are significantly underestimated. This makes the use of parametric SFH models for understanding mass assembly in galaxies challenging. In a companion paper, we consider nonparametric SFH models. [ABSTRACT FROM AUTHOR]