MACS0647$-$JD is a triply-lensed $z\sim11$ galaxy originally discovered with
the Hubble Space Telescope. Here we report new JWST imaging, which clearly
resolves MACS0647$-$JD as having two components ...that are either merging
galaxies or stellar complexes within a single galaxy. Both are very small, with
stellar masses $\sim10^8\,M_\odot$ and radii $r<100\,\rm pc$. The brighter
larger component "A" is intrinsically very blue ($\beta\sim-2.6$), likely due
to very recent star formation and no dust, and is spatially extended with an
effective radius $\sim70\,\rm pc$. The smaller component "B" appears redder
($\beta\sim-2$), likely because it is older ($100-200\,\rm Myr$) with mild dust
extinction ($A_V\sim0.1\,\rm mag$), and a smaller radius $\sim20\,\rm pc$. We
identify galaxies with similar colors in a high-redshift simulation, finding
their star formation histories to be out of phase. With an estimated stellar
mass ratio of roughly 2:1 and physical projected separation $\sim400\,\rm pc$,
we may be witnessing a galaxy merger 400 million years after the Big Bang. We
also identify a candidate companion galaxy C $\sim3\,{\rm kpc}$ away, likely
destined to merge with galaxies A and B. The combined light from galaxies A+B
is magnified by factors of $\sim$8, 5, and 2 in three lensed images JD1, 2, and
3 with F356W fluxes $\sim322$, $203$, $86\,\rm nJy$ (AB mag 25.1, 25.6, 26.6).
MACS0647$-$JD is significantly brighter than other galaxies recently discovered
at similar redshifts with JWST. Without magnification, it would have AB mag
27.3 ($M_{UV}=-20.4$). With a high confidence level, we obtain a photometric
redshift of $z=10.6\pm0.3$ based on photometry measured in 6 NIRCam filters
spanning $1-5\rm\mu m$, out to $4300\,\r{A}$ rest-frame. JWST NIRSpec
observations planned for January 2023 will deliver a spectroscopic redshift and
a more detailed study of the physical properties of MACS0647$-$JD.
For studies of galaxy formation and evolution, one of the major benefits of the James Webb Space Telescope is that space-based IFUs like those on its NIRSpec and MIRI instruments will enable ...spatially resolved spectroscopy of distant galaxies, including spectroscopy at the scale of individual star-forming regions in galaxies that have been gravitationally lensed. In the meantime, there is only a very small subset of lensed sources where work like this is possible even with the Hubble Space Telescope's Wide Field Camera 3 infrared channel grisms. We examine two of these sources, SDSS J1723+3411 and SDSS J2340+2947, using HST WFC3/IR grism data and supporting spatially-unresolved spectroscopy from several ground-based instruments to explore the size of spatial variations in observed strong emission line ratios like O32, R23, which are sensitive to ionization parameter and metallicity, and the Balmer decrement as an indicator of reddening. We find significant spatial variation in the reddening and the reddening-corrected O32 and R23 values which correspond to spreads of a few tenths of a dex in ionization parameter and metallicity. We also find clear evidence of a negative radial gradient in star formation in SDSS J2340+2947 and tentative evidence of one in SDSS J1723+3411, though its star formation is quite asymmetric. Finally, we find that reddening can vary enough spatially to make spatially-resolved reddening corrections necessary in order to characterize gradients in line ratios and the physical conditions inferred from them, necessitating the use of space-based IFUs for future work on larger, more statistically robust samples.
Recent studies have shown that galaxies at cosmic noon are redder in the center and bluer in the outskirts, mirroring results in the local universe. These color gradients could be caused by either ...gradients in the stellar age or dust opacity; however, distinguishing between these two causes is impossible with rest-frame optical photometry alone. Here we investigate the underlying causes of the gradients from spatially-resolved rest-frame \(U-V\) vs. \(V-J\) color-color diagrams, measured from early observations with the James Webb Space Telescope. We use \(1\, \mu m - 4\, \mu m\) NIRCam photometry from the CEERS survey of a sample of 54 galaxies with \(M_* / M_\odot>10\) at redshifts \(1.7<z<2.3\) selected from the 3D-HST catalog. We model the light profiles in the F115W, F200W and F356W NIRCam bands using \texttt{imcascade}, a Bayesian implementation of the Multi-Gaussian expansion (MGE) technique which flexibly represents galaxy profiles using a series of Gaussians. We construct resolved rest-frame \(U-V\) and \(V-J\) color profiles. The majority of star-forming galaxies have negative gradients (i.e. redder in the center, bluer in the outskirts) in both \(U-V\) and \(V-J\) colors consistent with radially decreasing dust attenuation. A smaller population (roughly 15\%) of star-forming galaxies have positive \(U-V\) but negative \(V-J\) gradients implying centrally concentrated star-formation. For quiescent galaxies we find a diversity of UVJ color profiles, with roughly one-third showing star-formation in their center. This study showcases the potential of JWST to study the resolved stellar populations of galaxies at cosmic noon.
We examine the wavelength dependence of radial light profiles based on Sérsic index \(n\) measurements of 1067 galaxies with M\(_*\geq\) 10\(^{9.5}\)M\(_\odot\) and in the redshift range \(0.5 < z < ...3\). The sample and rest-frame optical light profiles are drawn from CANDELS\(+\)3D-HST; rest-frame near-infrared light profiles are inferred from CEERS JWST/NIRCam imaging. \(n\) shows only weak dependence on wavelength, regardless of redshift, galaxy mass and type: on average, star-forming galaxies have \(n = 1-1.5\) and quiescent galaxies have \(n = 3-4\) in the rest-frame optical and near-infrared. The strong correlation at all wavelengths between \(n\) and star-formation activity implies a physical connection between the radial stellar mass profile and star-formation activity. The main caveat is that the current sample is too small to discern trends for the most massive galaxies (M\(_* > 10^{11}M_\odot\)).
We present the results of our Keck/DEIMOS spectroscopic follow-up of candidate galaxies of i-band-dropout protocluster candidate galaxies at \(z\sim6\) in the COSMOS field. We securely detect ...Lyman-\(\alpha\) emission lines in 14 of the 30 objects targeted, 10 of them being at \(z=6\) with a signal-to-noise ratio of \(5-20\), the remaining galaxies are either non-detections or interlopers with redshift too different from \(z=6\) to be part of the protocluster. The 10 galaxies at \(z\approx6\) make the protocluster one of the riches at \(z>5\). The emission lines exhibit asymmetric profiles with high skewness values ranging from 2.87 to 31.75, with a median of 7.37. This asymmetry is consistent with them being Ly\(\alpha\), resulting in a redshift range of \(z=5.85-6.08\). Using the spectroscopic redshifts, we re-calculate the overdensity map for the COSMOS field and find the galaxies to be in a significant overdensity at the \(4\sigma\) level, with a peak overdensity of \(\delta=11.8\) (compared to the previous value of \(\delta=9.2\)). The protocluster galaxies have stellar masses derived from Bagpipes SED fits of \(10^{8.29}-10^{10.28} \rm \,M_{\rm \odot}\) and star formation rates of \(2-39\,\rm M_{\rm \odot}\rm\,yr^{-1}\), placing them on the main sequence at this epoch. Using a stellar-to-halo-mass relationship, we estimate the dark matter halo mass of the most massive halo in the protocluster to be \(\sim 10^{12}\rm M_{\rm \odot}\). By comparison with halo mass evolution tracks from simulations, the protocluster is expected to evolve into a Virgo- or Coma-like cluster in the present day.
We report the spectroscopic confirmation of a massive (\(\log(M_\star/M_\odot)=10.34 \pm_{0.07}^{0.06}\)), HST-dark (\(m_\mathrm{F150W} - m_\mathrm{F444W} = 3.6\)) quiescent galaxy at ...\(z_{spec}=3.97\) in the UNCOVER survey. NIRSpec/PRISM spectroscopy and a non-detection in deep ALMA imaging surprisingly reveals that the galaxy is consistent with a low (\(<\)10 \(M_\odot \ \mathrm{yr^{-1}}\)) star formation rate despite evidence for moderate dust attenuation. The F444W image is well modeled with a two component \sersic fit that favors a compact, \(r_e\sim200\) pc, \(n\sim2.9\) component and a more extended, \(r_e\sim1.6\) kpc, \(n\sim1.7\) component. The galaxy exhibits strong color gradients: the inner regions are significantly redder than the outskirts. Spectral energy distribution models that reproduce both the red colors and low star formation rate in the center of UNCOVER 18407 require both significant (\(A_v\sim1.4\) mag) dust attenuation and a stellar mass-weighted age of 900 Myr, implying 50\% of the stars in the core already formed by \(z=7.5\). Using spatially resolved annular mass-to-light measurements enabled by the galaxy's moderate magnification (\(\mu=2.12\pm_{0.01}^{0.05}\)) to reconstruct a radial mass profile from the best-fitting two-component \sersic model, we infer a total mass-weighted \(r_\mathrm{eff} = 0.72 \pm_{0.11}^{0.15}\) kpc and log\((\Sigma_\mathrm{1 kpc} \ \mathrm{M_\odot/kpc^2}) = 9.61 \pm_{0.10}^{0.08}\). The early formation of a dense, low star formation rate, and dusty core embedded in a less attenuated stellar envelope suggests an evolutionary link between the earliest-forming massive galaxies and their elliptical descendants. Furthermore, the disparity between the global, integrated dust properties and the spatially resolved gradients highlights the importance of accounting for radially varying stellar populations when characterizing the early growth of galaxy structure.
Observations of high-redshift galaxies provide a critical direct test to the theories of early galaxy formation, yet to date, only three have been spectroscopically confirmed at \(z>12\). Due to ...strong gravitational lensing over a wide area, the galaxy cluster field A2744 is ideal for searching for the earliest galaxies. Here we present JWST/NIRSpec observations of two galaxies: a robust detection at \(z_{\rm spec} = 12.393^{+0.004}_{-0.001}\), and a plausible candidate at \(z_{\rm spec} = 13.079^{+0.013}_{-0.001}\). The galaxies are discovered in JWST/NIRCam imaging and their distances are inferred with JWST/NIRSpec spectroscopy, all from the JWST Cycle 1 UNCOVER Treasury survey. Detailed stellar population modeling using JWST NIRCam and NIRSpec data corroborates the primeval characteristics of these galaxies: low mass (\(\sim 10^8~{\rm M_\odot}\)), young, rapidly-assembling, metal-poor, and star-forming. Interestingly, both galaxies are spatially resolved, having lensing-corrected rest-UV effective radii on the order of 300-400 pc, which are notably larger than other spectroscopically confirmed systems at similar redshifts. The observed dynamic range of \(z \gtrsim 10\) sizes spans over 1 order of magnitude, implying a significant scatter in the size-mass relation at early times. Deep into the epoch of reionization, these discoveries elucidate the emergence of the first galaxies.
We present ALMA deep spectroscopy for a lensed galaxy at \(z_{\rm spec}=8.496\) with \(\log(M_{\rm star}/M_{\odot})\sim7.8\) whose optical nebular lines and stellar continuum are detected by ...JWST/NIRSpec and NIRCam Early Release Observations in SMACS0723. Our ALMA spectrum shows OIII88\(\mu\)m and CII158\(\mu\)m line detections at \(4.0\sigma\) and \(4.5\sigma\), respectively. The redshift and position of the OIII line coincide with those of the JWST source, while the CII line is blue-shifted by 90 km s\(^{-1}\) with a spatial offset of \(0.''5\) (\(\approx0.5\) kpc in source plane) from the JWST source. The NIRCam F444W image, including OIII\(\lambda\)5007 and H\(\beta\) line emission, spatially extends beyond the stellar components by a factor of \(>8\). This indicates that the \(z=8.5\) galaxy has already experienced strong outflows whose oxygen and carbon produce the extended OIII\(\lambda\)5007 and the offset CII emission, which would promote ionizing photon escape and facilitate reionization. With careful slit-loss corrections and removals of emission spatially outside the galaxy, we evaluate the OIII88\(\mu\)m/\(\lambda\)5007 line ratio, and derive the electron density \(n_{\rm e}\) by photoionization modeling to be \(220^{+170}_{-100}\) cm\(^{-3}\), which is comparable with those of \(z\sim2-3\) galaxies. We estimate an OIII88\(\mu\)m/CII158\(\mu\)m line ratio in the galaxy of \(>4\), as high as those of known \(z\sim6-9\) galaxies. This high OIII88\(\mu\)m/CII158\(\mu\)m line ratio is generally explained by the high \(n_{\rm e}\) as well as the low metallicity (\(Z_{\rm gas}/Z_{\odot}=0.04^{+0.02}_{-0.02}\)), high ionization parameter (\(\log U > -2.27\)), and low carbon-to-oxygen abundance ratio (\(\log\)(C/O) \(=-0.52:-0.24\)) obtained from the JWST/NIRSpec data; further CII follow-up observations will constrain the covering fraction of photodissociation regions.
The onset of galaxy formation is thought to be initiated by the infall of neutral, pristine gas onto the first protogalactic halos. However, direct constraints on the abundance of neutral atomic ...hydrogen (HI) in galaxies have been difficult to obtain at early cosmic times. Here we present spectroscopic observations with JWST of three galaxies at redshifts \(z=8.8 - 11.4\), about \(400-600\) Myr after the Big Bang, that show strong damped Lyman-\(\alpha\) absorption (\(N_{\rm HI} > 10^{22}\) cm\(^{-2}\)) from HI in their local surroundings, an order of magnitude in excess of the Lyman-\(\alpha\) absorption caused by the neutral intergalactic medium at these redshifts. Consequently, these early galaxies cannot be contributing significantly to reionization, at least at their current evolutionary stages. Simulations of galaxy formation show that such massive gas reservoirs surrounding young galaxies so early in the history of the universe is a signature of galaxy formation in progress.
JWST is revealing a new population of dust-reddened broad-line active galactic nuclei (AGN) at redshifts \(z\gtrsim5\). Here we present deep NIRSpec/Prism spectroscopy from the Cycle 1 Treasury ...program UNCOVER of 15 AGN candidates selected to be compact, with red continua in the rest-frame optical but with blue slopes in the UV. From NIRCam photometry alone, they could have been dominated by dusty star formation or AGN. Here we show that the majority of the compact red sources in UNCOVER are dust-reddened AGN: \(60\%\) show definitive evidence for broad-line H\(\alpha\) with FWHM\(\, >2000\) km/s, for \(20\%\) current data are inconclusive, and \(20\%\) are brown dwarf stars. We propose an updated photometric criterion to select red \(z>5\) AGN that excludes brown dwarfs and is expected to yield \(>80\%\) AGN. Remarkably, among all \(z_{\rm phot}>5\) galaxies with F277W\(-\)F444W\(>1\) in UNCOVER at least \(33\%\) are AGN regardless of compactness, climbing to at least \(80\%\) AGN for sources with F277W\(-\)F444W\(>1.6\). The confirmed AGN have black hole masses of \(10^7-10^9\) M\(_{\odot}\). While their UV-luminosities (\(-16>M_{\rm UV}>-20\) AB mag) are low compared to UV-selected AGN at these epochs, consistent with percent-level scattered AGN light or low levels of unobscured star formation, the inferred bolometric luminosities are typical of \(10^7-10^9\) M\(_{\odot}\) black holes radiating at \(\sim 10-40\%\) of Eddington. The number densities are surprisingly high at \(\sim10^{-5}\) Mpc\(^{-3}\) mag\(^{-1}\), 100 times more common than the faintest UV-selected quasars, while accounting for \(\sim1\%\) of the UV-selected galaxies. While their UV-faintness suggest they may not contribute strongly to reionization, their ubiquity poses challenges to models of black hole growth.
