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We present size measurements of 78 high-redshift (z ≥ 5.5) galaxy candidates from the Reionization Lensing Cluster Survey (RELICS). These distant galaxies are well resolved due to the gravitational lensing power of foreground galaxy clusters, imaged by the Hubble Space Telescope and the Spitzer Space Telescope. We compute sizes using the forward-modelling code lenstruction and account for magnification using public lens models. The resulting size–magnitude measurements confirm the existence of many small galaxies with effective radii Reff < 200 pc in the early Universe, in agreement with previous studies. In addition, we highlight compact and highly star-forming sources with star formation rate surface densities $\Sigma _\text{SFR}\gt 10\, \mathrm{M}_\odot \, \text{yr}^{-1}\, \text{kpc}^{-2}$ as possible Lyman continuum leaking candidates that could be major contributors to the process of reionization. Future spectroscopic follow-up of these compact galaxies (e.g. with the James Webb Space Telescope) will further clarify their role in reionization and the physics of early star formation.

 

We present results of [Cii] 158μm emission line observations, and report the spectroscopic redshift confirmation of a strongly lensed (μ∼ 20) star-forming galaxy, MACS0308-zD1 atz= 6.2078 ± 0.0002. The [Cii] emission line is detected with a signal-to-noise ratio >6 within the rest-frame UV-bright clump of the lensed galaxy (zD1.1) and exhibits multiple velocity components; the narrow [Cii] has a velocity full width half maximum (FWHM) of 110 ± 20 km s⁻¹, while broader [Cii] is seen with an FWHM of 230 ± 50 km s⁻¹. The broader [Cii] component is blueshifted (−80 ± 20 km s⁻¹) with respect to the narrow [Cii] component, and has a morphology that extends beyond the UV-bright clump. We find that, while the narrow [Cii] emission is most likely associated with zD1.1, the broader component is possibly associated with a physically distinct gas component from zD1.1 (e.g., outflowing or inflowing gas). Based on the nondetection ofλ_158μmdust continuum, we find that MACS0308-zD1's star formation activity occurs in a dust-free environment indicated by a strong upper limit of infrared luminosity ≲9 × 10⁸L_⊙. Targeting this strongly lensed faint galaxy for follow-up Atacama Large Millimeter/submillimeter Array and JWST observations will be crucial to characterize the details of typical galaxy growth in the early Universe.

 

We report the discovery of a double-peaked Lyman-α (Ly α) emitter (LAE) at z = 3.2177 ± 0.0001 in VLT/MUSE data. The galaxy is strongly lensed by the galaxy cluster RXC J0018.5+1626 recently observed in the RELICS survey, and the double-peaked Ly α emission is clearly detected in the two counter images in the MUSE field of view. We measure a relatively high Ly α rest-frame equivalent width (EW) of EWLy α, 0 = (63 ± 2) Å. Additional spectroscopy with Gemini/GNIRS in the near-infrared (NIR) allows us to measure the H β, [O iii] λ4959 Å, and [O iii] λ5007 Å emission lines, which show moderate rest-frame EWs of the order of a few ∼10–100 Å, an [O iii] λ5007 Å/H β ratio of 4.8 ± 0.7, and a lower limit on the [O iii]/[O ii] ratio of >9.3. The galaxy has very blue UV-continuum slopes of βFUV = −2.23 ± 0.06 and βNUV = −3.0 ± 0.2, and is magnified by factors μ ∼ 7–10 in each of the two images, thus enabling a view into a low-mass ($M_{\star }\simeq 10^{7.5}\, \mathrm{M}_{\odot }$) high-redshift galaxy analogue. Notably, the blue peak of the Ly α profile is significantly stronger than the red peak, which suggests an inflow of matter and possibly very low H i column densities in its circumgalactic gas. To the best of our knowledge, this is the first detection of such a Ly α profile. Combined with the high lensing magnification and image multiplicity, these properties make this galaxy a prime candidate for follow-up observations to search for LyC emission and constrain the LyC photon escape fraction.

 

The Reionization Cluster Survey imaged 41 galaxy clusters with the Hubble Space Telescope (HST), in order to detect lensed and high-redshift galaxies. Each cluster was imaged to about 26.5 AB mag in three optical and four near-infrared bands, taken in two distinct visits separated by varying time intervals. We make use of the multiple near-infrared epochs to search for transient sources in the cluster fields, with the primary motivation of building statistics for bright caustic crossing events in gravitational arcs. Over the whole sample, we do not find any significant (≳5σ) caustic crossing events, in line with expectations from semi-analytical calculations but in contrast to what may be naively expected from previous detections of some bright events or from deeper transient surveys that do find high rates of such events. Nevertheless, we find six prominent supernova (SN) candidates over the 41 fields: three of them were previously reported and three are new ones reported here for the first time. Out of the six candidates, four are likely core-collapse SNe – three in cluster galaxies, and among which only one was known before, and one slightly behind the cluster at z ∼ 0.6–0.7. The other two are likely Ia – both of them previously known, one probably in a cluster galaxy and one behind it at z ≃ 2. Our study supplies empirical bounds for the rate of caustic crossing events in galaxy cluster fields to typical HST magnitudes, and lays the groundwork for a future SN rate study.

 

We report the discovery of a triply imaged active galactic nucleus (AGN), lensed by the galaxy cluster MACS J0035.4−2015 (zd = 0.352). The object is detected in Hubble Space Telescope imaging taken for the RELICS program. It appears to have a quasi-stellar nucleus consistent with a point-source, with a de-magnified radius of re ≲ 100 pc. The object is spectroscopically confirmed to be an AGN at zspec = 2.063 ± 0.005 showing broad rest-frame UV emission lines, and detected in both X-ray observations with Chandra and in ALCS ALMA band 6 (1.2 mm) imaging. It has a relatively faint rest-frame UV luminosity for a quasar-like object, MUV, 1450 = −19.7 ± 0.2. The object adds to just a few quasars or other X-ray sources known to be multiply lensed by a galaxy cluster. Some diffuse emission from the host galaxy is faintly seen around the nucleus, and there is a faint object nearby sharing the same multiple-imaging symmetry and geometric redshift, possibly an interacting galaxy or a star-forming knot in the host. We present an accompanying lens model, calculate the magnifications and time delays, and infer the physical properties of the source. We find the rest-frame UV continuum and emission lines to be dominated by the AGN, and the optical emission to be dominated by the host galaxy of modest stellar mass $M_{\star }\simeq 10^{9.2}\, \mathrm{M}_{\odot }$ . We also observe some variation in the AGN emission with time, which may suggest that the AGN used to be more active. This object adds a low-redshift counterpart to several relatively faint AGN recently uncovered at high redshifts with HST and JWST.

 

Abstract The gravitationally lensed star WHL 0137–LS, nicknamed Earendel, was identified with a photometric redshift z phot = 6.2 ± 0.1 based on images taken with the Hubble Space Telescope. Here we present James Webb Space Telescope (JWST) Near Infrared Camera images of Earendel in eight filters spanning 0.8–5.0 μ m. In these higher-resolution images, Earendel remains a single unresolved point source on the lensing critical curve, increasing the lower limit on the lensing magnification to μ > 4000 and restricting the source plane radius further to r < 0.02 pc, or ∼4000 au. These new observations strengthen the conclusion that Earendel is best explained by an individual star or multiple star system and support the previous photometric redshift estimate. Fitting grids of stellar spectra to our photometry yields a stellar temperature of T eff ≃ 13,000–16,000 K, assuming the light is dominated by a single star. The delensed bolometric luminosity in this case ranges from log ( L ) = 5.8 to 6.6 L ⊙ , which is in the range where one expects luminous blue variable stars. Follow-up observations, including JWST NIRSpec scheduled for late 2022, are needed to further unravel the nature of this object, which presents a unique opportunity to study massive stars in the first billion years of the universe.