分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The massive galaxy cluster El Gordo ($z=0.87$) imprints multitudes of gravitationally lensed arcs onto James Webb Space Telescope (JWST) Near-Infrared Camera (NIRCam) images. Eight bands of NIRCam imaging were obtained in the Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) program (GTO #1176). PSF-matched photometry across a suite of Hubble Space Telescope (HST) and NIRCam filters gives new photometric redshifts. We confirm 54 known image multiplicities and find two new ones and construct a lens model based on the light-traces-mass method. The mass within 500kpc estimated from the lens model is $\sim$$7.0\times10^{14}$M$_{\odot}$ with a mass ratio between the southeastern and northwestern components of $\sim$unity, similar to recent works. A statistical search for substructures recovers only these two components, which are each tightly bound kinematically and are separated in radial velocity by ~300 km s$^{-1}$. We identify a candidate member of a known 4-member $z=4.32$ galaxy overdensity by its model-predicted and photometric redshifts. These five members span a physical extent of $\sim$60 kpc and exhibit multiple components consistent with satellite associations. Thirteen additional candidates selected by spectroscopic/photometric constraints are small and faint, with a mean apparent brightness corrected for lensing magnification that is $\sim$2.2 mag fainter than M*. NIRCam imaging admits a wide range of brightnesses and morphologies for these candidates, suggesting a more diverse galaxy population may be underlying this rare view of a strongly-lensed galaxy overdensity.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of two extremely magnified lensed star candidates behind the galaxy cluster MACS J0647.7+7015, in recent multi-band James Webb Space Telescope (JWST) NIRCam observations. The candidates are seen in a previously known, $z_{phot}\simeq4.8$ dropout giant arc that straddles the critical curve. The candidates lie near the expected critical curve position but lack clear counter images on the other side of it, suggesting these are possibly stars undergoing caustic crossings. We present revised lensing models for the cluster, including multiply imaged galaxies newly identified in the JWST data, and use them to estimate a background macro-magnification of at least $\gtrsim90$ and $\gtrsim50$ at the positions of the two candidates, respectively. With these values, we expect effective, caustic-crossing magnifications of $10^4-10^5$ for the two star candidates. The Spectral Energy Distributions (SEDs) of the two candidates match well spectra of B-type stars with best-fit surface temperatures of $\sim10,000$ K, and $\sim12,000$ K, respectively, and we show that such stars with masses $\gtrsim20$ M$_{\odot}$ and $\gtrsim50$ M$_{\odot}$, respectively, can become sufficiently magnified to be observed. We briefly discuss other alternative explanations and conclude these are likely lensed stars, but also acknowledge that the less magnified candidate may instead be or reside in a star cluster. These star candidates constitute the second highest-redshift examples to date after Earendel at $z_{phot}\simeq6.2$, establishing further the potential of studying extremely magnified stars to high redshifts with the JWST. Planned visits including NIRSpec observations will enable a more detailed view of the candidates already in the near future.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus `external shear', which notionally accounts for neighbouring galaxies and cosmic shear. We show that it does not. Except in a handful of rare systems, the best-fit values of external shear do not correlate with independent measurements of shear: from weak lensing in 45 Hubble Space Telescope images, or in 50 mock images of lenses with complex distributions of mass. Instead, the best-fit shear is aligned with the major or minor axis of 88% of lens galaxies; and the amplitude of the external shear increases if that galaxy is disky. We conclude that `external shear' attached to a power law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Reionization Cluster Survey (RELICS) 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 ($\gtrsim5 \sigma$) caustic crossing events, in line with expectations from semi-analytic 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 (CC) SNe -- three in cluster galaxies, and among which only one was known before, and one slightly behind the cluster at $z\sim0.6-0.7$. The other two are likely Ia -- both of them previously known, one probably in a cluster galaxy, and one behind it at $z\simeq2$. 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.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We derive the spatial and wavelength behavior of dust attenuation in the multiple-armed spiral galaxy VV191b using backlighting by the superimposed elliptical system VV191a in a pair with an exceptionally favorable geometry for this measurement. Imaging using JWST and HST spans the wavelength range 0.3-4.5 microns with high angular resolution, tracing the dust in detail from 0.6 to 1.5 microns. Distinct dust lanes continue well beyond the bright spiral arms, and trace a complex web, with a very sharp radial cutoff near 1.7 Petrosian radii. We present attenuation profiles and coverage statistics in each band at radii 14-21 kpc. We derive the attenuation law with wavelength; the data both within and between the dust lanes clearly favor a stronger reddening behavior (R ~ 2.0 between 0.6 and 0.9 microns, approaching unity by 1.5 microns) than found for starbursts and star-forming regions of galaxies. Power-law extinction behavior lambda^(-beta) gives beta=2.1 from 0.6-0.9 microns. R decreases at increasing wavelengths (R~1.1 between 0.9 and 1.5 microns), while beta steepens to 2.5. Mixing regions of different column density flattens the wavelength behavior, so these results suggest a different grain population than in our vicinity. The NIRCam images reveal a lens arc and counterimage from a background galaxy at z~1, spanning 90 degrees azimuthally at 2.8" from the foreground elliptical galaxy nucleus, and an additional weakly-lensed galaxy. The lens model and imaging data give a mass/light ratio 7.6 in solar units within the Einstein radius 2.0 kpc.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The first deep field images from the James Webb Space Telescope (JWST) of the galaxy cluster SMACS~J0723.3-7327 reveal a wealth of new lensed images at uncharted infrared wavelengths, with unprecedented depth and resolution. Here we securely identify 14 new sets of multiply imaged galaxies totalling 42 images, adding to the five sets of bright and multiply-imaged galaxies already known from Hubble Space Telescope data. We find examples of arcs crossing critical curves, allowing detailed community follow-up, such as JWST spectroscopy for precise redshift determinations, and measurements of the chemical abundances and of the detailed internal gas dynamics of very distant, young galaxies. One such arc contains a pair of compact knots that are magnified by a factor of hundreds, and features a microlensed transient. We also detect an Einstein cross candidate only visible thanks to JWST's superb resolution. Our parametric lens model is available through the following link: https://www.dropbox.com/sh/gwup2lvks0jsqe5/AAC2RRSKce0aX-lIFCc9vhBXa?dl=0, and will be regularly updated using additional spectroscopic redshifts. The model is constrained by 16 of these sets of multiply imaged galaxies, three of which have spectroscopic redshifts, and reproduces the multiple images to better than an rms of $0.5^{\prime \prime}$, allowing for accurate magnification estimates of high-redshift galaxies. The intracluster light extends beyond the cluster members, exhibiting large-scale features that suggest a significant past dynamical disturbance. This work represents a first taste of the enhanced power JWST will have for lensing-related science.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of a triply-imaged active galactic nucleus (AGN), lensed by the galaxy cluster MACS J0035.4-2015 ($z_{\mathrm{d}}=0.352$). The object is detected in Hubble Space Telescope (HST) 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 $r_e\lesssim100$ pc. The object is spectroscopically confirmed to be an AGN at $z_{\mathrm{spec}}=2.063\pm0.005$ showing broad rest-frame UV emission lines, and is detected in both X-ray observations with \textit{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, $M_{\mathrm{UV},1450}=-19.7\pm0.2$. The object adds to just a few quasars or other X-ray sources known to be multiply lensed by a galaxy cluster. Some faint, diffuse emission from the host galaxy is also seen around the nucleus, and nearby there is another fainter object sharing the same multiple-imaging symmetry and geometric redshift, which may be 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 physical properties for 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 relatively young ($\sim100$ Myr) host galaxy of modest stellar mass $M_{\star}\simeq10^{9.2} \mathrm{M}_{\odot}$. We also observe variations in the AGN's emission, 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.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of an extremely magnified star at redshift $z=2.65$ in James Webb Space Telescope (JWST) NIRISS pre-imaging of the Abell 2744 galaxy-cluster field. The star's background host galaxy lies on a fold caustic of the foreground lens, and the cluster creates a pair of images of the region close to the lensed star. We identified the bright transient in one of the merging images at a distance of $\sim 0.15"$ from the critical curve, by subtracting the JWST F115W and F150W imaging from coadditions of archival Hubble Space Telescope (HST) F105W and F125W images and F140W and F160W images, respectively. Since the time delay between the two images should be only hours, the transient must be the microlensing event of an individual star, as opposed to a luminous stellar explosion which would persist for days to months. Analysis of individual exposures suggests that the star's magnification is not changing rapidly during the observations. From photometry of the point source through the F115W, F150W, and F200W filters, we identify a strong Balmer break, and modeling allows us to constrain the star's temperature to be approximately 7,000--12,000 K.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The PASSAGES ($Planck$ All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts) collaboration has recently defined a sample of 30 gravitationally-lensed dusty star-forming galaxies (DSFGs). These rare, submillimeter-selected objects enable high-resolution views of the most extreme sites of star formation in galaxies at Cosmic Noon. Here, we present the first major compilation of strong lensing analyses using LENSTOOL for PASSAGES, including 15 objects spanning $z=1.1-3.3$, using complementary information from $0.6^{\prime\prime}$-resolution 1 mm Atacama Large Millimeter/submillimeter Array (ALMA) and $0.4^{\prime\prime}$ 5 cm Jansky Very Large Array continuum imaging, in tandem with 1.6$\mu$m $Hubble$ and optical imaging with Gemini-S. Magnifications range from $\mu = 2 - 28$ (median $\mu=7$), yielding intrinsic infrared luminosities of $L_{\rm IR} = 0.2 - 5.9 \times 10^{13}~L_\odot$ (median ${1.4}\times 10^{13}~L_\odot$) and inferred star formation rates of $170-6300~M_\odot~{\rm yr}^{-1}$ (median $1500~M_\odot~{\rm yr}^{-1}$). These results suggest that the PASSAGES objects comprise some of the most extreme known starbursts, rivaling the luminosities of even the brightest unlensed objects, further amplified by lensing. The intrinsic sizes of far-infrared continuum regions are large ($R_{\rm e} = {1.7 - 4.3}$ kpc; median $3.0$ kpc) but consistent with $L_{\rm IR}-R_{\rm e}$ scaling relations for $z>1$ DSFGs, suggesting a widespread spatial distribution of star formation. With modestly-high angular resolution, we explore if these objects might be maximal starbursts. Instead of approaching Eddington-limited surface densities, above which radiation pressure will disrupt further star formation, they are safely sub-Eddington -- at least on global, galaxy-integrated scales.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Gradients in the mass-to-light ratio of distant galaxies impede our ability to characterize their size and compactness. The long-wavelength filters of $JWST$'s NIRCam offer a significant step forward. For galaxies at Cosmic Noon ($z\sim2$), this regime corresponds to the rest-frame near-infrared, which is less biased towards young stars and captures emission from the bulk of a galaxy's stellar population. We present an initial analysis of an extraordinary lensed dusty star-forming galaxy (DSFG) at $z=2.3$ behind the $El~Gordo$ cluster ($z=0.87$), named $El~Anzuelo$ ("The Fishhook") after its partial Einstein-ring morphology. The FUV-NIR SED suggests an intrinsic star formation rate of $81^{+7}_{-2}~M_\odot~{\rm yr}^{-1}$ and dust attenuation $A_V\approx 1.6$, in line with other DSFGs on the star-forming main sequence. We develop a parametric lens model to reconstruct the source-plane structure of dust imaged by the Atacama Large Millimeter/submillimeter Array, far-UV to optical light from $Hubble$, and near-IR imaging with 8 filters of $JWST$/NIRCam, as part of the Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) program. The source-plane half-light radius is remarkably consistent from $\sim 1-4.5~\mu$m, despite a clear color gradient where the inferred galaxy center is redder than the outskirts. We interpret this to be the result of both a radially-decreasing gradient in attenuation and substantial spatial offsets between UV- and IR-emitting components. A spatial decomposition of the SED reveals modestly suppressed star formation in the inner kiloparsec, which suggests that we are witnessing the early stages of inside-out quenching.