arXiv subjects

Wenlei Chen

Publications and source records attributed to Wenlei Chen.

47 records · Page 3Linked to original sources

arXiv2023

Unscrambling the lensed galaxies in JWST images behind SMACS0723

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.

astro-ph.GA↗

arXiv2023

Two lensed star candidates at $z\simeq4.8$ behind the galaxy cluster MACS J0647.7+7015

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.

astro-ph.GA↗

arXiv2022

JWST Imaging of Earendel, the Extremely Magnified Star at Redshift $z=6.2$

The gravitationally lensed star WHL0137-LS, nicknamed Earendel, was identified with a photometric redshift $z_{phot} = 6.2 \pm 0.1$ based on images taken with the Hubble Space Telescope. Here we present James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) images of Earendel in 8 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 $\sim 4000$ 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 $T_{\mathrm{eff}} \simeq 13000$--16000 K assuming the light is dominated by a single star. The delensed bolometric luminosity in this case ranges from $\log(L) = 5.8$--6.6 $L_{\odot}$, 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.

astro-ph.GA↗

arXiv2022

Flashlights: More than A Dozen High-Significance Microlensing Events of Extremely Magnified Stars in Galaxies at Redshifts z=0.7-1.5

Once only accessible in nearby galaxies, we can now study individual stars across much of the observable universe aided by galaxy-cluster gravitational lenses. When a star, compact object, or multiple such objects in the foreground galaxy-cluster lens become aligned, they can magnify a background individual star, and the timescale of a magnification peak can limit its size to tens of AU. The number and frequency of microlensing events therefore opens a window into the population of stars and compact objects, as well as high-redshift stars. To assemble the first statistical sample of stars in order to constrain the initial mass function (IMF) of massive stars at redshift z=0.7-1.5, the abundance of primordial black holes in galaxy-cluster dark matter, and the IMF of the stars making up the intracluster light, we are carrying out a 192-orbit program with the Hubble Space Telescope called "Flashlights," which is now two-thirds complete owing to scheduling challenges. We use the ultrawide F200LP and F350LP long-pass WFC3 UVIS filters and conduct two 16-orbit visits separated by one year. Having an identical roll angle during both visits, while difficult to schedule, yields extremely clean subtraction. Here we report the discovery of more than a dozen bright microlensing events, including multiple examples in the famous "Dragon Arc" discovered in the 1980s, as well as the "Spocks" and "Warhol" arcs that have hosted already known supergiants. The ultradeep observer-frame ultraviolet-through-optical imaging is sensitive to hot stars, which will complement deep James Webb Space Telescope infrared imaging. We are also acquiring Large Binocular Telescope LUCI and Keck-I MOSFIRE near-infrared spectra of the highly magnified arcs to constrain their recent star-formation histories.

astro-ph.CO↗

arXiv2022

Early results from GLASS-JWST VIII: An Extremely Magnified Blue Supergiant Star at Redshift 2.65 in the Abell 2744 Cluster Field

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.

astro-ph.GA↗

arXiv2019

All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission concept that will provide essential contributions to multimessenger astrophysics in the late 2020s and beyond. AMEGO combines high sensitivity in the 200 keV to 10 GeV energy range with a wide field of view, good spectral resolution, and polarization sensitivity. Therefore, AMEGO is key in the study of multimessenger astrophysical objects that have unique signatures in the gamma-ray regime, such as neutron star mergers, supernovae, and flaring active galactic nuclei. The order-of-magnitude improvement compared to previous MeV missions also enables discoveries of a wide range of phenomena whose energy output peaks in the relatively unexplored medium-energy gamma-ray band.

astro-ph.IM↗

arXiv2019

Searching for Highly Magnified Stars at Cosmological Distances: Discovery of a Redshift 0.94 Blue Supergiant in Archival Images of the Galaxy Cluster MACS J0416.1-2403

Individual highly magnified stars have been recently discovered at lookback times of more than half the age of the Universe, in lensed galaxies that straddle the critical curves of massive galaxy clusters. Having confirmed their detectability, it is now important to carry out systematic searches for them in order to establish their frequency, and in turn learn about the statistical properties of high-redshift stars and of the granularity of matter in the foreground deflector. Here we report the discovery of a highly magnified star at redshift $z = 0.94$ in a strongly lensed arc behind a Hubble Frontier Field galaxy cluster, MACS J0416.1-2403, discovered as part of a systematic archival search. The bright transient (dubbed "Warhol") was discovered in Hubble Space Telescope data taken on 2014 September 15 and 16. This single image faded over a period of two weeks, and observations taken on 2014 September 1 show that the duration of the microlensing event was at most four weeks in total. The light curve may also exhibit slow changes over a period of years consistent with the level of microlensing expected from stars responsible for the intracluster light (ICL) of the cluster. Optical and infrared observations taken near peak brightness can be fit by a stellar spectrum with moderate host-galaxy extinction. A blue supergiant matches the measured spectral energy distribution near peak, implying a temporary magnification of at least several thousand. While the spectrum of an O-type star would also fit the transient's spectral energy distribution, extremely luminous O-type stars are much less common than blue supergiants. The short timescale of the event and the estimated effective temperature indicate that the lensed source is an extremely magnified star.

astro-ph.GA↗

arXiv2018

Novel Search for TeV-Initiated Pair Cascades in the Intergalactic Medium

TeV emission from blazars can be used to probe the intergalactic magnetic fields and measure their intensity, coherence length, and helicity. Intergalactic magnetic fields deflect the electron-positron pairs produced by very-high-energy gamma-rays from blazars, resulting in broadened beams of cascade gamma-rays developing along the projected direction of the blazar jet. We present an analysis that uses for the first time the jet orientation of 12 high-synchrotron-peaked (HSP) BL~Lac-type blazars from VLBA radio observations to search for signatures of pair haloes in the Fermi-LAT data. Our search improves the sensitivity of previous studies by taking the asymmetry of the pair haloes into account, increasing the signal to noise and reducing the possibility of systematics in determining the point spread function. Although there is no significant detection, a hint for an offset halo with a global significance of 2$σ$ is found in the $30-300$ GeV energy range, corresponding to an intergalactic magnetic field with $\sim 10^{-15}$ Gauss, consistent with the inferred field from a prior study using stacked HSP BL Lac objects.

astro-ph.HE↗

arXiv2015

Evidence for GeV Pair Halos around Low Redshift Blazars

We report on the results of a search for $γ$-ray pair halos with a stacking analysis of low-redshift blazars using data from the Fermi Large Area Telescope. For this analysis we used a number of a-priori selection criteria, including the spatial and spectral properties of the Fermi sources. The angular distribution of $\sim$ 1GeV photons around 24 stacked isolated high-synchrotron-peaked BL Lacs with redshift $z<0.5$ shows an excess over that of point-like sources. A statistical analysis yields a Bayes factor of $\mathrm{log}_{10}B_{10}>2$, providing evidence in favor of extended emission against the point-source hypothesis, consistent with expectations for pair halos produced in the IGMF with strength $B_{\mathrm{IGMF}}\sim 10^{-17}-10^{-15}\mathrm{G}$.

astro-ph.HE↗

arXiv2014

Intergalactic magnetic field spectra from diffuse gamma rays

Non-vanishing parity-odd correlators of gamma ray arrival directions observed by Fermi-LAT indicate the existence of a helical intergalactic magnetic field (Tashiro et al.2013). We successfully test this hypothesis using more stringent cuts of the data, Monte Carlo simulations with Fermi-LAT time exposure information, separate analyses for the northern and southern galactic hemispheres, and confirm predictions made in Tashiro & Vachaspati (2014). With some further technical assumptions, we show how to reconstruct the magnetic helicity spectrum from the parity-odd correlators.

astro-ph.CO↗

arXiv2014

Search for CP Violating Signature of Intergalactic Magnetic Helicity in the Gamma Ray Sky

The existence of a cosmological magnetic field could be revealed by the effects of non-trivial helicity on large scales. We evaluate a CP odd statistic, $Q$, using gamma ray data obtained from Fermi satellite observations at high galactic latitudes to search for such a signature. Observed values of $Q$ are found to be non-zero; the probability of a similar signal in Monte Carlo simulations is $\sim 0.2\%$. Contamination from the Milky Way does not seem to be responsible for the signal since it is present even for data at very high galactic latitudes. Assuming that the signal is indeed due to a helical cosmological magnetic field, our results indicate left-handed magnetic helicity and field strength $\sim 10^{-14}~{\rm G}$ on $\sim 10~{\rm Mpc}$ scales.

astro-ph.CO↗