Search for: All records

ABSTRACT We explore for the first time the possibility of detecting lensed star transients in galaxy–galaxy strong lensing systems upon repeated, deep imaging using the James Webb Space Telescope (JWST). Our calculation predicts that the extremely high recent star formation rate of $${\sim} 140\ {\rm M}_{\odot } \, \textrm {yr}^{-1}$$ over the last 50 Myr (not accounting for image multiplicity) in the ‘Cosmic Horseshoe’ lensed system ($z = 2.381$) generates many young, bright stars, of which their large abundance is expected to lead to a detection rate of $${\sim} 60$$ transients per pointing in JWST observations with a $$5\sigma$$ limiting magnitude of $${\sim} 29\, m_{\rm AB}$$. With the high expected detection rate and little room for uncertainty for the lens model compared with cluster lenses, our result suggests that the Cosmic Horseshoe could be an excellent tool to test the nature of dark matter based on the spatial distribution of transients, and can be used to constrain axion mass if dark matter is constituted of ultra-light axions. We also argue that the large distance modulus of $${\sim} 46.5\,$$ mag at $$z \approx 2.4$$ can act as a filter to screen out less massive stars as transients and allow one to better constrain the high-mass end of the stellar initial mass function based on the transient detection rate. Follow-up JWST observations of the Cosmic Horseshoe would allow one to better probe the nature of dark matter and the star formation properties, such as the initial mass function at the cosmic noon, via lensed star transients. 

The extreme magnification from galaxy clusters and microlenses therein allows for the detection of individual, luminous stars in lensed galaxies as transient events, and hence provides a valuable window into the high-mass stellar population inz > 1 galaxies. As these bright stars can only be formed at specific ages, the relative abundance of transient events at blue (B) and red (R) optical and near-infrared wavelengths (B/Rratio) can provide insights into the recent star formation history of galaxies that are not well constrained by their spectral energy distributions (SEDs). Here, we forward model the transient detection rates in an idealized mock scenario to find that theB/Rratio of strongly lensedz > 1 galaxies decreases quickly with increasing age. This ratio has a moderate sensitivity to metallicity and a comparatively low sensitivity to dust attenuation, with no significant dependency on the stellar initial mass function. Fitting model stellar populations to either the SED orB/Rratio alone of the “Warhol” arc (z = 0.94), we find that neither a simple single starburst nor more complex star formation can simultaneously reproduce both constraints. We then demonstrate that a best-fit model constrained by both theB/Rratio and SED requires a star formation rate that has varied quite dramatically over the past ∼50 Myr, for which the total stellar mass formed over this time is a factor of 10 (with 2 − 3σsignificance) different from the best-fit models to the SED alone. Our work shows that the transientB/Rratio can be used as an additional powerful constraint on the recent star formation history of higher-redshift galaxies in future works that are strongly lensed by galaxy clusters. 

Abstract Our understanding of galaxy properties and evolution is contingent on knowing the initial mass function (IMF), and yet to date the IMF is constrained only to local galaxies. Individual stars are now becoming routinely detected at cosmological distances, where luminous stars such as supergiants in background galaxies strongly lensed by galaxy clusters are temporarily further magnified by huge factors (up to 10⁴) by intracluster stars, thus being detected as transients. The detection rate of these events depends on the abundance of luminous stars in the background galaxy and is thus sensitive to the IMF and the star formation history (SFH), especially for the blue supergiants detected as transients in the rest-frame ultraviolet/optical filters. As a proof of concept, we use simple SFH and IMF models constrained by spectral energy distributions (SEDs) to see how well we can predict the Hubble Space Telescope and James Webb Space Telescope transient detection rate in a lensed arc dubbed “Spock” (z= 1.0054). We find that demanding a simultaneous fit of the SED and the transient detection rate places constraints on the IMF, independent of the assumed simple SFH model. We conclude that our likelihood analysis indicates that the data definitively prefers the “Spock” galaxy to have a Salpeter IMF (α = 2.35) rather than a top-heavy IMF (α = 1)—which is thought to be the case in the early universe—with no clear excess of supergiants above the standard IMF. 

Recent observations of caustic-crossing galaxies at redshift 0.7 ≲ z ≲ 1 show a wealth of transient events. Most of them are believed to be microlensing events of highly magnified stars. Earlier work predicts such events should be common near the critical curves (CCs) of galaxy clusters (“near region”), but some are found relatively far away from these CCs (“far region”). We consider the possibility that substructure on milliarcsecond scales (few parsecs in the lens plane) is boosting the microlensing signal in the far region. We study the combined magnification from the macrolens, millilenses, and microlenses (“3M lensing”), when the macromodel magnification is relatively low (common in the far region). After considering realistic populations of millilenses and microlenses, we conclude that the enhanced microlensing rate around millilenses is not sufficient to explain the high fraction of observed events in the far region. Instead, we find that the shape of the luminosity function (LF) of the lensed stars combined with the amount of substructure in the lens plane determines the number of microlensing events found near and far from the CC. By measuringβ(the exponent of the adopted power law LF,dN/dL = ϕ(L)∝(1/L)^β), and the number density of microlensing events at each location, one can create a pseudoimage of the underlying distribution of mass on small scales. We identify two regimes: (i) positive-imaging regime whereβ > 2 and the number density of events is greater around substructures, and (ii) negative-imaging regime whereβ < 2 and the number density of microlensing events is reduced around substructures. This technique opens a new window to map the distribution of dark-matter substructure down to ∼10³ M_⊙. We study the particular case of seven microlensing events found in the Flashlights program in the Dragon arc (z = 0.725). A population of supergiant stars having a steep LF withβ = 2.55_−0.56^+0.72fits the distribution of these events in the far and near regions. We also find that the new microlensing events from JWST observations in this arc imply a surface mass density substructure of Σ_∗= 54M_⊙pc⁻², consistent with the expected population of stars from the intracluster medium. We identify a small region of high density of microlensing events, and interpret it as evidence of a possible invisible substructure, for which we derive a mass of ∼1.3 × 10⁸ M_⊙(within its Einstein radius) in the galaxy cluster. 

Abstract We  present new high-spectral-resolution observations (R=λ/Δλ= 7000) of the filamentary nebula surrounding NGC 1275, the central galaxy of the Perseus cluster. These observations have been obtained with SITELLE, an imaging Fourier transform spectrometer installed on the Canada–France–Hawai Telescope with a field of view of $11\prime \times 11\prime$, encapsulating the entire filamentary structure of ionized gas despite its large size of 80 kpc × 50 kpc. Here, we present renewed fluxes, velocities, and velocity dispersion maps that show in great detail the kinematics of the optical nebula at [Sii]λ6716, [Sii]λ6731, [Nii]λ6584, Hα(6563 Å), and [Nii]λ6548. These maps reveal the existence of a bright flattened disk-shaped structure in the core extending tor∼10 kpc and dominated by a chaotic velocity field. This structure is located in the wake of X-ray cavities and characterized by a high mean velocity dispersion of 134 km s⁻¹. The disk-shaped structure is surrounded by an extended array of filaments spread out tor∼ 50 kpc that are 10 times fainter in flux, remarkably quiescent, and have a uniform mean velocity dispersion of 44 km s⁻¹. This stability is puzzling given that the cluster core exhibits several energetic phenomena. Based on these results, we argue that there are two mechanisms that form multiphase gas in clusters of galaxies: a first triggered in the wake of X-ray cavities leading to more turbulent multiphase gas and a second, distinct mechanism, that is gentle and leads to large-scale multiphase gas spreading throughout the core. 

In this work, we present a constraint on the abundance of supergiant (SG) stars at redshiftz ≈ 1, based on recent observations of a strongly lensed arc at this redshift. First we derived a free-form model of MACS J0416.1-2403 using data from the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) program. The new lens model is based on 72 multiply lensed galaxies that produce 214 multiple images, making it the largest sample of spectroscopically confirmed lensed galaxies on this cluster. The larger coverage in BUFFALO allowed us to measure the shear up to the outskirts of the cluster, and extend the range of lensing constraints up to ∼1 Mpc from the central region, providing a mass estimate up to this radius. As an application, we make predictions for the number of high-redshift multiply lensed galaxies detected in future observations with theJames WebbSpace Telescope (JWST). Then we focus on a previously known lensed galaxy atz = 1.0054, nicknamed Spock, which contains four previously reported transients. We interpret these transients as microcaustic crossings of SG stars and explain how we computed the probability of such events. Based on simplifications regarding the stellar evolution, we find that microlensing (by stars in the intracluster medium) of SG stars atz = 1.0054 can fully explain these events. The inferred abundance of SG stars is consistent with either (1) a number density of stars with bolometric luminosities beyond the Humphreys-Davidson (HD) limit (L_max ≈ 6 × 10⁵ L_⊙for red stars), which is below ∼400 stars kpc⁻², or (2) the absence of stars beyond the HD limit but with a SG number density of ∼9000 kpc⁻²for stars with luminosities between 10⁵ L_⊙and 6 × 10⁵ L_⊙. This is equivalent to one SG star per 10 × 10 pc². Finally, we make predictions for future observations with JWST’s NIRcam. We find that in observations made with theF200Wfilter that reach 29 mag AB, if cool red SG stars exist atz ≈ 1 beyond the HD limit, they should be easily detected in this arc. 

Abstract Using the first epoch of four-band NIRCam observations obtained by the James Webb Space Telescope (JWST) Prime Extragalactic Areas for Reionization and Lensing Science Program in the Spitzer IRAC Dark Field, we search for F150W and F200W dropouts. In 14.2 arcmin², we have found eight F150W dropouts and eight F200W dropouts, all brighter than 27.5 mag (the brightest being ∼24 mag) in the band to the red side of the break. As they are detected in multiple bands, these must be real objects. Their nature, however, is unclear, and characterizing their properties is important for realizing the full potential of JWST. If the observed color decrements are due to the Lyman break, these objects should be atz≳ 11.7 andz≳ 15.4, respectively. The color diagnostics show that at least four F150W dropouts are far away from the usual contaminators encountered in dropout searches (red galaxies at much lower redshifts or brown dwarf stars). While the diagnostics of the F200W dropouts are less certain due to the limited number of passbands, at least one of them is likely not a known type of contaminant, and the rest are consistent with either high-redshift galaxies with evolved stellar populations or old galaxies atz≈ 3–8. If a significant fraction of our dropouts are indeed atz≳ 12, we have to face the severe problem of explaining their high luminosities and number densities. Spectroscopic identifications of such objects are urgently needed. 

Abstract We give an overview and describe the rationale, methods, and first results from NIRCam images of the JWST “Prime Extragalactic Areas for Reionization and Lensing Science” (PEARLS) project. PEARLS uses up to eight NIRCam filters to survey several prime extragalactic survey areas: two fields at the North Ecliptic Pole (NEP); seven gravitationally lensing clusters; two high redshift protoclusters; and the iconic backlit VV 191 galaxy system to map its dust attenuation. PEARLS also includes NIRISS spectra for one of the NEP fields and NIRSpec spectra of two high-redshift quasars. The main goal of PEARLS is to study the epoch of galaxy assembly, active galactic nucleus (AGN) growth, and First Light. Five fields—the JWST NEP Time-Domain Field (TDF), IRAC Dark Field, and three lensing clusters—will be observed in up to four epochs over a year. The cadence and sensitivity of the imaging data are ideally suited to find faint variable objects such as weak AGN, high-redshift supernovae, and cluster caustic transits. Both NEP fields have sightlines through our Galaxy, providing significant numbers of very faint brown dwarfs whose proper motions can be studied. Observations from the first spoke in the NEP TDF are public. This paper presents our first PEARLS observations, their NIRCam data reduction and analysis, our first object catalogs, the 0.9–4.5 μ m galaxy counts and Integrated Galaxy Light. We assess the JWST sky brightness in 13 NIRCam filters, yielding our first constraints to diffuse light at 0.9–4.5 μ m. PEARLS is designed to be of lasting benefit to the community.