arXiv subjects

Ashish Kumar Meena

Publications and source records attributed to Ashish Kumar Meena.

34 records · Page 2Linked to original sources

arXiv2023

Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies -- IV. Elliptical NFW Lenses and Hyperbolic Umbilics

A source lying near hyperbolic umbilic (HU) leads to a ring-like image formation, constituting four images with high magnification factors and lying in a small region of the lens plane. Since (based on our earlier work) the observed number of HU image formations in cluster lenses is expected to increase in future, it is timely to investigate them in more detail. Like fold and cusp, HU also satisfies the magnification relation, i.e., the signed magnification sum of the four images equals zero. This work presents a detailed study of HU magnification relation ($R_{\rm hu}$) considering the elliptical Navarro-Frenk-White (eNFW) lens profile suitable for cluster scale dark matter halos. Our results show that for an isolated eNFW lens, $R_{\rm hu}$ is more sensitive to ellipticity than its mass or concentration parameter. An ellipticity greater than 0.3 results in $R_{\rm hu}$ lying close to zero with a small scatter around it. A substructure near the HU image formation causes the average $R_{\rm hu}$ value to deviate from zero and increases the scatter, with the amount of deviation depending on the image type near which the substructure lies. However, a population of substructures in the lens plane (equivalent to the galaxy lenses inside the cluster) does not significantly shift the average $R_{\rm hu}$ value from zero but increases the scatter around it. We find that $R_{\rm hu} \simeq 0$ for HU image formation in the Abell 1703 cluster. Repeating this test in other clusters where HU formations are discovered can be a useful indicator of substructure in cluster halos.

astro-ph.CO↗

arXiv2023

JWST reveals a possible $z \sim 11$ galaxy merger in triply-lensed MACS0647$-$JD

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 ($β\sim-2.6$), 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 ($β\sim-2$), 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 $1-5\rmμm$, out to $4300\,Å$ 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.

astro-ph.GA↗

arXiv2023

Flashlights: An Off-Caustic Lensed Star at Redshift $z$ = 1.26 in Abell 370

We report the discovery of a transient seen in a strongly lensed arc at redshift $z_{\rm s}=1.2567$ in \emph{Hubble Space Telescope} imaging of the Abell 370 galaxy cluster. The transient is detected at $29.51\pm0.14$ AB mag in a WFC3/UVIS F200LP difference image made using observations from two different epochs, obtained in the framework of the \emph{Flashlights} program, and is also visible in the F350LP band ($m_{\rm F350LP} \approx 30.53\pm0.76$ AB mag). The transient is observed on the negative-parity side of the critical curve at a distance of $\sim 0.6"$ from it, greater than previous examples of lensed stars. The large distance from the critical curve yields a significantly smaller macromagnification, but our simulations show that bright, O/B-type supergiants can reach sufficiently high magnifications to be seen at the observed position and magnitude. In addition, the observed transient image is a trailing image with an observer-frame time delay of $\sim+0.8$ days from its expected counterpart, so that any transient lasting for longer than that should have also been seen on the minima side and is thus excluded. This, together with the blue colour we measure for the transient ($m_{\rm F200LP} - m_{\rm F350LP} \approx [-0.3,-1.6]$ AB), rules out most other transient candidates such as (kilo)novae, for example, and makes a lensed star the prime candidate. Assuming the transient is indeed a lensed star as suggested, many more such events should be detected in the near future in cluster surveys with the \emph{Hubble Space Telescope} and \emph{James Webb Space Telescope}.

astro-ph.CO↗

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↗

arXiv2023

Spatially Resolved Stellar Populations of $0.3<z<6.0$ Galaxies in WHL0137-08 and MACS0647+70 Clusters as Revealed by JWST: How do Galaxies Grow and Quench Over Cosmic Time?

We study the spatially resolved stellar populations of 444 galaxies at $0.3<z<6.0$ in two clusters (WHL0137-08 and MACS0647+70) and a blank field, combining imaging data from HST and JWST to perform spatially resolved spectral energy distribution (SED) modeling using pixedfit. The high spatial resolution of the imaging data combined with magnification from gravitational lensing in the cluster fields allows us to resolve some galaxies to sub-kpc scales (for 109 of our galaxies). At redshifts around cosmic noon and higher ($2.5\lesssim z\lesssim 6.0$), we find mass doubling times to be independent of radius, inferred from flat specific star formation rate (sSFR) radial profiles and similarities between the half-mass and half-SFR radii. At lower redshifts ($1.5\lesssim z\lesssim 2.5$), a significant fraction of our star-forming galaxies show evidence for nuclear starbursts, inferred from centrally elevated sSFR, and a much smaller half-SFR radius compared to the half-mass radius. At later epochs, we find more galaxies suppress star formation in their center but are still actively forming stars in the disk. Overall, these trends point toward a picture of inside-out galaxy growth consistent with theoretical models and simulations. We also observe a tight relationship between the central mass surface density and global stellar mass with $\sim 0.38$ dex scatter. Our analysis demonstrates the potential of spatially resolved SED analysis with JWST data. Future analysis with larger samples will be able to further explore the assembly of galaxy mass and the growth of their structures

astro-ph.GA↗

arXiv2022

JWST's PEARLS: Prime Extragalactic Areas for Reionization and Lensing Science: Project Overview and First Results

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 proto-clusters; 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, AGN growth, and First Light. Five fields, the JWST NEP Time-Domain Field (TDF), IRAC Dark Field (IDF), 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 μm. PEARLS is designed to be of lasting benefit to the community.

astro-ph.CO↗

arXiv2022

An Efficient Method for Simulating Light Curves of Cosmological Microlensing and Caustic Crossing Events

A new window to observing individual stars and other small sources at cosmological distances was opened recently, with the detection of several caustic-crossing events in galaxy cluster fields. Many more such events are expected soon from dedicated campaigns with the \emph{Hubble Space Telescope} and from the \emph{James Webb Space Telescope}. These events can teach us not only about the lensed sources themselves, such as individual high-redshift stars, star clusters, or accretion disks, but through their light-curves they also hold information about the point-mass function of the lens and thus, potentially, the composition of dark matter. We present here a simple method for simulating light curves of such events, i.e., the change in apparent magnitude of the source as it sweeps over the net of caustics generated by microlenses embedded around the critical region of the lens. The method is recursive and so any reasonably sized small source can be accommodated, down to sub-solar scales, in principle. We compare the method, which we dub \emph{Adaptive Boundary Method}, with other common methods such as simple inverse ray shooting, and demonstrate that it is significantly more efficient and accurate in the small-source and high-magnification regime of interest. A \textsc{python} version of the code is made publicly available in an open-source fashion for simulating future events.

astro-ph.IM↗

arXiv2022

Gravitational Lensing of Gravitational Waves: Probability of Microlensing in Galaxy-Scale Lens Population

With the increase in the number of observed gravitational wave (GW) signals, detecting strongly lensed GWs by galaxies has become a real possibility. Lens galaxies also contain microlenses (e.g., stars and black holes), introducing further frequency-dependent modulations in the strongly lensed GW signal within the LIGO frequency range. The multiple lensed signals in a given lens system have different underlying macro-magnifications ($|μ|$) and are located in varied microlens densities ($Σ_\bullet$), leading to different levels of microlensing distortions. This work quantifies the fraction of strong lens systems affected by microlensing using realistic mock observations. We study 50 quadruply imaged systems (quads) by generating 50 realizations for each lensed signal. However, our conclusions are equally valid for lensed signals in doubly imaged systems (doubles). The lensed signals studied here have $|μ|\sim[0.5, 10]$ and $Σ_\bullet\sim[10, 10^3]~{\rm M}_\odot/{\rm pc^2}$. We find that the microlensing effects are more sensitive to the macro-magnification than the underlying microlens density, even if the latter exceeds $10^3~{\rm M}_\odot/{\rm pc^2}$. The mismatch between lensed and unlensed GW signals rarely exceeds $1\%$ for nearly all binary black hole sources in the total mass range [10 M$_\odot$, 200 M$_\odot$]. This implies that microlensing is not expected to affect the detection or the parameter estimation of such signals and does not pose any further challenges in identifying the different lensed counterparts when macro-magnification is ${\leq}10$. Such a magnification cut is expected to be satisfied by ${\sim}50\%$ of the detectable pairs in quads and ${\sim}90\%$ of the doubles in the fourth observing run of the LIGO-Virgo detector network.

astro-ph.GA↗

arXiv2022

Gravitational Lensing of Core Collapse Supernova Gravitational Wave Signals

We discuss the prospects of gravitational lensing of gravitational waves (GWs) coming from core-collapse supernovae (CCSN). As the CCSN GW signal can only be detected from within our own Galaxy and the local group by current and upcoming ground-based GW detectors, we focus on microlensing. We introduce a new technique based on analysis of the power spectrum and association of peaks of the power spectrum with the peaks of the amplification factor to identify lensed signals. We validate our method by applying it on the CCSN-like mock signals lensed by a point mass lens. We find that the lensed and unlensed signal can be differentiated using the association of peaks by more than one sigma for lens masses M$_{\rm L} {>} 150{\rm M}_{\odot}$. We also study the correlation integral between the power spectra and corresponding amplification factor. This statistical approach is able to differentiate between unlensed and lensed signals for lenses as small as M$_{\rm L} {\sim} 15{\rm M}_{\odot}$. Further, we demonstrate that this method can be used to estimate the mass of a lens in case the signal is lensed. The power spectrum based analysis is general and can be applied to any broad band signal and is especially useful for incoherent signals.

gr-qc↗

arXiv2021

Wave Effects in Double-Plane Lensing

We discuss the wave optical effects in gravitational lens systems with two point mass lenses in two different lens planes. We identify and vary parameters (i.e., lens masses, related distances, and their alignments) related to the lens system to investigate their effects on the amplification factor. We find that due to a large number of parameters, it is not possible to make generalized statements regarding the amplification factor. We conclude by noting that the best approach to study two-plane and multi-plane lensing is to study various possible lens systems case by case in order to explore the possibilities in the parameter space instead of hoping to generalize the results of a few test cases. We present a preliminary analysis of the parameter space for a two-lens system here.

astro-ph.CO↗

arXiv2021

Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies -- III: Statistics with HUDF

We study the image formation near point singularities (swallowtail and umbilics) in the simulated strongly lensed images of Hubble Ultra Deep Field (HUDF) by the Hubble Frontier Fields (HFF) clusters. In this work, we only consider nearly half of the brightest (a total of 5271) sources in the HUDF region. For every HFF cluster, we constructed 11 realizations of strongly lensed HUDF with an arbitrary translation of the cluster centre within the central region of HUDF and an arbitrary rotation. In each of these realizations, we visually identify the characteristic/exotic image formation corresponding to the different point singularities. We find that our current results are consistent with our earlier results based on different approaches. We also study time delay in these exotic image formations and compare it with typical five-image geometries. We find that the typical time delay in exotic image formations is an order of magnitude smaller than the typical time delay in a generic five-image geometry.

astro-ph.CO↗

arXiv2021

Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies -- II: Uncertainties

Due to the finite amount of observational data, the best-fit parameters corresponding to the reconstructed cluster mass have uncertainties. In turn, these uncertainties affect the inferences made from these mass models. Following our earlier work, we have studied the effect of such uncertainties on the singularity maps in simulated and actual galaxy clusters. The mass models for both simulated and real clusters have been constructed using grale. The final best-fit mass models created using grale give the simplest singularity maps and a lower limit on the number of point singularities that a lens has to offer. The simple nature of these singularity maps also puts a lower limit on the number of three image (tangential and radial) arcs that a cluster lens has. Hence, we estimate the number of galaxy sources giving rise to the three image arcs, which can be observed with the James Webb Space Telescope (JWST). We find that we expect to observe at least 20-30 tangential and 5-10 radial three-image arcs in the Hubble Frontier Fields cluster lenses with the JWST.

astro-ph.CO↗

arXiv2021

Gravitational Lensing of Gravitational Waves: Effect of Microlens Population in Lensing Galaxies

With increasing sensitivities of the current ground-based gravitational-wave (GW) detectors, the prospects of detecting a strongly lensed GW signal are going to be high in the coming years. When such a signal passes through an intervening lensing galaxy or galaxy cluster, the embedded stellar-mass microlenses lead to interference patterns in the signal that may leave observable signatures. In this work, we present an extensive study of these wave effects in the LIGO/Virgo frequency band ($10$-$10^4$ Hz) due to the presence of the microlens population in galaxy-scale lenses for the first time. We consider a wide range of strong lensing (macro) magnifications and the corresponding surface microlens densities found in lensing galaxies and use them to generate realisations of the amplification factor. The methodologies for simulating amplification curves for both types of images (minima and saddle points) are also discussed. We then study how microlensing is broadly affected by the parameters like macro-magnifications, stellar densities, the initial mass function (IMF), types of images, and microlens distribution around the source. In general, with increasing macro-magnification values, the effects of microlensing become increasingly significant regardless of other parameters. Mismatch analysis between the lensed and the unlensed GW waveforms from chirping binaries suggests that, while inferring the source parameters, microlensing can not be neglected for macro-magnification $\gtrsim 15$. Furthermore, for extremely high macro-magnifications $\gtrsim 100$, the mismatch can even exceed $5\%$, which can result in both a missed detection and, consequently, a missed lensed signal.

astro-ph.CO↗

arXiv2019

Finding Singularities in Gravitational Lensing

The number of strong lens systems is expected to increase significantly in ongoing and upcoming surveys. With an increase in the total number of such systems we expect to discover many configurations that correspond to unstable caustics. In such cases, the instability can be used to our advantage for constraining the lens model. We have implemented algorithms for detection of different types of singularities in gravitational lensing. We test our approach on a variety of lens models and then go on to apply it to the inferred mass distribution for Abell 697 as an example application. We propose to represent lenses using A3-lines and singular points (A4 and D4) in the image plane. We propose this as a compact representation of complex lens systems that can capture all the details in a single snapshot.

astro-ph.CO↗

arXiv2019

Gravitational Waves from merging binaries

We discuss gravitational waves from merging binaries using a Newtonian approach with some inputs from the Post-Newtonian formalism. We show that it is possible to understand the key features of the signal using fundamental physics and also demonstrate that an approximate calculation gives us the correct order of magnitude estimate of the parameters describing the merging binary system. We build on this analysis to understand the range for different types of sources for given detector sensitivity. We also consider known binary pulsar systems and discuss the expected gravitational wave signal from these.

physics.pop-ph↗

arXiv2019

Gravitational lensing of gravitational waves: wave nature and prospects for detection

We discuss the gravitational lensing of gravitational wave signals from coalescing binaries. We delineate the regime where wave effects are significant from the regime where geometric limit can be used. Further, we focus on the effect of micro-lensing and the combined effect of strong lensing and micro-lensing. We find that micro-lensing combined with strong lensing can introduce time varying phase shift in the signal and hence can lead to detectable differences in the signal observed for different images produced by strong lensing. This, coupled with the coarse localization of signal source in the sky for gravitational wave detections, can make it difficult to identify the common origin of signal corresponding to different images and use observables like time delay. In case we can reliably identify corresponding images, micro-lensing of individual images can be used as a tool to constrain properties of micro-lenses. Sources of gravitational waves can undergo microlensing due to lenses in the disk/halo of the Galaxy, or due to lenses in an intervening galaxy even in absence of strong lensing. In general the probability for this is small with one exception: Extragalactic sources of gravitational waves that lie in the galactic plane are highly likely to be micro-lensed. Wave effects are extremely important for such cases. In case of detections of such sources with low SNR, the uncertainty of occurrence of microlensing or otherwise introduces an additional uncertainty in the parameters of the source.

astro-ph.CO↗