arXiv subjects

Jordi Miralda-Escudé

Publications and source records attributed to Jordi Miralda-Escudé.

At least 19 recordsLinked to original sources

arXiv2026

Supermagnified Stars in Lensing Clusters and Small-Scale Structure in the Dark Matter

Supermagnified stars, discovered by HST and JWST in lensing clusters of galaxies, are luminous stars in source galaxies lying near lensing caustics that are magnified by large factors ($\sim 1000$), making them detectable at cosmological distances. Intracluster stars in the lens modify the large-scale caustic into a corrugated network of micro-caustics, causing frequent microlensing events of the supermagnified stars. The frequency and lightcurves of these micro-caustic crossings are extremely sensitive to any small-scale, low amplitude surface density fluctations in the lensing cluster, making them a unique probe to dark matter minihalos or any small-scale irregularities. Furthermore, the disks of supermagnified stars can be resolved (at cosmological distances!) from photometric monitoring of micro-caustic crossing events, exploiting the limb darkening effect. Examples are shown of model lightcurves to fit the Kelly et al. (2018) observations of the first supermagnified star discovered. The unique sensitivity and angular resolution of the Habitable Worlds Observatory enables photometry of microlensing events of supermagnified stars sensitive to small-scale structure different from intracluster stars, a probe to the nature of dark matter that is not accessible to other observations at present.

astro-ph.CO↗

arXiv2026

Rethinking Resonance Detectability during Binary Neutron Star Inspiral: Accurate Mismatch Computations for Low-lying Dynamical Tides

We compute deviations from observed gravitational wave signals, where the amplitude of the signal is unchanged. As an example, we consider the detectability of low lying dynamical tides in binary neutron star or neutron star black hole mergers. Tidal forces can excite oscillatory modes of one or both of the stars in the binary when the orbital frequency of the binary system sweeps through the resonant mode frequency dissipating energy into the vibrational mode. The orbital energy loss to the vibrational mode extracts energy from the orbital motion, advancing the time to merger. The inspiral then continues with an excess phase and a time advance. Both will cause a mismatch when fitting to a system that has not gone through the resonance. To resolve this effect, we compute the mismatch for current and planned detectors using both a quasi-analytical approach that relies on the computation of moment integrals and an optimized version of the standard numerical match function. We conclude that detectability can occur for time advances of the order of 1 ms with advanced LVK detectors for an excess energy-flux that is a few percent of the gravitational wave emission. Our results contrast with previous work, which model this effect solely as a phase shift of the waveform or by using the difference in the number of cycles induced by the resonant behavior. We show that tidal resonance effects primarily cause a time advance of the merger, rather than a phase difference, and that the single-frequency approximation commonly used in the literature significantly overestimates the detectability of this effect.

gr-qc↗

arXiv2026

First Statistical Study of Over 100 Magnified Stellar Events at Redshift $z \approx 0.725$ with JWST

Highly magnified stars at cosmological distances ($z \gtrsim 0.7$) become detectable thanks to microlensing by intracluster stars near the critical curves of galaxy clusters. Multi-epoch photometric campaigns targeting caustic crossing galaxies magnified by massive galaxy clusters enable the detection of these objects as transient events. Such stars provide unique opportunities to study stellar populations at early cosmic times, probe the nature of dark matter, reveal small-scale structure in the cluster, and improve lens models. To date, only a few dozen high-redshift stars have been reported, with a single lensed galaxy, the Dragon, holding the current record of 44 detections. These numbers, however, remain insufficient to exploit their full potential. In this paper, owing to the inclusion of new observations, we report the identification of more than 100 magnified stellar events in the Dragon, behind the massive galaxy cluster Abell 370. The relatively low redshift of the Dragon ($z\approx0.725$) facilitates the detection of its most massive stars. Using imaging data from three different cycles (2022--2024) with the James Webb Space Telescope, we apply a time-domain technique to identify flux variations associated with caustic-crossing events. From the spatial distribution of stellar events we constrain the high-end slope of the stellar luminosity function, finding $β=2.18^{+0.20}_{-0.30}$. Alternatively, assuming a fixed slope, we constrain the microlens surface mass density. In addition, we examine the parity asymmetry of the detected caustic-crossing events, a proposed probe of wave dark matter, and find that it remains present. We also use the events to trace the regions of highest magnification, offering an alternative way to map the system critical curves.

astro-ph.GA↗

arXiv2025

Constraining the environment of compact binary mergers with self-lensing signatures

Gravitational waves (GWs) from coalescing binary black holes (BBHs) can come from different environments. GWs interact gravitationally with astrophysical objects, which makes it possible to use gravitational lensing by objects in the same gravitational system (self-lensing) to learn about their environments. We quantify the probability of self-lensing through the optical depth $τ$ for the main channels of detectable GWs at frequencies $f_{\rm GW}\sim (1-10^3)\,{\rm Hz}$. We then analyze the detectability of the lensing effect (imprint). In star clusters, the probability of self-lensing by stellar-mass black holes (BHs) is low, $τ\simeq10^{-7}$, even when taking into account nearby BHs in resonant interactions, $τ\simeq 10^{-5}$. Additionally, the lensing imprint of a stellar-mass lens (diffraction and interference) is too marginal to be detectable by the LIGO-Virgo-KAGRA detectors and most Einstein Telescope signals. For a massive BH lens in the center of a cluster, the probability can reach $τ\simeq 10^{-4}$ either via von Zeipel-Lidov-Kozai induced mergers of BBHs orbiting a central massive BH, or BBHs formed as GW captures in single-single interactions in the Bahcall-Wolf cusp of a nuclear cluster. For self-lensing by a supermassive BH for BBHs in the migration trap of an active galactic nucleus (AGN) disk, $τ\simeq 10^{-2}$. The imprint of these massive lenses are multiple images that are already detectable. Moreover, self-lensed signals from AGN disks have a distinct linear polarization. The probability depends on the extent of the detectability through the threshold impact parameter $y_{\rm max}$, which can increase for future detectors. We conclude that constraining the environment of BBHs is possible by combining self-lensing imprints with other waveform signatures such as eccentricity and polarization.

astro-ph.HE↗

arXiv2025

Limits on Dark Matter Compact Objects implied by Supermagnified Stars in Lensing Clusters

Supermagnified stars are gravitationally lensed individual stars that are located close to a caustic of a lensing galaxy cluster, and have their flux magnified by a large enough factor (typically ~ 1000) to make them detectable with present telescopes. The maximum magnification is limited by microlensing caused by intracluster stars or other compact objects, which create a network of corrugated critical lines with an angular width proportional to the surface density of microlenses. We consider a set of 9 cases of supermagnified stars reported in the literature, and derive an upper limit on the surface density of compact objects, such as primordial black holes, that might be present as a fraction of the dark matter in addition to known intracluster stars. Any such additional compact objects would widen the corrugated critical line network and therefore the width of the distribution of supermagnified stars around the modeled critical lines of the lens. We find that any compact objects, including primordial black holes, with masses above $\sim 10^{-6}\, M_{\odot}$ (below which the microcaustics are closer together than the typical angular size of supermagnified stars) cannot account for more than ~ 2% of the dark matter.

astro-ph.CO↗

arXiv2025

Medium-band Astrophysics with the Grism of NIRCam In Frontier fields (MAGNIF): Spectroscopic Census of H$α$ Luminosity Functions and Cosmic Star Formation at $z\sim 4.5$ and 6.3

We measure H$α$ luminosity functions (LFs) at redshifts $z \sim 4.5$ and 6.3 using the JWST MAGNIF (Medium-band Astrophysics with the Grism of NIRCam In Frontier fields) survey. MAGNIF obtained NIRCam grism spectra with the F360M and F480M filters in four Frontier Fields. We identify 248 H$α$ emitters based on the grism spectra and photometric redshifts from combined HST and JWST imaging data. The numbers of the H$α$ emitters show a large field-to-field variation, highlighting the necessity of multiple fields to mitigate cosmic variance. We calculate both observed and dust-corrected H$α$ LFs in the two redshift bins. Thanks to the gravitational lensing, the measured H$α$ LFs span three orders of magnitude in luminosity, and the faint-end luminosity reaches $L_{\mathrm{H}α} \sim 10^{40.3} \mathrm{erg} \mathrm{s}^{-1}$ at $z \sim 4.5$ and $10^{41.5} \mathrm{erg} \mathrm{s}^{-1}$ at $z \sim 6.3$, corresponding to star-formation rates (SFRs) of $\sim$ 0.1 and 1.7 $\mathrm{M}_\odot \mathrm{yr}^{-1}$. We conclude no or weak redshift evolution of the faint-end slope of H$α$ LF across $z\simeq0.4-6.3$, and the comparison with the faint-end slopes of UV LF indicates stochastic star formation history among low-mass H$α$ emitters. The derived cosmic SFR densities are $0.058^{+0.008}_{-0.006}\ \ M_\odot\ \mathrm{yr}^{-1}\ \mathrm{Mpc}^{-3}$ at $z \sim 4.5$ and $0.025^{+0.009}_{-0.007}\ \ M_\odot\ \mathrm{yr}^{-1}\ \mathrm{Mpc}^{-3}$ at $z \sim 6.3$. These are approximately 2.2 times higher than previous estimates based on dust-corrected UV LFs, but consistent with recent measurements from infrared surveys. We discuss uncertainties in the H$α$ LF measurements, including those propagate from the lens models, cosmic variance, and AGN contribution.

astro-ph.GA↗

arXiv2024

Microlensing near macro-caustics

Microlensing near macro-caustics is a complex phenomenon in which swarms of micro-images produced by micro-caustics form on both sides of a macro-critical curve. Recent discoveries of highly magnified images of individual stars in massive galaxy cluster lenses, predicted to be formed by these micro-image swarms, have stimulated studies on this topic. In this Chapter, we explore microlensing near macro-caustics using both simulations and analytic calculations. We show that the mean total magnification of the micro-image swarms follows that of an extended source in the absence of microlensing. Micro-caustics join into a connected network in a region around the macro-critical line of a width proportional to the surface density of microlenses; within this region, the increase of the mean magnification toward the macro-caustic is driven by the increase of the number of micro-images rather than individual magnifications of micro-images. The maximum achievable magnification in micro-caustic crossings decreases with the mass fraction in microlenses. We conclude with a review of applications of this microlensing phenomenon, including limits to the fraction of dark matter in compact objects, and searches of Population III stars and dark matter subhalos. We argue that the discovered highly magnified stars at cosmological distances already imply that less than $\sim$ 10\% of the dark matter may be in the form of compact objects with mass above $\sim 10^{-6}\, M_{\odot}$.

astro-ph.GA↗

arXiv2023

The oblateness of the Milky Way dark matter halo from the stellar streams of NGC 3201, M68, and Palomar 5

We explore constraints on the Milky Way dark matter halo oblateness using three stellar streams from globular clusters NGC 3201, M68, and Palomar 5. Previous constraints on the gravitational potential from dynamical equilibrium of stellar populations and distant Milky Way satellites are included. We model the dark halo as axisymmetric with axis ratio $q_ρ^{\rm h}$ and four additional free parameters of a two power-law density profile. The halo axis ratio, while barely constrained by the NGC 3201 stream alone, is required to be close to spherical by the streams of Palomar 5 ($q_ρ^{\rm h}=1.01\pm0.09$) and M68 ($q_ρ^{\rm h}=1.14^{+0.21}_{-0.14}$), the latter allowing a more prolate shape. The three streams together are well fitted with a halo axis ratio $q_ρ^{\rm h}=1.06\pm0.06$ and core radius $\sim$ 20 kpc. Our estimate of the halo shape agrees with previous studies using other observational data and is in tension with cosmological simulations predicting that most spiral galaxies have oblate dark matter halos with the short axis perpendicular to the disc. We discuss why the impact of the Magellanic Clouds tide is too small to change our conclusion on the halo axis ratio. We note that dynamical equilibrium of a spherical halo in the oblate disk potential implies an anisotropic dark matter velocity dispersion, larger along the vertical direction than the horizontal ones, which should relate to the assembly history of the Milky Way.

astro-ph.GA↗

arXiv2022

Primordial black holes capture by stars and induced collapse to low-mass stellar black holes

Primordial black holes in the asteroid-mass window ($\sim 10^{-16}$ to $10^{-11} \rm M_{\odot}$), which might constitute all the dark matter, can be captured by stars when they traverse them at low enough velocity. After being placed on a bound orbit during star formation, they can repeatedly cross the star if the orbit happens to be highly eccentric, slow down by dynamical friction and end up in the stellar core. The rate of these captures is highest in halos of high dark matter density and low velocity dispersion, when the first stars form at redshift $z \sim 20$. We compute this capture rate for low-metallicity stars of $0.3$ to $1\rm M_{\odot}$, and find that a high fraction of these stars formed in the first dwarf galaxies would capture a primordial black hole, which would then grow by accretion up to a mass that may be close to the total star mass. We show the capture rate of primordial black holes does not depend on their mass over this asteroid-mass window, and should not be much affected by external tidal perturbations. These low-mass stellar black holes could be discovered today in low-metallicity, old binary systems in the Milky Way containing a surviving low-mass main-sequence star or a white dwarf, or via gravitational waves emitted in a merger with another compact object. No mechanisms in standard stellar evolution theory are known to form black holes of less than a Chandrasekhar mass, so detecting a low-mass black hole would fundamentally impact our understanding of stellar evolution, dark matter and the early Universe.

astro-ph.GA↗

arXiv2022

The Canfranc Axion Detection Experiment (CADEx): Search for axions at 90 GHz with Kinetic Inductance Detectors

We propose a novel experiment, the Canfranc Axion Detection Experiment (CADEx), to probe dark matter axions with masses in the range 330-460 $μ$eV, within the W-band (80-110 GHz), an unexplored parameter space in the well-motivated dark matter window of Quantum ChromoDynamics (QCD) axions. The experimental design consists of a microwave resonant cavity haloscope in a high static magnetic field coupled to a highly sensitive detecting system based on Kinetic Inductance Detectors via optimized quasi-optics (horns and mirrors). The experiment is in preparation and will be installed in the dilution refrigerator of the Canfranc Underground Laboratory. Sensitivity forecasts for axion detection with CADEx, together with the potential of the experiment to search for dark photons, are presented.

hep-ex↗

arXiv2022

Design of new resonant haloscopes in the search for the darkmatter axion: a review of the first steps in the RADES collaboration

Within the increasing interest in the dark matter axion detection through haloscopes, in which different international groups are currently involved, the RADES group was established in 2016 with the goal of developing very sensitive detection systems to be operated in dipole magnets. This review deals with the work developed by this collaboration during its first five years, from the first designs, based on the multi-cavity concept, aiming to increase the haloscope volume and, so, to improve its sensitivity, their evolution, the data acquisition design, and, finally, the first experimental run. Moreover, the envisaged work within RADES, for both dipole and solenoid magnets, in the short and medium term is also presented.

physics.ins-det↗

arXiv2021

Maximum amplitude of the high-redshift 21-cm absorption feature

We examine the maximum possible strength of the global 21-cm absorption dip on the Cosmic Background Radiation at high-redshift caused by the atomic intergalactic medium, when the Lyman-$α$ coupling is maximum, assuming no exotic cooling mechanisms from interactions with dark matter. This maximum absorption is limited by three inevitable factors that need to be accounted for: $(a)$ heating by energy transferred from the Cosmic Background Radiation to the hydrogen atoms via 21-cm transitions, dubbed as 21-cm heating; $(b)$ Ly$α$ heating by scatterings of Ly$α$ photons from the first stars; $(c)$ the impact of the expected density fluctuations in the intergalactic gas in standard Cold Dark Matter theory, which reduces the mean 21-cm absorption signal. Inclusion of this third novel effect reduces the maximum global 21-cm absorption by $\sim 10\%$. Overall, the three effects studied here reduce the 21-cm global absorption by $\sim 20\%$ at $z \simeq 17$.

astro-ph.CO↗

arXiv2020

Axion Gegenschein: Probing Back-scattering of Astrophysical Radio Sources Induced by Dark Matter

We investigate a novel technique for the astrophysical detection of axions or axion-like particles in the dark matter halo of the Milky Way based on stimulated decay of axions, which we call axion gegenschein emission. Photons from the brightest known radio sources with a frequency equal to half the axion mass stimulate axion decay while propagating through the dark matter halo, causing radio emission in a direction precisely opposite to the incoming photon in the axion rest-frame and creating a countersource for every radio source, with an image smoothed by the dark matter velocity dispersion. We calculate the flux of the axion gegenschein countersource of Cygnus A, the brightest extragalactic radio source, and the limits that can be set with SKA to the axion-photon coupling constant $g_{aγ}$. We find this method to be more powerful than previous proposals based on searching for radio emission from axion decay in nearby dwarf galaxies or the Milky Way. The forecasted limits remain considerably higher than predictions from QCD axion models, and limits that can be set with laboratory searches of radio waves generated in resonant cavities with strong magnetic fields similar to the ADMX experiment, although this observation would directly measure a column density of dark matter through the Galactic halo and is therefore not affected by possible substructure in the dark matter distribution.

astro-ph.CO↗

arXiv2020

J-PLUS: Unveiling the brightest-end of the Lyα luminosity function at 2.0<z<3.3 over 1000 deg^2

We present the photometric determination of the bright-end (L_Lya>10^43.5 erg/s) of the Lya luminosity function (LF) within four redshifts windows in the interval 2.2 10^43.3 erg/s) Lya-emitting candidates. We test our selection with two spectroscopic follow-up programs at the GTC telescope, confirming ~89% of the targets as line-emitting sources, with ~64% being genuine z~2.2 QSOs. We extend the 2.2 2 AGN/QSOs (likely dominant in our samples) are described by a structurally different LF than z>2 star-forming LAEs, namely with L^*_QSOs ~ 100 L^*_LAEs and Phi^*_QSOs ~ 10^-3 Phi^*_LAEs. Finally, our method identifies very efficiently as high-z line-emitters sources without previous spectroscopic confirmation, currently classified as stars (~2000 objects in each redshift bin, on average). Assuming a large predominance of Lya-emitting AGN/QSOs in our samples, this supports the scenario by which these are the most abundant class of z>2 Lya emitters at L_Lya>10^43.3 erg/s.

astro-ph.GA↗

arXiv2020

Asymmetric Surface Brightness Structure of Caustic Crossing Arc in SDSS J1226+2152: A Case for Dark Matter Substructure

We study the highly magnified arc SGAS J122651.3+215220 caused by a star-forming galaxy at $z_s=2.93$ crossing the lensing caustic cast by the galaxy cluster SDSS J1226+2152 ($z_l=0.43$), using Hubble Space Telescope observations. We report in the arc several asymmetric surface brightness features whose angular separations are a fraction of an arcsecond from the lensing critical curve and appear to be highly but unequally magnified image pairs of underlying compact sources, with one brightest pair having clear asymmetry consistently across four filters. One explanation of unequal magnification is microlensing by intracluster stars, which induces independent flux variations in the images of individual or groups of source stars in the lensed galaxy. For a second possibility, intracluster dark matter subhalos invisible to telescopes effectively perturb lensing magnifications near the critical curve and give rise to persistently unequal image pairs. Our modeling suggests, at least for the most prominent identified image pair, that the microlensing hypothesis is in tension with the absence of notable asymmetry variation over a six-year baseline, while subhalos of $\sim 10^6$--$10^8\,M_\odot$ anticipated from structure formation with Cold Dark Matter typically produce stationary and sizable asymmetries. We judge that observations at additional times and more precise lens models are necessary to stringently constrain temporal variability and robustly distinguish between the two explanations. The arc under this study is a scheduled target of a Director's Discretionary Early Release Science program of the James Webb Space Telescope, which will provide deep images and a high-resolution view with integral field spectroscopy.

astro-ph.GA↗

arXiv2019

Gravitational Lensing Signatures of Axion Dark Matter Minihalos in Highly Magnified Stars

Axions are a viable candidate for Cold Dark Matter (CDM) which should generically form minihalos of sub-planetary masses from white-noise isocurvature density fluctuations if the Peccei-Quinn phase transition occurs after inflation. Despite being denser than the larger halos formed out of adiabatic fluctuations from inflation, axion minihalos have surface densities much smaller than the critical value required for gravitational lensing to produce multiple images or high magnification, and hence are practically undetectable as lenses in isolation. However, their lensing effect can be enhanced when superposed near critical curves of other lenses. We propose a method to detect them through photometric monitoring of recently discovered caustic transiting stars behind cluster lenses, under extreme magnification factors $μ\gtrsim 10^3$--$10^4$ as the lensed stars cross microlensing caustics induced by intracluster stars. For masses of the first gravitationally collapsed minihalos in the range $\sim 10^{-15}$--$10^{-8}\,h^{-1}\,M_\odot$, we show that axion minihalos in galaxy clusters should collectively produce subtle surface density fluctuations of amplitude $\sim 10^{-4}$--$10^{-3}$ on projected length scales of $\sim 10$--$10^4\,$AU, which imprint irregularities in the microlensing light curves of caustic transiting stars. We estimate that, inside a cluster halo and over the age of the Universe, most of these minihalos are likely to avoid dynamic disruption by encounters with stars or other minihalos.

astro-ph.CO↗

arXiv2019

Highly Magnified Stars in Lensing Clusters: New Evidence in a Galaxy Lensed by MACS J0416.1-2403

We examine a caustic-straddling arc at $z=0.9397$ in the field of the galaxy cluster MACS J0416.1-2403 ($z=0.397$) using archival multiband HST images and show that its surface brightness exhibits anomalies that can be explained by a single highly magnified star undergoing microlensing. First, we show that the surface brightness pattern is not perfectly symmetric across the cluster critical curve, which is inconsistent with a locally smooth lens model; the location of the candidate star exhibits the most significant asymmetry. Second, our analysis indicates that the asymmetric feature has $\sim 30\%$ higher flux in the 2012 visits compared to the Frontier Fields program visits in 2014. Moreover, the variable asymmetric feature shows an anomalous color between the F814W and F105W filters in 2014. These anomalies are naturally explained by microlensing induced variability of a caustic-transiting blue supergiant in a star-forming region, with a mean magnification factor around $μ\sim 200$. We extend this study to a statistical analysis of the whole arc image and find tentative evidence of the increased mismatch of the two images in the proximity of the critical line. Robust detection of one or multiple caustic-transiting stars in this arc will enable detailed follow-up studies that can shed light on the small-scale structure of the dark matter inside the cluster halo.

astro-ph.GA↗

arXiv2018

Probing Dark Matter Subhalos in Galaxy Clusters Using Highly Magnified Stars

Luminous stars in background galaxies straddling the lensing caustic of a foreground galaxy cluster can be individually detected due to extreme magnification factors of $\sim 10^2$--$10^3$, as recently observed in deep HST images. We propose a direct method to probe the presence of dark matter subhalos in galaxy clusters by measuring the astrometric perturbation they induce on the image positions of magnified stars or bright clumps: lensing by subhalos breaks the symmetry of a smooth critical curve, traced by the midpoints of close image pairs. For the giant arc at $z = 0.725$ behind the lensing cluster Abell 370 at $z = 0.375$, a promising target for detecting image pairs of stars, we find that subhalos of masses in the range $10^6$--$10^8\,M_\odot$ with the abundance predicted in the cold dark matter theory should typically imprint astrometric distortions at the level of $20$--$80\,{\rm mas}$. We estimate that $\sim 10\,$hr integrations with JWST at $\sim 1$--$3\,μ{\rm m}$ may uncover several magnified stars whose image doublets will reveal the subhalo-induced structures of the critical curve. This method can probe a dynamic range in the subhalo to cluster halo mass ratio $m/M \sim 10^{-7}$--$10^{-9}$, thereby placing new constraints on the nature of dark matter.

astro-ph.CO↗