arXiv subjects

Jenny Wagner

Publications and source records attributed to Jenny Wagner.

At least 19 recordsLinked to original sources

arXiv2026

How much dark matter really matters?

Strong gravitational lensing is a key probe to trace dark matter. It assumes that mass curves spacetime so that light from a background source is deflected on its way to the observer. If dark matter contributes the major part to a massive cosmic structure, reconstructing the latter from strong-lensing observables allows us to infer characteristics of dark matter. Standard reconstructions fit a pre-defined mass-density model to the data. In this essay, I show how these mass models over-estimate the dark-matter contents of light-deflecting masses. Eliminating these models from the reconstruction reveals that observations directly constrain local properties of light-deflecting masses. How much dark matter is really needed in strong-gravitational-lensing effects and how much do we make up by our model choices?

gr-qc↗

arXiv2026

Hamilton's Object Revisited: A challenging source redshift of a strong lensing configuration

Low-resolution spectrographs used to have difficulties in determining redshifts of galaxies at $z\approx1$ and $z\approx3$. Spectral emission and absorption lines of magnesium and iron redshifted to $z\approx1$ fall close to hydrogen, silicon, and oxygen lines at $z\approx3$. Here, we demonstrate that even with modern, integrated field unit spectrographs, this task remains challenging. Hamilton's Object, a blue star-forming galaxy, gravitationally lensed into three multiple images by the galaxy cluster SDSS J223010.47-081017.8, is such a case. Using the Blue Keck Cosmic Web Imager (KCWI) its redshift was determined as $z=0.82$, while its MOIRCS spectrum hinted at $z=3.201$. To resolve the ambiguity, we completely reanalyse the Blue KCWI spectra of all three multiple images including the star-forming region in the outskirts. We employ a new data reduction pipeline, PypeIt, signal enhancement, and line fitting by Python-routines. The reevaluation confirms the previous result based on six absorption features, $z=0.820 \pm 0.001$, and four emission features, $z=0.821 \pm 0.002$. The alternative $z=3.199\pm 0.003$, based on six absorption and two emission lines, is a worse fit, also compared to other spectra. Moreover, we find the MOIRCS spectrum inconclusive: observations cover two of three multiple images, with the slit for image C only covering its central bulge; furthermore the pixel-to-wavelength calibration requires a nightsky emission-line calibration due to a missing calibration arc lamp. New MOIRCS observations are needed to verify that Hamilton's Object has the smallest separation in angular diameter distance between lensing cluster and source galaxy among the known cluster-scale strong lenses.

astro-ph.GA↗

arXiv2025

The Binary Ballet: Mapping Local Expansion Around M81 & M82

This study of the M81 complex and its Hubble flow delivers new and improved Tip of the Red Giant Branch (TRGB)-based distances for nine member galaxies, yielding a total of 58 galaxies with high-precision TRGB distances. With those, we perform a systematic analysis of the group's dynamics in the core and its embedding in the local cosmic environment. Our analysis confirms that the satellite galaxies of the M81 complex exhibit a flattened, planar distribution almost perpendicular to the supergalactic pole and thus aligned with a larger-scale filamentary structure in the Local Universe. We demonstrate that the properties of the group's barycentre are robustly constrained by the two brightest members, M81 and M82, and that correcting heliocentric velocities for the solar motion in the Local Group decreases the velocity dispersion of the group. Then applying minor and major infall models, we fit the local Hubble flow to constrain the Hubble Constant and the total mass of the M81 complex. The joint best-fit parameters from both models yield $H_0 = \left(63 \pm 6 \right)$ km/s/Mpc and total mass of $(2.28\pm 0.49) \times 10^{12} M_{\odot}$. We thus arrive at an increased mass estimate compared to prior work but reach a higher consistency with virial, $(2.74 \pm 0.36)\times 10^{12}\,M_\odot$, and projected-mass estimates, $(3.11 \pm 0.69)\times 10^{12} M_\odot$. Moreover, our $H_0$ estimate shows an agreement with Planck, consistent with other TRGB-based Local-Universe inferences of $H_0$ and still within a 2-$σ$ agreement with Cepheid-based Local-Universe probes.

astro-ph.CO↗

arXiv2025

Galaxy infall models for arbitrary velocity directions

For most galaxies in the cosmos, our knowledge of their motion is limited to line-of-sight velocities from redshift observations. To determine the radial velocity between two galaxies the minor and major infall models were established by Karachentsev & Kashibadze (2006). Regardless of the background cosmology, our derivations reveal that these infall models approximate the total radial velocity between two galaxies by two different projections employing different information about the system. For galaxies having small angular separations $θ$, all infall models agree that the radial velocity is the difference of their line-of-sight components. Applying these models to ca. $500$ halos of the Illustris-3 simulation, we find the perpendicular and tangential velocity parts to be non-negligible for more than 90% of all, more than 5000 infalling subhalos. Thus, even for $θ< 10$ deg, the infall-model velocities deviate from the true radial velocity. Only for 30% we found the true one lay between the minor and major infall velocity. However, the infall models yield robust upper and lower bounds to the true radial velocity dispersion. Observed under $θ< 10$ deg the velocity dispersion inferred from the sole difference of line-of-sight velocity components even coincides with the true one, justifying this approach for high-redshift groups and clusters. Based on these findings, we predict the radial velocity dispersion of the M81-group from the minor infall model (upper bound) $σ_{\mathrm{r,min}} = (180 \pm 42)~\mbox{km}/\mbox{s}$, from the major infall model (lower bound) $σ_{\mathrm{r,maj}} = (142 \pm 64) ~\mbox{km}/\mbox{s}$ and $σ_\mathrm{r,Δv} = (99 \pm 36)~\mbox{km}/\mbox{s}$ from the line-of-sight-velocity difference.

astro-ph.GA↗

arXiv2025

Unveiling the Coma Cluster Structure: From the Core to the Hubble Flow

The Coma cluster, embedded in a cosmic filament, is a complex and dynamically active structure in the local Universe. Applying a density-based member selection dbscan to data from the Sloan Digital Sky Survey (SDSS), we identify cluster member galaxies from its virialised core out to the zero-velocity boundary in the least model-dependent way. From dbscan, we infer a projected virial radius of $r_{\rm vir} = \left(1.95 \pm 0.12\right)\,h^{-1}~\text{Mpc}$ and projected zero-velocity radius of $r_{\rm ta} \geq 4.87~{h}^{-1}~\mbox{Mpc}$. Assuming that the barycentre of Coma has zero peculiar velocity, its distance from us is $r_\mathrm{c}=(69.959 \pm 0.012_\mathrm{stat}) \, h^{-1}~\text{Mpc}$ determined from the redshifts of 1092 member galaxies. Cross-correlating with the Cosmicflows-4 (CF4) catalogue enables a velocity-distance analysis. This reveals, for the first time, the Hubble flow surrounding Coma, a first step to investigate the entanglement between Coma's dark matter halo and the dark energy driving the expansion of the surroundings. If $v_\mathrm{c}$ is moving with the cosmic expansion, the CF4 distances yield a Hubble constant $H_0 = (73 \pm {1_\mathrm{stat} \pm 7_\mathrm{sys}})~\mbox{km}/\mbox{s}/\mbox{Mpc}$ with a dominating systematic error from different calibrations for the distance moduli. Mass estimates via caustics, the virial theorem, and the Hubble-flow method yield $M = [0.77, 2.0] \times 10^{15}\,h^{-1}\,M_{\odot}$ consistent with prior mass estimates. Our mass estimates are based on fewer model assumptions in the member selection and require $\sim20\%$ members to attain the same precision. Our approach maps the structure of Coma into its Hubble flow and shows degeneracies between the Hubble constant, the virial radius, and the total mass only using data and models from the single line-of-sight towards Coma.

astro-ph.CO↗

arXiv2024

Observables of super-extremal black holes: challenging Cosmic Censorship to comprehend the Cosmological Constant

Einstein's Field Equations have proven applicable across many scales, from black holes to cosmology. Even the mysterious Cosmological Constant found a physical interpretation in the so-called ``dark energy'' causing the accelerated cosmic expansion as inferred from multiple observables. Yet, we still lack a material source for this dark fluid. Probing the local universe to find it yields complementary information to the one from the cosmic microwave background. Could dark energy be sourced by super-extremal charged black holes? Contrary to intuition, such objects could exist with only weak observational signatures. The latter are introduced here to outline how sky surveys can identify individual candidates which challenge Cosmic Censorship on the one hand but may explain the physical origin of the Cosmological Constant on the other.

gr-qc↗

arXiv2024

Strong Lensing by Galaxy Clusters

Galaxy clusters as gravitational lenses play a unique role in astrophysics and cosmology: they permit mapping the dark matter distribution on a range of scales; they reveal the properties of high and intermediate redshift background galaxies that would otherwise be unreachable with telescopes; they constrain the particle nature of dark matter and are a powerful probe of global cosmological parameters, like the Hubble constant. In this review we summarize the current status of cluster lensing observations and the insights they provide, and offer a glimpse into the capabilities that ongoing, and the upcoming next generation of telescopes and surveys will deliver. While many open questions remain, cluster lensing promises to remain at the forefront of discoveries in astrophysics and cosmology.

astro-ph.CO↗

arXiv2024

Essentials of strong gravitational lensing

Of order one in 10^3 quasars and high-redshift galaxies appears in the sky as multiple images as a result of gravitational lensing by unrelated galaxies and clusters that happen to be in the foreground. While the basic phenomenon is a straightforward consequence of general relativity, there are many non-obvious consequences that make multiple-image lensing systems (aka strong gravitational lenses) remarkable astrophysical probes in several different ways. This article is an introduction to the essential concepts and terminology in this area, emphasizing physical insight. The key construct is the Fermat potential or arrival-time surface: from it the standard lens equation, and the notions of image parities, magnification, critical curves, caustics, and degeneracies all follow. The advantages and limitations of the usual simplifying assumptions (geometrical optics, small angles, weak fields, thin lenses) are noted, and to the extent possible briefly, it is explained how to go beyond these. Some less well-known ideas are discussed at length: arguments using wavefronts show that much of the theory carries over unchanged to the regime of strong gravitational fields; saddle-point contours explain how even the most complicated image configurations are made up of just two ingredients. Orders of magnitude, and the question of why strong lensing is most common for objects at cosmological distance, are also discussed. The challenges of lens modeling, and diverse strategies developed to overcome them, are discussed in general terms, without many technical details.

astro-ph.CO↗

arXiv2024

Dark Energy as a Critical Period in Binary Motion: Bounds from Multi-scale Binaries

The two-body problem under the influence of both dark energy and post-Newtonian modifications is studied. In this unified framework, we demonstrate that dark energy plays the role of a critical period with $T_Λ = 2π/c \sqrtΛ \approx 60~\text{Gyr}$. We also show that the ratio between orbital and critical period naturally emerges from the Kretschmann scalar, which is a quadratic curvature invariant characterizing all binary systems effectively represented by a de Sitter-Schwarzschild spacetime. The suitability of a binary system to constrain dark energy is determined by the ratio between its Keplerian orbital period $T_\text{K}$ and the critical period $T_Λ$. Systems with $T_\text{K} \approx T_Λ$ are optimal for constraining the cosmological constant $Λ$, such as the Local Group and the Virgo Cluster. Systems with $T_{\text{K}} \ll T_Λ$ are dominated by attractive gravity (which are best suited for studying modified gravity corrections). Systems with $T_{\text{K}} \gg T_Λ$ are dominated by repulsive dark energy and can thus be used to constrain $Λ$ from below. We use our unified framework of post-Newtonian and dark-energy modifications to calculate the precession of bounded and unbounded astrophysical systems and infer constraints on $Λ$ from them. Pulsars, the solar system, S stars around Sgr A*, the Local Group, and the Virgo Cluster, having orbital periods of days to gigayears, are analyzed. The results reveal that the upper bound on the cosmological constant decreases when the orbital period of the system increases, emphasizing that $Λ$ is a critical period in binary motion.

astro-ph.CO↗

arXiv2023

Observables for moving, stupendously charged and massive primordial black holes

Stupendously large black holes exceeding $10^{11} M_\odot$ could exist, supported by recent observations of unexpectedly massive black holes at high redshifts. These objects may constitute a part of dark matter or even dark energy. One possibility to explain the cosmic accelerated expansion could be to consider charged black holes whose mutual repulsion overcomes their gravitational attraction. However, the extreme charge required turns these black holes into naked singularities, whose existence is questioned by the cosmic censorship hypothesis. Since the latter is driven by theoretical assumptions, we work out the most promising observables which are least cosmology-dependent to test their existence. We derive the electro-magnetic and gravitational lensing effects caused by such extreme objects at distances much larger than their extent to investigate possible ways for a discovery. Restricting searches to black holes between $10^{12}$ to $10^{14} M_\odot$, we show that such objects do not cause totally disruptive catastrophes, like dissociation of neutral hydrogen clouds or proton decay induced by strong electro-magnetic fields. Einstein rings of the order of 10" and rotation measures of plasma clouds subject to the magnetic fields induced by the moving black holes are identified as optimum observable signatures for now. Future space-based black-hole telescopes will follow up on these candidates and finally check the cosmic censorship hypothesis by their strong-field strong-lensing signatures, like an additional sub-arcsecond inner Einstein ring. Observable effects are so surprisingly moderate that a violation of cosmic censorship is hard to detect and even explaining cosmic expansion with moving naked singularities might be possible.

astro-ph.CO↗

arXiv2023

Solving the Hubble tension à la Ellis & Stoeger 1987

The discrepancy between the value of the Hubble constant $H_0$ in the late, local universe and the one obtained from the Planck collaboration representing an all-sky value for the early universe reached the 5-$σ$ level. Approaches to alleviate the tension contain a wide range of ansatzes: increasing uncertainties in data acquisition, reducing biases in the astrophysical models that underly the probes, or taking into account observer-dependent variances in the parameters of the cosmological background model. Yet, early and late universe probes are often treated as independent, they live on different length scales, and require different perturbations to be subtracted. Hence, fitting a flat Friedmann-Lemaître-Robertson-Walker cosmology to different probes at different cosmic epochs can yield different sets of cosmological parameter values. Tensions arise if these background fits and perturbing biases are not consistently calibrated or synchronised with respect to each other. This consistent model-fitting calibration is lacking between the two $H_0$ values mentioned above, thus causing a tension. As shown here, this interpretation resolves the $H_0$tension, if 15% of the matter-density parameter obtained from the fit to the cosmic microwave background, $Ω_m = 0.315$, are assigned to decoupled perturbations yielding $Ω_m = 0.267$ for the fit at redshifts of the supernova observations. Existing theoretical analyses and data evaluations which support this solution are given.

astro-ph.CO↗

arXiv2023

Much ado about no offset -- Characterising the anomalous multiple-image configuration and the model-driven displacement between light and mass in the multi-plane strong lens Abell 3827

Abell 3827 is a unique galaxy cluster with a dry merger in its core causing a highly-resolved multiple-image configuration of a blue spiral galaxy at $z_\mathrm{s}=1.24$. The surface brightness profiles of four merging galaxies around $z_\mathrm{d}=0.099$ complicate a clear identification of the number of images and finding corresponding small-scale features across them. The entailed controversies about offsets between luminous and dark matter have never been settled and dark-matter characteristics in tension with bounds from complementary probes and simulations seemed necessary to explain this multiple-image configuration. We resolve these issues with a systematic study of possible feature matchings across all images and their impact on the reconstructed mass density distribution. From the local lens properties directly constrained by these feature matchings without imposing any global lens model, we conclude that none of them are consistent with expected local characteristics from standard single-lens-plane lensing, nor can they be motivated by the light distribution in the cluster. Inspecting complementary spectroscopic data, we show that all these results originate from an insufficient constraining power of the data and seem to hint at a thick lens and not at exotic forms of dark matter or modified gravity. If the thick-lens hypothesis can be corroborated with follow-up multi-plane lens modelling, A3827 suffers from a full three-dimensional degeneracy in the distribution of dark matter because combinations of shearings and scalings in a single lens plane can also be represented by an effective shearing and a rotation caused by multiple lens planes.

astro-ph.CO↗

arXiv2023

Is the Observable Universe Consistent with the Cosmological Principle?

The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive $Λ$-Cold-Dark-Matter ($Λ$CDM) model. Yet, tensions have emerged within the $Λ$CDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, $H_0$. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the $H_0$ tension may extend beyond $Λ$CDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.

astro-ph.CO↗

arXiv2022

Generalised model-independent characterisation of strong gravitational lenses VII: impact of source properties and higher-order lens properties on the local lens reconstruction

We investigate the impact of higher-order gravitational lens properties and properties of the background source on our approach to directly infer local lens properties from observables in multiple images of strong gravitationally lensed extended, static background sources developed in papers I to VI. As the degeneracy between local lens and source properties only allows to determine relative local lens properties between the multiple image positions, we cannot distinguish common scalings and distortions caused by lensing from intrinsic source characteristics. The consequences of this degeneracy for lens modelling and our approach and ways to break it are detailed here. We also set up quantitative measures around the critical curve to find clear limits on the validity of the approximation that source properties are negligible to infer local lens properties at critical points. The impact of the source on the local lens properties depends on the reduced shear at the image position and the amplitude and orientation of the source ellipticity, as we derive in this paper. Similarly, we investigate the role of third-order lens properties (flexion), in two galaxy-cluster simulations and in the Lenstool-reconstruction of the galaxy-cluster lens CL0024. In all three cases, we find that flexion is negligible in over 90% of all pixels of the lensing region for our current imprecision of local lens properties of about 10%. Decreasing the imprecision to 2%, higher-order terms start to play a role, especially in regions with shear components close to zero.

astro-ph.CO↗

arXiv2022

Generalised model-independent characterisation of strong gravitational lenses VIII. automated multi-band feature detection to constrain local lens properties

As established in previous papers of this series, observables in highly distorted and magnified multiple images caused by the strong gravitational lensing effect can be used to constrain the distorting properties of the gravitational lens at the image positions. If the background source is extended and contains substructure, like star forming regions, which is resolved in multiple images, all substructure that can be matched across a minimum of three multiple images can be used to infer the local distorting properties of the lens. In this work, we replace the manual feature selection by an automated feature extraction based on SExtractor for Python and show its superior performance. Despite its aimed development to improve our lens reconstruction, it can be employed in any other approach, as well. Valuable insights on the definition of an `image position' in the presence of noise are gained from our calibration tests. Applying it to observations of a five-image configuration in galaxy cluster CL0024 and the triple-image configuration containing Hamilton's object, we determine local lens properties for multiple wavebands separately. Within current confidence bounds, all of them are consistent with each other, corroborating the wavelength-independence of strong lensing and offering a tool to detect deviations caused by micro-lensing and dust in further examples.

astro-ph.CO↗

arXiv2021

On the double-plane plasma lensing

Plasma lensing is the refraction of low-frequency electromagnetic rays due to cold free electrons in the universe. For sources at a cosmological distance, there is observational evidence of elongated, complex plasma structures along the line of sight requiring a multi-lens-plane description. To investigate the limits of single-plane plasma lensing, we set up a double-plane lens with a projected Gaussian electron density in each lens plane. We compare double-plane scenarios with corresponding effective single-plane configurations. Our results show how double-plane lenses can be distinguished from single-plane lenses by observables, i.e. resolved multiple image positions, relative magnifications, time delays, and pulse shapes. For plasma lensing of fast radio bursts, the observed pulse shape may be dominated by the lensing effect, allowing us to neglect the intrinsic source pulse shape to distinguish different lensing configurations. The time-domain observables turn out to be the most salient features to tell multi- and single-plane lenses apart.

astro-ph.HE↗

arXiv2021

Hamilton's Object -- a clumpy galaxy straddling the gravitational caustic of a galaxy cluster : Constraints on dark matter clumping

We report the discovery of a 'folded' gravitationally lensed image, 'Hamilton's Object', found in a HST image of the field near the AGN SDSS J223010.47-081017.8 ($z=0.62$). The lensed images are sourced by a galaxy at a spectroscopic redshift of 0.8200$\pm0.0005$ and form a fold configuration on a caustic caused by a foreground galaxy cluster at a photometric redshift of 0.526$\pm0.018$ seen in the corresponding Pan-STARRS PS1 image and marginally detected as a faint ROSAT All-Sky Survey X-ray source. The lensed images exhibit properties similar to those of other folds where the source galaxy falls very close to or straddles the caustic of a galaxy cluster. The folded images are stretched in a direction roughly orthogonal to the critical curve, but the configuration is that of a tangential cusp. Guided by morphological features, published simulations and similar fold observations in the literature, we identify a third or counter-image, confirmed by spectroscopy. Because the fold-configuration shows highly distinctive surface brightness features, follow-up observations of microlensing or detailed investigations of the individual surface brightness features at higher resolution can further shed light on kpc-scale dark matter properties. We determine the local lens properties at the positions of the multiple images according to the observation-based lens reconstruction of Wagner et al. (2019). The analysis is in accordance with a mass density which hardly varies on an arc-second scale (6 kpc) over the areas covered by the multiple images.

astro-ph.CO↗

arXiv2020

papaya2: 2D Irreducible Minkowski Tensor computation

A common challenge in scientific and technical domains is the quantitative description of geometries and shapes, e.g. in the analysis of microscope imagery or astronomical observation data. Frequently, it is desirable to go beyond scalar shape metrics such as porosity and surface to volume ratios because the samples are anisotropic or because direction-dependent quantities such as conductances or elasticity are of interest. Minkowski Tensors are a systematic family of versatile and robust higher-order shape descriptors that allow for shape characterization of arbitrary order and promise a path to systematic structure-function relationships for direction-dependent properties. Papaya2 is a software to calculate 2D higher-order shape metrics with a library interface, support for Irreducible Minkowski Tensors and interpolated marching squares. Extensions to Matlab, JavaScript and Python are provided as well. While the tensor of inertia is computed by many tools, we are not aware of other open-source software which provides higher-rank shape characterization in 2D.

cs.GR↗