arXiv subjects

Charlie T. Mpetha

Publications and source records attributed to Charlie T. Mpetha.

10 recordsLinked to original sources

arXiv2026

Measuring satellite galaxy subhalo masses in redMaPPer clusters with UNIONS weak lensing data

In this work, we present the observed galaxy-galaxy lensing signal of satellite galaxies in rich clusters obtained from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). Such observations are crucial for understanding dark matter halo physics and interaction dynamics in the cluster environment. Our goal is to investigate the tidal stripping of cluster subhalos. Theoretical predictions, supported by recent studies, suggest that as satellite galaxies fall into clusters, their dark matter halos are stripped and dispersed into the cluster's main halo, while their stellar mass remains relatively intact. A robust method to probe this phenomenon is through statistical measurements of the excess surface mass density of satellite galaxies at different cluster-centric distances. Our results reveal a significant lensing signal, with strong statistical power from the large number of lens-source pairs. A simple model used in the fitting process to constrain subhalo masses effectively reproduces the observed lensing signal at relevant satellite-centric distances. However, it lacks the complexity to accurately fit the excess surface density at larger distances, where the host cluster halo dominates. We measure the subhalo-to-stellar mass ratio as a function of cluster-centric radius and find a strong positive trend. These results confirm tidal stripping or other environmental processes that impact the relative masses of a satellite galaxy's stellar component and its dark matter halo.

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arXiv2026

Revisiting the 'Lensing is Low' Problem with UNIONS

We present new measurements of the galaxy-galaxy lensing (GGL) signal around Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies using background sources from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). With high-quality imaging of background sources and a survey overlap of approximately 2650 square degrees, we obtain precise large-scale GGL measurements. Building on these new measurements, we revisit the so-called 'lensing is low' problem, wherein galaxy-halo connection models calibrated on galaxy clustering (GC) data over-predict the GGL signal by 20-40% assuming CMB-based cosmological parameters. We model the galaxy-halo connection using a halo occupation distribution (HOD), and perform joint fits to both GGL and GC signals across a wide range of scales, as well as a GC-only fit. In contrast to previous work, we do not find a significant 'lensing is low' effect in the CMASS sample, although the best joint fits are achieved by decreasing the amplitude of the matter power spectrum slightly relative to the Planck cosmological parameters. Overall, we find that two models describe our observables similarly well: one where HOD and cosmological parameters are free, and one where HOD, cosmological, and feedback parameters are free. Importantly, we emphasise the role of large scales in constraining the lensing is low effect, shifting the narrative away from an exclusively small-scale issue.

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arXiv2026

Tracing the dynamical states and mass accretion histories of galaxy clusters in IllustrisTNG

As the largest and most recently formed stage of hierarchical structure, present-day galaxy clusters are predicted to have a broad range of late-time assembly histories. This diversity may explain much of the scatter in scaling relations and other cluster properties. Observationally, systems with more or less recent accretion should appear as unrelaxed and relaxed clusters, respectively. However, it is unclear which of the many possible structural measures best correlate with assembly history. Using the IllustrisTNG simulations, we explore the correlation between structural parameters and assembly history. To assess the effectiveness of different structural selection criteria, we define subsamples of the most and least relaxed clusters based on the values of various intrinsic, projected, and stellar structural parameters, and then compare the median assembly history of the subsamples in each case. We find that several observable quantities, including the magnitude gap between the brightest galaxies and the asymmetry of the stellar mass distribution, are very effective in selecting cluster samples with more or less recent accretion, even when applied in projection. Given the strong correlations between assembly history and present-day cluster structure, we suggest that structural classification be included explicitly in any analysis of catalogue completeness, scaling relations, or mean density profiles.

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arXiv2025

Cluster properties as a function of dynamical state in the DESI Legacy x UNIONS surveys

We investigate how the dynamical state of galaxy clusters influences their galaxy populations and mass distributions. Using photometrically selected clusters from the DESI Legacy Imaging Survey cross-matched with the UNIONS galaxy shear catalogue, we classify clusters as evolved or evolving based on their rest-frame r-band magnitude gaps and stellar mass ratios between the brightest cluster galaxies (BCGs) and bright satellites. We measure the stellar mass functions, weak-lensing profiles, and radial number density and red-fraction profiles of stacked clusters in both subsamples. Evolved clusters exhibit more concentrated lensing profiles, bimodal stellar mass functions dominated by massive BCGs, and a deficit of intermediate-mass satellites, while evolving clusters show flatter central lensing signals and an excess of massive satellites. Applying the same selection to IllustrisTNG clusters reproduces these trends and links the observed differences to distinct mass accretion histories. These results demonstrate the close link between cluster galaxy populations and the overall dynamical state of their underlying dark matter halo.

astro-ph.GA↗

arXiv2025

Cosmology from UNIONS weak lensing profiles of galaxy clusters

Cosmological information is encoded in the structure of galaxy clusters. In Universes with less matter and larger initial density perturbations, clusters form earlier and have more time to accrete material, leading to a more extended infall region. Thus, measuring the mean mass distribution in the infall region provides a novel cosmological test. The infall region is largely insensitive to baryonic physics, and provides a cleaner structural test than other measures of cluster assembly time such as concentration. We consider cluster samples from three publicly available galaxy cluster catalogues: the Spectroscopic Identification of eROSITA Sources (SPIDERS) catalogue, the X-ray and Sunyaev-Zeldovich effect selected clusters in the meta-catalogue M2C, and clusters identified in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey. Using a preliminary shape catalogue from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we derive excess surface mass density profiles for each sample. We then compare the mean profile for the DESI Legacy sample, which is the most complete, to predictions from a suite of simulations covering a range of $Ω_{\rm m}$ and $σ_8$, obtaining constraints of $Ω_{\rm m}=0.34\pm 0.06$ and $σ_8=0.77 \pm 0.04$. We also measure mean (comoving) splashback radii for SPIDERS, M2C and DESI Legacy Imaging Survey clusters of $1.39^{+0.21}_{-0.18} {\rm cMpc}$, $1.77^{+0.20}_{-0.18} {\rm cMpc}/h$ and $1.42^{+0.11}_{-0.12} {\rm cMpc}/h$ respectively. Performing this analysis with the final UNIONS shape catalogue and the full sample of spectroscopically observed clusters in DESI, we can expect to improve on the best current constraints from cluster abundance studies by a factor of 2 or more.

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arXiv2025

UNIONS: The Ultraviolet Near-Infrared Optical Northern Survey

The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a "collaboration of collaborations" that is using the Canada-France-Hawai'i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain $ugriz$ images of a core survey region of 6250 deg$^2$ of the northern sky. The $10σ$ point source depth of the data, as measured within a 2-arcsecond diameter aperture, are $[u,g,r,i,z] = [23.7, 24.5, 24.2, 23.8, 23.3]$\ in AB magnitudes. UNIONS is addressing some of the most fundamental questions in astronomy, including the properties of dark matter, the growth of structure in the Universe from the very smallest galaxies to large-scale structure, and the assembly of the Milky Way. It is set to become the major ground-based legacy survey for the northern hemisphere for the next decade and provides an essential northern complement to the static-sky science of the Vera C. Rubin Observatory's Legacy Survey of Space and Time. UNIONS supports the core science mission of the {\it Euclid} space mission by providing the data necessary in the northern hemisphere for the calibration of the wavelength dependence of the {\it Euclid} point-spread function and derivation of photometric redshifts in the North Galactic Cap. This region contains the highest quality sky for {\it Euclid}, with low backgrounds from the zodiacal light, stellar density, extinction, and emission from Galactic cirrus. Here, we describe the UNIONS survey components, science goals, data products, and the current status of the overall program.

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arXiv2024

The infall region as a complementary probe to cluster abundance

Galaxy cluster abundance measurements provide a classic test of cosmology. They are most sensitive to the evolved amplitude of fluctuations, usually expressed as $S_8 = σ_8\sqrt{Ω_m/0.3}$. Thus, abundance constraints exhibit a strong degeneracy between $σ_8$ and $Ω_{\rm m}$, as do other similar low-redshift tests such as cosmic shear. The mass distribution in the infall region around galaxy clusters, where material is being accreted from the surrounding field, also exhibits a cosmological dependence, but in this case it is nearly orthogonal to the $S_8$ direction in the $Ω_m$--$σ_8$ plane, making it highly complementary to halo abundance or cosmic shear studies. We explore how weak lensing measurements of the infall region might be used to complement abundance studies, considering three different tests. The splashback radius is a prominent feature of the infall region; we show that detection of this feature in lensing data from the Euclid survey could independently constrain $Ω_{\rm m}$ and $σ_8$ to $\pm 0.05$. Another feature, the depletion radius where the bias reaches a minimum, also shows cosmological dependence, though it is challenging to observe in practice. The strongest constraints come from direct measurements of the shear profile in the infall region at $2$--$4\,r_{200{\rm c}}$. Combining the latter with abundance constraints such as those reported from SRG$/$eROSITA should reduce the area of the error contours by an estimated factor of $1.2$ using a sample of clusters observed by the UNIONS survey, or a factor of $3$ using clusters observed by the Euclid Wide survey over a broader range of redshift.

astro-ph.CO↗

arXiv2024

Impact of weak lensing on bright standard siren analyses

Gravitational waves from binary mergers at cosmological distances will experience weak lensing by large scale structure. This causes a (de-)magnification, $μ$, of the wave amplitude, and a degenerate modification to the inferred luminosity distance $d_L$. To address this the uncertainty on $d_L$ is increased according to the dispersion of the magnification distribution at the source redshift, $σ_μ$. But this term is dependent on cosmological parameters that are being constrained by gravitational wave "standard sirens", such as the Hubble parameter $H_0$, and the matter density fraction $Ω_m$. $σ_μ$ is also sensitive to the resolution of the simulation used for its calculation. Tension in the measured value of $H_0$ from independent datasets, and the present use of outdated cosmological simulations, suggest $σ_μ$ could be underestimated. We consider two classes of standard siren, supermassive black hole binary and binary neutron star mergers. Underestimating $H_0$ and $Ω_m$ when calculating $σ_μ$ increases the probability of finding a residual lensing bias on these parameters greater than $1σ$ by 1.5-3 times. Underestimating $σ_μ$ by using low resolution/small sky-area simulations can also significantly increase the probability of biased results. For neutron star mergers, the spread of possible biases is 0.25 km/s/Mpc, comparable to the forecasted uncertainty. Left uncorrected this effect limits the use of BNS mergers for precision cosmology. For supermassive black hole binaries, the spread of possible biases on $H_0$ is significant, 5 km/s/Mpc, but $O(200)$ observations are needed to reduce the variance below the bias. To achieve accurate sub-percent level precision on cosmological parameters using standard sirens, first much improved knowledge on the form of the magnification distribution and its dependence on cosmology is needed.

astro-ph.CO↗

arXiv2024

Constraining cosmological parameters using the splashback radius of galaxy clusters

Cosmological parameters such as $Ω_{\rm{M}}$ and $σ_{8}$ can be measured indirectly using various methods, including galaxy cluster abundance and cosmic shear. These measurements constrain the composite parameter $S_{8}$, leading to degeneracy between $Ω_{\rm{M}}$ and $σ_{8}$. However, some structural properties of galaxy clusters also correlate with cosmological parameters, due to their dependence on a cluster's accretion history. In this work, we focus on the splashback radius, an observable cluster feature that represents a boundary between a cluster and the surrounding Universe. Using a suite of cosmological simulations with a range of values for $Ω_{\rm{M}}$ and $σ_{8}$, we show that the position of the splashback radius around cluster-mass halos is greater in cosmologies with smaller values of $Ω_{\rm{M}}$ or larger values of $σ_{8}$. This variation breaks the degeneracy between $Ω_{\rm{M}}$ and $σ_{8}$ that comes from measurements of the $S_{8}$ parameter. We also show that this variation is, in principle, measurable in observations. As the splashback radius can be determined from the same weak lensing analysis already used to estimate $S_{8}$, this new approach can tighten low-redshift constraints on cosmological parameters, either using existing data, or using upcoming data such as that from Euclid and LSST.

astro-ph.CO↗

arXiv2024

Future prospects on testing extensions to $Λ$CDM through the weak lensing of gravitational waves

With planned space-based and 3rd generation ground-based gravitational wave detectors (LISA, Einstein Telescope, Cosmic Explorer), and proposed DeciHz detectors (DECIGO, Big Bang Observer), it is timely to explore statistical cosmological tests that can be employed with the forthcoming plethora of data, $10^4-10^6$ mergers a year. We forecast the combination of the standard siren measurement with the weak lensing of gravitational waves from binary mergers. For 10 years of 3rd generation detector runtime, this joint analysis will constrain the dark energy equation of state with marginalised $1σ$ uncertainties of $σ(w_0)$~0.005 and $σ(w_a)$~0.04. This is comparable to or better than forecasts for future galaxy/intensity mapping surveys, and better constraints are possible when combining these and other future probes with gravitational waves. We find that combining mergers with and without an electromagnetic counterpart helps break parameter degeneracies. Using DeciHz detectors in the post-LISA era, we demonstrate for the first time how merging binaries could achieve a precision on the sum of neutrino masses of $σ(Σm_ν)$~0.05 eV using $3\times10^6$ sources up to $z=3.5$ with a distance uncertainty of $1\%$, and ~percent or sub-percent precision also on curvature, dark energy, and other parameters, independently from other probes. Finally, we demonstrate how the cosmology dependence in the redshift distribution of mergers can be exploited to improve dark energy constraints if the cosmic merger rate is known, instead of relying on measured distributions as is standard in cosmology. In the coming decades gravitational waves will become a formidable probe of both geometry and large scale structure.

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