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Register a new userHamiltons Object -- a clumpy galaxy straddling the gravitational caustic of a galaxy cluster : Constraints on dark matter clumping
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We report the discovery of a folded gravitationally lensed image, Hamiltons 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.
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Cosmological simulations predict that galaxies are embedded into triaxial dark matter haloes, which appear approximately elliptical in projection. Weak gravitational lensing allows us to constrain these halo shapes and thereby test the nature of dark matter. Weak lensing has already provided robust detections of the signature of halo flattening at the mass scales of groups and clusters, whereas results for galaxies have been somewhat inconclusive. Here we combine data from five surveys (NGVSLenS, KiDS/KV450, CFHTLenS, CS82, and RCSLenS) in order to tighten observational constraints on galaxy-scale halo ellipticity for photometrically selected lens samples. We constrain $f_rm{h}$, the average ratio between the aligned component of the halo ellipticity and the ellipticity of the light distribution, finding $f_rm{h}=0.303^{+0.080}_{-0.079}$ for red lenses and $f_rm{h}=0.217^{+0.160}_{-0.159}$ for blue lenses when assuming elliptical NFW density profiles and a linear scaling between halo ellipticity and galaxy ellipticity. Our constraints for red galaxies constitute the currently most significant ($3.8sigma$) systematics-corrected detection of the signature of halo flattening at the mass scale of galaxies. Our results are in good agreement with expectations from the Millennium Simulation that apply the same analysis scheme and incorporate models for galaxy-halo misalignment. Assuming these misalignment models and the analysis assumptions stated above are correct, our measurements imply an average dark matter halo ellipticity for the studied red galaxy samples of $langle|epsilon_rm{h}|rangle=0.174pm 0.046$, where $|epsilon_{h}|=(1-q)/(1+q)$ relates to the ratio $q=b/a$ of the minor and major axes of the projected mass distribution. Similar measurements based on larger upcoming weak lensing data sets can help to calibrate models for intrinsic galaxy alignments. [abridged]
A defining prediction of the cold dark matter (CDM) cosmological model is the existence of a very large population of low-mass haloes. This population is absent in models in which the dark matter particle is warm (WDM). These alternatives can, in principle, be distinguished observationally because halos along the line-of-sight can perturb galaxy-galaxy strong gravitational lenses. Furthermore, the WDM particle mass could be deduced because the cut-off in their halo mass function depends on the mass of the particle. We systematically explore the detectability of low-mass haloes in WDM models by simulating and fitting mock lensed images. Contrary to previous studies, we find that halos are harder to detect when they are either behind or in front of the lens. Furthermore, we find that the perturbing effect of haloes increases with their concentration: detectable haloes are systematically high-concentration haloes, and accounting for the scatter in the mass-concentration relation boosts the expected number of detections by as much as an order of magnitude. Haloes have lower concentration for lower particle masses and this further suppresses the number of detectable haloes beyond the reduction arising from the lower halo abundances alone. Taking these effects into account can make lensing constraints on the value of the mass function cut-off at least an order of magnitude more stringent than previously appreciated.
We study the shapes of galaxy dark matter haloes by measuring the anisotropy of the weak gravitational lensing signal around galaxies in the second Red-sequence Cluster Survey (RCS2). We determine the average shear anisotropy within the virial radius for three lens samples: all galaxies with 19<m_r<21.5, and the `red and `blue samples, whose lensing signals are dominated by massive low-redshift early-type and late-type galaxies, respectively. To study the environmental dependence of the lensing signal, we separate each lens sample into an isolated and clustered part and analyse them separately. We also measure the azimuthal dependence of the distribution of physically associated galaxies around the lens samples. We find that these satellites preferentially reside near the major axis of the lenses, and constrain the angle between the major axis of the lens and the average location of the satellites to <theta>=43.7 deg +/- 0.3 deg for the `all lenses, <theta>=41.7 deg +/- 0.5 deg for the `red lenses and <theta>=42.0 deg +/- 1.4 deg for the `blue lenses. For the `all sample, we find that the anisotropy of the galaxy-mass cross-correlation function <f-f_45>=0.23 +/- 0.12, providing weak support for the view that the average galaxy is embedded in, and preferentially aligned with, a triaxial dark matter halo. Assuming an elliptical Navarro-Frenk-White (NFW) profile, we find that the ratio of the dark matter halo ellipticity and the galaxy ellipticity f_h=e_h/e_g=1.50+1.03-1.01, which for a mean lens ellipticity of 0.25 corresponds to a projected halo ellipticity of e_h=0.38+0.26-0.25 if the halo and the lens are perfectly aligned. For isolated galaxies of the `all sample, the average shear anisotropy increases to <f-f_45>=0.51+0.26-0.25 and f_h=4.73+2.17-2.05, whilst for clustered galaxies the signal is consistent with zero. (abridged)
We assess how much unused strong lensing information is available in the deep emph{Hubble Space Telescope} imaging and VLT/MUSE spectroscopy of the emph{Frontier Field} clusters. As a pilot study, we analyse galaxy cluster MACS,J0416.1-2403 ($z$$=$$0.397$, $M(R<200,{rm kpc})$$=$$1.6$$times$$10^{14}msun$), which has 141 multiple images with spectroscopic redshifts. We find that many additional parameters in a cluster mass model can be constrained, and that adding even small amounts of extra freedom to a model can dramatically improve its figures of merit. We use this information to constrain the distribution of dark matter around cluster member galaxies, simultaneously with the clusters large-scale mass distribution. We find tentative evidence that some galaxies dark matter has surprisingly similar ellipticity to their stars (unlike in the field, where it is more spherical), but that its orientation is often misaligned. When non-coincident dark matter and baryonic halos are allowed, the model improves by 35%. This technique may provide a new way to investigate the processes and timescales on which dark matter is stripped from galaxies as they fall into a massive cluster. Our preliminary conclusions will be made more robust by analysing the remaining five emph{Frontier Field} clusters.
The nearby Perseus galaxy cluster is a key target for indirect detection searches for decaying dark matter. We use the C-EAGLE simulations of galaxy clusters to predict the flux, width and shape of a dark matter decay line, paying particular attention to the unexplained 3.55keV line detected in the spectra of some galaxies and clusters, and the upcoming XRISM X-ray observatory mission. We show that the line width in C-EAGLE clusters similar to Perseus is typically [600-800]$mathrm{kms^{-1}}$, and therefore narrower than the amplitude of the velocity dispersion of galaxies in the cluster. Haloes that are significantly disturbed can, however, exhibit galaxy velocity dispersions higher than $1000mathrm{kms^{-1}}$, and in this case will show a large difference between the line profiles of on- and off-center observations. We show that the line profile is likely to be slightly asymmetric, but still well approximated by a Gaussian at the 10% level, and that the halo asymmetry can lead to fluxes that vary by a factor of two. In summary, we predict that, if the previously reported 3.55keV line detections do originate from dark matter decay, the XRISM mission will detect a line with a roughly Gaussian profile at a rest frame energy of 3.55keV, with a width $>600mathrm{kms^{-1}}$ and flux approximately in the range $[4-9]times10^{-8}mathrm{counts/sec/cm^{2}}$.
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