Multi-wavelength strong lensing analyses of baryons and dark matter in galaxy clusters

The observed dark matter abundance in the Universe can be explained with non-thermal, heavy dark matter models. In order for dark matter to still be present today, its lifetime has to far exceed the age of the Universe. In these scenarios, dark matter decay can produce highly energetic neutrinos, along with other Standard Model particles. To date, the IceCube Neutrino Observatory is the world’s largest neutrino telescope, located at the geographic South Pole. In 2013, the IceCube collaboration reported the first observation of high-energy astrophysical neutrinos. Since then, IceCube has collected a large amount of astrophysical neutrino data with energies up to tens of PeV, allowing us to probe the heavy dark matter models using neutrinos. We search the IceCube data for neutrinos from decaying dark matter in galaxy clusters and galaxies. The targeted dark matter masses range from 10 TeV to 10 PeV. In this contribution, we present the method and sensitivities of the analysis.

Studies of dark matter using gravitational lenses are reviewed, with emphasis on recent maps of dark matter in galaxy clusters. Although there are several QSOs which are multiply imaged by isolated intervening galaxies, distant galaxies are 4 million times more numerous than QSOs and thus can sample a lens at multiple positions. To a surface brightness of 29 B mag arcsec−2 there are about 100 background galaxies per square arcmin, which is sufficient to map statistically the dark matter distribution in a foreground galaxy cluster. Automated pattern recognition software generates a 2‐d mass density map. There is evidence for a soft core of about 100 kpc radius for the dark matter in the rich clusters studied so far. These lens studies confirm the large mass in rich galaxy clusters which were implied by virial calculations using velocity dispersion. The dark matter distribution in a cluster is found to be similar to the distribution of the bright galaxies.

We investigate the numerical modelling of relativistic protons, so-called cosmic rays, in a magnetised plasma. For the first time we combine two different components of a cosmological simulation code that so far have only been tested and employed independently of each other. By means of magnetohydrodynamic shock tube calculations in a gas that contains a constant fraction of relativistic particles we check for the correct physical behaviour of the combined numerical models. For this purpose we derive an analytical expression for the magnetosonic shock and rarefaction waves in the MHD Riemann problem and solve for the resulting system of equations using an iterative scheme. Comparing the theoretical and numerical solutions of a number of shock tube calculations we assess the physical correctness of the simulation. After successful testing we simulate the structure formation of three galaxy clusters including the consistent modelling of magnetic fields and cosmic rays. By means of analytical models we compute the X-ray and radio emission of the simulated clusters and reveal the temporal correlation between both quantities. Through a comparison with the strong lensing cross sections we demonstrate that the observable radio emission of galaxy clusters is directly connected to the occurrence of substructure in their dark matter halos and is thus triggered by strong merger shocks.

[Abridged] We consider two clusters (A496 and Coma) that are representative of the two classes of cool-core and non-cool-core clusters. We first refer to a two-component dynamical model that ignores the contribution from the galaxy density distribution and study the condition of hydrostatic equilibrium for the hot intracluster medium (ICM) under the assumption of spherical symmetry, in the presence of dark matter. We model the ICM density distribution in terms of a standard $\beta$-model with $\beta=2/3$, i.e. with a distribution similar to that of a regular isothermal sphere (RIS), and fit the observed X-ray brightness profiles. With the explicit purpose of ignoring cosmological arguments, we na\ively assume that dark matter, if present, has an analogous density distribution, with the freedom of two different density and length scales. The relative distribution of visible and dark matter is then derived by fitting the temperature data for the ICM under conditions of hydrostatic equilibrium. For both clusters, we find that dark matter is more concentrated with respect to visible matter. We then test whether the conclusion changes significantly when dark matter is taken to be distributed according to cosmologically favored density profiles and when the contribution of the mass contained in galaxies is taken into account. Although the qualitative conclusions remain unchanged, we find that the contribution of galaxies to the mass budget is more important than generally assumed. We also show that, without resorting to additional information on the small scale, it is not possible to tell whether a density cusp is present or absent in these systems. [Abridged]

In this study, we explore how a non-minimal coupling between dark matter and gravity can affect the behavior of dark matter in galaxy clusters. We have considered the case of a disformal coupling, which leads to a modification of the Poisson equation. Building on an earlier work, we expand the analysis considering all possible disformal coupling scenarios and employing various dark matter density profiles. In doing so, we aim to constrain the key parameter in our model, the characteristic coupling length. To achieve this, we analyze data from a combination of strong and weak lensing using three statistical approaches: a single cluster fitting procedure, a joint analysis, and one with stacked profiles. Our findings show that the coupling length is typically very small, thus being fully consistent with general relativity, although with an upper limit at 1σ which is of the order of 100 kpc.

The environment in which a galaxy resides plays a key role in how both the stellar component and the dark matter halo evolve. Satellites of massive halos provide important clues as to how accreted halos develop in a dense environment, and are subjected to unique effects such as tidal disruption and gas stripping. Isolated galaxies are shielded from effects induced by other galaxies, and so represent a `clean' test of how galaxies grow in the context of the $____Lambda$CDM paradigm. In this work, I examine the structure of a number of dwarf galaxies in a range of environments, from the most isolated members of the Local Group to satellites of the Milky Way, and consider what this suggests about the evolution of the galaxy halo. I first present new spectroscopy of the isolated dwarf spheroidal galaxy Tucana. Tucana displays a high velocity dispersion consistent with residing in a dense subhalo. I perform Jeans modelling of the density profile of Tucana, using the line--of--sight velocities of its member stars. I find that it favours a high central density consistent with pristine $____Lambda$CDM subhalos, and a massive dark matter halo consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, and it is plausible that the dearth of recent star formation has allowed it to retain its central mass. I then present new chemo--kinematics of Hercules, a Milky Way satellite which is purported to be tidally disrupting on the basis of its elongated shape and apparent debris surrounding the central galaxy. I find little evidence for tidal disruption in the kinematics of Hercules, with no significant velocity gradient across the galaxy. The proper motion of Hercules is shown to be misaligned with the position angle of the elongation, contrary to expectations from simulations, although this may be resolved by future astrometry. With current data, the orbital path of Hercules is too uncertain to determine the extent to which Hercules has been affected by the tidal influence of the Milky Way. In the final chapter I introduce a new survey of isolated, low mass dwarf galaxies in the Local Group. Under $____Lambda$CDM, galaxies at all mass scales are believed to grow through hierarchical merging of smaller bodies. Traces of this accretion history should be observable as substructures in the virial halo of galaxies. Using photometry of isolated dwarfs across a range of low halo masses, this survey aims to quantify whether such structured halos are observed at all scales, or whether there is a mass scale at which galaxies no longer accrete smaller systems.

The electron population inferred to be responsible for the mini-halo within the Ophiuchus galaxy cluster is a steep power law in energy with a slope of 3.8. This is substantially different to that predicted by dark matter (DM) annihilation models. In this work, we present a method of indirect comparison between the observed electron spectrum and that predicted for indirect DM emissions. This method utilizes differences in the consequences of a given electron distribution on the subsequent spectral features of synchrotron emissions. To fully exploit this difference, by leveraging the fact that the peak and cut-off synchrotron frequencies are substantially different to hard power-law cases for WIMP masses above ∼50 GeV, we find that we need μJy sensitivities at frequencies above 10 GHz while being sensitive to arcminute scales. We explore the extent to which this electron spectrum comparison can be validated with the up-coming next-generation Very Large Array (ngVLA) instrument. We show that, with the ngVLA, this method allows us to produce far stronger constraints than existing VLA data, indeed these exceed the Fermi-LAT dwarf searches in a wide variety of annihilation channels and for all studied magnetic field scenarios.

In this paper a family of two-component non-isothermal hydrostatic models are elaborated in order to simulate clusters of galaxies in which a dark background exists. The influence of an invisible component on the dynamical analysis is examined. We show that the mass-to-light ratio MLR is not linearly related to the actual amount of the unseen mass, leading to the conclusion that the MLR is not a reliable indicator of the amount of dark matter in clusters of galaxies. The determination of the mass depends strongly on the relative concentration of both the luminous and the dark component. This is the so-called Limber effect which is quantified on the ground of our two-component hydrostatic models. The asymptotic behaviour of the density profile of both components is investigated. We found that they allow useful information to be obtained on the distribution of the unseen component and on its thermodynamical state. We also show that intra-cluster medium (ICM) can not mimic the dark matter and consequently one has to invoke the dark matter to explain the high MLRs in galaxy clusters.

We assess how much unused strong lensing information is available in the deep\n\\emph{Hubble Space Telescope} imaging and VLT/MUSE spectroscopy of the\n\\emph{Frontier Field} clusters. As a pilot study, we analyse galaxy cluster\nMACS\\,J0416.1-2403 ($z$$=$$0.397$, $M(R<200\\,{\\rm\nkpc})$$=$$1.6$$\\times$$10^{14}\\msun$), which has 141 multiple images with\nspectroscopic redshifts. We find that many additional parameters in a cluster\nmass model can be constrained, and that adding even small amounts of extra\nfreedom to a model can dramatically improve its figures of merit. We use this\ninformation to constrain the distribution of dark matter around cluster member\ngalaxies, simultaneously with the cluster's large-scale mass distribution. We\nfind tentative evidence that some galaxies' dark matter has surprisingly\nsimilar ellipticity to their stars (unlike in the field, where it is more\nspherical), but that its orientation is often misaligned. When non-coincident\ndark matter and baryonic halos are allowed, the model improves by 35\\%. This\ntechnique may provide a new way to investigate the processes and timescales on\nwhich dark matter is stripped from galaxies as they fall into a massive\ncluster. Our preliminary conclusions will be made more robust by analysing the\nremaining five \\emph{Frontier Field} clusters.\n

The dark matter distribution in the very inner region of our Galaxy is still\nin debate. In the N-body simulations a cuspy dark matter halo density profile\nis favored. Several dissipative baryonic processes however are found to be able\nto significantly flatten dark matter distribution and a cored dark matter halo\ndensity profile is possible. The baryons dominate the gravitational potential\nin the inner Galaxy, hence a direct constrain on the abundance of the dark\nmatter particles is rather challenging. Recently, a few groups have identified\na tentative 130 GeV line signal in the Galactic center, which could be\ninterpreted as the signal of the dark matter annihilation. With current 130 GeV\nline data and adopting the generalized Navarro-Frenk-White profile of the dark\nmatter halo, for local dark matter density \\rho_0=0.4 GeV cm^{-3} and r_s=20\nkpc we obtain a 95% confidence level lower (upper) limit on the inner slope of\ndark matter density distribution \\alpha = 1.06 (the cross section of dark\nmatter annihilation into gamma-rays <\\sigma v >_{\\chi\\chi -> \\gamma\\gamma}=\n1.3\\times 10^{-27} cm^3 s^{-1}). Such a slope is consistent with the results of\nsome N-body simulations, and if the signal is due to dark matter, suggests that\nbaryonic processes may be unimportant.\n

We compile a sample of 38 galaxy clusters which have both X-ray and strong lensing observations, and study for each cluster the projected offset between the dominant component of baryonic matter center (measured by X-rays) and the gravitational center (measured by strong lensing). Among the total sample, 45% clusters have offsets >10". The >10" separations are significant, considering the arcsecond precision in the measurement of the lensing/X-ray centers. This suggests that it might be a common phenomenon in unrelaxed galaxy clusters that gravitational field is separated spatially from the dominant component of baryonic matter. It also has consequences for lensing models of unrelaxed clusters since the gas mass distribution may differ from the dark matter distribution and give perturbations to the modeling. Such offsets can be used as a statistical tool for comparison with the results of Lambda-CDM simulations and to test the modified dynamics.

Gravitational lensing originates from the deflection of light by masses, irrespective of their physical state or composition. Since it appears inescapable that most of the matter in the universe is dark, gravitational lensing has developed into one of the primary tools to learn about the amount, composition and distribution of masses in the universe.The review will summarise the theory of gravitational lensing, starting from Fermat’s principle. This will first be applied to isolated lenses like compact objects, galaxies, and galaxy clusters. Cosmologically relevant applications will be described, such as searches for compact dark-matter objects in galactic halos, measurements of the Hubble constant in galaxy lenses, and methods for mapping the dark matter in galaxy clusters. Next, the theory of cosmological lensing will be introduced. The concepts of lensing by large-scale structures and its measurement will be discussed, concluding with an overview of results which have so far been obtained, and an outlook at what can be expected in the near future.

The resolution of the cosmic far-infrared background at long wavelengths has uncovered a population of high-redshift, highly infrared-luminous galaxies that indicate that a significant amount of highly obscured star formation occurred in the early history of the Universe, which has not been taken into account in studies of the cosmic star formation and mass assembly histories. Since such studies are important tests of hierarchical galaxy formation models in a cold dark matter- dominated universe, it is vital that the contribution of these luminous submillimeter-selected galaxies to the buildup of stellar mass in the Universe be understood and included when testing galaxy formation models. Progress in understanding the nature of submillimeter-selected galaxies has been slow, however, due to the faintness of the population outside of the submillimeter bands and the coarse resolution of the single-dish submillimeter surveys in which the galaxies were discovered. The slow progress has at least two important consequences: (1) there is still much that is unknown about submillimeterselected galaxies at all wavelengths, so the properties of and evolutionary predictions for the population are often inferred by making analogies to local galaxies of similar luminosity and assuming the high-redshift galaxies represent a uniform population of galaxies; and (2) the relationships of the submillimeter-selected galaxies to other high-redshift galaxy populations selected at other wavelengths remain poorly understood. In this work we capitalize on recently-available observing resources of the Spitzer Space Telescope, at near- and mid-infrared wavelengths, and the Green Bank Telescope, at 1 cm, to improve the characterization of the largest representative sample of submillimeter-selected galaxies with spectroscopic redshifts. We combine our new data with data from the literature at a variety of wavelengths to place new constraints on the fundamental properties of gas mass, stellar mass, infrared luminosity, and spectral energy distribution, and use our new constraints to test a variety of assumptions which have been used in the past to predict and infer characteristics of submillimeter-selected galaxies. We also use our new data to compare different populations of high-redshift galaxies selected at different wavelengths in an effort to understand the relationships between the different types of galaxies. First, we use observations of CO rotational line emission with the Green Bank Telescope to constrain the cold gas mass and gas conditions in several submillimeter-selected galaxies. We obtain the first detection of CO(1→0) emission from a submillimeter-selected galaxy, finding that the CO(4→3)/CO(1→0) brightness temperature ratio of ~ 0.26 suggests n(H₂) > 3 − 10 × 10² cm⁻³ and the presence of sub-thermally excited gas. The integrated line flux implies a cold molecular gas mass 4 times larger than the mass predicted from the CO(4→3) line, assuming a brightness temperature ratio of 1.0, suggesting that extrapolating molecular gas masses from J upper ≥ 3 transitions of CO, which is the primary method of estimating molecular gas masses of high-z galaxies in the literature, leads to considerable uncertainties. Next, we use deep imaging with the Multiband Imaging Photometer for Spitzer (MIPS) on the Spitzer Space Telescope of the spectroscopic sample of radio-detected submillimeter-selected galaxies of Chapman et al. (2005) to derive new estimates of the infrared luminosity for these objects. Our Spitzer data constrain the Wien side of the infrared spectral energy distribution peak of high-redshift submillimeter-selected galaxies, and thus are extremely important to determine the contribution of hot dust emission to the total infrared luminosity. We find that most submillimeter-selected galaxies do not have dominant contributions from hot dust at rest-frame mid-infrared wavelengths. We also find that the spectral energy distribution of the nearest ultraluminous infrared galaxy, Arp 220, is significantly different on average from most high-z submillimeter-selected galaxies and is thus a poor template with which to predict properties of submillimeter-selected galaxies even though it has been often used in the past. Using our new infrared luminosity estimates constrained by multiple infrared and submillimeter data points, we show that submillimeter-selected galaxies display a relatively tight, almost linear correlation between total infrared luminosity and radio luminosity, which is not largely different from the far-infrared–radio correlation of local galaxies selected by the Infrared Astronomical Satellite (IRAS). We examine the rest-frame ultraviolet through near-infrared spectral energy distributions of the same sample of radio-detected submillimeter-selected galaxies with spectroscopic redshifts, obtained using measurements from imaging with the Infrared Array Camera (IRAC) on Spitzer in combination with observed-frame optical and near-infrared data from the literature. We find from these spectral energy distributions, which trace the stellar light from a galaxy in the absence of an active nucleus, that submillimeter-selected galaxies suffer significant extinction at rest-frame optical wavelengths, and that both stars and dust emission contribute to the near-infrared luminosity in many z > 2 submillimeter-selected galaxies. We estimate stellar masses for the individual galaxies in the sample using restframe H-band luminosities interpolated from the observed spectral energy distributions, obtaining a median stellar mass for the sample of 6 − 7 × 10¹⁰ Msun. By comparing our stellar mass estimates to molecular gas and dynamical mass estimates for 13 individual submillimeter-selected galaxies in our sample observed in CO emission lines, we determine that the molecular gas fraction in submillimeter-selected galaxies declines with increasing stellar mass, which is suggestive of an evolutionary trend. If the molecular gas masses for the 13 galaxies for which gas mass estimates are available are typical of the entire radiodetected submillimeter-selected galaxy population, then a typical lower limit to the total baryonic mass of submillimeter-selected galaxies is ~ 10¹¹ Msun, and these galaxies are unlikely to significantly increase their stellar mass in the current epoch of activity which is the source of their enormous infrared luminosity. Lastly, we compare the IRAC and MIPS properties of submillimeter-selected galaxies and their stellar masses to those of high-redshift ultraviolet- and optically-selected galaxies, 24 μm-selected galaxies, and powerful radio galaxies. In the IRAC bands, submillimeter-selected galaxies are brighter and redder than ultraviolet-selected galaxies, suggesting they have higher dust content, higher stellar mass, a higher contribution from an active nucleus, or some combination of these factors. The near-infrared colors of submillimeterselected galaxies are most similar to those of high-z radio galaxies, objects which are known to contain powerful, obscured active nuclei. However, submillimeter-selected galaxies are fainter in the MIPS 24 μm band than high-z radio galaxies, suggesting that the dust in the submillimeter-selected galaxies is not heated to such high temperatures as in the radio sources. We find that the typical stellar mass of submillimeter-selected galaxies is larger by a factor of 3–4 than that of high-redshift ultraviolet-selected galaxies, roughly similar to the typical stellar mass of optically-selected high-z galaxies, and lower than the typical stellar mass of powerful high-z radio galaxies. These comparisons suggest that submillimeter-selected galaxies are among the more massive galaxies of their epoch, but not necessarily the most massive, as has been suggested in the literature. However, systematic errors in the stellar masses of any of the high-redshift galaxy samples of a factor of a few, which are certainly possible, can alter this conclusion.

`Galaxy groups' have hardly been realized as a separate class of objects with specific characteristics in the structural hierarchy of the universe. The presumption that the self-similarity of dark matter structures is a valid prescription for the baryonic universe also at all scales has rendered smaller structures undetectable by current observational facilities, leading to lesser dedicated studies on them. Some recent reports on deviation of $\rm{L_x}$-T scaling in groups from that of clusters have motivated us to study their physical properties in depth. In this article, we report the extensive study on physical properties of groups in comparison with clusters through cosmological hydrodynamic plus N-body simulations using ENZO 2.2 code. We have included cooling and heating physics and star formation feedback in the simulation. And produced a mock sample of 362 objects with mass ranging from $5\times10^{12}\; \rm{M_{\odot}}$ to 2.5$\times 10^{15}\; \rm{M_{\odot}}$. Strikingly, we have found that objects with a mass below $\sim$ $8\times 10^{13}\;\rm{M_{\odot}}$ do not follow any of the cluster self-similar laws in hydrostatics, not even in thermal and non-thermal regimes. Two distinct scaling laws are observed to be followed with breaks at $\sim$ $6-8\times 10^{13}\;\rm{M_{\odot}}$ for mass, $\sim$1 keV for temperature and $\sim$1 Mpc for radius. This places groups as a distinct entity in the hierarchical structures, well demarcated from clusters. This study reveals that groups are mostly far away from virialization, suggesting the need for formulating new models for deciphering their physical parameters. They are also shown to have high turbulence and more non-thermal energy stored, indicating better visibility in the non-thermal regime.

Theories of structure formation in a cold dark matter dominated universe predict that massive clusters of galaxies assemble from the hierarchical merging of lower mass subhalos. Exploiting strong and weak gravitational lensing signals inferred from panoramic Hubble Space Telescope imaging data, we present a high-resolution reconstruction of the mass distribution in the massive, lensing cluster Cl 0024+16 at z = 0.39. Applying galaxy–galaxy lensing techniques we track the fate of dark matter subhalos as a function of projected cluster-centric radius out to 5 Mpc, well beyond the virial radius. We report the first detection of the statistical lensing signal of dark matter subhalos associated with late-type galaxies in clusters. The mass of a fiducial dark matter halo that hosts an early-type L* galaxy varies from M = 6.3+2.7−2.0 × 1011 M☉ within r < 0.6 Mpc, 1.3+0.8−0.6 × 1012 M☉ within r < 2.9 Mpc, and increases further to M = 3.7+1.4−1.1 × 1012 M☉ in the outskirts. The mass of a typical dark matter subhalo that hosts an L* galaxy increases with projected cluster-centric radius in line with expectations from the tidal stripping hypothesis. The mass of a dark matter subhalo that hosts a late-type L* galaxy is 1.06+0.52−0.41 × 1012 M☉. Early-type galaxies appear to be hosted on average in more massive dark matter subhalos compared to late-type galaxies. Early-type galaxies also trace the overall mass distribution of the cluster whereas late-type galaxies are biased tracers. We interpret our findings as evidence for the active assembly of mass via tidal stripping in galaxy clusters. The mass function of dark matter subhalos as a function of projected cluster-centric radius is compared with an equivalent mass function derived from clusters in the Millennium Run simulation populated with galaxies using semianalytic models. The shape of the observationally determined mass functions based on an I-band-selected sample of cluster members and lensing data are in agreement with the shapes of the subhalo mass functions derived from the Millennium Run simulation. However, simulated subhalos appear to be more efficiently stripped than lensing observations suggest. This is likely an artifact of comparison with a dark matter only simulation. Future simulations that simultaneously follow the detailed evolution of the baryonic component during cluster assembly will be needed for a more detailed comparison.