arXiv subjects

David Wittman

Publications and source records attributed to David Wittman.

At least 37 records · Page 2Linked to original sources

arXiv2019

Brightest Cluster Galaxy Alignments in Merging Clusters

The orientations of brightest cluster galaxies (BCGs) and their host clusters tend to be aligned, but the mechanism driving this is not clear. To probe the role of cluster mergers in this process, we quantify alignments of 38 BCGs in 22 clusters undergoing major mergers (up to $\sim 1$ Gyr after first pericenter). We find alignments entirely consistent with those of clusters in general. This suggests that alignments are robust against major cluster mergers. If, conversely, major cluster mergers actually help orient the BCG, such a process is acting quickly because the orientation is in place within $\sim 1$ Gyr after first pericenter.

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arXiv2018

The Mismeasure of Mergers: Revised Limits on Self-interacting Dark Matter in Merging Galaxy Clusters

In an influential recent paper, Harvey et al (2015) derive an upper limit to the self-interaction cross section of dark matter ($σ_{\rm DM} < 0.47$ cm$^2$/g at 95\% confidence) by averaging the dark matter-galaxy offsets in a sample of merging galaxy clusters. Using much more comprehensive data on the same clusters, we identify several substantial errors in their offset measurements. Correcting these errors relaxes the upper limit on $σ_{\rm DM}$ to $\lesssim 2$ cm$^2$/g, following the Harvey et al prescription for relating offsets to cross sections in a simple solid body scattering model. Furthermore, many clusters in the sample violate the assumptions behind this prescription, so even this revised upper limit should be used with caution. Although this particular sample does not tightly constrain self-interacting dark matter models when analyzed this way, we discuss how merger ensembles may be used more effectively in the future. We conclude that errors inherent in using single-band imaging to identify mass and light peaks do not necessarily average out in a sample of this size, particularly when a handful of substructures constitute a majority of the weight in the ensemble.

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arXiv2018

Constraints on Cosmology and Baryonic Feedback with the Deep Lens Survey Using Galaxy-Galaxy and Galaxy-Mass Power Spectra

We present cosmological parameter measurements from the Deep Lens Survey (DLS) using galaxy-mass and galaxy-galaxy power spectra in the multipole range $\ell=250\sim2000$. We measure galaxy-galaxy power spectra from two lens bins centered at $z\sim0.27$ and $0.54$ and galaxy-mass power spectra by cross-correlating the positions of galaxies in these two lens bins with galaxy shapes in two source bins centered at $z\sim0.64$ and $1.1$. We marginalize over a baryonic feedback process using a single-parameter representation and a sum of neutrino masses, as well as photometric redshift and shear calibration systematic uncertainties. For a flat $Λ$CDM cosmology, we determine $S_8\equivσ_8\sqrt{Ω_m/0.3}=0.810^{+0.039}_{-0.031}$, in good agreement with our previous DLS cosmic shear and the Planck Cosmic Microwave Background (CMB) measurements. Without the baryonic feedback marginalization, $S_8$ decreases by $\sim0.05$ because the dark matter-only power spectrum lacks the suppression at the highest $\ell$'s due to Active Galactic Nuclei (AGN) feedback. Together with the Planck CMB measurement, we constrain the baryonic feedback parameter to $A_{baryon}=1.07^{+0.31}_{-0.39}$, which suggests an interesting possibility that the actual AGN feedback might be stronger than the recipe used in the OWLS simulations. The interpretation is limited by the validity of the baryonic feedback simulation and the one-parameter representation of the effect.

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arXiv2018

Simulated Analogs of Merging Galaxy Clusters Constrain the Viewing Angle

A key uncertainty in interpreting observations of bimodal merging galaxy clusters is the unknown angle between the subcluster separation vector and the plane of the sky. We present a new method for constraining this key parameter. We find analogs of observed systems in cosmological n-body simulations and quantify their likelihood of matching the observed projected separation and relative radial velocities between subclusters, as a function of viewing angle. We derive constraints on the viewing angle of many observed bimodal mergers including the Bullet Cluster (1E 0657-558) and El Gordo (ACT-CL J0102-4915). We also present more generic constraints as a function of projected separation and relative radial velocity, which can be used to assess additional clusters as information about them becomes available. The constraints from these two observables alone are weak (typically $\gtrsim 70-75^\circ$ at 68\% confidence and $\gtrsim 55-60^\circ$ at 95\% confidence) but incorporate much more cosmological context than the classical timing argument, marginalizing over many realizations of substructure, peculiar velocities, and so on. Compared to the MCMAC code, which implements the timing argument on NFW halos, our constraints generally predict subcluster separation vectors closer to the plane of the sky. This is because in realistic mergers the subcluster velocity vectors are not entirely parallel to the separation vector (i.e, the mergers are not perfectly head-on). As a result, observation of a nonzero relative radial velocity does not exclude a separation vector in the plane of the sky, as it does in the head-on timing argument employed by MCMAC.

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arXiv2017

MC$^2$: Subaru and Hubble Space Telescope Weak-Lensing Analysis of the Double Radio Relic Galaxy Cluster PLCK G287.0+32.9

The second most significant detection of the Planck Sunyaev Zel'dovich survey, PLCK~G287.0+32.9 ($z=0.385$) boasts two similarly bright radio relics and a radio halo. One radio relic is located $\sim 400$ kpc northwest of the X-ray peak and the other $\sim 2.8$ Mpc to the southeast. This large difference suggests that a complex merging scenario is required. A key missing puzzle for the merging scenario reconstruction is the underlying dark matter distribution in high resolution. We present a joint Subaru Telescope and {\it Hubble Space Telescope} weak-lensing analysis of the cluster. Our analysis shows that the mass distribution features four significant substructures. Of the substructures, a primary cluster of mass $M_{200\text{c}}=1.59^{+0.25}_{-0.22}\times 10^{15} \ h^{-1}_{70} \ \text{M}_{\odot}$ dominates the weak-lensing signal. This cluster is likely to be undergoing a merger with one (or more) subcluster whose mass is approximately a factor of 10 lower. One candidate is the subcluster of mass $M_{200\text{c}}=1.16^{+0.15}_{-0.13}\times 10^{14} \ h^{-1}_{70} \ \text{M}_{\odot}$ located $\sim 400$ kpc to the southeast. The location of this subcluster suggests that its interaction with the primary cluster could be the source of the NW radio relic. Another subcluster is detected $\sim 2$ Mpc to the SE of the X-ray peak with mass $M_{200\text{c}}=1.68^{+0.22}_{-0.20}\times 10^{14} \ h^{-1}_{70} \ \text{M}_{\odot}$. This SE subcluster is in the vicinity of the SE radio relic and may have created the SE radio relic during a past merger with the primary cluster. The fourth subcluster, $M_{200\text{c}}=1.87^{+0.24}_{-0.22}\times 10^{14} \ h^{-1}_{70} \ \text{M}_{\odot}$, is northwest of the X-ray peak and beyond the NW radio relic.

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arXiv2017

MC$^2$: Multi-wavelength and dynamical analysis of the merging galaxy cluster ZwCl 0008.8+5215: An older and less massive Bullet Cluster

We analyze a rich dataset including Subaru/SuprimeCam, HST/ACS and WFC3, Keck/DEIMOS, Chandra/ACIS-I, and JVLA/C and D array for the merging galaxy cluster ZwCl 0008.8+5215. With a joint Subaru/HST weak gravitational lensing analysis, we identify two dominant subclusters and estimate the masses to be M$_{200}=\text{5.7}^{+\text{2.8}}_{-\text{1.8}}\times\text{10}^{\text{14}}\,\text{M}_{\odot}$ and 1.2$^{+\text{1.4}}_{-\text{0.6}}\times10^{14}$ M$_{\odot}$. We estimate the projected separation between the two subclusters to be 924$^{+\text{243}}_{-\text{206}}$ kpc. We perform a clustering analysis on confirmed cluster member galaxies and estimate the line of sight velocity difference between the two subclusters to be 92$\pm$164 km s$^{-\text{1}}$. We further motivate, discuss, and analyze the merger scenario through an analysis of the 42 ks of Chandra/ACIS-I and JVLA/C and D polarization data. The X-ray surface brightness profile reveals a remnant core reminiscent of the Bullet Cluster. The X-ray luminosity in the 0.5-7.0 keV band is 1.7$\pm$0.1$\times$10$^{\text{44}}$ erg s$^{-\text{1}}$ and the X-ray temperature is 4.90$\pm$0.13 keV. The radio relics are polarized up to 40$\%$. We implement a Monte Carlo dynamical analysis and estimate the merger velocity at pericenter to be 1800$^{+\text{400}}_{-\text{300}}$ km s$^{-\text{1}}$. ZwCl 0008.8+5215 is a low-mass version of the Bullet Cluster and therefore may prove useful in testing alternative models of dark matter. We do not find significant offsets between dark matter and galaxies, as the uncertainties are large with the current lensing data. Furthermore, in the east, the BCG is offset from other luminous cluster galaxies, which poses a puzzle for defining dark matter -- galaxy offsets.

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arXiv2017

X-ray Temperatures, Luminosities, and Masses From XMM-Newton Follow-up of the First Shear-selected Galaxy Cluster Sample

We continue the study of the first sample of shear-selected clusters (Wittman et al. 2006) from the initial 8.6 square degrees of the Deep Lens Survey (DLS, Wittman et al. 2002); a sample with well-defined selection criteria corresponding to the highest ranked shear peaks in the survey area. We aim to characterize the weak lensing selection by examining the sample's X-ray properties. There are multiple X-ray clusters associated with nearly all the shear peaks: 14 X-ray clusters corresponding to seven DLS shear peaks. An additional three X-ray clusters cannot be definitively associated with shear peaks, mainly due to large positional offsets between the X-ray centroid and the shear peak. Here we report on the X-ray properties of the 17 X-ray clusters. The X-ray clusters display a wide range of luminosities and temperatures; the Lx-Tx relation we determine for the shear-associated X-ray clusters is consistent with X-ray cluster samples selected without regard to dynamical state, while it is inconsistent with self-similarity. For a subset of the sample, we measure X-ray masses using temperature as a proxy, and compare to weak lensing masses determined by the DLS team (Abate et al. 2009; Wittman et al. 2014). The resulting mass comparison is consistent with equality. The X-ray and weak lensing masses show considerable intrinsic scatter (~48%), which is consistent with X-ray selected samples when their X-ray and weak lensing masses are independently determined.

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arXiv2017

Offsets between member galaxies and dark matter in clusters: a test with the Illustris simulation

Dark matter with a non-zero self-interacting cross section ($σ_{\rm SIDM}$) has been posited as a solution to a number of outstanding astrophysical mysteries. Many studies of merging galaxy clusters have given constraints on $σ_{\rm SIDM}$ based on the spatial offset between the member galaxy population and the dark matter distribution. Assuming $σ_{\rm SIDM} = 0$, how likely is it for us to see the galaxy-DM offset values observed in merging clusters of galaxies? To answer this question, we formulate a hypothesis test using data from Illustris, a $Λ$CDM cosmological simulation. We select 43 Illustris clusters and their galaxy members at z~0 and examine the accuracy of commonly used galaxy summary statistics, including kernel-density-estimation (KDE) luminosity peak, KDE number density peak, shrinking aperture, centroid and the location of the brightest cluster galaxy (BCG). We use the dark-matter particles to reproduce commonly adopted methods to identify dark-matter peaks based on gravitational lensing cluster maps. By analysing each cluster in 768 projections, we determine the optimistic noise floor in the measurements of the galaxy-DM offsets. We find that the choice of the galaxy summary statistics affects the inferred offset values substantially, with the BCG and the luminosity peak giving the tightest 68-th percentile offset levels, $\lesssim$ 4 kpc and $\lesssim$ 32 kpc, respectively. Shrinking aperture, number density and centroid give a large offset scatter of about 50-100 kpc at the 68-th percentile level, even for clusters with only one dominant mass component. Out of the 15 reported offsets from observed merging clusters that we examined, 13 of them are consistent with Illustris unrelaxed cluster offsets at the 2-sigma (95-th percentile) level, i.e. consistent with the hypothesis that $Λ$CDM is the true underlying physical model.

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arXiv2017

The Case for Electron Re-Acceleration at Galaxy Cluster Shocks

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-size radio sources have been found, so-called radio relics. These relics are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach number collisionless shocks generated by cluster-cluster merger events. A long-standing problem is how low-Mach number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here we report on the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

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arXiv2016

In the Wake of Dark Giants: New Signatures of Dark Matter Self Interactions in Equal Mass Mergers of Galaxy Clusters

Merging galaxy clusters have been touted as one of the best probes for constraining self-interacting dark matter, but few simulations exist to back up this claim. We simulate equal mass mergers of 10$^{15}$ M$_\odot$ halos, like the El Gordo and Sausage clusters, with cosmologically-motivated halo and merger parameters, and with velocity-independent dark-matter self-interactions. Although the standard lore for merging clusters is that self-interactions lead to large separations between the galaxy and dark-matter distributions, we find that maximal galaxy-dark matter offsets of $\lesssim~20$~kpc form for a self-interaction cross section of $σ_\text{SI}/m_χ$ = 1 cm$^2$/g. This is an order of magnitude smaller than those measured in observed equal mass and near equal mass mergers, and is likely to be even smaller for lower-mass systems. While competitive cross-section constraints are thus unlikely to emerge from offsets, we find other signatures of self-interactions which are more promising. Intriguingly, we find that after dark matter halos coalesce, the collisionless galaxies (and especially the Brightest Cluster Galaxy [BGC]) oscillate around the center of the merger remnant on stable orbits of 100 kpc for $σ_\text{SI}/m_χ= 1$~cm$^2$/g for at least several Gyr, well after the clusters have relaxed. If BCG miscentering in relaxed clusters remains a robust prediction of SIDM under the addition of gas physics, substructure, merger mass ratios (e.g., 10:1 like the Bullet Cluster), and complex cosmological merger histories, the observed BCG offsets may constrain $σ_\text{SI}/m_χ\lesssim$ 0.1 cm$^2$/g---the tightest constraint yet.

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arXiv2016

MC$^2$: Dynamical Analysis of the Merging Galaxy Cluster MACS J1149.5+2223

We present an analysis of the merging cluster MACS J1149.5+2223 using archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from Keck/DEIMOS and Gemini/GMOS. We employ two and three dimensional substructure tests and determine that MACS J1149.5+2223 is composed of two separate mergers between three subclusters occurring $\sim$1 Gyr apart. The primary merger gives rise to elongated X-ray morphology and a radio relic in the southeast. The brightest cluster galaxy is a member of the northern subcluster of the primary merger. This subcluster is very massive (16.7$^{+\text{1.25}}_{-\text{1.60}}\times\text{10}^{\text{14}}$ M$_{\odot}$). The southern subcluster is also very massive (10.8$^{+\text{3.37}}_{-\text{3.54}}\times\text{10}^{\text{14}}$ M$_{\odot}$), yet it lacks an associated X-ray surface brightness peak, and it has been unidentified previously despite the detailed study of this \emph{Frontier Field} cluster. A secondary merger is occurring in the north along the line of sight with a third, less massive, subcluster (1.20$^{+\text{0.19}}_{-\text{0.34}}\times\text{10}^{\text{14}}$ M$_{\odot}$). We perform a Monte Carlo dynamical analysis on the main merger and estimate a collision speed at pericenter of 2770$^{+\text{610}}_{-\text{310}}$ km s$^{-\text{1}}$. We show the merger to be returning from apocenter with core passage occurring 1.16$^{+\text{0.50}}_{-\text{0.25}}$ Gyr before the observed state. We identify the line of sight merging subcluster in a strong lensing analysis in the literature and show that it is likely bound to MACS J1149 despite having reached an extreme collision velocity of $\sim$4000 km s$^{-\text{1}}$.

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arXiv2016

Cosmic Shear Results from the Deep Lens Survey - II: Full Cosmological Parameter Constraints from Tomography

We present a tomographic cosmic shear study from the Deep Lens Survey (DLS), which, providing a limiting magnitude r_{lim}~27 (5 sigma), is designed as a pre-cursor Large Synoptic Survey Telescope (LSST) survey with an emphasis on depth. Using five tomographic redshift bins, we study their auto- and cross-correlations to constrain cosmological parameters. We use a luminosity-dependent nonlinear model to account for the astrophysical systematics originating from intrinsic alignments of galaxy shapes. We find that the cosmological leverage of the DLS is among the highest among existing >10 sq. deg cosmic shear surveys. Combining the DLS tomography with the 9-year results of the Wilkinson Microwave Anisotropy Probe (WMAP9) gives Omega_m=0.293_{-0.014}^{+0.012}, sigma_8=0.833_{-0.018}^{+0.011}, H_0=68.6_{-1.2}^{+1.4} km/s/Mpc, and Omega_b=0.0475+-0.0012 for LCDM, reducing the uncertainties of the WMAP9-only constraints by ~50%. When we do not assume flatness for LCDM, we obtain the curvature constraint Omega_k=-0.010_{-0.015}^{+0.013} from the DLS+WMAP9 combination, which however is not well constrained when WMAP9 is used alone. The dark energy equation of state parameter w is tightly constrained when Baryonic Acoustic Oscillation (BAO) data are added, yielding w=-1.02_{-0.09}^{+0.10} with the DLS+WMAP9+BAO joint probe. The addition of supernova constraints further tightens the parameter to w=-1.03+-0.03. Our joint constraints are fully consistent with the final Planck results and also the predictions of a LCDM universe.

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arXiv2016

MC^2: Mapping the Dark Matter Distribution of the "Toothbrush" Cluster RX J0603.3+4214 with Hubble Space Telescope and Subaru Weak-lensing

The galaxy cluster RX J0603.3+4214 at z=0.225 is one of the rarest clusters boasting an extremely large (~2 Mpc) radio-relic. Because of the remarkable morphology of the relic, the cluster is nicknamed "Toothbrush Cluster". Although the cluster's underlying mass distribution is one of the critical pieces of information needed to reconstruct the merger scenario responsible for the puzzling radio-relic morphology, its proximity to the Galactic plane b~10 deg has imposed significant observational challenges. We present a high-resolution weak-lensing study of the cluster with Subaru/Suprime Cam and Hubble Space Telescope imaging data. Our mass reconstruction reveals that the cluster is comprised of complicated dark matter substructures closely tracing the galaxy distribution, however in contrast with the relatively simple binary X-ray morphology. Nevertheless, we find that the cluster mass is still dominated by the two most massive clumps aligned north-south with a ~3:1 mass ratio (M_{200}=6.29_{-1.62}^{+2.24} x 10^{14} Msun and 1.98_{-0.74}^{+1.24} x 10^{14} Msun for the northern and southern clumps, respectively). The southern mass peak is ~2' offset toward the south with respect to the corresponding X-ray peak, which has a "bullet"-like morphology pointing south. Comparison of the current weak-lensing result with the X-ray, galaxy, and radio-relic suggests that perhaps the dominant mechanism responsible for the observed relic may be a high-speed collision of the two most massive subclusters, although the peculiarity of the morphology necessitates involvement of additional sub-clusters. Careful numerical simulations should follow in order to obtain more complete understanding of the merger scenario utilizing all existing observations.

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arXiv2016

Overconfidence in Photometric Redshift Estimation

We describe a new test of photometric redshift performance given a spectroscopic redshift sample. This test complements the traditional comparison of redshift {\it differences} by testing whether the probability density functions $p(z)$ have the correct {\it width}. We test two photometric redshift codes, BPZ and EAZY, on each of two data sets and find that BPZ is consistently overconfident (the $p(z)$ are too narrow) while EAZY produces approximately the correct level of confidence. We show that this is because EAZY models the uncertainty in its spectral energy distribution templates, and that post-hoc smoothing of the BPZ $p(z)$ provides a reasonable substitute for detailed modeling of template uncertainties. Either remedy still leaves a small surplus of galaxies with spectroscopic redshift very far from the peaks. Thus, better modeling of low-probability tails will be needed for high-precision work such as dark energy constraints with the Large Synoptic Survey Telescope and other large surveys.

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arXiv2015

MC$^2$: Boosted AGN and star-formation activity in CIZA J2242.8+5301, a massive post-merger cluster at z=0.19

Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. (2014) revealed unexpected over-densities of candidate H$α$ emitters near the ~1 Mpc-wide shock fronts of the massive (~2x10$^{15}$M$_{\odot}$) "Sausage" merging cluster, CIZA J2242.8+5301. We used Keck/DEIMOS and WHT/AF2 to confirm 83 H$α$ emitters in and around the merging cluster. We find that cluster star-forming galaxies in the hottest X-ray gas and/or in the cluster sub-cores (away from the shock fronts) show high [SII]6716/[SII]6761 and high [SII]6716/H$α$, implying very low electron densities (<30x lower than all other star-forming galaxies outside the cluster) and significant contribution from supernovae, respectively. All cluster star-forming galaxies near the cluster centre show evidence of significant outflows (blueshifted Na D~200-300km/s), likely driven by supernovae. Strong outflows are also found for the cluster H$α$ AGN. H$α$ star-forming galaxies in the merging cluster follow the z~0 mass-metallicity relation, showing systematically higher metallicity (~0.15-0.2 dex) than H$α$ emitters outside the cluster (projected R>2.5 Mpc). This suggests that the shock front may have triggered remaining metal-rich gas which galaxies were able to retain into forming stars. Our observations show that the merger of impressively massive (~10$^{15}$M$_\odot$) clusters can provide the conditions for significant star-formation and AGN activity, but, as we witness strong feedback by star-forming galaxies and AGN (and given how massive the merging cluster is), such sources will likely quench in a few 100 Myrs.

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arXiv2014

The rise and fall of star-formation in $\bf z\sim0.2$ merging galaxy clusters

CIZA J2242.8+5301 (`Sausage') and 1RXS J0603.3+4213 (`Toothbrush') are two low-redshift ($z\sim0.2$), massive ($\sim2\times10^{15}M_\odot$), post-core passage merging clusters, which host shock waves traced by diffuse radio emission. To study their star-formation properties, we uniformly survey the `Sausage' and `Toothbrush' clusters in broad and narrow band filters and select a sample of $201$ and $463$ line emitters, down to a rest-frame equivalent width ($13$Å). We robustly separate between H$α$ and higher redshift emitters using a combination of optical multi-band (B, g, V, r, i, z) and spectroscopic data. We build H$α$ luminosity functions for the entire cluster region, near the shock fronts, and away from the shock fronts and find striking differences between the two clusters. In the dynamically younger, $1$ Gyr old `Sausage' cluster we find numerous ($59$) H$α$ emitters above a star-formation rate (SFR) of $0.17$ M$_{\sun}$ yr$^{-1}$ surprisingly located in close proximity to the shock fronts, embedded in very hot intra-cluster medium plasma. The SFR density for the cluster population is at least at the level of typical galaxies at $z\sim2$. Down to the same star-formation rate, the possibly dynamically more evolved `Toothbrush' cluster has only $9$ H$α$ galaxies. The cluster H$α$ galaxies fall on the SFR-stellar mass relation $z\sim0.2$ for the field. However, the `Sausage' cluster has an H$α$ emitter density $>20$ times that of blank fields. If the shock passes through gas-rich cluster galaxies, the compressed gas could collapse into dense clouds and excite star-formation for a few $100$ Myr. This process ultimately leads to a rapid consumption of the molecular gas, accelerating the transformation of gas-rich field spirals into cluster S0s or ellipticals.

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arXiv2014

MC^2: Constraining the Dark Matter Distribution of the Violent Merging Galaxy Cluster CIZA J2242.8+5301 by Piercing through the Milky Way

CIZA J2242.8+5301 is a merging system with a prominent (~2 Mpc long) radio relic, which together with the morphology of the X-ray emission provides strong evidence for a violent collision along the N-S axis. We present our constraints on the dark matter distribution of this unusual system using Subaru and CFHT imaging data. Measuring a high S/N lensing signal from this cluster is potentially a challenging task because of its proximity to the Milky Way plane (|b|~5 deg). We overcome this challenge with careful observation planning and systematics control, which enables us to successfully map the dark matter distribution of the cluster with high fidelity. The resulting mass map shows that the mass distribution is highly elongated along the N-S merger axis inferred from the orientation of the radio relics. Based on our mass reconstruction, we identify two sub-clusters, which coincide with the cluster galaxy distributions. We determine their masses using MCMC analysis by simultaneously fitting two NFW halos without fixing their centroids. The resulting masses of the northern and southern systems are $M_{200}=11.0_{-3.2}^{+3.7}\times10^{14} M_{\sun}$ and $9.8_{-2.5}^{+3.8}\times10^{14} M_{\sun}$, respectively, indicating that we are witnessing a post-collision of two giant systems of nearly equal mass. When the mass and galaxy centroids are compared in detail, we detect ~ 1' (~190 kpc) offsets in both northern and southern sub-clusters. We find that the galaxy luminosity-mass offset for the northern clump is statistically significant at the ~2 sigma level whereas the detection is only marginal for the southern sub-cluster in part because of a relatively large mass centroid error. We conclude that it is yet premature to uniquely attribute the galaxy-mass misalignment to SIDM and discuss caveats.

astro-ph.CO↗

arXiv2014

MC^2: Galaxy Imaging and Redshift Analysis of the Merging Cluster CIZA J2242.8+5301

X-ray and radio observations of CIZA J2242.8+5301 suggest that it is a major cluster merger. Despite being well studied in the X-ray, and radio, little has been presented on the cluster structure and dynamics inferred from its galaxy population. We carried out a deep (i<25) broad band imaging survey of the system with Subaru SuprimeCam (g & i bands) and the Canada France Hawaii Telescope (r band) as well as a comprehensive spectroscopic survey of the cluster area (505 redshifts) using Keck DEIMOS. We use this data to perform a comprehensive galaxy/redshift analysis of the system, which is the first step to a proper understanding the geometry and dynamics of the merger, as well as using the merger to constrain self-interacting dark matter. We find that the system is dominated by two subclusters of comparable richness with a projected separation of 6.9'^{+0.7}_{-0.5} (1.3^{+0.13}_{-0.10} Mpc). We find that the north and south subclusters have similar redshifts of z=0.188 with a relative line-of-sight velocity difference of 69+/-190 km/s. We also find that north and south subclusters have velocity dispersions of 1160^{+100}_{-90} km/s and 1080^{+100}_{-70} km/s, respectively. These correspond to masses of 16.1^{+4.6}_{-3.3}x10^14 M_sun and 13.0^{+4.0}_{-2.5}x10^14 M_sun, respectively. While velocity dispersion measurements of merging clusters can be biased we believe the bias in this system to be minor due to the large projected separation and nearly plane-of-sky merger configuration. CIZA J2242.8+5301 is a relatively clean dissociative cluster merger with near 1:1 mass ratio, which makes it an ideal merger for studying merger associated physical phenomena.

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