arXiv subjects

D. Wittman

Publications and source records attributed to D. Wittman.

At least 19 recordsLinked to original sources

arXiv2023

Abell 746: A highly disturbed cluster undergoing multiple mergers

We present deep XMM-Newton, Karl Jansky Very Large Array, and upgraded Giant Metrewave Radio Telescope observations of Abell 746, a cluster that hosts a plethora of diffuse emission sources that provide evidence for the acceleration of relativistic particles. Our new XMM-Newton images reveal a complex morphology of the thermal gas with several substructures. We observe an asymmetric temperature distribution across the cluster: the southern regions exhibit higher temperatures, reaching ~9 keV, while the northern regions have lower temperatures (below 4 keV), likely due to a complex merger. We find evidence of four surface brightness edges, of which three are merger-driven shock fronts. Combining our new data with the published LOw-Frequency ARray observations has unveiled the nature of diffuse sources in this system. The bright northwest relic shows thin filaments and high degree of polarization with aligned magnetic field vectors. We detect a density jump, aligned with the fainter relic to the north. To the south, we detect high-temperature regions, consistent with shock-heated regions and density jump coincident with the northern tip of the southern radio structure. Its integrated spectrum shows a high-frequency steepening. Lastly, we find that the cluster hosts large-scale radio halo emission. The comparison of the thermal and nonthermal emission reveals an anticorrelation between the bright radio and X-ray features at the center. Our findings suggest that Abell 746 is a complex system that involves multiple mergers.

astro-ph.CO↗

arXiv2017

On the absence of radio halos in clusters with double relics

Pairs of radio relics are believed to form during cluster mergers, and are best observed when the merger occurs in the plane of the sky. Mergers can also produce radio halos, through complex processes likely linked to turbulent re-acceleration of cosmic-ray electrons. However, only some clusters with double relics also show a radio halo. Here, we present a novel method to derive upper limits on the radio halo emission, and analyse archival X-ray Chandra data, as well as galaxy velocity dispersions and lensing data, in order to understand the key parameter that switches on radio halo emission. We place upper limits on the halo power below the $P_{\rm 1.4 \, GHz}\, M_{500}$ correlation for some clusters, confirming that clusters with double relics have different radio properties. Computing X-ray morphological indicators, we find that clusters with double relics are associated with the most disturbed clusters. We also investigate the role of different mass-ratios and time-since-merger. Data do not indicate that the merger mass ratio has an impact on the presence or absence of radio halos (the null hypothesis that the clusters belong to the same group cannot be rejected). However, the data suggests that the absence of radio halos could be associated with early and late mergers, but the sample is too small to perform a statistical test. Our study is limited by the small number of clusters with double relics. Future surveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to better address this issue.

astro-ph.CO↗

arXiv2016

Star Formation In The Cluster Merger DLSCL J0916.2+2953

We investigate star formation in DLSCL J0916.2+2953, a dissociative merger of two clusters at z=0.53 that has progressed $1.1^{+1.3}_{-0.4}$ Gyr since first pass-through. We attempt to reveal the effects a collision may have had on the evolution of the cluster galaxies by tracing their star formation history. We probe current and recent activity to identify a possible star formation event at the time of the merger using EW(Hd), EW([OII]), and D$_{n}$4000 measured from the composite spectra of 64 cluster and 153 coeval field galaxies. We supplement $Keck$ DEIMOS spectra with DLS and $HST$ imaging to determine the color, stellar mass, and morphology of each galaxy and conduct a comprehensive study of the populations in this complex structure. Spectral results indicate the average cluster and cluster red sequence galaxies experienced no enhanced star formation relative to the surrounding field during the merger, ruling out a predominantly merger-quenched population. We find that the average blue galaxy in the North cluster is currently active and in the South cluster is currently post-starburst having undergone a recent star formation event. While the North activity could be latent or long-term merger effects, a young blue stellar population and irregular geometry suggest the cluster was still forming prior the collision. While the South activity coincides with the time of the merger, the blue early-type population could be a result of secular cluster processes. The evidence suggests that the dearth or surfeit of activity is indiscernible from normal cluster galaxy evolution.

astro-ph.GA↗

arXiv2015

A powerful double radio relic system discovered in PSZ1 G108.18-11.53: evidence for a shock with non-uniform Mach number?

Diffuse radio emission in the form of radio halos and relics has been found in a number of merging galaxy clusters. These structures indicate that shock and turbulence associated with the merger accelerate electrons to relativistic energies. We report the discovery of a radio relic + radio halo system in PSZ1 G108.18-11.53 (z=0.335). This cluster hosts the second most powerful double radio relic system ever discovered. We observed PSZ1 G108.18-11.53 with the Giant Meterwave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). We obtained radio maps at 147, 323, 607 and 1380 MHz. We also observed the cluster with the Keck telescope, obtaining the spectroscopic redshift for 42 cluster members. From the injection index we obtained the Mach number of the shocks generating the two radio relics. For the southern shock we found M = 2.33^{+0.19}_{-0.26}, while the northern shock Mach number goes from M = 2.20^{+0.07}_{-0.14} in the north part down to M = 2.00^{+0.03}_{-0.08} in the southern region. If the relation between the injection index and the Mach number predicted by diffusive shock acceleration (DSA) theory holds, this is the first observational evidence for a gradient in the Mach number along a galaxy cluster merger shock.

astro-ph.HE↗

arXiv2015

The return of the merging galaxy subclusters of El Gordo?

Merging galaxy clusters with radio relics provide rare insights to the merger dynamics as the relics are created by the violent merger process. We demonstrate one of the first uses of the properties of the radio relic to reduce the uncertainties of the dynamical variables and determine the 3D configuration of a cluster merger, ACT-CL J0102-4915, nicknamed El Gordo. From the double radio relic observation and the X-ray observation of a comet-like gas morphology induced by motion of the cool core, it is widely believed that El Gordo is observed shortly after the first core-passage of the subclusters. We employ a Monte Carlo simulation to investigate the three-dimensional (3D) configuration and dynamics of El Gordo. Using the polarization fraction of the radio relic, we constrain the estimate of the angle between the plane of the sky and the merger axis to be $α= 21~{\rm degree} \pm^9_{11}$. We find the relative 3D merger speed of El Gordo to be $2400\pm^{400}_{200}~{\rm km}~{\rm s}^{-1}$ at pericenter. The two possible estimates of the time-since-pericenter are $0.46\pm^{0.09}_{0.16}$ Gyr and $0.91\pm^{0.22}_{0.39}$ Gyr for the outgoing and returning scenario respectively. We put our estimates of the time-since-pericenter into context by showing that if the time-averaged shock velocity is approximately equal to or smaller than the pericenter velocity of the corresponding subcluster in the center of mass frame, the two subclusters are more likely to be moving towards, rather than away, from each other, post apocenter. We compare and contrast the merger scenario of El Gordo with that of the Bullet Cluster, and show that this late-stage merging scenario explains why the southeast dark matter lensing peak of El Gordo is closer to the merger center than the southeast cool core.

astro-ph.CO↗

arXiv2013

Shedding Light on the Matter of Abell 781

The galaxy cluster Abell 781 West has been viewed as a challenge to weak gravitational lensing mass calibration, as Cook and dell'Antonio (2012) found that the weak lensing signal-to-noise in three independent sets of observations was consistently lower than expected from mass models based on X-ray and dynamical measurements. We correct some errors in statistical inference in Cook and dell'Antonio (2012) and show that their own results agree well with the dynamical mass and exhibit at most 2.2--2.9$σ$ low compared to the X-ray mass, similar to the tension between the dynamical and X-ray masses. Replacing their simple magnitude cut with weights based on source photometric redshifts eliminates the tension between lensing and X-ray masses; in this case the weak lensing mass estimate is actually higher than, but still in agreement with, the dynamical estimate. A comparison of lensing analyses with and without photometric redshifts shows that a 1--2$σ$ chance alignment of low-redshift sources lowers the signal-to-noise observed by all previous studies which used magnitude cuts rather than photometric redshifts. The fluctuation is unexceptional, but appeared to be highly significant in Cook and dell'Antonio (2012) due to the errors in statistical interpretation.

astro-ph.CO↗

arXiv2012

Constraining Source Redshift Distributions with Gravitational Lensing

We introduce a new method for constraining the redshift distribution of a set of galaxies, using weak gravitational lensing shear. Instead of using observed shears and redshifts to constrain cosmological parameters, we ask how well the shears around clusters can constrain the redshifts, assuming fixed cosmological parameters. This provides a check on photometric redshifts, independent of source spectral energy distribution properties and therefore free of confounding factors such as misidentification of spectral breaks. We find that ~40 massive ($σ_v=1200$ km/s) cluster lenses are sufficient to determine the fraction of sources in each of six coarse redshift bins to ~11%, given weak (20%) priors on the masses of the highest-redshift lenses, tight (5%) priors on the masses of the lowest-redshift lenses, and only modest (20-50%) priors on calibration and evolution effects. Additional massive lenses drive down uncertainties as $N_{lens}^0.5$, but the improvement slows as one is forced to use lenses further down the mass function. Future large surveys contain enough clusters to reach 1% precision in the bin fractions if the tight lens mass priors can be maintained for large samples of lenses. In practice this will be difficult to achieve, but the method may be valuable as a complement to other more precise methods because it is based on different physics and therefore has different systematic errors.

astro-ph.CO↗

arXiv2012

Ubercalibration of the Deep Lens Survey

We describe the internal photometric calibration of the Deep Lens Survey, which consists of five widely separated fields observed by two different observatories. Adopting the global linear least-squares ("ubercal") approach developed for the Sloan Digital Sky Survey (SDSS), we derive flatfield corrections for all observing runs, which indicate that the original sky flats were nonuniform by up to 0.13 mag peak to valley in $\z$ band, and by up to half that amount in {\it BVR}. We show that application of these corrections reduces spatial nonuniformities in corrected exposures to the 0.01-0.02 mag level. We conclude with some lessons learned in applying ubercal to a survey structured very differently from SDSS, with isolated fields, multiple observatories, and shift-and-stare rather than drift-scan imaging. Although the size of the error caused by using sky or dome flats is instrument- and wavelength-dependent, users of wide-field cameras should not assume that it is small. Pipeline developers should facilitate routine application of this procedure, and surveys should include it in their plans from the outset.

astro-ph.IM↗

arXiv2010

Shaping Attitudes Toward Science in an Introductory Astronomy Class

At many universities, astronomy is a popular way for non-science majors to fulfill a general education requirement. Because general-education astronomy may be the only college-level science course taken by these students, it is the last chance to shape the science attitudes of these future journalists, teachers, politicians, and voters. I report on an attempt to measure and induce changes in science attitudes in my general-education astronomy course. I describe construction of the attitude survey, classroom activities designed to influence attitudes, and give numerical results indicating a significant improvement. In contrast, the literature on attitudes in introductory physics courses generally reports stagnation or decline. I briefly comment on some plausible explanations for this difference.

physics.ed-ph↗

arXiv2009

Shear-Selected Clusters From the Deep Lens Survey III: Masses from Weak Lensing

We present weak lensing mass estimates of seven shear-selected galaxy cluster candidates from the Deep Lens Survey. The clusters were previously identified as mass peaks in convergence maps of 8.6 sq. deg of R band imaging, and followed up with X-ray and spectroscopic confirmation, spanning a redshift range 0.19 - 0.68. Most clusters contained multiple X-ray peaks, yielding 17 total mass concentrations. In this paper, we constrain the masses of these X-ray sources with weak lensing, using photometric redshifts from the full set of BVRz' imaging to properly weight background galaxies according to their lensing distance ratios. We fit both NFW and singular isothermal sphere profiles, and find that the results are insensitive to the assumed profile. We also show that the results do not depend significantly on the assumed prior on the position of the mass peak, but that this may become an issue in future larger samples. The inferred velocity dispersions for the extended X-ray sources range from 250-800 km/s, with the exception of one source for which no lensing signal was found. This work further establishes shear selection as a viable technique for finding clusters, but also highlights some unresolved issues such as determination of the mass profile center without biasing the mass estimate, and fully accounting for line-of-sight projections. A follow-up paper will examine the mass-X-ray scaling relations of these clusters.

astro-ph.CO↗

arXiv2009

What Lies Beneath: Using p(z) to Reduce Systematic Photometric Redshift Errors

We use simulations to demonstrate that photometric redshift "errors" can be greatly reduced by using the photometric redshift probability distribution p(z) rather than a one-point estimate such as the most likely redshift. In principle this involves tracking a large array of numbers rather than a single number for each galaxy. We introduce a very simple estimator that requires tracking only a single number for each galaxy, while retaining the systematic-error-reducing properties of using the full p(z) and requiring only very minor modifications to existing photometric redshift codes. We find that using this redshift estimator (or using the full p(z)) can substantially reduce systematics in dark energy parameter estimation from weak lensing, at no cost to the survey.

astro-ph.CO↗

arXiv2009

Dark Matter Structures in the Universe: Prospects for Optical Astronomy in the Next Decade

The Cold Dark Matter theory of gravitationally-driven hierarchical structure formation has earned its status as a paradigm by explaining the distribution of matter over large spans of cosmic distance and time. However, its central tenet, that most of the matter in the universe is dark and exotic, is still unproven; the dark matter hypothesis is sufficiently audacious as to continue to warrant a diverse battery of tests. While local searches for dark matter particles or their annihilation signals could prove the existence of the substance itself, studies of cosmological dark matter in situ are vital to fully understand its role in structure formation and evolution. We argue that gravitational lensing provides the cleanest and farthest-reaching probe of dark matter in the universe, which can be combined with other observational techniques to answer the most challenging and exciting questions that will drive the subject in the next decade: What is the distribution of mass on sub-galactic scales? How do galaxy disks form and bulges grow in dark matter halos? How accurate are CDM predictions of halo structure? Can we distinguish between a need for a new substance (dark matter) and a need for new physics (departures from General Relativity)? What is the dark matter made of anyway? We propose that the central tool in this program should be a wide-field optical imaging survey, whose true value is realized with support in the form of high-resolution, cadenced optical/infra-red imaging, and massive-throughput optical spectroscopy.

astro-ph.CO↗

arXiv2008

LSST and the Dark Sector: Image Processing Challenges

Next generation probes of dark matter and dark energy require high precision reconstruction of faint galaxy shapes from hundreds of dithered exposures. Current practice is to stack the images. While valuable for many applications, this stack is a highly compressed version of the data. Future weak lensing studies will require analysis of the full dataset using the stack and its associated catalog only as a starting point. We describe a "Multi-Fit" algorithm which simultaneously fits individual galaxy exposures to a common profile model convolved with each exposure's point spread function at that position in the image. This technique leads to an enhancement of the number of usable small galaxies at high redshift and, more significantly, a decrease in systematic shear error.

astro-ph↗

arXiv2007

Photometric Redshifts and Photometry Errors

We examine the impact of non-Gaussian photometry errors on photometric redshift performance. We find that they greatly increase the scatter, but this can be mitigated to some extent by incorporating the correct noise model into the photometric redshift estimation process. However, the remaining scatter is still equivalent to that of a much shallower survey with Gaussian photometry errors. We also estimate the impact of non-Gaussian errors on the spectroscopic sample size required to verify the photometric redshift rms scatter to a given precision. Even with Gaussian {\it photometry} errors, photometric redshift errors are sufficiently non-Gaussian to require an order of magnitude larger sample than simple Gaussian statistics would indicate. The requirements increase from this baseline if non-Gaussian photometry errors are included. Again the impact can be mitigated by incorporating the correct noise model, but only to the equivalent of a survey with much larger Gaussian photometry errors. However, these requirements may well be overestimates because they are based on a need to know the rms, which is particularly sensitive to tails. Other parametrizations of the distribution may require smaller samples.

astro-ph↗

arXiv2006

First Results On Shear-Selected Clusters From the Deep Lens Survey: Optical Imaging, Spectroscopy, and X-ray Followup

We present the first sample of galaxy clusters selected on the basis of their weak gravitational lensing shear. The shear induced by a cluster is a function of its mass profile and its redshift relative to the background galaxies being sheared; in contrast to more traditional methods of selecting clusters, shear selection does not depend on the cluster's star formation history, baryon content, or dynamical state. Because mass is the property of clusters which provides constraints on cosmological parameters, the dependence on these other parameters could induce potentially important biases in traditionally-selected samples. Comparison of a shear-selected sample with optically and X-ray selected samples is therefore of great importance. Here we present the first step toward a new shear-selected sample: the selection of cluster candidates from the first 8.6 deg$^2$ of the 20 deg$^2$ Deep Lens Survey (DLS), and tabulation of their basic properties such as redshifts and optical and X-ray counterparts.

astro-ph↗

arXiv2006

Three Gravitational Lenses for the Price of One: Enhanced Strong Lensing through Galaxy Clustering

We report the serendipitous discovery of two strong gravitational lens candidates (ACS J160919+6532 and ACS J160910+6532) in deep images obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, each less than 40 arcsec from the previously known gravitational lens system CLASS B1608+656. The redshifts of both lens galaxies have been measured with Keck and Gemini: one is a member of a small galaxy group at z~0.63, which also includes the lensing galaxy in the B1608+656 system, and the second is a member of a foreground group at z~0.43. By measuring the effective radii and surface brightnesses of the two lens galaxies, we infer their velocity dispersions based on the passively evolving Fundamental Plane (FP) relation. Elliptical isothermal lens mass models are able to explain their image configurations within the lens hypothesis, with a velocity dispersion compatible with that estimated from the FP for a reasonable source-redshift range. Based on the large number of massive early-type galaxies in the field and the number-density of faint blue galaxies, the presence of two additional lens systems around CLASS B1608+656 is not unlikely in hindsight. Gravitational lens galaxies are predominantly early-type galaxies, which are clustered, and the lensed quasar host galaxies are also clustered. Therefore, obtaining deep high-resolution images of the fields around known strong lens systems is an excellent method of enhancing the probability of finding additional strong gravitational lens systems.

astro-ph↗

arXiv2005

Spurious Shear from the Atmosphere in Ground-Based Weak Lensing Observations

Weak lensing observations have the potential to be even more powerful than cosmic microwave background (CMB) observations in constraining cosmological parameters. However, the practical limits to weak lensing observations are not known. Most theoretical studies of weak lensing constraints on cosmology assume that the only limits are shot noise on small scales, and cosmic variance on large scales. For future large surveys, shot noise will be so low that other, systematic errors will likely dominate. Here we examine a potential source of additive systematic error for ground-based observations: spurious power induced by the atmosphere. We show that this limit will not be a significant factor even in future massive surveys such as LSST.

astro-ph↗

arXiv2003

Weak Lensing Discovery and Tomography of a Cluster at z=0.68

We report the weak lensing discovery, spectroscopic confirmation, and weak lensing tomography of a massive cluster of galaxies at $z=0.68$, demonstrating that shear selection of clusters works at redshifts high enough to be cosmologically interesting. The mass estimate from weak lensing, $11.1 +- 2.8 x 10^{14} (r/Mpc)$ solar masses within projected radius r, agrees with that derived from the spectroscopy ($σ_v = 980 km s^{-1}$), and with the position of an arc which is likely to be a strongly lensed background galaxy. The redshift estimate from weak lensing tomography is consistent with the spectroscopy, demonstrating the feasibility of baryon-unbiased mass surveys. This tomographic technique will be able to roughly identify the redshifts of any dark clusters which may appear in shear-selected samples, up to z ~ 1.

astro-ph↗