Weak lensing study of Abell 2029

Abell 2029 is one of the most studied clusters due to its proximity (z=0.07), its strong X-ray brightness and its giant cD galaxy which is one of the biggest stellar aggregates we know. We present here the first weak lensing mass reconstruction of this cluster made from a deep I-band image of 28.5'x28.5' centered on the cluster cD galaxy. This preliminary result allows us already to show the shape similarities between the cD galaxy and the cluster itself, suggesting that they form actually a single structure. We find a lower estimate of the total mass of 1.8 10^14 h^-1 solar masses within a radius of 0.3 h^-1 Mpc. We finally compute the mass-to-cD-light ratio and its evolution as a function of scale.Comment: 5 pages, to appear in the proceedings of "Matter and Energy in Clusters of Galaxies", ASP Conference Serie

GaBoDS: The Garching-Bonn Deep Survey -- IX. A sample of 158 shear-selected mass concentration candidates

The aim of the present work is the construction of a mass-selected galaxy cluster sample based on weak gravitational lensing methods. This sample will be subject to spectroscopic follow-up observations. We apply the mass aperture statistics and a derivative of it to 19 square degrees of high quality, single colour wide field imaging data obtained with the WFI@MPG/ESO 2.2m telescope. For the statistics a family of filter functions is used that approximates the expected tangential radial shear profile and thus allows for the efficient detection of mass concentrations. We identify 158 possible mass concentrations. This is the first time that such a large and blindly selected sample is published. 72 of the detections are associated with concentrations of bright galaxies. For about 22 of those we found spectra in the literature, indicating or proving that the galaxies seen are indeed spatially concentrated. 15 of those were previously known to be clusters or have meanwhile been secured as such. We currently follow-up a larger number of them spectroscopically to obtain deeper insight into their physical properties. The remaining 55% of the possible mass concentrations found are not associated with any optical light, or could not be classified unambiguously. We show that those "dark" detections are to a significant degree due to noise, and appear preferentially in shallow data.Comment: 25 pages, 18 figures, submitted to A&A; for a better print version, see http://www.astro.uni-bonn.de/~mischa/astroph_0607022.pd

Studying the Nature of Dark Energy with Galaxy Clusters

We report on the status of our effort to constrain the nature of dark energy through the evolution of the cluster mass function. Chandra temperature profiles for 31 clusters from a local cluster sample are shown. The X-ray appearance of the proto supermassive binary black hole at the center of the cluster Abell 400 is described. Preliminary weak lensing results obtained with Megacam@MMT for a redshift z=0.5 cluster from a distant cluster sample are given.Comment: 5 pages, to appear in: Aschenbach, B., Burwitz, V., Hasinger, G., Leibundgut, B. (eds.), Relativistic Astrophysics and Cosmology - Einstein's Legacy. ESO Astrophysics Symposia, Springer Verlag, Berlin, German

A New X-ray Selected Sample of Very Extended Galaxy Groups from the ROSAT All-Sky Survey

Some indications for tension have long been identified between cosmological constraints obtained from galaxy clusters and primary CMB measurements. Typically, assuming the matter density and fluctuations, as parameterized with Omega_m and sigma_8, estimated from CMB measurements, many more clusters are expected than those actually observed. One possible explanation could be that certain types of galaxy groups or clusters were missed in samples constructed in previous surveys, resulting in a higher incompleteness than estimated. We aim to determine if a hypothetical class of very extended, low surface brightness, galaxy groups or clusters have been missed in previous X-ray cluster surveys based on the ROSAT All-Sky Survey (RASS). We applied a dedicated source detection algorithm sensitive also to more unusual group or cluster surface brightness distributions. We found many known but also a number of new group candidates, which are not included in any previous X-ray / SZ cluster catalogs. In this paper, we present a pilot sample of 13 very extended groups discovered in the RASS at positions where no X-ray source has been detected previously and with clear optical counterparts. The X-ray fluxes of at least 5 of these are above the nominal flux-limits of previous RASS cluster catalogs. They have low mass ($10^{13} - 10^{14} M_{\odot}$; i.e., galaxy groups), are at low redshift (z<0.08), and exhibit flatter surface brightness distributions than usual. We demonstrate that galaxy groups were missed in previous RASS surveys, possibly due to the flat surface brightness distributions of this potential new population. Analysis of the full sample will show if this might have a significant effect on previous cosmological parameter constraints based on RASS cluster surveys. (This is a shortened version of the abstract - full text in the article)Comment: 18 pages, 7 figures, accepted by A&

The 400d Galaxy Cluster Survey weak lensing programme: II: Weak lensing study of seven clusters with MMT/Megacam

Evolution in the mass function of galaxy clusters sensitively traces both the expansion history of the Universe and cosmological structure formation. Robust cluster mass determinations are a key ingredient for a reliable measurement of this evolution, especially at high redshift. Weak gravitational lensing is a promising tool for, on average, unbiased mass estimates. This weak lensing project aims at measuring reliable weak lensing masses for a complete X-ray selected sample of 36 high redshift (0.35<z<0.9) clusters. The goal of this paper is to demonstrate the robustness of the methodology against commonly encountered problems, including pure instrumental effects, the presence of bright (8--9 mag) stars close to the cluster centre, ground based measurements of high-z (z~0.8) clusters, and the presence of massive unrelated structures along the line-sight. We select a subsample of seven clusters observed with MMT/Megacam. Instrumental effects are checked in detail by cross-comparison with an archival CFHT/MegaCam observation. We derive mass estimates for seven clusters by modelling the tangential shear with an NFW profile, in two cases with multiple components to account for projected structures in the line-of-sight. We firmly detect lensing signals from all seven clusters at more than $3.5\sigma$ and determine their masses, ranging from $10^{14} M_{\odot}$ to $10^{15} M_{\odot}$, despite the presence of nearby bright stars. We retrieve the lensing signal of more than one cluster in the CL 1701+6414 field, while apparently observing CL 1701+6414 through a massive foreground filament. We also find a multi-peaked shear signal in CL 1641+4001. Shear structures measured in the MMT and CFHT images of CL 1701+6414 are highly correlated.Comment: Accepted for publication in Astronomy & Astrophysics; A&A 546, A7

Arbeit und Einkommen in der Solidarischen Landwirtschaft

Eine Solidarische Landwirtschaft (Solawi) ist ein Modell der verbindlichen Zusammenarbeit zwischen ErzeugerInnen und VerbraucherInnen, die sich als eine Gemeinschaft aus aktiven und nicht-aktiven Landwirten verstehen, Verantwortung für die Produktion übernehmen und das Risiko sowie die Ernte teilen. Trotz des großen Potentials für eine bedürfnisorientierte Gestaltung von Arbeitsplätzen und Einkommen geben neuere Untersuchungen Hinweise auf niedrige Einkommen und Arbeitsbedingungen, die nicht den Bedürfnissen der ErzeugerInnen entsprechen. Ein Ziel der vorliegenden Arbeit ist es, einen Überblick über derzeitige Situation der deutschsprachigen Solawi-Betriebe hinsichtlich Einkommensniveau, Arbeitszeiten und Arbeitszufriedenheit der ErzeugerInnen zu bekommen. Dies wurde mit standardisierten Online-Fragebogen untersucht. Parallel wurden qualitative Interviews mit ausgewählten ErzeugerInnen geführt, um Einblicke in ihre Erfahrungen und Zukunftsperpektiven für die Arbeits-Gestaltung in Solawis zu gewinnen. Die Ergebnisse zeigen, dass die Einkommen der BewirtschafterInnen zwar über dem Mindestlohn liegen, aber z.T. deutlich unter denen von BetriebsleiterInnen und ArbeitnehmerInnen in normalen landwirtschaftlichen Betrieben. Die befragten MitarbeiterInnen bewerten ihre Einkommen je nach Arbeitsverhältnis als ausreichend bis nicht ausreichend, während sie bei der Arbeitszeit zwar Veränderungsbedarf äußern, das Verhältnis zur Freizeit jedoch als relativ ausgeglichen ansehen. Aus den Interviews wird deutlich, dass die MitarbeiterInnen die Finanzierung in Solawis als grundlegende soziale Herausforderung und Entwicklungschance begreifen, die Vertrauen, Gemeinschaft und Kommunikation voraussetzen. An einzelnen Beispielen zeigt sich, dass mangelnde Klarheit über die soziale Dimension zu durchaus prekären Situationen und sozialer Distanz führen kann. Andererseits können die BewirtschafterInnen durch selbstbewusste und klare Formulierung ihres finanziellen Bedarfs bei gleichzeitiger Dialogbereitschaft zu einer für beide Seiten durchaus zufriedenstellenden Einigung beitragen. Eine Grundvoraussetzung für den Dialog über Landwirtschaft und solidarische Finanzierung ist aus Sicht vieler InterviewpartnerInnen die gezielte Einbindung und Sensibilisierung der VerbraucherInnen durch jede Art von Bildungsarbeit. Bei einzelnen Solawis wird deutlich, dass eine verbindliche Vereinbarung den Umfang der Mitarbeit erhöhen, die Kosten deutlich senken und gleichzeitig die Begegnung fördern kann. Im Dialog und der Bildungsarbeit sehen die Befragten die größten Potentiale für den Aufbau sozialer Beziehungen zwischen VerbraucherInnen und ErzeugerInnen, die wiederum die Basis für das Verständnis und die Wertschätzung der jeweiligen Bedürfnisse darstellt. Die Verwirklichung sozialer Grundwerte in der Solidarischen Landwirtschaft sollte vor dem Hintergrund des derzeit rasanten Wachstums der Bewegung aufmerksam beobachtet werden, um dem Ideal einer fairen Bezahlung der LandwirtInnen gerecht zur werden

Cluster Magnification & the Mass-Richness Relation in CFHTLenS

Gravitational lensing magnification is measured with a significance of 9.7 sigma on a large sample of galaxy clusters in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). This survey covers ~154 deg^2 and contains over 18,000 cluster candidates at redshifts 0.2 <= z <= 0.9, detected using the 3D-Matched Filter cluster-finder of Milkeraitis et al. (2010). We fit composite-NFW models to the ensemble, accounting for cluster miscentering, source-lens redshift overlap, as well as nearby structure (the 2-halo term), and recover mass estimates of the cluster dark matter halos in range of ~10^13 M_sun to 2*10^14 M_sun. Cluster richness is measured for the entire sample, and we bin the clusters according to both richness and redshift. A mass-richness relation M_200 = M_0 (N_200 / 20)^beta is fit to the measurements. For two different cluster miscentering models we find consistent results for the normalization and slope, M_0 = (2.3 +/- 0.2)*10^13 M_sun, beta = 1.4 +/- 0.1 and M_0 = (2.2 +/- 0.2)*10^13 M_sun, beta = 1.5 +/- 0.1. We find that accounting for the full redshift distribution of lenses and sources is important, since any overlap can have an impact on mass estimates inferred from flux magnification.Comment: 11 pages, 8 figures, Accepted to MNRA

The RedGOLD cluster detection algorithm and its cluster candidate catalogue for the CFHT-LS W1

We present RedGOLD (Red-sequence Galaxy Overdensity cLuster Detector), a new optical/NIR galaxy cluster detection algorithm, and apply it to the CFHT-LS W1 field. RedGOLD searches for red-sequence galaxy overdensities while minimizing contamination from dusty star-forming galaxies. It imposes an Navarro–Frenk–White profile and calculates cluster detection significance and richness. We optimize these latter two parameters using both simulations and X-ray-detected cluster catalogues, and obtain a catalogue ∼80 per cent pure up to z ∼ 1, and ∼100 per cent (∼70 per cent) complete at z ≤ 0.6 (z ≲ 1) for galaxy clusters with M ≳ 10^(14) M_⊙ at the CFHT-LS Wide depth. In the CFHT-LS W1, we detect 11 cluster candidates per deg^2 out to z ∼ 1.1. When we optimize both completeness and purity, RedGOLD obtains a cluster catalogue with higher completeness and purity than other public catalogues, obtained using CFHT-LS W1 observations, for M ≳ 10^(14) M_⊙. We use X-ray-detected cluster samples to extend the study of the X-ray temperature–optical richness relation to a lower mass threshold, and find a mass scatter at fixed richness of σ_(lnM|λ) = 0.39 ± 0.07 and σ_(lnM|λ) = 0.30 ± 0.13 for the Gozaliasl et al. and Mehrtens et al. samples. When considering similar mass ranges as previous work, we recover a smaller scatter in mass at fixed richness. We recover 93 per cent of the redMaPPer detections, and find that its richness estimates is on average ∼40–50 per cent larger than ours at z > 0.3. RedGOLD recovers X-ray cluster spectroscopic redshifts at better than 5 per cent up to z ∼ 1, and the centres within a few tens of arcseconds