How does the grouping scheme affect the Wiener Filter reconstruction of the local Universe?

High quality reconstructions of the three dimensional velocity and density fields of the local Universe are essential to study the local Large Scale Structure. In this paper, the Wiener Filter reconstruction technique is applied to galaxy radial peculiar velocity catalogs to understand how the Hubble constant (H0) value and the grouping scheme affect the reconstructions. While H0 is used to derive radial peculiar velocities from galaxy distance measurements and total velocities, the grouping scheme serves the purpose of removing non linear motions. Two different grouping schemes (based on the literature and a systematic algorithm) as well as five H0 values ranging from 72 to 76 km/s/Mpc are selected. The Wiener Filter is applied to the resulting catalogs. Whatever grouping scheme is used, the larger H0 is, the larger the infall onto the local Volume is. However, this conclusion has to be strongly mitigated: a bias minimization scheme applied to the catalogs after grouping suppresses this effect. At fixed H0, reconstructions obtained with catalogs grouped with the different schemes exhibit structures at the proper location in both cases but the latter are more contrasted in the less aggressive scheme case: having more constraints permits an infall from both sides onto the structures to reinforce their overdensity. Such findings highlight the importance of a balance between grouping to suppress non linear motions and preserving constraints to produce an infall onto structures expected to be large overdensities. Such an observation is promising to perform constrained simulations of the local Universe including its massive clusters.Comment: Accepted for publication in MNRAS, 10 pages, 6 figures, 3 table

The Role of Community Land Trusts in Preserving and Creating Commercial Assets: A Dual Cae Study of Rondo CLT in St. Paul, Minnesota and Crescent City CLT in New Orleans, Louisiana

As the community land trust (CLT) movement in the United States approaches its 50th anniversary, CLT members, practitioners and researchers are exploring and pushing the boundaries of the model. CLTs offer an alternative model of land use tenure that permanently removes properties from the speculative market for the ongoing common good of the community. Most frequently associated with the provision of affordable housing in strong real estate markets, several CLTs across the country are now expanding into the commercial realm. This thesis compares the incipient commercial development efforts underway in St. Paul, Minnesota and New Orleans, Louisiana in order to better understand the potential role of CLTs in helping communities preserve and create commercial assets under a wide range of market forces

Stuck With the Algorithm: Algorithmic Consciousness and Repertoire in Fridays for Future's Data Contention

By focusing on the transnational youth climate movement Fridays for Future, this article explores how activists understand algorithms and how they try to use them in their digital campaigns. A qualitative case study, this article provides insights from nine virtual in-depth semi-structured interviews with organizers in social media roles from Fridays for Future country collectives across the globe, giving youth activists the opportunity to tell stories about their understandings and experiences in working in datafied spaces. Four central themes emerge via a three-step qualitative data analysis: algorithmic consciousness (understanding, functions, issues, pitfalls, and misinterpretations), algorithm as stake (contentious importance, tactical politics), algorithm as repertoire (role in activism, algorithmic campaigning), and data contention (data analysis, digital contentious tactics, uncritical uses). The interviews show that activists are stuck with the algorithm in two ways: They have to engage with them but are often unsure how. In that sense, activists frame algorithms as a stakeholder in their campaign but are often unclear on how they work. While organizers recognize algorithmic dependency on campaign success, they lack specific mobilization strategies, which prevents them from leveraging algorithms as a contentious tactic. Data contention includes conducting analytics and tailoring strategies to platforms; yet, datafied spaces are used largely uncritically. This article prompts scholars to go beyond textual analyses of digital activism and conduct research that centers on the experiences and practices of activists in dealing with algorithms and data as structural conditions for digital activism

The Mid-Infrared Tully-Fisher Relation: Calibration of the SNIa Scale and Ho

This paper builds on a calibration of the SNIa absolute distance scale begun with a core of distances based on the correlation between galaxy rotation rates and optical Ic band photometry. This new work extends the calibration through the use of mid-infrared photometry acquired at 3.6 microns with Spitzer Space Telescope. The great virtue of the satellite observations is constancy of the photometry at a level better than 1% across the sky. The new calibration is based on 39 individual galaxies and 8 clusters that have been the sites of well observed SNIa. The new 3.6 micron calibration is not yet as extensively based as the Ic band calibration but is already sufficient to justify a preliminary report. Distances based on the mid-infrared photometry are 2% greater in the mean than reported at Ic band. This difference is only marginally significant. The Ic band result is confirmed with only a small adjustment. Incorporating a 1% decrease in the LMC distance, the present study indicates Ho = 75.2 +/- 3.0 km/s/Mpc.Comment: Accepted for publication in The Astrophysical Journal Letters, 6 pages, 2 figure