40,438 research outputs found
Revealing the z~2.5 Cosmic Web With 3D Lyman-Alpha Forest Tomography: A Deformation Tensor Approach
Studies of cosmological objects should take into account their positions
within the cosmic web of large-scale structure. Unfortunately, the cosmic web
has only been extensively mapped at low-redshifts (), using galaxy
redshifts as tracers of the underlying density field. At , the required
galaxy densities are inaccessible for the foreseeable future, but 3D
reconstructions of Lyman- forest absorption in closely-separated
background QSOs and star-forming galaxies already offer a detailed window into
large-scale structure. We quantify the utility of such maps for
studying the cosmic web by using realistic Ly forest
simulations matched to observational properties of upcoming surveys. A
deformation tensor-based analysis is used to classify voids, sheets, filaments
and nodes in the flux, which is compared to those determined from the
underlying dark matter field. We find an extremely good correspondence, with
of the volume in the flux maps correctly classified relative to the dark
matter web, and classified to within 1 eigenvalue. This compares
favorably to the performance of galaxy-based classifiers with even the highest
galaxy densities at low-redshift. We find that narrow survey geometries can
degrade the cosmic web recovery unless the survey is or on the sky. We also
examine halo abundances as a function of the cosmic web, and find a clear
dependence as a function of flux overdensity, but little explicit dependence on
the cosmic web. These methods will provide a new window on cosmological
environments of galaxies at this very special time in galaxy formation, "high
noon", and on overall properties of cosmological structures at this epoch.Comment: 10 pages, 8 figures. Accepted by Ap
The Minimum of Solar Cycle 23: As Deep as It Could Be?
In this work we introduce a new way of binning sunspot group data with the
purpose of better understanding the impact of the solar cycle on sunspot
properties and how this defined the characteristics of the extended minimum of
cycle 23. Our approach assumes that the statistical properties of sunspots are
completely determined by the strength of the underlying large-scale field and
have no additional time dependencies. We use the amplitude of the cycle at any
given moment (something we refer to as activity level) as a proxy for the
strength of this deep-seated magnetic field.
We find that the sunspot size distribution is composed of two populations:
one population of groups and active regions and a second population of pores
and ephemeral regions. When fits are performed at periods of different activity
level, only the statistical properties of the former population, the active
regions, is found to vary.
Finally, we study the relative contribution of each component (small-scale
versus large-scale) to solar magnetism. We find that when hemispheres are
treated separately, almost every one of the past 12 solar minima reaches a
point where the main contribution to magnetism comes from the small-scale
component. However, due to asymmetries in cycle phase, this state is very
rarely reached by both hemispheres at the same time. From this we infer that
even though each hemisphere did reach the magnetic baseline, from a
heliospheric point of view the minimum of cycle 23 was not as deep as it could
have been
The GALEX Extended Mission: Surveying UV Tracers of the Hidden Side of Galaxy Evolution
The Galaxy Evolution Explorer (GALEX) continues its surveys of the ultraviolet sky. GALEX surveys have supported the following galaxy evolution investigations: calibrating UV as a star formation rate tracer, using wide and deep surveys to measure star formation history, studying the evolution of dust extinction and metallicity, selecting and analyzing galaxies in transitory states, finding local analogs to Lyman Break Galaxies, probing and time-dating star formation in a wide variety of physical regimes. Our continuing mission is focussed on relating star formation history and galaxy evolution paths to the properties of dark matter halos and their assembly history, and on beginning to relate the evolution of galaxies to that of black holes and the intergalactic medium. GALEX has proven that the UV is an ideal band to find and map star formation in low mass, low density objects, and potentially in primordial gas. With future UV missions it may be possible to map emission from the intergalactic and circum-galactic medium, and make a definitive connection between galaxy evolution and the cooling, accretion, heating, and enrichment of gas in the cosmic web
A Broad Evaluation of the Tor English Content Ecosystem
Tor is among most well-known dark net in the world. It has noble uses,
including as a platform for free speech and information dissemination under the
guise of true anonymity, but may be culturally better known as a conduit for
criminal activity and as a platform to market illicit goods and data. Past
studies on the content of Tor support this notion, but were carried out by
targeting popular domains likely to contain illicit content. A survey of past
studies may thus not yield a complete evaluation of the content and use of Tor.
This work addresses this gap by presenting a broad evaluation of the content of
the English Tor ecosystem. We perform a comprehensive crawl of the Tor dark web
and, through topic and network analysis, characterize the types of information
and services hosted across a broad swath of Tor domains and their hyperlink
relational structure. We recover nine domain types defined by the information
or service they host and, among other findings, unveil how some types of
domains intentionally silo themselves from the rest of Tor. We also present
measurements that (regrettably) suggest how marketplaces of illegal drugs and
services do emerge as the dominant type of Tor domain. Our study is the product
of crawling over 1 million pages from 20,000 Tor seed addresses, yielding a
collection of over 150,000 Tor pages. We make a dataset of the intend to make
the domain structure publicly available as a dataset at
https://github.com/wsu-wacs/TorEnglishContent.Comment: 11 page
- …