754 research outputs found
Spirality: A Novel Way to Measure Spiral Arm Pitch Angle
We present the MATLAB code Spirality, a novel method for measuring spiral arm
pitch angles by fitting galaxy images to spiral templates of known pitch.
Computation time is typically on the order of 2 minutes per galaxy, assuming at
least 8 GB of working memory. We tested the code using 117 synthetic spiral
images with known pitches, varying both the spiral properties and the input
parameters. The code yielded correct results for all synthetic spirals with
galaxy-like properties. We also compared the code's results to two-dimensional
Fast Fourier Transform (2DFFT) measurements for the sample of nearby galaxies
defined by DMS PPak. Spirality's error bars overlapped 2DFFT's error bars for
26 of the 30 galaxies. The two methods' agreement correlates strongly with
galaxy radius in pixels and also with i-band magnitude, but not with redshift,
a result that is consistent with at least some galaxies' spiral structure being
fully formed by z=1.2, beyond which there are few galaxies in our sample. The
Spirality code package also includes GenSpiral, which produces FITS images of
synthetic spirals, and SpiralArmCount, which uses a one-dimensional Fast
Fourier Transform to count the spiral arms of a galaxy after its pitch is
determined. The code package is freely available online; see Comments for URL.Comment: 19 pages, 9 figures, 3 tables. The code package is available at
http://dafix.uark.edu/~doug/SpiralityCode
Persistence of Various Alfalfa Populations in South Dakota Rangeland
Inclusion of alfalfa (Medicago sativa L.) in grasslands has long been valued to increase forage production and quality. Persistence of alfalfa in semiarid rangeland has generally been poor when nonâadapted and/or conventional hayâtype cultivars are utilized, however. Demand exists for alfalfa cultivars that establish readily and persist, particularly under grazing, in semiarid rangelands. A wild population of predominantly yellowâflowered alfalfa (Medicago sativa subsp. falcata) was found growing and reproducing naturally in the Grand River National Grassland in northwestern South Dakota. This predominantly falcata alfalfa therefore demonstrates persistence in this semiarid environment. We initiated a study in May 2006 at the SDSU Antelope Livestock & Range Field Station near Buffalo, SD to evaluate persistence and vigor of eleven alfalfa populations transplanted into mixedâgrass prairie. Populations consisted of four predominantly falcata experimental populations (three are naturally selected and locally adapted; one is artificially selected), one pure falcata experimental population, one pure falcata cultivar, two pastureâtype cultivars, and three conventional hayâtype cultivars. Greenhouse grown seedlings were transplanted on 1 mâcenters within three exclosures (35 m X 35 m) divided into two sections; one exposed to grazing, the other protected from grazing. Grazing by cattle was initiated in August 2007. During the 2008 and 2009 growing seasons, intense grazing of alfalfa plants and associated vegetation occurred monthly for 1â2 days. Survival, height, and canopy volume of grazed and protected alfalfa plants were measured before each grazing event. Despite a harsh winter with persistent ice cover, data from May 2009 revealed that falcataâbased populations had the highest survival under grazing (mean survival = 36%). Pastureâtype cultivars and conventional hayâtype cultivars experienced substantial mortality losses under grazing (mean survival = 8%). Low mortality and high vigor of all protected plant populations indicates that grazing weakened the grazed plants, greatly increasing the risk of winterkill and winter injury. These findings reveal that environmental adaptation, in addition to a degree of grazing tolerance, is necessary for persistence under grazing in this semiarid region. Populations that exhibit high persistence under both grazing and severe winter conditions offer great potential for being utilized in the northern Great Plains
Impacts of Mowing Treatments on Smooth Bromegrass (Bromus inermis) Belowground Bud Bank
Introduced in the 1880s for improving forage production and controlling soil erosion, smooth bromegrass (Bromus inermis Leyss) has invaded and is threating numerous native prairie ecosystems and wildlife habitats in the Northern Great Plains. Land managers of the mixed-grass prairie ecosystems currently spend significant resources attempting to control invasive species and restore native grasslands with various management strategies including grazing, prescribed burning, herbicide application and seeding native species. Unfortunately, many studies have showed that such management efforts have minimal short-term effects. Without sustained effort, persistence and resurgence of smooth bromegrass is inevitable
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