6,353 research outputs found
Accounting for the dispersion in the x ray properties of early-type galaxies
The x ray luminosities of early-type galaxies are correlated with their optical (e.g., blue) luminosities (L sub X approx. L sub B exp 1.6), but the x ray luminosities exhibit considerable scatter for a given optical luminosity L sub B. This dispersion in x ray luminosity is much greater than the dispersion of other properties of early-type galaxies (for a given L sub B), such as luminosity scale-length, velocity dispersion, color, and metallicity. Here, researchers consider several possible sources for the dispersion in x ray luminosity. Some of the scatter in x ray luminosity may result from stellar population variations between galaxies with similar L sub B. Since the x ray emitting gas is from accumulated stellar mass loss, the L sub X dispersion may be due to variations in integrated stellar mass loss rates. Another possible cause of the L sub X dispersion may be variations in the amount of cool material in the galaxies; cool gas may act as an energy sink for the hot gas. Infrared emission may be used to trace such cool material, so researchers look for a correlation between the infrared emission and the x ray emission of early-type galaxies at fixed L sub B. Velocity dispersion variations between galaxies of similar L sub B may also contribute to the L sub X dispersion. The most likely a priori source of the dispersion in L sub X is probably the varying amount of ram-pressure stripping in a range of galaxy environments. The hot gaseous halos of early-type galaxies can be stripped in encounters with other galaxies or with ambient cluster gas if the intracluster gas is sufficiently dense. Researchers find that the most likely cause of dispersion in the x ray properties of early type galaxies is probably the ram-pressure stripping of gaseous halos from galaxies. For a sample of 81 early-type galaxies with x ray luminosities or upper limits derived from Einstein Observatory observations (CFT) researchers calculated the cumulative distribution of angular distances between the x ray sample members and bright galaxies from the Revised Shapley - Ames catalog. Collectively, galaxies with low x ray luminosities (for a given L sub B) tend to be in denser environments than galaxies with higher x ray luminosities
X-ray Binaries and Globular Clusters in Elliptical Galaxies
The X-ray emission from normal elliptical galaxies has two major components:
soft emission from diffuse gas and harder emission from populations of
accreting (low-mass) stellar X-ray binaries (LMXB). If LMXB populations are
tied to the field stellar populations in galaxies, their total X-ray
luminosities should be proportional to the optical luminosities of galaxies.
However, recent ASCA and Chandra X-ray observations show that the global
luminosities of LMXB components in ellipticals exhibit significant scatter at a
given optical luminosity. This scatter may reflect a range of evolutionary
stages among LMXB populations in ellipticals of different ages. If so, the
ratio of the global LMXB X-ray luminosity to the galactic optical luminosity,
L_LMXB/L_opt, may be used to determine when the bulk of stars were formed in
individual ellipticals. To test this, we compare variations in L_LMXB/L_opt for
LMXB populations in ellipticals to optically-derived estimates of stellar ages
in the same galaxies. We find no correlation, implying that L_LMXB/L_opt
variations are not good age indicators for ellipticals. Alternatively, LMXBs
may be formed primarily in globular clusters (through stellar tidal
interactions), rather than in the stellar fields of galaxies. Since elliptical
galaxies exhibit a wide range of globular cluster populations for a given
galaxian luminosity, this may induce a dispersion in the LMXB populations of
ellipticals with similar optical luminosities. Indeed, we find that
L_LMXB/L_opt ratios for LMXB populations are strongly correlated with the
specific globular cluster frequencies in elliptical galaxies. This suggests
that most LMXBs were formed in globular clusters.Comment: 5 pages, emulateapj5 style, 2 embedded EPS figures, to appear in ApJ
Letter
Historical Fire In Longleaf Pine (\u3ci\u3ePinus palustris\u3c/i\u3e) Forests of South Mississippi and Its Relation to Land Use and Climate
We characterized historical fire regimes in Pinus palustris (longleaf pine) forests of southern Mississippi with regard to global and regional coupled climate systems (e.g., El NiñoâSouthern Oscillation) and past human activity. The composite fire chronology spanned 1756â2013 with 132 individual scars representing 89 separate fire events. The mean fire interval was 2.9 yr, and mean intervals were significantly different between identified time periods (e.g., settlement period vs. management period). Evidence of biannual fire activity (up to three fires occurring within a 12â to 15âmonth period) was found coeval with a peak in livestock grazing and logging from the 1850s through the 1880s. Connections were also found between historical fire and Pacific climate variability (e.g., El NiñoâSouthern Oscillation and Pacific Decadal Oscillation; P \u3c 0.05), yet the fireâclimate linkage was likely at least partially masked by substantial human land use activities over the past several centuries. Coupled climate and human land use activity controlled the historical fire regime over the past ca. 240 yr. Although the many fire adaptions of P. palustris yield limitations in treeâringâbased fire history studies (e.g., thick bark), we highlight the efficacy of considering the height at which fire scars are analyzed along the bole as a way to glean a more accurate depiction of historical fire occurrence, especially in ecosystems characterized by a frequent, lowâseverity fire regime. This study suggests growingâseason fire prescribed at a 2â to 3âyr interval would be the first step toward simulating historical landscape conditions and fire activity, should that be the goal by land managers
Missouri farmland values
Individuals and businesses often need an estimate of the value of a piece of land. A properly done appraisal provides the most accurate estimate of a piece of landâs value. Appraisals attempt to estimate value from comparable sales. Comparable sales should be nearby, have sold recently and have similar characteristics. It is not always feasible to get an appraisal on a piece of land, particularly when a historical value is sought. A historical land value is often needed to calculate taxes and settle estates. When appraisals are not feasible, estimates from the U.S. Department of Agriculture (USDA) data might be an acceptable option.Ryan Milhollin (Assistant Extension Professor, Agricultural Business and Policy), Raymond Massey (Extension Professor, Agricultural Business and Policy). Original authors: Ronald L. Plain and Joyce White.Includes bibliographical reference
Seeing Galaxies Through Thick and Thin: II. Direct Measures of Extinction in Spiral Disks Through Spectroscopy of Overlapping Galaxies
We use slit spectroscopy of overlapping pairs of galaxies to directly
determine the extinction in disks of foreground spiral galaxies. The Doppler
shifts of pair members are determined via cross-correlation and their relative
correlation amplitudes are used to separate their contributions to the combined
spectra in regions of overlap. This spectroscopic approach is less subject to
stringent symmetry constraints than our previous purely photometric analyses.
Extinctions of foreground members were obtained for 6 of the candidates in our
sample of 18 mostly spiral/spiral pairs, when the signal to noise and velocity
difference were suitable. In agreement with our previous imaging results, we
find that the extinction in interarm regions is very modest, typically A_B=0.1
mag (corrected to face on), while spiral arms exhibit higher extinctions of 0.3
mag.Comment: 14 figures, 3 tables, Accepted to Astrophysical Journa
Time-Dependence of the Mass Accretion Rate in Cluster Cooling Flows
We analyze two time-dependent cluster cooling flow models in spherical
symmetry. The first assumes that the intracluster gas resides in a static
external potential, and includes the effects of optically thin radiative
cooling and mass deposition. This corresponds to previous steady-state cooling
flow models calculated by White & Sarazin (1987). Detailed agreement is found
between steady-state models and time-dependent models at fixed times in the
simulations. The mass accretion rate is found either to increase or remain
nearly constant once flows reach a steady state. The time rate of change of the
accretion rate is strongly sensitive to the value of the mass deposition
parameter q, but only mildly sensitive to the ratio beta of gravitational
binding energy to gas temperature. We show that previous scaling arguments
presented by Bertschinger (1988) and White (1988) are valid only for mature
cooling flows with weak mass deposition (q ~< 1). The second set of models
includes the effects of a secularly deepening cluster potential and secondary
infall of gas from the Hubble flow. We find that such heating effects do not
prevent the flows from reaching a steady state within an initial central
cooling time.Comment: 22 pages (AASTeX) with 16 EPS figures; accepted for publication in
The Astrophysical Journa
Forecasting US Army Enlistment Contract Production in Complex Geographical Marketing Areas
Purpose: The purpose of this paper is to demonstrate an improved method for forecasting the US Army recruiting. Design/methodology/approach: Time series methods, regression modeling, principle components and marketing research are included in this paper. Findings: This paper found the unique ability of multiple statistical methods applied to a forecasting context to consider the effects of inputs that are controlled to some degree by a decision maker. Research limitations/implications: This work will successfully inform the US Army recruiting leadership on how this improved methodology will improve their recruitment process. Practical implications: Improved US Army analytical technique for forecasting recruiting goals
Spin-tunnel investigation of a 1/15-scale model of an Australian trainer airplane
An investigation was conducted in the Langley Spin Tunnel of the spin and spin-recovery characteristics of a 1/15-scale model of an Australian trainer airplane. The invesigation included erect and inverted spins; configuration variables such as a long tail, fuselage strakes, 20 deg. elevator cutouts, and rudder modifications; and determination of the parachute size for emergency spin recovery. Also included in the investigation were wing leading-edge modifications to evaluate Reynolds number effects. Results indicate that the basic configuration will spin erect at an angle of attack of about 63 deg. at about 2 to 2.3 seconds per turn. Recovery from this spin was unsatisfactory by rudder reversal or by rudder reversal and ailerons deflected to full with the spin. The elevators had a pronounced effect on the recovery characteristics. The elevators-down position was very adverse to recoveries, whereas the elevators-up position provided favorable recovery effects. Moving the vertical tail aft (producing a long tail configuration) improved the spin characteristics, but the recoveries were still considered marginal. An extension to the basic rudder chord and length made a significant improvement in the spin and recovery characteristics. Satisfactory recoveries were obtained by deflecting the rudder to full against the spin and the elevators and ailerons to neutral
Prevalence and Properties of Dark Matter in Elliptical Galaxies
Given the recently deduced relationship between X-ray temperatures and
stellar velocity dispersions (the "T-sigma relation") in an optically complete
sample of elliptical galaxies (Davis & White 1996), we demonstrate that L>L_*
ellipticals contain substantial amounts of dark matter in general. We present
constraints on the dark matter scale length and on the dark-to-luminous mass
ratio within the optical half-light radius and within the entire galaxy. For
example, we find that minimum values of dark matter core radii scale as r_dm >
4(L_V/3L_*)^{3/4}h^{-1}_80 kpc and that the minimum dark matter mass fraction
is >~20% within one optical effective radius r_e and is >~39-85% within 6r_e,
depending on the stellar density profile and observed value of beta_spec. We
also confirm the prediction of Davis & White (1996) that the dark matter is
characterized by velocity dispersions that are greater than those of the
luminous stars: sigma_dm^2 ~ 1.4-2 sigma_*^2. The T-sigma relation implies a
nearly constant mass-to-light ratio within six half-light radii: M/L_V ~ 25h_80
M_sun/L_V_sun. This conflicts with the simplest extension of CDM theories of
large scale structure formation to galactic scales; we consider a couple of
modifications which can better account for the observed T-sigma relation.Comment: 27 pages AASTeX; 15 PostScript figures; to appear in Ap
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