150 research outputs found
Recent relative sea-level change in Eastern North America
Thesis (M.S.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987.Supervised by David G. Aubrey.Includes bibliographical references (leaves 57-61).by Barbara Vanston Braatz.M.S
The density, the cosmic microwave background and the proton-to-electron mass ratio in a cloud at redshift 0.9
Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented toward the gravitational lens system PKS 1830–211, which is by far the best known target to study dense cool gas in absorption at intermediate redshift. Determining average radial velocities and performing Large Velocity Gradient radiative transfer calculations, the aims of this study are (1) to determine the density of the gas, (2) to constrain the temperature of the cosmic microwave background (CMB), and (3) to evaluate the proton-to-electron mass ratio at redshift z ∼ 0.89. Analyzing data from six rotational HC_3N transitions (this includes the J = 7 ← 6 line, which is likely detected for the first time in the interstellar medium) we obtain n(H_2) ∼ 2600 cm^(−3) for the gas density of the south-western absorption component, assuming a background source covering factor, which is independent of frequency. With a possibly more realistic frequency dependence proportional to ν^(0.5) (the maximal exponent permitted by observational boundary conditions), n(H2) ∼ 1700 cm^(−3). Again toward the south-western source, excitation temperatures of molecular species with optically thin lines and higher rotational constants are, on average, consistent with the expected temperature of the cosmic microwave background, T^(CMB) = 5.14 K. However, individually, there is a surprisingly large scatter which far surpasses expected uncertainties. A comparison of CS J = 1 ← 0 and 4 ← 3 optical depths toward the weaker north-western absorption component results in T_(ex) = 11 K and a 1-σ error of 3 K. For the main component, a comparison of velocities determined from ten optically thin NH_3 inversion lines with those from five optically thin rotational transitions of HC_3N, observed at similar frequencies, constrains potential variations of the proton-to-electron mass ratio μ to Δμ/μ < 1.4 × 10^(−6) with 3-σ confidence. Also including optically thin rotational lines from other molecular species, it is emphasized that systematic errors are ΔV < 1 kms^(−1), corresponding to Δμ/μ < 1.0 × 10^(−6)
The Megamaser Cosmology Project. V. An Angular Diameter Distance to NGC 6264 at 140 Mpc
We present the direct measurement of the Hubble constant, yielding the direct
measurement of the angular-diameter distance to NGC 6264 using the HO
megamaser technique. Our measurement is based on sensitive observations of the
circumnuclear megamaser disk from four observations with the Very Long Baseline
Array, the Green Bank Telescope and the Effelsberg Telescope. We also monitored
the maser spectral profile for 2.3 years using the Green Bank Telescope to
measure accelerations of maser lines by tracking their line-of-sight velocities
as they change with time. The measured accelerations suggest that the systemic
maser spots have a significantly wider radial distribution than in the
archetypal megamaser in NGC 4258. We model the maser emission as arising from a
circumnuclear disk with orbits dominated by the central black hole. The best
fit of the data gives a Hubble constant of 689 km s
Mpc, which corresponds to an angular-diameter distance of 14419
Mpc. In addition, the fit also gives a mass of the central black hole of
(3.090.42) . The result demonstrates the
feasibility of measuring distances to galaxies located well into the Hubble
flow by using circumnuclear megamaser disks.Comment: 29 pages, 8 figures, Accepted by Ap
The Megamaser Cosmology Project: IV. A Direct Measurement of the Hubble Constant from UGC 3789
In Papers I and II from the Megamaser Cosmology Project (MCP), we reported
initial observations of water masers in an accretion disk of a supermassive
black hole at the center of the galaxy UGC 3789, which gave an angular-diameter
distance to the galaxy and an estimate of Ho with 16% uncertainty. We have
since conducted more VLBI observations of the spatial-velocity structure of
these water masers, as well as continued monitoring of its spectrum to better
measure maser accelerations. These more extensive observations, combined with
improved modeling of the masers in the accretion disk of the central
supermassive black hole, confirm our previous results, but with signifcantly
improved accuracy. We find Ho = 68.9 +/- 7.1 km/s/Mpc; this estimate of Ho is
independent of other methods and is accurate to +/-10%, including sources of
systematic error. This places UGC 3789 at a distance of 49.6 +/- 5.1 Mpc, with
a central supermassive black hole of (1.16 +/- 0.12) x 10^7 Msun.Comment: to appear in Ap
How to increase the mobilization of Climate Financing Instruments
The session on Climate Financing Instruments in Mediterranean landscapes at the IV Mediterranean Forest Week attracted several participants, project partners and international donors. The discussion was inspired by several presentations focusing on the initiatives undertaken at the pilot site, national and regional level in the framework of two regional projects. They clearly showed the potential for accessing mitigation and adaptation financing mechanisms for the Mediterranean forest sector but also the complexity of the subject in the region. The discussion flagged the synergies between adaptation and mitigation in Mediterranean forests and the opportunities presented by financing mechanisms to support the sustainable management of Mediterranean forests
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
Water vapour at high redshift: Arecibo monitoring of the megamaser in MG J0414+0534
The study of water masers at cosmological distances would allow us to
investigate the parsec-scale environment around powerful radio sources, to
probe the physical conditions of the molecular gas in the inner parsecs of
quasars, and to estimate their nuclear engine masses in the early universe. To
derive this information, the nature of the maser source, jet or disk-maser,
needs to be assessed through a detailed investigation of the observational
characteristics of the line emission. We monitored the maser line in the lensed
quasar MGJ0414+0534 at z = 2.64 with the 300-m Arecibo telescope for ~15 months
to detect possible additional maser components and to measure a potential
velocity drift of the lines. In addition, we follow the maser and continuum
emissions to reveal significant variations in their flux density and to
determine correlation or time-lag, if any, between them. The main maser line
profile is complex and can be resolved into a number of broad features with
line widths of 30-160 km/s. A new maser component was tentatively detected in
October 2008 that is redshifted by 470 km/s w.r.t the systemic velocity of the
quasar. The line width of the main maser feature increased by a factor of two
between the Effelsberg and EVLA observations reported by Impellizzeri et al.
(2008) and the first epoch of the Arecibo monitoring campaign. After correcting
for the lens magnification, we find that the total H2O isotropic luminosity of
the maser in MGJ0414+0534 is now ~30,000 Lsun, making this source the most
luminous ever discovered.[Abridged]Comment: 8 pages, 6 figures, accepted for publication in A&
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