New observational Constraints on the Growth of the First Supermassive Black Holes

We constrain the total accreted mass density in supermassive black holes at z>6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. Studying galaxies obtained from the deepest Hubble Space Telescope images combined with the Chandra 4 Msec observations of the Chandra Deep Field South, we achieve the most restrictive constraints on total black hole growth in the early Universe. We estimate an accreted mass density <1000Mo Mpc^-3 at z~6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observations. These results place interesting constraints on early black growth and mass assembly by accretion and imply one or more of the following: (1) only a fraction of the luminous galaxies at this epoch contain active black holes; (2) most black hole growth at early epochs happens in dusty and/or less massive - as yet undetected - host galaxies; (3) there is a significant fraction of low-z interlopers in the galaxy sample; (4) early black hole growth is radiatively inefficient, heavily obscured and/or is due to black hole mergers as opposed to accretion or (5) the bulk of the black hole growth occurs at late times. All of these possibilities have important implications for our understanding of high redshift seed formation models.Comment: ApJ Accepted, 10 pages, 7 figures, 1 table, in emulateapj forma

Absolute frequency measurements of the D2D_2 line and fine-structure interval in 39^{39}K

We report a value for the $D_2$-line frequency of $^{39}$K with 0.25 ppb uncertainty. The frequency is measured using an evacuated ring-cavity resonator whose length is calibrated against a reference laser. The $D_2$ line presents a problem in identifying the line center because the closely-spaced energy levels of the excited state are not resolved. We use computer modelling of the measured spectrum to extract the line center and obtain a value of 391 015 578.040(75) MHz. In conjunction with our previous measurement of the $D_1$ line, we determine the fine-structure interval in the $4P$ state to be 1 729 997.132(90) MHz. The results represent significant improvement over previous values.Comment: 4 pages, 3 figure

Local measurements of velocity fluctuations and diffusion coefficients for a granular material flow

Measurements were made of two components of the average and fluctuating velocities, and of the local self-diffusion coefficients in a flow of granular material. The experiments were performed in a 1 m-high vertical channel with roughened sidewalls and with polished glass plates at the front and the back to create a two-dimensional flow. The particles used were glass spheres with a nominal diameter of 3 mm. The flows were high density and were characterized by the presence of long-duration frictional contacts between particles. The velocity measurements indicated that the flows consisted of a central uniform regime and a shear regime close to the walls. The fluctuating velocities in the transverse direction increased in magnitude from the centre towards the walls. A similar variation was not observed for the streamwise fluctuations. The self-diffusion coefficients showed a significant dependence on the fluctuating velocities and the shear rate. The velocity fluctuations were highly anistropic with the streamwise components being 2 to 2.5 times the transverse components. The self-diffusion coefficients for the streamwise direction were an order-of-magnitude higher than those for the transverse direction. The surface roughness of the particles led to a decrease in the self-diffusion coefficients

Direct measurement of the fine-structure interval in alkali atoms using diode lasers

We demonstrate a technique for directly measuring the fine-structure interval in alkali atoms using two frequency-stabilized diode lasers. Each laser has a linewidth of order 1 MHz and precise tunability: one laser is tuned to a hyperfine transition in the D_1 line, and the other laser to a hyperfine transition in the D_2 line. The outputs of the lasers are fed into a scanning Michelson interferometer that measures the ratio of their wavelengths accurately. To illustrate the technique, we measure the fine-structure interval in Rb, and obtain a value of 237.6000(3)(5) cm^-1 for the hyperfine-free 5P_{3/2} - 5P_{1/2} interval.Comment: 3 pages, 2 figures, to be published in Applied Physics Letters, 20 May 2002 editio