Multispeckle diffusing-wave spectroscopy: a tool to study slow relaxation and time-dependent dynamics

A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from non-ergodic and/or non-stationary systems is described. The method employs a CCD camera as a multispeckle light detector and a computer-based correlator, and permits the simultaneous calculation of up to 500 correlation functions, where each correlation function is started at a different time. The correlation functions are calculated in real time and are referenced to a unique starting time. The multispeckle nature of the CCD camera detector means that a true ensemble average is calculated; no time averaging is necessary. The technique thus provides a "snapshot" of the dynamics, making it particularly useful for non-stationary systems where the dynamics are changing with time. Delay times spanning the range from 1 ms to 1000 s are readily achieved with this method. The technique is demonstrated in the multiple scattering limit where diffusing-wave spectroscopy theory applies. The technique can also be combined with a recently-developed two-cell technique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in soft glassy materials, which exhibit both short and long relaxation times, non-ergodic dynamics, and slowly-evolving transient behavior.Comment: 11 pages 13 figures Accepted in Review of Scientific Instrument (june 02

Neutrinos And Big Bang Nucleosynthesis

The early universe provides a unique laboratory for probing the frontiers of particle physics in general and neutrino physics in particular. The primordial abundances of the relic nuclei produced during the first few minutes of the evolution of the Universe depend on the electron neutrinos through the charged-current weak interactions among neutrons and protons (and electrons and positrons and neutrinos), and on all flavors of neutrinos through their contributions to the total energy density which regulates the universal expansion rate. The latter contribution also plays a role in determining the spectrum of the temperature fluctuations imprinted on the Cosmic Background Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer and helium-4 as a chronometer, the predictions of BBN and the CBR are compared to observations. The successes of, as well as challenges to the standard models of particle physics and cosmology are identified. While systematic uncertainties may be the source of some of the current tensions, it could be that the data are pointing the way to new physics. In particular, BBN and the CBR are used to address the questions of whether or not the relic neutrinos were fully populated in the early universe and, to limit the magnitude of any lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129, "Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O. Botner, P. Carlson, P. O. Hulth, and T. Ohlsso

Dynamical Mass Estimates for Five Young Massive Stellar Clusters

We have obtained high-dispersion spectra for four massive star clusters in the dwarf irregular galaxies NGC 4214 and NGC 4449, using the HIRES spectrograph on the Keck I telescope. Combining the velocity dispersions of the clusters with structural parameters and photometry from images taken with HST, we estimate mass-to-light ratios and compare these with simple stellar population (SSP) models in order to constrain the stellar mass functions (MFs). For all clusters we find mass-to-light ratios which are similar to or slightly higher than for a Kroupa MF, and thereby rule out any MF which is deficient in low-mass stars compared to a Kroupa-type MF. The four clusters have virial masses ranging between 2.1E5 Msun and 1.5E6 Msun, half-light radii between 3.0 and 5.2 pc, estimated core densities in the range 2E3 Msun pc^-3 to 2E5 Msun pc^-3 and ages between 200 Myr and 800 Myr. We also present new high-dispersion near-infrared spectroscopy for a luminous young (about 15 Myr) cluster in the nearby spiral galaxy NGC 6946, which we have previously observed with HIRES. The new measurements in the infrared agree well with previous estimates of the velocity dispersion, yielding a mass of about 1.7E6 Msun. The properties of the clusters studied here are all consistent with the clusters being young versions of the old globular clusters found around all major galaxies.Comment: 30 pages, including 7 figures and 9 tables. Corrected an error in Table 2: The colors listed for N6946-1447 were not reddening corrected. This also affected Table 9 and Fig 2, 6 and

The influence of massive stars in the interstellar medium of IC 1613: the supernova remnant S8 and the nebula S3 associated with a WO star

We present a detailed kinematical analysis of two selected nebulae in the Local Group irregular galaxy IC 1613. The nebulae are: S8, the only known supernova remnant in this galaxy, and S3, a Wolf-Rayet nebula associated with the only WO star in this galaxy. For S8, we have obtained and analyzed its radial velocity field, where we found complex profiles which can be fitted by several velocity components. These profiles also show the presence of high velocity, low density gas. From this, we have obtained the expansion velocity, estimated the preshock density and calculated the basic kinematical parameters of this SNR. We suggest that in S8 we are seing a SNR partially hidden by dust. This suggestion comes from the fact that the SNR is located between two superbubbles where a ridge of obscured material unveils the existence of dust. Moreover, we show that this hypothesis prevails when energetic arguments are taken into account. In the case of S3, this nebula shows bipolar structure. By means of its kinematics, we have analyzed its two lobes, the ``waist'', as well as its relation with the nearest superbubbles. For the first time we are able to see closed the NW lobe, showing a clover leaf shape. This fact allows a better quantitative knowledge of the nebula as a whole. Furthermore, we found evidence of an expansion motion in the NW lobe. In the light of our results, we can express that these nebulae are the product of very massive stellar evolution. It is surprising the influence these stars still have in shaping their surrounding gas, and on the energy liberation towards the interstellar medium of this galaxy.Comment: Accepted for publication in the Astronomical Journal, July issue. 11 pages, 12 figures. High resolution figures can be found at http://www.inaoep.mx/~mago/PAPERS/AJ

Lyman Alpha Emitters in the Hierarchically Clustering Galaxy Formation

We present a new theoretical model for the luminosity functions (LFs) of Lyman alpha (Lya) emitting galaxies in the framework of hierarchical galaxy formation. We extend a semi-analytic model of galaxy formation that reproduces a number of observations for local and high-z galaxies, without changing the original model parameters but introducing a physically-motivated modelling to describe the escape fraction of Lya photons from host galaxies (f_esc). Though a previous study using a hierarchical clustering model simply assumed a constant and universal value of f_esc, we incorporate two new effects on f_esc: extinction by interstellar dust and galaxy-scale outflow induced as a star formation feedback. It is found that the new model nicely reproduces all the observed Lya LFs of the Lya emitters (LAEs) at different redshifts in z ~ 3-6. Especially, the rather surprisingly small evolution of the observed LAE Lya LFs compared with the dark halo mass function is naturally reproduced. Our model predicts that galaxies with strong outflows and f_esc ~ 1 are dominant in the observed LFs. This is also consistent with available observations, while the simple universal f_esc model requires f_esc << 1 not to overproduce the brightest LAEs. On the other hand, we found that our model significantly overpredicts LAEs at z > 6, and absorption of Lya photons by neutral hydrogen in intergalactic medium (IGM) is a reasonable interpretation for the discrepancy. This indicates that the IGM neutral fraction x_HI rapidly evolves from x_HI << 1 at z < 6 to a value of order unity at z ~ 6-7, which is broadly consistent with other observational constraints on the reionization history.Comment: 14 pages, 7 figures, 1 table; accepted to ApJ; the html abstract is replaced to match the accepted version, the .ps and .pdf files are strictly identical between the 2nd and the 3rd version

The Effect of Star Formation on Molecular Clouds in Dwarf Irregular Galaxies: IC 10 and NGC 6822

We have observed the 13CO J=2-1, 12CO J=2-1 and 12CO J=3-2 lines at a few locations in the dwarf irregular galaxies IC 10 and NGC 6822 using the James Clerk Maxwell Telescope. In addition, we report the first detection of the 13CO J=3-2 transition in a Local Group galaxy. These low metallicity environments appear to be porous to UV radiation and allow for more efficient heating of molecular gas by nearby HII regions. The high 12CO J=3-2/J=2-1 ratio in NGC 6822 suggests that the 12CO emission is optically thin in this region. This high line ratio is likely the result of its location inside a large HII region with low metallicity and low gas content. In IC 10 we observe structures on a variety of size scales that all appear to be gravitationally bound. This effect may help explain the rather high star formation rate in IC 10.Comment: 20 pages with 6 ps figures, accepted for publication in The Astrophysical Journa

The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter

Chemical evolution models and Monte Carlo simulation techniques have been combined for the first time to study the distribution of blue compact galaxies on the N/O plateau. Each simulation comprises 70 individual chemical evolution models. For each model, input parameters relating to a galaxy's star formation history (bursting or continuous star formation, star formation efficiency), galaxy age, and outflow rate are chosen randomly from ranges predetermined to be relevant. Predicted abundance ratios from each simulation are collectively overplotted onto the data to test its viability. We present our results both with and without observational scatter applied to the model points. Our study shows that most trial combinations of input parameters, including a simulation comprising only simple models with instantaneous recycling, are successful in reproducing the observed morphology of the N/O plateau once observational scatter is added. Therefore simulations which include delay of nitrogen injection are no longer favored over those which propose that most nitrogen is produced by massive stars, if only the plateau morphology is used as the principal constraint. The one scenario which clearly cannot explain plateau morphology is one in which galaxy ages are allowed to range below 250 Myr. We conclude that the present data for the N/O plateau are insufficient by themselves for identifying the portion of the stellar mass spectrum most responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200

Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption

We study the column densities of neutral atomic, molecular, and warm ionized Galactic gas through their continuous absorption of extragalactic X-ray spectra at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This is significantly less than the anticpated ratio of 1.23, which would occur if He were half He I and half He II in the warm ionized component. We suggest that the ionized component out of the plane is highly ionized, with He being mainly He II and He III. In the limiting case that H is entirely HI, we place an upper limit on the He abundance in the ISM of He/H <= 0.103. At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This excess column cannot be due to the warm ionized component, even if He were entirely He I, so it must be due to a molecular component. This result implies that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas is common and with a column density comprable to N(H,21cm). This work bears upon the far infrared background, since a warm ionized component, anticorrelated with N(H,21cm), might produce such a background. Not only is such an anticorrelation absent, but if the dust is destroyed in the warm ionized gas, the far infrared background may be slightly larger than that deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the TeX fil

The DEEP2 Galaxy Redshift Survey: Discovery of Luminous, Metal-poor, Sta r-forming Galaxies at Redshifts z~0.7

We have discovered a sample of 17 metal-poor, yet luminous, star-forming galaxies at redshifts z~0.7. They were selected from the initial phase of the DEEP2 survey of 3900 galaxies and the Team Keck Redshift Survey (TKRS) of 1536 galaxies as those showing the temperature-sensitive [OIII]l4363 auroral line. These rare galaxies have blue luminosities close to L*, high star formation rates of 5 to 12 solar masses per year, and oxygen abundances of 1/3 to 1/10 solar. They thus lie significantly off the luminosity-metallicity relation found previously for field galaxies with strong emission lines at redshifts z~0.7. The prior surveys relied on indirect, empirical calibrations of the R23 diagnostic and the assumption that luminous galaxies are not metal-poor. Our discovery suggests that this assumption is sometimes invalid. As a class, these newly-discovered galaxies are: (1) more metal-poor than common classes of bright emission-line galaxies at z~0.7 or at the present epoch; (2) comparable in metallicity to z~3 Lyman Break Galaxies but less luminous; and (3) comparable in metallicity to local metal-poor eXtreme Blue Compact Galaxies (XBCGs), but more luminous. Together, the three samples suggest that the most-luminous, metal-poor, compact galaxies become fainter over time.Comment: This is a .tgz file. It should create the following files: texto.tex, tab1.tex, f1.eps and f2.eps. The LaTeX style used is emulateapj.cls, version November 26, 2004. This submission is 5 pages long, one table and two figures. To appear in ApJ

Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the ^12CO and ^13CO isotopologues from their ultraviolet A--X absorption bands detected in archival Space Telescope Imaging Spectrograph data with lambda/Deltalambda geq 46,000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are not significantly different from the local value of 70 for ^12C/^13C, which is based on mm-wave observations of rotational lines in emission from CO and H_2CO inside dense molecular clouds, as well as on results from optical measurements of CH^+. Five of the 25 sight lines are found to be fractionated toward lower 12-to-13 values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the ^12CO-to-^13CO ratio with physical conditions such as gas temperature or density, yet ^12CO/^13CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H_2 show that all sight lines with detected amounts of ^13CO pass through gas that is on average colder by 20 K than the gas without ^13CO. This colder gas is also sampled by CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of ^12CO/^13CO.Comment: 1-column emulateapj, 23 pages, 9 figure