Engineering topics

Spergel, David N

Publications and source records attributed to Spergel, David N.

5 records

DOE OSTI2012

POWER-LAW TEMPLATE FOR INFRARED POINT-SOURCE CLUSTERING

We perform a combined fit to angular power spectra of unresolved infrared (IR) point sources from the Planck satellite (at 217, 353, 545, and 857 GHz, over angular scales 100 {approx}< l {approx}< 2200), the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST; 250, 350, and 500 {mu}m; 1000 {approx}< l {approx}< 9000), and from correlating BLAST and Atacama Cosmology Telescope (ACT; 148 and 218 GHz) maps. We find that the clustered power over the range of angular scales and frequencies considered is well fitted by a simple power law of the form C{sup clust}{sub l}{proportional_to}l{sup -n} with n = 1.25 {+-} 0.06. While the IR sources are understood to lie at a range of redshifts, with a variety of dust properties, we find that the frequency dependence of the clustering power can be described by the square of a modified blackbody, {nu}{sup {beta}} B({nu}, T{sub eff}), with a single emissivity index {beta} = 2.20 {+-} 0.07 and effective temperature T{sub eff} = 9.7 K. Our predictions for the clustering amplitude are consistent with existing ACT and South Pole Telescope results at around 150 and 220 GHz, as is our prediction for the effective dust spectral index, which we find to be {alpha}{sub 150-220} = 3.68 {+-} 0.07 between 150 and 220 GHz. Our constraints on the clustering shape and frequency dependence can be used to model the IR clustering as a contaminant in cosmic microwave background anisotropy measurements. The combined Planck and BLAST data also rule out a linear bias clustering model.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

MEAN-FLUX-REGULATED PRINCIPAL COMPONENT ANALYSIS CONTINUUM FITTING OF SLOAN DIGITAL SKY SURVEY Ly{alpha} FOREST SPECTRA

Continuum fitting is an important aspect of Ly{alpha} forest science, since errors in the derived optical depths scale with the fractional continuum error. However, traditional methods of estimating continua in noisy and moderate-resolution spectra (e.g., Sloan Digital Sky Survey, SDSS; S/N {approx}< 10 pixel{sup -1} and R {approx} 2000), such as power-law extrapolation or dividing by the mean spectrum, achieve no better than {approx}15% rms accuracy. To improve on this, we introduce mean-flux-regulated principal component analysis (MF-PCA) continuum fitting. In this technique, PCA fitting is carried out redward of the quasar Ly{alpha} line in order to provide a prediction for the shape of the Ly{alpha} forest continuum. The slope and amplitude of this continuum prediction is then corrected using external constraints for the Ly{alpha} forest mean flux. This requires prior knowledge of the mean flux, (F), but significantly improves the accuracy of the flux transmission, F {identical_to} exp (- {tau}), estimated from each pixel. From tests on mock spectra, we find that MF-PCA reduces the errors to 8% rms in S/N {approx} 2 spectra, and <5% rms in spectra with S/N {approx}> 5. The residual Fourier power in the continuum is decreased by a factor of a few in comparison with dividing by the mean continuum, enabling Ly{alpha} flux power spectrum measurements to be extended to {approx}2 Multiplication-Sign larger scales. Using this new technique, we make available continuum fits for 12,069 z > 2.3 Ly{alpha} forest spectra from SDSS Data Release 7 for use by the community. This technique is also applicable to future releases of the ongoing Baryon Oscillations Spectroscopic Survey, which obtains spectra for {approx}150, 000 Ly{alpha} forest spectra at low signal-to-noise (S/N {approx} 2).

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

EVIDENCE AGAINST DARK MATTER HALOS SURROUNDING THE GLOBULAR CLUSTERS MGC1 AND NGC 2419

The conjecture that the ancient globular clusters (GCs) formed at the center of their own dark matter (DM) halos was first proposed by Peebles in 1984 and has recently been revived to explain the puzzling abundance patterns observed within many GCs. In this paper, we demonstrate that the outer stellar density profile of isolated GCs is very sensitive to the presence of an extended dark halo. The GCs NGC 2419, located at 90 kpc from the center of our Galaxy, and MGC1, located at {approx}200 kpc from the center of M31, are ideal laboratories for testing the scenario that GCs formed at the centers of massive dark halos. Comparing analytic models to observations of these GCs, we conclude that these GCs cannot be embedded within dark halos with a virial mass greater than 10{sup 6} M{sub sun}, or, equivalently, the DM halo-mass-to-stellar mass ratio must be M{sub DM}/M{sub *} < 1. If these GCs have indeed orbited within weak tidal fields throughout their lifetimes, then these limits imply that these GCs did not form within their own dark halos. Recent observations of an extended stellar halo in the GC NGC 1851 are also interpreted in the context of our analytic models. Implications of these results for the formation of GCs are briefly discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

ON THE FORMATION OF MULTIPLE STELLAR POPULATIONS IN GLOBULAR CLUSTERS

Nearly all globular clusters (GCs) studied to date show evidence for multiple stellar populations, in stark contrast to the conventional view that GCs are a mono-metallic, coeval population of stars. This generic feature must therefore emerge naturally within massive star cluster formation. Building on earlier work, we propose a simple physical model for the early evolution (several 10{sup 8} yr) of GCs. We consider the effects of stellar mass loss, Type II supernovae (SNe II) and prompt Type Ia supernovae (SNe Ia), ram pressure, and accretion from the ambient interstellar medium (ISM) on the development of a young GC's own gas reservoir. In our model, SNe II from a first generation of star formation clears the GC of its initial gas reservoir. Over the next several 10{sup 8} yr, mass lost from asymptotic giant branch stars and matter accreted from the ambient ISM collect at the center of the GC. This material must remain quite cool (T {approx} 10{sup 2} K), but does not catastrophically cool on a crossing time because of the high Lyman-Werner flux density in young GCs. The collection of gas within the GC must compete with ram pressure from the ambient ISM. After several 10{sup 8} yr, the Lyman-Werner photon flux density drops by more than three orders of magnitude, allowing molecular hydrogen and then stars to form. After this second generation of star formation, SNe II from the second generation and then prompt SNe Ia associated with the first generation maintain a gas-free GC, thereby ending the cycle of star formation events. Our model makes clear predictions for the presence or absence of multiple stellar populations within GCs as a function of GC mass and formation environment. While providing a natural explanation for the approximately equal number of first- and second-generation stars in GCs, substantial accretion from the ambient ISM may produce fewer chemically peculiar second-generation stars than are observed. Analyzing intermediate-age LMC clusters, we find for the first time evidence for a mass threshold of {approx}10{sup 4} M{sub sun} below which LMC clusters appear to be truly coeval. This threshold mass is consistent with our predictions for the mass at which ram pressure is capable of clearing gas from clusters in the LMC at the present epoch. Recently, claims have been made that multiple populations within GCs require that GCs form at the center of their own dark matter halos. We argue that such a scenario is implausible. Observations of the young and intermediate-age clusters in the LMC and M31 will provide strong constraints on our proposed scenario.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

Constraining fundamental physics with future CMB experiments

The Planck experiment will soon provide a very accurate measurement of cosmic microwave background anisotropies. This will let cosmologists determine most of the cosmological parameters with unprecedented accuracy. Future experiments will improve and complement the Planck data with better angular resolution and better polarization sensitivity. This unexplored region of the CMB power spectrum contains information on many parameters of interest, including neutrino mass, the number of relativistic particles at recombination, the primordial helium abundance, and the injection of additional ionizing photons by dark matter self-annihilation. We review the imprint of each parameter on the CMB and forecast the constraints achievable by future experiments by performing a Monte Carlo analysis on synthetic realizations of simulated data. We find that next generation satellite missions such as CMBPol could provide valuable constraints with a precision close to that expected in current and near future laboratory experiments. Finally, we discuss the implications of this intersection between cosmology and fundamental physics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗