arXiv subjects

David Spergel

Publications and source records attributed to David Spergel.

61 records · Page 4Linked to original sources

arXiv1999

Studying Evolution of the Galactic Potential and Halo Streamers with Future Astrometric Satellites

Future astrometric satellites, such as SIM (NASA's Space Interferometric Mission) and GAIA (ESA's Global Astrometric Interferometer for Astrophysics), hold the promise of mapping out the detailed phase space structure of the Galactic halo by providing unprecedented annual proper motion and parallax of $1-10μ$as astrometric accuracy. Here we show that proper motions of hundred or so giant branch stars in a tidal debris torn from a small satellite (a $10^{5-7}L_\odot$ Galactic dwarf galaxy or globular cluster) in the halo is sensitive to the current Galactic potential and its past evolution. We follow the evolution of a cold (velocity dispersion of 10 km/s) stream on a nearby (between 8-50 kpc) polar orbit in a variety of histories of the potential of the Galaxy, and observe the bright ($V<18$mag) members of the debris tail with GAIA accuracy. We simulate effects due to the growing or flipping of the Galactic disk over the past 4 Gyrs or the perturbation from a massive accreted lump such as the progenitor of the Magellanic Clouds. We study various factors influencing our ability to identify streams, including contamination from field stars, accuracy of radial velocity and distance data and evolution and non-axial symmetry of the potential. Our simulations suggest that nearby, cold streams could be detected with GAIA if these cousins of the Sagittarius stream exist. Results of Johnston, Zhao, Spergel & Hernquist (1999) and Helmi, Zhao & de Zeeuw (1999) for static Galactic potentials are likely to be largely generalizable to moderately time-evolving potentials. SIM and GAIA measurements of debris stars might be used to probe both Galactic structure and Galactic history.

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arXiv1998

The Submillimeter Frontier: A Space Science Imperative

A major goal of modern astrophysics is to understand the processes by which the universe evolved from its initial simplicity, as seen in measurements of the Cosmic Microwave Background, to the universe we see today, with complexity on all scales. While the diffuse background measurements of COBE reveal the importance of the far infrared and submillimeter in early galaxy and star formation, the understanding of the development of complex structure requires high resolution imaging and spectroscopy. We present a concept for a space mission called SPECS, the Submillimeter Probe of the Evolution of Cosmic Structure, which provides these capabilities. SPECS is a cold, spatial and spectral Michelson interferometer with adjustable baselines ranging up to 1 km. It has Hubble sensitivity and angular resolution in the far IR and submillimeter, spectral resolution up to 10,000, and a 14' field of view. SPECS will be able to image thermal dust continuum and infrared cooling and diagnostic line emission over a wide range of redshifts, providing extinction-free astrophysical probes of young galaxies and early cosmic structures, and measures of the luminosity and heavy element formation history of the universe. SPECS would also have the potential to improve vastly our knowledge of protostars, protoplanetary systems, Active Galactic Nuclei and other objects in the local universe. We recommend that a concerted effort be made during the next decade to develop and test certain critical technologies (photon-counting far IR detectors, formation flying spacecraft, cold, lightweight mirrors, and active coolers), so that SPECS can be deployed in about 2015.

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arXiv1998

Can COBE see the shape of the universe?

In recent years, the large angle COBE--DMR data have been used to place constraints on the size and shape of certain topologically compact models of the universe. Here we show that this approach does not work for generic compact models. In particular, we show that compact hyperbolic models do not suffer the same loss of large angle power seen in flat or spherical models. This follows from applying a topological theorem to show that generic hyperbolic three manifolds support long wavelength fluctuations, and by taking into account the dominant role played by the integrated Sachs-Wolfe effect in a hyperbolic universe.

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arXiv1998

Circles in the Sky: Finding Topology with the Microwave Background Radiation

If the universe is finite and smaller than the distance to the surface of last scatter, then the signature of the topology of the universe is writ large on the microwave background sky. We show that the microwave background will be identified at the intersections of the surface of last scattering as seen by different ``copies'' of the observer. Since the surface of last scattering is a two-sphere, these intersections will be circles, regardless of the background geometry or topology. We therefore propose a statistic that is sensitive to all small, locally homogeneous topologies. Here, small means that the distance to the surface of last scatter is smaller than the ``topology scale'' of the universe.

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arXiv1997

The Delayed Formation of Dwarf Galaxies

One of the largest uncertainties in understanding the effect of a background UV field on galaxy formation is the intensity and evolution of the radiation field with redshift. This work attempts to shed light on this issue by computing the quasi-hydrostatic equilibrium states of gas in spherically symmetric dark matter halos (roughly corresponding to dwarf galaxies) as a function of the amplitude of the background UV field. We integrate the full equations of radiative transfer, heating, cooling and non-equilibrium chemistry for nine species: H, H^+, H^-,H_2, H_2^+, He, He^+, He^{++}, and e^-. As the amplitude of the UV background is decreased the gas in the core of the dwarf goes through three stages characterized by the predominance of ionized (H^+), neutral (H) and molecular (H_2) hydrogen. Characterizing the gas state of a dwarf galaxy with the radiation field allows us to estimate its behavior for a variety of models of the background UV flux. Our results indicate that a typical radiation field can easily delay the collapse of gas in halos corresponding to 1-$σ$ CDM perturbations with circular velocities less than 30 km/s.

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arXiv1996

Cosmology in a String-Dominated Universe

The string-dominated universe locally resembles an open universe, and fits dynamical measures of power spectra, cluster abundances, redshift distortions, lensing constraints, luminosity and angular diameter distance relations and microwave background observations. We show examples of networks which might give rise to recent string-domination without requiring any fine-tuned parameters. We discuss how future observations can distinguish this model from other cosmologies.

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arXiv1996

The photometric structure of the inner Galaxy

The light distribution in the inner few kiloparsecs of the Milky Way is recovered non-parametrically from a dust-corrected near-infrared COBE/DIRBE surface brightness map of the inner Galaxy. The best fits to the photometry are obtained when the Sun is assumed to lie $\sim14\pm4\pc$ below the plane. The recovered density distributions clearly show an elongated three-dimensional bulge set in a highly non-axisymmetric disk. In the favoured models, the bulge has axis ratios $1{:}0.6{:}0.4$ and semi-major axis length $\sim2\kpc$. Its nearer long axis lies in the first quadrant. The bulge is surrounded by an elliptical disk that extends to $\sim2\kpc$ on the minor axis and $\sim3.5\kpc$ on the major axis. In all models there is a local density minimum $\sim2.2\kpc$ down the minor axis. The subsequent maximum $\sim3\kpc$ down the minor axis (corresponding to $l\simeq-22°$ and $l\simeq 17°$) may be associated with the Lagrange point L$_4$. From this identification and the length of the bulge-bar, we infer a pattern speed $Ω_b\simeq 60-70\kms\kpc^{-1}$ for the bar. Experiments in which pseudo-data derived from models with spiral structure were deprojected under the assumption that the Galaxy is either eight-fold or four-fold symmetric, indicate that the highly non-axisymmetric disks recovered from the COBE data could reflect spiral structure within the Milky Way if that structure involves density contrasts greater than $\gta 3$ at NIR wavelengths. These experiments indicate that the angle $ϕ_0$ between the Sun--centre line and a major axis of the bulge lies near $20°$.

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