arXiv subjects

David M. Goldberg

Publications and source records attributed to David M. Goldberg.

32 records · Page 2Linked to original sources

arXiv2004

The Mass Function of Void Galaxies in the SDSS Data Release 2

We estimate the Mass Function of void galaxies in the second public data release of the Sloan Digital Sky Survey from a sample of 1000 galaxies with local density contrasts of delta_v < -0.6. The galaxy sample is split into ellipticals and spirals using a color-Sersic index criteria. We estimate the virial masses of ellipticals using the measured spectral line-widths along with the observed size. Projection effects and uncertainties in halo properties make mass estimates of spirals more difficult. We use an inversion of the Tully-Fisher relation to estimate the isothermal rotational velocity, and introduce a scaling factor to estimate the halo extent. We then fit the measured mass function against a theoretical Press-Schechter model, and find that the distribution of galaxies in voids appears to be nearly unbiased compared to the mass.

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arXiv2004

Galaxy-Galaxy Flexion: Weak Lensing to Second Order

In this paper, we develop a new gravitational lensing inversion technique. While traditional approaches assume that the lensing field varies little across a galaxy image, we note that this variation in the field can give rise to a ``Flexion'' or bending of a galaxy image, which may then be used to detect a lensing signal with increased signal to noise. Since the significance of the Flexion signal increases on small scales, this is ideally suited to galaxy-galaxy lensing. We develop an inversion technique based on the ``Shapelets'' formalism of Refregier (2003). We then demonstrate the proof of this concept by measuring a Flexion signal in the Deep Lens Survey. Assuming an intrinsically isothermal distribution, we find from the Flexion signal alone a velocity width of v_c=221\pm 12 km/s for lens galaxies of r < 21.5, subject to uncertainties in the intrinsic Flexion distribution.

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arXiv2004

Mapping the Cosmic Web with the Sloan Digital Sky Survey

Wide-angle, moderately deep redshift surveys such as that conducted as part of the Sloan Digital Sky Survey (SDSS) allow study of the relationship between the structural elements of the large-scale distribution of galaxies -- including groups, cluster, superclusters, and voids -- and the dependence of galaxy formation and evolution on these enviroments. We present a progress report on mapping efforts with the SDSS and discuss recently constructed catalogs of clusters, voids, and void galaxies, and evidence for a 420Mpc/h supercluster or ``Great Wall.'' Analysis of multi-band photometry and moderate-resolution spectroscopy from the SDSS reveals environmental dependence of the star formation history of galaxies that extends over more than a factor of 100 in density, from clusters all the way to the deep interiors of voids. On average, galaxies in the rarified environments of voids exhibit bluer colors, higher specific star formation rates, lower dust content, and more disk-like morphology than objects in denser regions. This trend persists in comparisons of samples in low vs. high-density regions with similar luminosity and morphology, thus this dependence is not simply an extension of the morphology-density relation. Large-scale modulation of the halo mass function and the temperature of the intergalactic medium might explain this dependence of galaxy evolution on the large-scale environment.

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arXiv2003

Simulating Voids

We present a novel method for simulation of the interior of large cosmic voids, suitable for study of the formation and evolution of objects lying within such regions. Following Birkhoff's theorem, void regions dynamically evolve as universes with cosmological parameters that depend on the underdensity of the void. We derive the values of $Ω_M$, $Ω_Λ$, and $H_0$ that describe this evolution. We examine how the growth rate of structure and scale factor in a void differ from the background universe. Together with a prescription for the power spectrum of fluctuations, these equations provide the initial conditions for running specialized void simulations. The increased efficiency of such simulations, in comparison with general-purpose simulations, allows an improvement of upwards of twenty in the mass resolution. As a sanity check, we run a moderate resolution simulation ($N=128^3$ particles) and confirm that the resulting mass function of void halos is consistent with other theoretical and numerical models.

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arXiv2000

Using Perturbative Least Action to Reconstruct the Local Group

In this paper, we apply the Perturbative Least Action Method to model the Local Group of galaxies for various Cosmological models. We show that though the galaxy masses are theoretically good discriminators of $Ω_M$ given some observed MW and M31 separation and radial velocity, current estimates of the masses are insufficient to make any cosmological claims. We then discuss additional complications to expand this analysis.

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arXiv2000

Using Perturbative Least Action to Reconstruct Redshift Space Distortions

In this paper, we present a redshift space reconstruction scheme which is analogous to and extends the Perturbative Least Action (PLA) method described by Goldberg & Spergel (2000). We first show that this scheme is effective in reconstructing even nonlinear observations. We then suggest that by varying the cosmology to minimize the quadrupole moment of a reconstructed density field, it may be possible to lower the errorbars on the redshift distortion parameter, $β$ as well as to break the degeneracy between the linear bias parameter, $b$, and $Ω_M$. Finally, we discuss how PLA might be applied to realistic redshift surveys.

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arXiv2000

Using Perturbative Least Action to Recover Cosmological Initial Conditions

We introduce a new method for generating initial conditions consistent with highly nonlinear observations of density and velocity fields. Using a variant of the Least Action method, called Perturbative Least Action (PLA), we show that it is possible to generate several different sets of initial conditions, each of which will satisfy a set of highly nonlinear observational constraints at the present day. We then discuss a code written to test and apply this method and present the results of several simulations.

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arXiv1999

A Comparison of Simple Mass Estimators for Galaxy Clusters

High-resolution N-body simulations are used to investigate systematic trends in the mass profiles and total masses of clusters as derived from 3 simple estimators: (1) the weak gravitational lensing shear field under the assumption of an isothermal cluster potential, (2) the dynamical mass obtained from the measured velocity dispersion under the assumption of an isothermal cluster potential, and (3) the classical virial estimator. The clusters consist of order 2.5e+05 particles of mass $m_p \simeq 10^{10} \Msun$, have triaxial mass distributions, and significant substructure exists within their virial radii. Not surprisingly, the level of agreement between the mass profiles obtained from the various estimators and the actual mass profiles is found to be scale-dependent. The virial estimator yields a good measurement of the total cluster mass, though it is systematically underestimated by of order 10%. This result suggests that, at least in the limit of ideal data, the virial estimator is quite robust to deviations from pure spherical symmetry and the presence of substructure. The dynamical mass estimate based upon a measurement of the cluster velocity dispersion and an assumption of an isothermal potential yields a poor measurement of the total mass. The weak lensing estimate yields a very good measurement of the total mass, provided the mean shear used to determine the equivalent cluster velocity dispersion is computed from an average of the lensing signal over the entire cluster (i.e. the mean shear is computed interior to the virial radius). [abridged]

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arXiv1999

The Microwave Background Bispectrum, Paper I: Basic Formalism

In this paper, we discuss the potential importance of measuring the CMB anisotropy bispectrum. We develop a formalism for computing the bispectrum and for measuring it from microwave background maps. As an example, we compute the bispectrum resulting from the 2nd order Rees-Sciama effect, and find that is undetectable with current and upcoming missions.

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arXiv1999

The Microwave Background Bispectrum, Paper II: A Probe of the Low Redshift Universe

Gravitational fluctuations along the line-of-sight from the surface of last scatter to the observer distort the microwave background in several related ways: The fluctuations deflect the photon path (gravitational lensing), the decay of the gravitational potential produces additional fluctuations (ISW effect) and scattering off of hot gas in clusters produce additional fluctuations (Sunyaev-Zel'dovich effect). Even if the initial fluctuations generated at the surface of last scatter were Gaussian, the combination of these effects produce non-Gaussian features in the microwave sky. We discuss the microwave bispectrum as a tool for measuring a studying this signal. For MAP, we estimate that these measurements will enable us to determine the fraction of ionized gas and to probe the time evolution of the gravitational potential.

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arXiv1997

Astrometric Shifts in the OGLE-1 Microlensing Events

We measure the astrometric shifts of the light centroids for the microlensing events in the OGLE-1 database. Of the 15 consistently detected events, 7 had significant shifts which we were able to measure with a fair degree of confidence. Those events with large shifts are also expected to be unresolved with a background ``blend'', and thus, we suggest that we have identified events in the OGLE-1 catalog which are strongly blended. Though we concentrate on the OGLE-1 database, and use the DoPHOT package in order to perform our analysis, we suggest that this shift is a generic effect, and should be observable in any crowded field.

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arXiv1997

Using Astrometry to Deblend Microlensing Events

We discuss the prospect of deblending microlensing events by observing astrometric shifts of the lensed stars. Since microlensing searches are generally performed in very crowded fields, it is expected that stars will be confusion limited rather than limited by photon statistics. By performing simulations of events in crowded fields, we find that if we assume a dark lens and that the lensed star obeys a power law luminosity function, $n(L)\propto L^{-β}$, over half the simulated events show a measurable astrometric shift. Our simulations included 20000 stars in a $256\times 256$ Nyquist sampled CCD frame. For $β=2$, we found that 58% of the events were significantly blended $(F_{\ast}/F_{tot}\leq 0.9)$, and of those, 73% had a large astrometric shift $(\geq 0.5 pixels)$. Likewise, for $β=3$, we found that 85% of the events were significantly blended, and that 85% of those had large shifts. Moreover, the shift is weakly correlated to the degree of blending, suggesting that it may be possible not only to detect the existence of a blend, but also to deblend events statistically using shift information.

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arXiv1997

Determination of the Baryon Density from Large Scale Galaxy Redshift Surveys

We estimate the degree to which the baryon density, $Ω_{b}$, can be determined from the galaxy power spectrum measured from large scale galaxy redshift surveys, and in particular, the Sloan Digital Sky Survey. A high baryon density will cause wiggles to appear in the power spectrum, which should be observable at the current epoch. We assume linear theory on scales $\geq 20h^{-1}Mpc$ and do not include the effects of redshift distortions, evolution, or biasing. With an optimum estimate of $P(k)$ to $k\sim 2π/(20 h^{-1} Mpc)$, the $1 σ$ uncertainties in $Ω_{b}$ are roughly 0.07 and 0.016 in flat and open ($Ω_{0}=0.3$) cosmological models, respectively. This result suggests that it should be possible to test for consistency with big bang nucleosynthesis estimates of $Ω_{b}$ if we live in an open universe.

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arXiv1997

High-Resolution Simulations of Cluster Formation

The formation history of rich clusters is investigated using a hybrid N-body simulation in which high spatial and mass resolution can be achieved self-consistently within a small region of a very large volume. The evolution of three massive clusters is studied via mass accretion, spherically-averaged density profiles, three-dimensional and projected shapes, and degree of substructure. Each cluster consists of order $4\times 10^5$ particles at the present epoch and in the case that rich cluster evolution is well-described by a 1-parameter family, the simulations have sufficient resolution to demonstrate this. At $z=0$ the clusters have similar masses, $M(r \le 1.5h^{-1} Mpc) \sim 2\times 10^{15} h^{-1} M_\odot$, and similar spherically-averaged density profiles, however markedly different formation histories are observed. No single, dominant pattern is apparent in the time variation of the mass accretion rate, the cluster shape, or the degree of substructure. Although not a statistically large sample, these objects suggest that the detailed formation history of rich clusters cannot be characterized by a simple 1-parameter family. These results suggest that the use of observations of rich clusters over a wide range of redshifts to constrain cosmological parameters may not be entirely straightforward.

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