arXiv subjects

Barbara Ryden

Publications and source records attributed to Barbara Ryden.

3 recordsLinked to original sources

arXiv2001

The Morphological Evolution of Merger Remnants

Elliptical galaxies formed in a major merger tend to become more nearly spherical with time, thanks to the gravitational effect of their central black hole (or black holes). Observational results indicate that elliptical galaxies with older stellar populations (t > 7.5 Gyr) have rounder central isophotes than ellipticals with younger stellar populations. In addition, older ellipticals tend to have core profiles, while younger ellipticals tend to have power-law profiles. Numerical simulations of galaxy mergers indicate that if one or both of the progenitors have a central black hole with a mass around 0.2 percent of the mass in stars, then the effect of the black hole(s) is to make the central regions of the remnant rounder, with a characteristic time of a few gigayears.

astro-ph↗

arXiv1995

Voids in Real Space and in Redshift Space

Using two-dimensional numerical simulations of gravitational clustering, with initial power spectra of a power-law form with index n, we compare the properties of voids in real space to their properties in redshift space. Both the void probability function (VPF) and the underdense probability function (UPF) are enhanced in redshift space. The enhancement is greatest in the limit n=-2. The VPF and UPF treat voids statistically and assume that two-dimensional voids are circular; we present an algorithm which detects individual voids. Voids found by this algorithm are ellipses whose enclosed density of galaxies falls below a threshold density. When voids are identified using this algorithm, the mean void size and the maximum void size both increase in going from real space to redshift space. The increase is greatest in the limit n=-2. In redshift space, the principal axes of the largest voids in the n=2 and n=0 simulations show a statistically significant tendency (at the 95% confidence level) to be distributed anisotropically, relative to the line of sight from the origin to the void center.

astro-ph↗

arXiv1995

Measuring q_0 from the Distortion of Voids in Redshift Space

Because the transformation from distance to redshift is nonlinear, maps in redshift space become increasingly distorted as the redshift z becomes greater. As noted by Alcock \& Paczyński (1979), observed redshift distortions can be used to estimate the deceleration parameter $q_0$. If $q_0$ is greater than -1, voids in redshift space will be elongated along the line of sight. In addition, distant voids will have a greater volume in redshift space than nearby voids. Accurate measurement of the volume and the axis ratio of voids, as a function of their central redshift, will provide an estimate of $q_0$. To test this method of estimating $q_0$, I create a two-dimensional toy universe, free of peculiar velocities, in which the galaxies are located near the walls of Voronoi cells. The galaxies are then mapped into redshift space, adopting different values of $q_0$. In redshift space, I estimate the area and the axis ratio of the voids by fitting ellipses within the voids, using an algorithm which maximizes the area of the empty ellipses and ensures that ellipses do not overlap. The accuracy of the estimated values of $q_0$ is limited by the intrinsic scatter in the size and shape of the voids. In the toy universe, distinguishing between a $q_0 = -1$ universe and a $q_0 = 1/2$ universe requires a survey which goes to a depth z > 0.1 in redshift space. Peculiar velocities will create an additional source of uncertainty for the values of $q_0$ measured in N-body simulations and in the real universe.

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