Dynamical Modeling of NGC 7252 and the Return of Tidal Material

Motivated by recent neutral hydrogen observations with the VLA, we have undertaken an investigation into the interaction that produced the well known merger remnant NGC 7252. Through fully self-consistent N-body simulations, we are able to reproduce the kinematic character of the HI observations quite well, including the velocity reversals observed along each tidal tail. In the simulation these reversals arise from particles which have turned around in their orbit and are moving to smaller radii. The bases of the tails fall back quickly to small pericentric distances, while the more distant regions fall back more slowly to ever increasing pericentric distances. The delayed return of tidally ejected material may extend over many Gyr. The evolution of this merger is followed numerically for 800 h^-1 Myr beyond the best fit time. We find that nearly half of the present tail material, or of order 10^9 h^-2 of neutral hydrogen and 2x10^9 h^-2 of starlight, will return to within 13 h^-1 kpc of the nucleus within this time span. While the collisionless stars of the tails will continue orbiting between their inner and outer turning points, the observations show the HI gas of the tails disappearing upon its return. We discuss this result in light of the lack of central HI in the main body of this merger remnant.Comment: 28 pages of uuencoded, compressed postscript. Accepted to AJ. 9 Postscript figures available upon request from [email protected]; postscript text also available from http://ucowww.ucsc.edu/~hos/home.htm

A search for HI in five elliptical galaxies with fine structure

We report on VLA H I spectral line observations of five early-type galaxies classified as optically peculiar because of the presence of jets, ripples, or other optical fine structure. We detect H I within the primary beam (30' half-power beamwidth) in four of the five systems. However, in only one case is this gas associated with the targeted elliptical galaxy. In the other cases the H I is associated with a nearby gas-rich disk or dwarf galaxy. The one H I detection is for NGC 7626, where we tentatively detect an H I cloud lying between 20 and 40 kpc southwest of the galaxy center. Its origin is unclear. Our failure to detect obvious tidal H I features suggests that if these fine-structure elliptical galaxies are remnants of disk galaxy mergers, either the progenitors were gas-poor or they are well evolved and any gaseous tidal features have dispersed and/or been converted into other phases. Our targeted systems all reside in groups or clusters, and it seems likely that tidal H I is shorter lived in these environments than suggested by studies of more isolated merger remnants

An HI Threshold for Star Cluster Formation in Tidal Debris

Super star clusters are young, compact star clusters found in the central regions of interacting galaxies. Recently, they have also been reported to preferentially form in certain tidal tails, but not in others. In this paper, we have used 21 cm HI maps and the Hubble Space Telescope Wide Field Planetary Camera 2 images of eight tidal tail regions of four merging galaxy pairs to compare the kiloparsec scale HI distribution with the location of super star clusters found from the optical images. For most of the tails, we find that there is an increase in super star cluster density with increasing projected HI column density, such that the star cluster density is highest when log N(HI) >= 20.6 cm^{-2}, but equal to the background count rate at lower HI column density. However, for two tails (NGC 4038/39 Pos A and NGC 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the N(HI) threshold is a necessary but not sufficient condition for cluster formation. Gas volume density is likely to provide a more direct criterion for cluster formation, and other factors such as gas pressure or strength of encounter may also have an influence. Comparison of HI thresholds needed for formation of different types of stellar structures await higher resolution HI and optical observations of larger numbers of interacting galaxies.Comment: 19 pages, 6 figures, 3 tables, accepted for publication in MNRA

Siting Power Plants: Recent Experience in California and Best Practices in Other States

Compares California's power plant siting with results in other states. Includes interviews with California state agency representatives, developers and process mediators. Part of a series of research reports that examines energy issues facing California

Luminosity Profiles of Merger Remnants

Using published luminosity and molecular gas profiles of the late-stage mergers NGC 3921, NGC 7252 and Arp 220, we examine the expected luminosity profiles of the evolved merger remnants, especially in light of the massive CO complexes that are observed in their nuclei. For NGC 3921 and NGC 7252 we predict that the resulting luminosity profiles will be characterized by an r^{1/4} law. In view of previous optical work on these systems, it seems likely that they will evolve into normal ellipticals as regards their optical properties. Due to a much higher central molecular column density, Arp 220 might not evolve such a ``seamless'' light profile. We conclude that ultraluminous infrared mergers such as Arp 220 either evolve into ellipticals with anomalous luminosity profiles, or do not produce many low-mass stars out of their molecular gas complexes.Comment: Final refereed version. Note new title. 4 pages, 2 encapsulated color figures, uses emulateapj.sty. Accepted to ApJL. Also available at http://www.cv.nrao.edu/~jhibbard/Remnants/remnants.htm

How Engaged Are Consumers in Their Health and Health Care, and Why Does It Matter?

Summarizes findings on U.S. patients' ability to manage their health and health care by income level, education, Medicaid status, and health status. Examines access to care by level of engagement among the chronically ill. Discusses policy implications