David S. De Young

Tucson Nighttime Scientific Staff

Areas of Interest

Active Galaxies, Galaxy Clusters, Galaxy Evolution, Hydrodynamics

Recent Research Results

Jet Induced Star Formation in Galaxy Clusters
Clusters of galaxies are the largest gravitationally bound objects in the Universe. In addition to containing hundreds of galaxies, these objects are also filled with non-luminous dark matter and hot x-ray emitting gas; the mass of these two components is comparable to or greater than that of the constituent galaxies. It has long been known that the hot gas will cool over the age of the cluster and fall to the center, but a severe problem with this idea is that no evidence has been found for the stars that would form from this cooling inflow. Recent observations of several clusters have found an excess of blue light, which is coincident with radio emission from jets emanating from an active galaxy in the center of the cluster. This blue light could come from massive young stars, and De Young has done a detailed calculation of how the shocks associated with the radio jet could trigger star formation in the surrounding intracluster medium near the galaxy. Good agreement is obtained with observations, both with regard to the color and luminosity of the light. The significance of this result is that this young stellar population will inject copious amounts of energy into the intracluster medium via supernovae and stellar winds, and this energy together with that injected by the jet can reheat the cool infalling gas, thus slowing the overall inflow and reducing the amount of mass that must be accounted for in the centers of clusters.

Mass Entrainment in Young Stellar Outflows
Very young stars are known to lose mass at a high rate during the earliest stages of their formation, either in the form of stellar winds or in highly collimated jets, which commonly produce bright knots (Herbig- Haro objects). Often associated with these jets are slower, less well collimated outflows of molecular gas, and it is not clear if these are also produced by the young star or are a by-product of the jet flow. If the former, then our ideas of star formation and early stellar evolution must include a mechanism for both slow molecular outflow and higher speed jet outflow. In order to address this issue, De Young has examined the interaction of stellar jets and their environment. In particular he has developed a theoretical description for mass entrainment and momentum transfer in the very complex and turbulent boundary layer between a jet and its surroundings. These calculations provide a minimum estimate of the mass and momentum transfer, and they show that under most conditions a major portion of the jet momentum is transferred to the ambient medium. Hence the molecular outflows are likely to be derived from the more fundamental jet outflow.

Future Research Plans

Galaxy Evolution
In collaboration with Colin Norman (STScI), De Young plans an investigation of the fate of hot, metal rich gas that is injected into galactic halos by OB associations and supernova remnants. It has been widely conjectured, but never demonstrated, that this debris causes the halo gas to cool and condense into clouds which then settle back into the galactic disk. This is the essential assumption of the "closed box" models of galaxy evolution which have been used for the last two decades. In order to see if this model has any credibility, one needs a firm calculation that answers the following questions: Is the hot debris, when mixed (or not) with the halo gas, thermally unstable? If so, is the instability damped or does it proceed to the nonlinear regime? Does the instability form dense sheets, filaments, or clouds? Do these objects then persist and become gravitationally bound? This project requires complex and accurate modeling of the thermal conductivity in the context of time dependent numerical hydrodynamics, and the requisite algorithms are being developed. In a related project with Tim Heckman (JHU) and Crystal Martin (UA), an extended study of mass loss from dwarf galaxies due to starburst activity is being initiated. The issue is that of possible recollapse of an inflated ISM in the galaxy versus complete dispersal of the ISM due to energy injection from the starburst event, and the object is to reproduce the observed low metallicities in these objects together with their observed stellar populations. Critical parameters are the degree of central concentration of the starburst, the ellipticity of the ISM distribution, the filling factor of cold dense gas, and the metallicity of the ambient ISM. The solution will require realistic modeling of a two phase ISM with radiative cooling, and the numerical algorithms are now in hand to do this.

Service Activities

De Young's service activities to NOAO include acting as Chairman of the two KPNO telescope Time Allocation Committees, membership on the NOAO IPAC Committee, membership on the WIYN Board of Directors, membership on the WIYN Scientific Advisory Committee, membership on the NOAO Management Committee, supervisor of the NOAO Tucson library, member or chairman of ad hoc KPNO Personnel and Post Doctoral Selection Committees, Chairman of the AURA Strategic Planning Committee, co-author of the AURA education and outreach proposal to the NSF, membership on the AURA interim team for the SOFIA proposal, and membership on various ad hoc NOAO committees such as NOAO 2000 and the AURA sponsored Albuquerque Workshop. In the past De Young has served as Associate Director of KPNO and as Associate Director of NOAO for KPNO. In addition, De Young has carried out extensive numerical modeling of airflow over Mauna Kea and Cerro Pachon in order to facilitate site selection for the Gemini Project, and he has done numerical simulations of airflow in telescope enclosures and around mirror cells to assist the Gemini project in enclosure and telescope design. He is currently using similar methods to evaluate the effects of large scale venting of the KPNO 4-m enclosure and to investigate "mirror seeing" as a function of the temperature difference between a mirror and the surrounding air. De Young also serves on the Board of Trustees of the Aspen Center for Physics and on the Executive and Steering Committees of the San Diego Supercomputer Center.

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Posted: 06Dec1996