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