1,549 research outputs found
The Nature of Lyman Break Galaxies in Cosmological Hydrodynamic Simulations
What type of objects are being detected as "Lyman break galaxies"?
Are they predominantly the most massive galaxies at that epoch, or are many of
them smaller galaxies undergoing a short-lived burst of merger-induced star
formation? We attempt to address this question using high-resolution
cosmological hydrodynamic simulations including star formation and feedback.
Our CDM simulation, together with Bruzual-Charlot population synthesis
models, reproduces the observed number density and luminosity function of Lyman
break galaxies when dust is incorporated. The inclusion of dust is crucial for
this agreement. In our simulation, these galaxies are predominantly the most
massive objects at this epoch, and have a significant population of older
stars. Nevertheless, it is possible that our simulations lack the resolution
and requisite physics to produce starbursts, despite having a physical
resolution of \la 700 pc at z=3. Thus we cannot rule out merger-induced
starburst galaxies also contributing to the observed population of
high-redshift objects.Comment: 5 pages, contribution to the Proceedings of Rencontres
Internationales de l'IGRAP, Clustering at High Redshift, Marseille 199
Theoretical Modeling of the High Redshift Galaxy Population
We review theoretical approaches to the study of galaxy formation, with
emphasis on the role of hydrodynamic simulations in modeling the high redshift
galaxy population. We present new predictions for the abundance of star-forming
galaxies in the Lambda + cold dark matter model (Omega_m=0.4, Omega_L=0.6),
combining results from several simulations to probe a wide range of redshift.
At a threshold density of one object per arcmin^2 per unit z, these simulations
predict galaxies with star formation rates of 2 msun/yr (z=10), 5 msun/yr
(z=8), 20 msun/yr (z=6), 70-100 msun/yr (z=4-2), and 30 msun/yr (z=0.5). For
galaxies selected at a fixed comoving space density n=0.003 h^3 Mpc^{-3], a (50
Mpc/h)^3 simulation predicts a galaxy correlation function (r/5 Mpc/h)^{-1.8}
in comoving coordinates, essentially independent of redshift from z=4 to z=0.5.
Different cosmological models predict global histories of star formation that
reflect their overall histories of mass clustering, but robust numerical
predictions of the comoving space density of star formation are difficult
because the simulations miss the contribution from galaxies below their
resolution limit. The LCDM model appears to predict a star formation history
with roughly the shape inferred from observations, but it produces too many
stars at low redshift, predicting Omega_* ~ 0.015 at z=0. We conclude with a
brief discussion of this discrepancy and three others that suggest gaps in our
current theory of galaxy formation: small disks, steep central halo profiles,
and an excess of low mass dark halos. While these problems could fade as the
simulations or observations improve, they could also guide us towards a new
understanding of galactic scale star formation, the spectrum of primordial
fluctuations, or the nature of dark matter.Comment: 12 pages, 3 figs. To be published in "Photometric Redshifts and High
Redshift Galaxies", eds. R. Weymann, L. Storrie-Lombardi, M. Sawicki & R.
Brunner, (San Francisco: ASP Conference Series
Calcium mobilization and Na+/H+ antiport activation by endothelin in human skin fibroblasts
AbstractEndothelin (ET-1) has been shown to exert vasoconstrictor activity in vivo and mobilize Ca2+ in vascular smooth muscle cells in culture. In this paper we show that the human skin fibroblast exhibits specific receptors to ET-1 and that activation of these receptors results in increased intracellular Ca2+ (Ca2+i) and accelerated Na+/H+ antiport activity. ET-1 raised Ca2+i in a dose-response manner; the peak Ca2+i rise was from basal levels of 112.2 ± 21.9 to 299.2 ± 49.7 nM at 300 nM ET-1. This rise was attenuated by removal of extracellular Ca2+i0. Although ET-1 did not alter basal intracellular pH, it enhanced Na+/H+ antiport activity of acidified cells. Fibroblasts demonstrated 156 ± 18 (mean ± SE) ET-1 receptors per unit cell and an equilibrium dissociation constant of 203.4 ± 35.6 pM. Inasmuch as ET-1 plays a role in the metabolism of cells such as the undifferentiated fibroblast, an important action of this peptide may be to act as a growth factor.Endothelin; Growth factor; (Human skin fibroblast
MIcromechanical modeling of composite materials in finite element analysis using an embedded cell approch
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1994.Includes bibliographical references (leaves 182-187).by Jeffrey P. Gardner.M.S
A Pilot Survey of HI in Field Galaxies at Redshift z~0.2
We present the first results of a targeted survey carried out with the 305 m
Arecibo telescope to detect HI-line emission from galaxies at redshift z>0.16.
The targets, selected from the Sloan Digital Sky Survey database, are
non-interacting disk galaxies in relatively isolated fields. We present here
the HI spectra and derived HI parameters for ten objects detected in this pilot
program. All are massive disk galaxies in the redshift interval 0.17-0.25 (i.e.
2-3 Gyr look-back time), with HI masses M_HI=3-8 x 10^10 Msun and high gas mass
fractions (HI - to - stellar mass ratios ~10-30%). Our results demonstrate the
efficacy of exploiting Arecibo's large collecting area to measure the HI mass
and rotational velocity of galaxies above redshift z=0.2. In particular, this
sample includes the highest redshift detections of HI emission from individual
galaxies made to date. Extension of this pilot program will allow us to study
the HI properties of field galaxies at cosmological distances, thus
complementing ongoing radio synthesis observations of cluster samples at z~0.2.Comment: Accepted for publication in ApJ Letter
The Population of Damped Lyman-alpha and Lyman Limit Systems in the Cold Dark Matter Model
Lyman limit and damped Lyman-alpha absorption systems probe the distribution
of collapsed, cold gas at high redshift. Numerical simulations that incorporate
gravity and gas dynamics can predict the abundance of such absorbers in
cosmological models. We develop a semi-analytical method to correct the
numerical predictions for the contribution of unresolved low mass halos, and we
apply this method to the Katz et al. (1996) simulation of the standard cold
dark matter model (, , , ). Using
this simulation and higher resolution simulations of individual low mass
systems, we determine the relation between a halo's circular velocity and
its cross section for producing Lyman limit or damped absorption. We combine
this relation with the Press-Schechter formula for the abundance of halos to
compute the number of absorbers per unit redshift. The resolution correction
increases the predicted abundances by about a factor of two at z=2, 3, and 4,
bringing the predicted number of damped absorbers into quite good agreement
with observations. Roughly half of the systems reside in halos with circular
velocities v_c>100\kms and half in halos with 35\kms. Halos
with v_c>150\kms typically harbor two or more systems capable of producing
damped absorption. Even with the resolution correction, the predicted abundance
of Lyman limit systems is a factor of three below observational estimates,
signifying either a failure of standard CDM or a failure of these simulations
to resolve the systems responsible for most Lyman limit absorption. By
comparing simulations with and without star formation, we find that depletion
of the gas supply by star formation affects absorption line statistics at
only for column densities exceeding .Comment: AASlatex, 17 pages w/ 3 embedded ps figures. Submitted to Ap
The Infuence of Omega_baryon on High-Redshift Structure
We analyze high-redshift structure in three hydrodynamic simulations that
have identical initial conditions and cosmological parameters and differ only
in the value of the baryon density parameter, Omega_b=0.02, 0.05, 0.125.
Increasing Omega_b does not change the fraction of baryons in the diffuse
(unshocked) phase of the intergalactic medium, but it increases cooling rates
and therefore transfers some baryons from the shocked intergalactic phase to
the condensed phase associated with galaxies. Predictions of Lyman-alpha forest
absorption are almost unaffected by changes of Omega_b provided that the UV
background intensity is adjusted so that the mean opacity of the forest matches
the observed value. The required UV background intensity scales as Omega_b^1.7,
and the higher photoionization rate increases the gas temperature in low
density regions. Damped Lyman-alpha absorption and Lyman limit absorption both
increase with increasing Omega_b, though the impact is stronger for damped
absorption and is weaker at z=4 than at z=2-3. The mass of cold gas and stars
in high-redshift galaxies increases faster than Omega_b but slower than
Omega_b^2, and the global star formation rate scales approximately as
Omega_b^1.5. In the higher Omega_b models, the fraction of baryonic material
within the virial radius of dark matter halos is usually higher than the
universal fraction, indicating that gas dynamics and cooling can lead to
over-representation of baryons in virialized systems. On the whole, our results
imply a fairly intuitive picture of the influence of Omega_b on high-redshift
structure, and we provide scalings that can be used to estimate the impact of
Omega_b uncertainties on the predictions of hydrodynamic simulations.Comment: 22 pages, 9 figures, submitted to ApJ, high-resolution version
availible at http://lahmu.phyast.pitt.edu/~gardner/publications/omegab.ps.g
Novel Barriers to Prevent Dogwood Borer (Lepidoptera: Sesiidae) and Rodent Damage in Apple Plantings
We evaluated a combination of noninsecticidal alternatives to control trunk-damaging dogwood borer, Synanthedon scitula (Harris), consisting of novel barrier technologies, used alone or in combination with mating disruption. Barrier formulations evaluated included fibrous barriers of nonwoven ethylene vinyl acetate (EVA) and nonfibrous barriers of rubberized paint (elastomer) used in building coatings. To examine efficacy of dogwood borer control in orchards, all barrier trials were replicated in field tests, both in combination with mating disruption and without it. Trunk inspections to determine whether mating disruption and barriers effectively reduced actual tree infestation showed pheromone disruption significantly reduced infestation compared with the untreated check, but was not as effective as trunk handgun sprays of chlorpyrifos. EVA trunk barriers were effective in preventing borer infestation compared with untreated trees. The elastomer did not differ from the check or the EVA treatment. There was no interaction between disruption and barrier treatments. Barrier field life and durability was assessed over 2 yr by comparing degradation over time due to weathering and other environmental effects including animal damage. The EVA persisted and remained more intact than the elastomer, but was in need of reapplication after 2 yr. Barriers were also screened for efficacy against voles in small-plot trials in nonorchard locations with known high vole pressure; they were tested either alone, combined with a repellent (thiram), or, in the case of the elastomer only, combined with an abrasive (sand). Only the EVA significantly lowered vole chewing damage relative to the untreated check
Cooling Radiation and the Lyman-alpha Luminosity of Forming Galaxies
We examine the cooling radiation from forming galaxies in hydrodynamic
simulations of the LCDM model (cold dark matter with a cosmological constant),
focusing on the Ly-alpha line luminosities of high-redshift systems. Primordial
composition gas condenses within dark matter potential wells, forming objects
with masses and sizes comparable to the luminous regions of observed galaxies.
As expected, the energy radiated in this process is comparable to the
gravitational binding energy of the baryons, and the total cooling luminosity
of the galaxy population peaks at z ~= 2. However, in contrast to the classical
picture of gas cooling from the \sim 10^6 K virial temperature of a typical
dark matter halo, we find that most of the cooling radiation is emitted by gas
with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation
emerges in the Ly-alpha line. While a galaxy's cooling luminosity is usually
smaller than the ionizing continuum luminosity of its young stars, the two are
comparable in the most massive systems, and the cooling radiation is produced
at larger radii, where the Ly-alpha photons are less likely to be extinguished
by dust. We suggest, in particular, that cooling radiation could explain the
two large (\sim 100 kpc), luminous (L_{Ly-alpha} \sim 10^{44} erg s^{-1})
``blobs'' of Ly-alpha emission found in Steidel et al.'s (1999) narrow band
survey of a z = 3 proto-cluster. Our simulations predict objects of the
observed luminosity at about the right space density, and radiative transfer
effects can account for the observed sizes and line widths. We discuss
observable tests of this hypothesis for the nature of the Ly-alpha blobs, and
we present predictions for the contribution of cooling radiation to the
Ly-alpha luminosity function of galaxies as a function of redshift.Comment: Submitted to ApJ. 28 pages including 9 PS figures. Version with color
figures available at
http://donald.astro.umass.edu/~fardal/papers/cooling/cooling.htm
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