14 research outputs found
Massive galaxies at redshift 2 in cosmological hydrodynamic simulations
We study the properties of galaxies at z=2 in a Lambda cold dark matter
universe, using two different types of hydrodynamic simulation methods --
Eulerian TVD and smoothed particle hydrodynamics (SPH) -- and a
spectrophotometric analysis in the U_n, G, R filter set. The simulated galaxies
at z=2 satisfy the color-selection criteria proposed by Adelberger et al.
(2004) and Steidel et al. (2004) when we assume Calzetti extinction with
E(B-V)=0.15. We find that the number density of simulated galaxies brighter
than R<25.5 at z=2 is about 1e-2 h^3 Mpc^-3 for E(B-V)=0.15, which is roughly
twice that of the number density found by Erb et al. (2004) for the UV bright
sample. This suggests that roughly half of the massive galaxies with M*>10^{10}
Msun/h at z=2 are UV bright population, and the other half is bright in the
infra-red wavelengths. The most massive galaxies at z=2 have stellar masses >=
10^{11-12} Msun. They typically have been continuously forming stars with a
rate exceeding 30 Msun/yr over a few Gyrs from z=10 to z=2, together with
significant contribution by starbursts reaching up to 1000 Msun/yr which lie on
top of the continuous component. TVD simulations indicate a more sporadic star
formation history than the SPH simulations. Our results do not imply that
hierarchical galaxy formation fails to account for the observed massive
galaxies at z>=1. The global star formation rate density in our simulations
peaks at z>=5, a much higher redshift than predicted by the semianalytic
models. This star formation history suggests early build-up of the stellar mass
density, and predicts that 70 (50, 30)% of the total stellar mass at z=0 had
already been formed by z=1 (2, 3). Upcoming observations by Spitzer and Swift
might help to better constrain the star formation history at high redshift.Comment: 4 pages, Kluwer style files included. To appear in "Starbursts - from
30 Doradus to Lyman break galaxies" (IoA, Cambridge UK, Sep 2004; talk
summary), Astrophysics & Space Science Library, eds. de Grijs R., Gonzalez
Delgado R.M. (Kluwer: Dordrecht
What fraction of stars formed in infrared galaxies at high redshift?
Star formation happens in two types of environment: ultraviolet-bright
starbursts (like 30 Doradus and HII galaxies at low redshift and Lyman-break
galaxies at high redshift) and infrared-bright dust-enshrouded regions (which
may be moderately star-forming like Orion in the Galaxy or extreme like the
core of Arp 220). In this work I will estimate how many of the stars in the
local Universe formed in each type of environment, using observations of
star-forming galaxies at all redshifts at different wavelengths and of the
evolution of the field galaxy population.Comment: 7 pages, 0 figs, to appear in proceedings of "Starbursts - From 30
Doradus to Lyman break galaxies", edited by Richard de Grijs and Rosa M.
Gonzalez Delgado, published by Kluwe
Properties of Ly-alpha and Gamma Ray Burst selected starbursts at high redshifts
Selection of starbursts through either deep narrow band imaging of redshifted
Ly-alpha emitters, or localisation of host galaxies of gamma-ray bursts both
give access to starburst galaxies that are significantly fainter than what is
currently available from selection techniques based on the continuum (such as
Lyman-break selection). We here present results from a survey for Ly-alpha
emitters at z=3, conducted at the European Southern Observatory's Very Large
Telescope. Furthermore, we briefly describe the properties of host galaxies of
gamma-ray bursts at z>2. The majority of both Ly-alpha and gamma-ray burst
selected starbursts are fainter than the flux limit of the Lyman-break galaxy
sample, suggesting that a significant fraction of the integrated star formation
at z~3 is located in galaxies at the faint end of the luminosity function.Comment: invited talk, 6 pages, 3 figures, to appear in ``Starbursts from 30
Doradus to Lyman Break Galaxies'', eds. R. de Grijs, R. M. Gonzalez Delgado,
Astrophysics & Space Science Library Series, Kluwer (in press
Observing the First Stars and Black Holes
The high sensitivity of JWST will open a new window on the end of the
cosmological dark ages. Small stellar clusters, with a stellar mass of several
10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun
should be directly detectable out to redshift z=10, and individual supernovae
(SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible
beyond this redshift. Dense primordial gas, in the process of collapsing from
large scales to form protogalaxies, may also be possible to image through
diffuse recombination line emission, possibly even before stars or BHs are
formed. In this article, I discuss the key physical processes that are expected
to have determined the sizes of the first star-clusters and black holes, and
the prospect of studying these objects by direct detections with JWST and with
other instruments. The direct light emitted by the very first stellar clusters
and intermediate-mass black holes at z>10 will likely fall below JWST's
detection threshold. However, JWST could reveal a decline at the faint-end of
the high-redshift luminosity function, and thereby shed light on radiative and
other feedback effects that operate at these early epochs. JWST will also have
the sensitivity to detect individual SNe from beyond z=10. In a dedicated
survey lasting for several weeks, thousands of SNe could be detected at z>6,
with a redshift distribution extending to the formation of the very first stars
at z>15. Using these SNe as tracers may be the only method to map out the
earliest stages of the cosmic star-formation history. Finally, we point out
that studying the earliest objects at high redshift will also offer a new
window on the primordial power spectrum, on 100 times smaller scales than
probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", Astrophysics & Space Science Library, Eds. H.
Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008
Modeling CMB lensing cross correlations with CLEFT
A new generation of surveys will soon map large fractions of sky to ever greater depths and their science goals can be enhanced by exploiting cross correlations between them. In this paper we study cross correlations between the lensing of the CMB and biased tracers of large-scale structure at high z. We motivate the need for more sophisticated bias models for modeling increasingly biased tracers at these redshifts and propose the use of perturbation theories, specifically Convolution Lagrangian Effective Field Theory (CLEFT). Since such signals reside at large scales and redshifts, they can be well described by perturbative approaches. We compare our model with the current approach of using scale independent bias coupled with fitting functions for non-linear matter power spectra, showing that the latter will not be sufficient for upcoming surveys. We illustrate our ideas by estimating σ8 from the auto- and cross-spectra of mock surveys, finding that CLEFT returns accurate and unbiased results at high z. We discuss uncertainties due to the redshift distribution of the tracers, and several avenues for future development
The effectiveness of communication groups in day nurseries
This project, run by a speech and language therapist and a clinical psychologist, took place in three inner city day nurseries. A group of children over whose communication there was concern were assessed on both formal (verbal and non‐verbal) and informal (observational) measures. At each nursery these children were divided into two matched groups: intervention and control. The intervention consisted of communication groups run jointly with nursery staff. These small groups focused on promoting communication through play. On reassessment the children attending the communication groups had made significantly more improvement than the controls on both formal and informal measures. In particular, the number and quality of children's interactions, their underlying ability to focus on tasks and their knowledge of basic concepts appeared to benefit from the intervention. The study, therefore, provides strong evidence for the efficacy of such interventions in enhancing communication development in a real‐life setting. Cette étude, effectuée par un orthophoniste et un psychologue clinique, s'est déroulée dans trois crèches de jour urbaines. Un groupe d'enfants, dont la capacité à communiquer causait des inquiétudes, a été évalué a la fois selon des critères formels (verbaux at non verbaux) et informels (fondés sur l'observation). Dans chaque crèche ces enfants ont été divisés en deux groupes semblables: un groupe d'intervention et un de contrǒle. L'intervention consistait en l'établissement de petits groupes de communication gérés en association avec le personnel des crèches. Ces petits groupes avient pour but de promouvoir la communication à travers l'activité ludique. Une évaluation ultérieure a permis de voir des améliorations significativement plus importantes chez les enfants qui avaient pris part aux groupes de communication qu'au sein des groupes de contrǒle, en ce qui concernait tant les critères formels qu'informels. En particulier, le nombre et la qualité des interactions entre enfants, leur capacité intrinsèque de s'appliquer à des tǎches et leurs connaissances de concepts fondamentaux semblent avoir bénéficié de l'intervention. Par conséquent l'étude confirme nettement l'efficacité de ce type d'interventions pour le développement de la communication dans le cadre de la vie réelle. Dieses Projekt wurde von einem Sprachtherapeuten und einem klinischen Psychologen in drei Kindergarten des inneren Stadtbereiches durchgeführt. Eine Gruppe von Kindern, deren Kommunikationsvermögen zur Sorge Anlaß gab, wurde hinsichtlich formaler (verbal und nichtverbal) und informeller Kriterien (Beobachtungen) bewertet. In jeder Tagesstätte wurden die Kinder in gepaarte Gruppen eingeteilt: Kontrollgruppe und Behandlungsgruppe. Die Behandlungfand in Kommunikationsgruppen statt, die wir zusammen mit dem Kindergartenpersonal leiteten. Ziel dieser kleinen Gruppen war die Förderung der Kommunikation beim Spiel. Eine Folgeuntersuchung zeigte eine signifikant stärkere Besserung der Kommunikation hinsichtlich der formalen und der informellen Bewertungsmaße bei den Kindern in den Kommunikationsgruppen. Insbesondere wurden die Anzahl und die Qualität der Interaktionen, die Fähigkeit der Kinder, sich auf Aufgaben zu konzentrieren, und das Verständnis von grundlegenden Begriffen durch die Behandlung positiv beeinflußt. Die Untersuchung bietet daher positive Evidenz für die Wirksamkeit solcher Behandlungsmethoden bei der Förderung der Kommunikationsentwicklung im Alltag. 1993 Royal College of Speech & Language Therapist