93 research outputs found
Lyman-alpha Emitters and Lyman-break Galaxies at z=3-6 in Cosmological SPH Simulations
We study the properties of Lyman-alpha emitters (LAEs) and Lyman-break
galaxies (LBGs) at z=3-6 using cosmological SPH simulations. We investigate two
simple scenarios for explaining the observed Ly-a and rest-frame UV luminosity
functions (LFs) of LAEs: (i) the "escape fraction" scenario, in which the
"effective" escape fraction (including the IGM attenuation) of Ly-a photons is
f_Lya ~0.1 (0.15) at z=3 (6), and (ii) the "stochastic" scenario, in which the
fraction of LAEs that are turned on at z=3 (6) is \Cstoc ~0.07 (0.2) after
correcting for the IGM attenuation. Our comparisons with a number of different
observations suggest that the stochastic scenario is preferred over the escape
fraction scenario. We find that the mean values of stellar mass, metallicity
and black hole mass hosted by LAEs are all smaller in the stochastic scenario
than in the escape fraction scenario. In our simulations, the galaxy stellar
mass function evolves rapidly, as expected in hierarchical structure formation.
However, its evolution is largely compensated by a beginning decline in the
specific star formation rate, resulting in little evolution of the rest-frame
UV LF from z=6 to 3. The rest-frame UV LF of both LAEs and LBGs at z=3 & 6 can
be described well by the stochastic scenario provided the extinction is
moderate, E(B-V) ~0.15, for both populations, although our simulation might be
overpredicting the number of bright LBGs at z=6. We also discuss the
correlation function and bias of LAEs. The Ly-a LFs at z=6 in a field-of-view
of 0.2 deg^2 show a significantly larger scatter owing to cosmic variance
relative to that in a 1 deg^2 field, and the scatter seen in the current
observational estimates of the Ly-a LF can be accounted for by cosmic variance.Comment: 21 pages, 9 figures, PASJ, in press, Dec 2010 issu
Tracing early structure formation with massive starburst galaxies and their implications for reionization
Cosmological hydrodynamic simulations have significantly improved over the
past several years, and we have already shown that the observed properties of
Lyman-break galaxies (LBGs) at z=3 can be explained well by the massive
galaxies in the simulations. Here we extend our study to z=6 and show that we
obtain good agreement for the LBGs at the bright-end of the luminosity function
(LF). Our simulations also suggest that the cosmic star formation rate density
has a peak at z= 5-6, and that the current LBG surveys at z=6 are missing a
significant number of faint galaxies that are dimmer than the current magnitude
limit. Together, our results suggest that the universe could be reionized at
z=6 by the Pop II stars in ordinary galaxies. We also estimate the LF of
Lyman-alpha emitters (LAEs) at z=6 by relating the star formation rate in the
simulation to the Ly-alpha luminosity. We find that the simulated LAE LFs agree
with the observed data provided that the net escape fraction of Ly-alpha photon
is f_{Ly-alpha} <= 0.1. We investigate two possible scenarios for this effect:
(1) all sources in the simulation are uniformly dimmer by a factor of 10
through attenuation, and (2) one out of ten LAEs randomly lights up at a given
moment. We show that the correlation strength of the LAE spatial distribution
can possibly distinguish the two scenarios.Comment: 9 pages, 4 figures. Summary of the talk given at the "First Light &
Reionization" workshop at UC Irvine, May 2005. The published article is
available from http://dx.doi.org/10.1016/j.newar.2005.11.00
N-best Response-based Analysis of Contradiction-awareness in Neural Response Generation Models
Avoiding the generation of responses that contradict the preceding context is
a significant challenge in dialogue response generation. One feasible method is
post-processing, such as filtering out contradicting responses from a resulting
n-best response list. In this scenario, the quality of the n-best list
considerably affects the occurrence of contradictions because the final
response is chosen from this n-best list. This study quantitatively analyzes
the contextual contradiction-awareness of neural response generation models
using the consistency of the n-best lists. Particularly, we used polar
questions as stimulus inputs for concise and quantitative analyses. Our tests
illustrate the contradiction-awareness of recent neural response generation
models and methodologies, followed by a discussion of their properties and
limitations.Comment: 8 pages, Accepted to The 23rd Annual Meeting of the Special Interest
Group on Discourse and Dialogue (SIGDIAL 2022
Probing Chemical Enrichment in Extremely Metal-Poor Galaxies and First Galaxies
The chemical composition of galaxies offers vital insights into their
formation and evolution. A key aspect of this study is the correlation between
helium abundance (He/H) and metallicity, which is instrumental in estimating
the primordial helium generated by Big Bang nucleosynthesis. We study the
chemical enrichment history of low-metallicity galaxies, specifically focusing
on extremely metal-poor galaxies (EMPGs) and the first galaxies, using the
one-zone model and cosmological hydrodynamic simulations. Our one-zone model,
using the Limongi & Chieffi (2018) yield, aligns well with observed high He/H
ratios at low metallicities and reproduces Fe/O ratios akin to EMPGs.
Conversely, the Nomoto et al. (2013) yield does not fully match the high Fe/O
ratios seen in EMPGs. Our cosmological hydrodynamic simulations of the first
galaxy successfully replicate the stellar mass and star formation rate of
galaxies like GN-z11 but fail to produce metallicity and high He/H at low O/H.
This is consistent with the results of the one-zone model, which shows that the
slope of the He/H-O/H relation is moderate in young, actively star-forming
galaxies, suggesting the importance of using galaxies with similar star
formation histories for the fit. These results highlight the need for
high-resolution simulations and expanded observational datasets to refine our
understanding of early galactic chemical evolution.Comment: 14 pages, 7 figures, 1 table, submitted to Ap
On the inconsistency between the estimates of cosmic star formation rate and stellar mass density of high redshift galaxies
There are mainly two different approaches to measure the cosmic star
formation history: direct star formation rate density (SFRD) and stellar mass
density rhostar as functions of redshift. Compilations of current observations
seem to show a disparity in the two quantities, in the sense that the integral
of SFRD is higher than the observed rhostar (after considering gas recycling).
Using cosmological smoothed particle hydrodynamics simulations based on the
concordance Lambda cold dark matter model, we show that the two quantities
become more consistent with each other when we consider the observed galaxy
mass limit. The comparison between simulations and (dust corrected) observed
cosmic SFRD shows a good agreement, while the observed rhostar is significantly
lower than the simulation results. This can be reconciled if the current
high- galaxy surveys are missing faint low-mass galaxies due to their flux
limit. Our simulated GSMFs have steep low-mass end slopes of alpha 3,
and when these numerous low-mass galaxies are included, the total rhostar
matches with the integral of SFRD.Comment: 6 pages, 1 table, and 2 figures, accepted for publication in MNRA
Definitive Identification of the Transition between Small- to Large-Scale Clustering for Lyman Break Galaxies
We report angular correlation function (ACF) of Lyman Break Galaxies (LBGs)
with unprecedented statistical quality on the basis of 16,920 LBGs at z=4
detected in the 1 deg^2 sky of the Subaru/XMM-Newton Deep Field. The ACF
significantly departs from a power law, and shows an excess on small scale.
Particularly, the ACF of LBGs with i'<27.5 have a clear break between the small
and large-scale regimes at the angular separation of ~7'' whose projected
length corresponds to the virial radius of dark halos with a mass of 10^11-12
Mo, indicating multiple LBGs residing in a single dark halo. Both on small
(2''<theta<3'') and large (40''<theta<400'') scales, clustering amplitudes
monotonically increase with luminosity for the magnitude range of i'=24.5-27.5,
and the small-scale clustering shows a stronger luminosity dependence than the
large-scale clustering. The small-scale bias reaches b~10-50, and the outskirts
of small-scale excess extend to a larger angular separation for brighter LBGs.
The ACF and number density of LBGs can be explained by the cold dark matter
model.Comment: Accepted for publication in ApJL. 5 pages, 4 figures. The text and
Figures 2-4 have been revised. There is no major change which affects to the
main discussion shown in the original preprint. This paper with high
resolution figures is available at
http://www-int.stsci.edu/~ouchi/work/astroph/sxds_z4LBG/ouchi_highres.pdf
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