1,994,217 research outputs found
Lyman Break Galaxies at z = 4 - 6 in cosmological SPH Simulations
We perform a spectrophotometric analysis of galaxies at redshifts z = 4 - 6
in cosmological SPH simulations of a Lambda CDM universe. Our models include
radiative cooling and heating by a uniform UV background, star formation,
supernova feedback, and a phenomenological model for galactic winds. Analysing
a series of simulations of varying boxsize and particle number allows us to
isolate the impact of numerical resolution on our results. Specifically, we
determine the luminosity functions in B, V, R, i', and z' filters, and compare
the results with observed galaxy surveys done with the Subaru telescope and the
Hubble Space Telescope. We find that the simulated galaxies have UV colours
consistent with observations and fall in the expected region of the
colour-colour diagrams used by the Subaru group. Assuming a uniform extinction
of E(B-V) = 0.15, we also find reasonable agreement between simulations and
observations in the space density of UV bright galaxies at z = 3 - 6, down to
the magnitude limit of each survey. For the same moderate extinction level of
E(B-V) ~ 0.15, the simulated luminosity functions match observational data, but
have a steep faint-end slope with alpha ~ -2.0. We discuss the implications of
the steep faint-end slope found in the simulations.Comment: 13 pages, 16 figures, MNRAS in pres
The Sunday Night Black & White 6
A compilation zine featuring black and white art and short form narrative.https://source.sheridancollege.ca/unique_collections_snb/1005/thumbnail.jp
Steep Faint-end Slopes of Galaxy Mass and Luminosity Functions at z>=6 and the Implications for Reionisation
We present the results of a numerical study comparing photometric and
physical properties of simulated z=6-9 galaxies to the observations taken by
the WFC3 instrument aboard the Hubble Space Telescope. Using cosmological
hydrodynamical simulations we find good agreement with observations in
color-color space at all studied redshifts. We also find good agreement between
observations and our Schechter luminosity function fit in the observable range,
Muv<= -18, provided that a moderate dust extinction effect exists for massive
galaxies. However beyond what currently can be observed, simulations predict a
very large number of low-mass galaxies and evolving steep faint-end slopes from
alpha_L = -2.15 at z=6 to alpha_L = -2.64 at z=9, with a dependence of
|alpha_L| \propto (1+z)^0.59. During the same epoch, the normalization phi*
increases and the characteristic magnitude Muv* becomes moderately brighter
with decreasing redshift. We find similar trends for galaxy stellar mass
function with evolving low-mass end slope from alpha_M = - 2.26 at z=6 to
alpha_M = -2.87 at z=9, with a dependence of |alpha_M| \propto (1+z)^0.65.
Together with our recent result on the high escape fraction of ionizing photons
for low-mass galaxies, our results suggest that the low-mass galaxies are
important contributor of ionizing photons for the reionisation of the Universe
at z>=6.Comment: Revised metadata, 16 pages, 5 tables, 17 figures. MNRAS, in pres
Constraints on Physical Properties of z~6 Galaxies Using Cosmological Hydrodynamic Simulations
We conduct a detailed comparison of broad-band spectral energy distributions
of six z >= 5.5 galaxies against galaxies drawn from cosmological hydrodynamic
simulations. We employ a new tool called SPOC, which constrains the physical
properties of observed galaxies through a Bayesian likelihood comparison with
model galaxies. For five out of six observed z>=5.5 objects, our simulated
galaxies match the observations at least as well as simple star formation
histories such as tau-models, with similar favored values obtained for the
intrinsic physical parameters such as stellar mass and star formation rate, but
with substantially smaller uncertainties. Our results are broadly insensitive
to simulation choices for galactic outflows and dust reddening. Hence the
existence of early galaxies as observed is broadly consistent with current
hierarchical structure formation models. However, one of the six objects has
photometry that is best fit by a bursty SFH unlike anything produced in our
simulations, driven primarily by a high K-band flux. These findings illustrate
how SPOC provides a robust tool for optimally utilizing hydrodynamic
simulations (or any model that predicts galaxy SFHs) to constrain the physical
properties of individual galaxies having only photometric data, as well as
identify objects that challenge current models. (abridged)Comment: 22 pages, 11 figures, accepted by MNRAS. Added discussions of dust,
numerical resolution; clarified conclusion
On the puzzling plateau in the specific star formation rate at z=2-7
The observational indications for a constant specific star-formation rate
(sSFR) in the redshift range z=2-7 are puzzling in the context of current
galaxy-formation models. Despite the tentative nature of the data, their marked
conflict with theory motivates a study of the possible implications. The
plateau at sSFR ~ 2 Gyr^-1 is hard to reproduce because (a) its level is low
compared to the cosmological specific accretion rate at z > 6, (b) it is higher
than the latter at z ~ 2, (c) the natural correlation between SFR and stellar
mass makes it difficult to manipulate their ratio, and (d) a low SFR at high z
makes it hard to produce enough massive galaxies by z ~ 2. Using a flexible
semi-analytic model, we explore ad-hoc modifications to the standard physical
recipes trying to obey the puzzling observational constraints. Successful
models involve non-trivial modifications, such as (a) a suppressed SFR at z > 4
in galaxies of all masses, by enhanced feedback or reduced SFR efficiency,
following an initial active phase at z > 7, (b) a delayed gas consumption into
stars, allowing the gas that was prohibited from forming stars or ejected at
high z to form stars later in more massive galaxies, and (c) enhanced growth of
massive galaxies, in terms of either faster assembly or more efficient
starbursts in mergers, or by efficient star formation in massive haloes.Comment: 17 pages, 11 figures. MNRAS accepted. References added, small changes
to text after referee report. Results and conclusions unchange
LCDM predictions for galaxy protoclusters I: the relation between galaxies, protoclusters and quasars at z~6
Motivated by recent observational studies of the environment of z~6 QSOs, we
have used the Millennium Run (MR) simulations to construct a very large (~20
deg^2) mock redshift survey of star-forming galaxies at z~6. We use this
simulated survey to study the relation between density enhancements in the
distribution of i-dropouts and Lya-emitters, and their relation to the most
massive halos and protocluster regions at z~6. Our simulation predicts
significant variations in surface density across the sky with some voids and
filaments extending over scales of 1 degree, much larger than probed by current
surveys. Approximately one third of all z~6 halos hosting i-dropouts brighter
than z=26.5 mag (~M* at z=6) become part of z=0 galaxy clusters. i-dropouts
associated with protocluster regions are found in regions where the surface
density is enhanced on scales ranging from a few to several tens of arcminutes
on the sky. We analyze two structures of i-dropouts and Lya-emitters observed
with the Subaru Telescope and show that these structures must be the seeds of
massive clusters-in-formation. In striking contrast, six z~6 QSO fields
observed with HST show no significant enhancements in their i-dropout number
counts. With the present data, we cannot rule out the QSOs being hosted by the
most massive halos. However, neither can we confirm this widely used
assumption. We conclude by giving detailed recommendations for the
interpretation and planning of observations by current and future ground- and
space based instruments that will shed new light on questions related to the
large-scale structure at z~6.Comment: Accepted by MNRAS (19 pages, 20 figures, 2 tables). High resolution
PDF available at http://www.mpa-garching.mpg.de/~overzier/overzierz6.pd
Evolutionary algorithm-based analysis of gravitational microlensing lightcurves
A new algorithm developed to perform autonomous fitting of gravitational
microlensing lightcurves is presented. The new algorithm is conceptually
simple, versatile and robust, and parallelises trivially; it combines features
of extant evolutionary algorithms with some novel ones, and fares well on the
problem of fitting binary-lens microlensing lightcurves, as well as on a number
of other difficult optimisation problems. Success rates in excess of 90% are
achieved when fitting synthetic though noisy binary-lens lightcurves, allowing
no more than 20 minutes per fit on a desktop computer; this success rate is
shown to compare very favourably with that of both a conventional (iterated
simplex) algorithm, and a more state-of-the-art, artificial neural
network-based approach. As such, this work provides proof of concept for the
use of an evolutionary algorithm as the basis for real-time, autonomous
modelling of microlensing events. Further work is required to investigate how
the algorithm will fare when faced with more complex and realistic microlensing
modelling problems; it is, however, argued here that the use of parallel
computing platforms, such as inexpensive graphics processing units, should
allow fitting times to be constrained to under an hour, even when dealing with
complicated microlensing models. In any event, it is hoped that this work might
stimulate some interest in evolutionary algorithms, and that the algorithm
described here might prove useful for solving microlensing and/or more general
model-fitting problems.Comment: 14 pages, 3 figures; accepted for publication in MNRA
Faint Lyman-Break galaxies as a crucial test for galaxy formation models
It has recently been shown that galaxy formation models within the LambdaCDM
cosmology predict that, compared to the observed population, small galaxies
(with stellar masses < 10^{11} M_sun) form too early, are too passive since z ~
3 and host too old stellar populations at z=0. We then expect an overproduction
of small galaxies at z > 4 that should be visible as an excess of faint
Lyman-break galaxies. To check whether this excess is present, we use the
MORGANA galaxy formation model and GRASIL spectro-photometric + radiative
transfer code to generate mock catalogues of deep fields observed with HST-ACS.
We add observational noise and the effect of Lyman-alpha emission, and perform
color-color selections to identify Lyman-break galaxies. The resulting mock
candidates have plausible properties that closely resemble those of observed
galaxies. We are able to reproduce the evolution of the bright tail of the
luminosity function of Lyman-break galaxies (with a possible underestimate of
the number of the brightest i-dropouts), but uncertainties and degeneracies in
dust absorption parameters do not allow to give strong constraints to the
model. Besides, our model shows a clear excess with respect to observations of
faint Lyman-break galaxies, especially of z_{850} ~ 27 V-dropouts at z ~ 5. We
quantify the properties of these "excess" galaxies and discuss the
implications: these galaxies are hosted in dark matter halos with circular
velocities in excess of 100 km s^{-1}, and their suppression may require a deep
re-thinking of stellar feedback processes taking place in galaxy formation.Comment: 17 pages, 13 figures, 1 table; accepted for publication by MNRA
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