741 research outputs found
The two-point correlation function of randomly distributed Lyman alpha clouds
It is often assumed that Ly-alpha forest clouds are randomly distributed, intergalactic objects that are highly ionized by the UV background produced by quasars. If these assumptions are true, fluctuations in the UV background should produce a nonzero two point correlation function in the Ly-alpha forest. This effect, which is really just a generalization of the proximity effect, is more significant at high redshift (z is approximately 3-4) because the mean free path for UV photons is smaller there, and the fluctuations correspondingly larger. This effect was studied using both the semi-analytic techniques of Zuo's recent papers and Monte Carlo simulations. The correlation function is expected to have a small yet potentially measurable amplitude that is consistent with current upper limits. Furthermore, the signature of this effect is distinctive because the nonzero correlation function extends over the photon mean free path, which is larger than the expected scale of large-scale structure. Observations or upper limits on this effect could provide information about the source of the ionizing background at high redshifts and the nature of the Ly-alpha forest clouds
On the Evolutionary History of Stars and their Fossil Mass and Light
The total extragalactic background radiation can be an important test of the
global star formation history (SFH). Using direct observational estimates of
the SFH, along with standard assumptions about the initial mass function (IMF),
we calculate the total extragalactic background radiation and the observed
stellar density today. We show that plausible SFHs allow a significant range in
each quantity, but that their ratio is very tightly constrained. Current
estimates of the stellar mass and extragalactic background are difficult to
reconcile, as long as the IMF is fixed to the Salpeter slope above 1 Msun. The
joint confidence interval of these two quantities only agrees with that
determined from the allowed range of SFH fits at the 3-sigma level, and for our
best-fit values the discrepancy is about a factor of two. Alternative energy
sources that contribute to the background, such as active galactic nuclei
(AGN), Population III stars, or decaying particles, appear unlikely to resolve
the discrepancy. However, changes to the IMF allow plausible solutions to the
background problem. The simplest is an average IMF with an increased
contribution from stars around 1.5--4 Msun. A ``paunchy'' IMF of this sort
could emerge as a global average if low mass star formation is suppressed in
galaxies experiencing rapid starbursts. Such an IMF is consistent with
observations of star-forming regions, and would help to reconcile the fossil
record of star formation with the directly observed SFH.Comment: 21 pages, 7 figures, 3 tables; submitted to Monthly Notice
Sub-mm Galaxies in Cosmological Simulations
We study the predicted sub-mm emission from massive galaxies in a Lambda-CDM
universe, using hydrodynamic cosmological simulations. Assuming that most of
the emission from newly formed stars is absorbed and reradiated in the
rest-frame far-IR, we calculate the number of galaxies that would be detected
in sub-mm surveys conducted with SCUBA. The predicted number counts are
strongly dependent on the assumed dust temperature and emissivity law. With
plausible choices for SED parameters (e.g., T=35 K, beta=1.0), the simulation
predictions reproduce the observed number counts above ~ 1 mJy. The sources
have a broad redshift distribution with median z ~ 2, in reasonable agreement
with observational constraints. However, the predicted count distribution may
be too steep at the faint end, and the fraction of low redshift objects may be
larger than observed.
In this physical model of the sub-mm galaxy population, the objects detected
in existing surveys consist mainly of massive galaxies (several M_*) forming
stars fairly steadily over timescales ~ 10^8-10^9 years, at moderate rates ~100
Msun/yr. The typical descendants of these sub-mm sources are even more massive
galaxies, with old stellar populations, found primarily in dense environments.
While the resolution of our simulations is not sufficient to determine galaxy
morphologies, these properties support the proposed identification of sub-mm
sources with massive ellipticals in the process of formation. The most robust
and distinctive prediction of this model, stemming directly from the long
timescale and correspondingly moderate rate of star formation, is that the
far-IR SEDs of SCUBA sources have a relative high 850 micron luminosity for a
given bolometric luminosity. [Abridged]Comment: Submitted to ApJ. 34 pages including 8 PS figure
Galactic outflows and the kinematics of damped Lyman alpha absorbers
The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to
reproduce in hierarchical galaxy formation models, particularly the
preponderance of wide systems. We investigate DLA kinematics at z=3 using
high-resolution cosmological hydrodynamical simulations that include a
heuristic model for galactic outflows. Without outflows, our simulations fail
to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA
kinematics are in much better agreement with observations. Comparing two
outflow models, we find that a model based on momentum-driven wind scalings
provides the best match to the observed DLA kinematic statistics of Prochaska &
Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that
would be identified in emission. Narrow DLAs can arise from any halo and galaxy
mass, but wide ones only arise in halos with mass >10^11 Mo, from either large
central or small satellite galaxies. This implies that the success of this
outflow model originates from being most efficient at pushing gas out from
small satellite galaxies living in larger halos. This increases the
cross-section for large halos relative to smaller ones, thereby yielding wider
kinematics. Our simulations do not include radiative transfer effects or
detailed metal tracking, and outflows are modeled heuristically, but they
strongly suggest that galactic outflows are central to understanding DLA
kinematics. An interesting consequence is that DLA kinematics may place
constraints on the nature and efficiency of gas ejection from high-z galaxies.Comment: submitted to MNRA
Facilitating Extended Reality in Museums through a Web-Based Application
Masteroppgave i Programvareutvikling samarbeid med HVLPROG399MAMN-PRO
Parametrising Star Formation Histories
We examine the star formation histories (SFHs) of galaxies in smoothed
particle hydrodynamics (SPH) simulations, compare them to parametric models
that are commonly used in fitting observed galaxy spectral energy
distributions, and examine the efficacy of these parametric models as practical
tools for recovering the physical parameters of galaxies. The commonly used
tau-model, with SFR ~ exp(-t/tau), provides a poor match to the SFH of our SPH
galaxies, with a mismatch between early and late star formation that leads to
systematic errors in predicting colours and stellar mass-to-light ratios. A
one-parameter lin-exp model, with SFR ~ t*exp(-t/tau), is much more successful
on average, but it fails to match the late-time behavior of the bluest, most
actively star-forming galaxies and the passive, "red and dead" galaxies. We
introduce a 4-parameter model, which transitions from lin-exp to a linear ramp
after a transition time, which describes our simulated galaxies very well. We
test the ability of these parametrised models to recover (at z=0, 0.5, and 1)
the stellar mass-to-light ratios, specific star formation rates, and stellar
population ages from the galaxy colours, computed from the full SPH star
formation histories using the FSPS code of Conroy et al. (2009). Fits with
tau-models systematically overestimate M/L by ~ 0.2 dex, overestimate
population ages by ~ 1-2 Gyr, and underestimate sSFR by ~ 0.05 dex. Fits with
lin-exp are less biased on average, but the 4-parameter model yields the best
results for the full range of galaxies. Marginalizing over the free parameters
of the 4-parameter model leads to slightly larger statistical errors than
1-parameter fits but essentially removes all systematic biases, so this is our
recommended procedure for fitting real galaxies.Comment: 28 pages, 18 figure
The Implications of Gunn-Peterson Troughs in the HeII Lyman-alpha Forest
Many experts believe that the z ~ 3 HeII Ly-alpha forest will suffer from the
same saturation issues as the z ~ 6 HI Ly-alpha forest and, therefore, will not
be a sensitive probe of HeII reionization. However, there are several factors
that make HeII Ly-alpha absorption more sensitive than HI Ly-alpha. We show
that observations of HeII Ly-alpha and Ly-beta Gunn-Peterson troughs can
provide a relatively model-independent constraint on the volume-averaged HeII
fraction of x_HeII >~ 0.1. This bound derives from first using the most
underdense regions in the HeII forest to constrain the local HeII fraction and,
then, assuming photoionization equilibrium with the maximum allowed
photoionization rate to calculate the ionization state of nearby gas. It is
possible to evade this constraint by a factor of ~2, but only if the HeII were
reionized recently. We argue that HeII Ly-alpha Gunn-Peterson troughs observed
in the spectra of Q0302-003 and HE2347-4342 signify the presence of >~ 10
comoving Mpc patches in which x_HeII > 0.03. This is a factor of 20 improvement
over previous constraints from these spectra and 100 times stronger than the
tightest constraint on the HI volume-filling fraction from the z>6 HI Lyman
forest.Comment: 5 pages, 4 figures, includes some supplementary text not in ApJ
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