7,214 research outputs found
Observational Tests of Intergalactic Enrichment Models
We summarize recent results assessing the carbon and silicon abundances of
the intergalactic medium (IGM) using the `pixel optical depth' technique. We
briefly discuss the implications of these results for models of intergalactic
enrichment, focusing on distinguishing `early' z >> 4 enrichment by the first
generations of stars and objects from `late' enrichment by 2 < z < 5 Ly-break
galaxies. We then discuss the comparison of observed QSO spectra to simulated
spectra generated from cosmological simulations that self-consistently include
enrichment, and draw qualitative implications for the general picture of
intergalactic enrichment at z > 2.Comment: 6 pages, to appear in proceedings of IAU 199 conference, "Probing
Galaxies through Quasar Absorption Lines," eds. Williams, Shu, Menar
Analysis of variance--why it is more important than ever
Analysis of variance (ANOVA) is an extremely important method in exploratory
and confirmatory data analysis. Unfortunately, in complex problems (e.g.,
split-plot designs), it is not always easy to set up an appropriate ANOVA. We
propose a hierarchical analysis that automatically gives the correct ANOVA
comparisons even in complex scenarios. The inferences for all means and
variances are performed under a model with a separate batch of effects for each
row of the ANOVA table. We connect to classical ANOVA by working with
finite-sample variance components: fixed and random effects models are
characterized by inferences about existing levels of a factor and new levels,
respectively. We also introduce a new graphical display showing inferences
about the standard deviations of each batch of effects. We illustrate with two
examples from our applied data analysis, first illustrating the usefulness of
our hierarchical computations and displays, and second showing how the ideas of
ANOVA are helpful in understanding a previously fit hierarchical model.Comment: This paper discussed in: [math.ST/0508526], [math.ST/0508527],
[math.ST/0508528], [math.ST/0508529]. Rejoinder in [math.ST/0508530
The origin of scatter in the star formation rate - stellar mass relation
Observations have revealed that the star formation rate (SFR) and stellar
mass (M) of star-forming galaxies follow a tight relation known as
the galaxy main sequence. However, what physical information is encoded in this
relation is under debate. Here, we use the EAGLE cosmological hydrodynamical
simulation to study the mass dependence, evolution and origin of scatter in the
SFR-M relation. At , we find that the scatter decreases
slightly with stellar mass from 0.35 dex at M
M to 0.30 dex at M M. The
scatter decreases from to by 0.05 dex at M M and by 0.15 dex for lower masses. We show that the scatter
at originates from a combination of fluctuations on short time-scales
(ranging from 0.2-2 Gyr) that are presumably associated with self-regulation
from cooling, star formation and outflows, but is dominated by long time-scale
( Gyr) variations related to differences in halo formation times.
Shorter time-scale fluctuations are relatively more important for lower-mass
galaxies. At high masses, differences in black hole formation efficiency cause
additional scatter, but also diminish the scatter caused by different halo
formation times. While individual galaxies cross the main sequence multiple
times during their evolution, they fluctuate around tracks associated with
their halo properties, i.e. galaxies above/below the main sequence at
tend to have been above/below the main sequence for Gyr.Comment: Accepted for publication in MNRAS. Updated comparison to
observations. More detailed investigation of the relative importance of
SFH-fluctuation time-scales on the SFR(Mstar) scatter (S 4.3, Figs. 6 & 7
The mean free path of hydrogen ionizing photons during the epoch of reionization
We use the Aurora radiation-hydrodynamical simulations to study the mean free
path (MFP) for hydrogen ionizing photons during the epoch of reionization. We
directly measure the MFP by averaging the distance 1 Ry photons travel before
reaching an optical depth of unity along random lines-of-sight. During
reionization the free paths tend to end in neutral gas with densities near the
cosmic mean, while after reionizaton the end points tend to be overdense but
highly ionized. Despite the increasing importance of discrete, over-dense
systems, the cumulative contribution of systems with suffices to drive the MFP at , while at
earlier times higher column densities are more important. After reionization
the typical size of HI systems is close to the local Jeans length, but during
reionization it is much larger. The mean free path for photons originating
close to galaxies, , is much smaller than the cosmic MFP. After
reionization this enhancement can remain significant up to starting distances
of comoving Mpc. During reionization, however, for
distances comoving kpc typically exceeds the cosmic MFP.
These findings have important consequences for models that interpret the
intergalactic MFP as the distance escaped ionizing photons can travel from
galaxies before being absorbed and may cause them to under-estimate the
required escape fraction from galaxies, and/or the required emissivity of
ionizing photons after reionization.Comment: 13 pages, 9 figures, 1 table; submitted to MNRA
Predictions for the relation between strong HI absorbers and galaxies at redshift 3
We combine cosmological, hydrodynamical simulations with accurate radiative
transfer corrections to investigate the relation between strong HI absorbers
(N_HI >~ 10^17 /cm^2) and galaxies at redshift z = 3. We find a strong
anti-correlation between the column density and the impact parameter that
connects the absorber to the nearest galaxy. The median impact parameters for
Lyman Limit (LL) and Damped Lyman-{\alpha} (DLA) systems are ~10 and ~1 proper
kpc, respectively. If normalized to the size of the halo of the nearest central
galaxy, the median impact parameters for LL and DLA systems become ~1 and
~10^-1 virial radii, respectively. At a given HI column density, the impact
parameter increases with the mass of the closest galaxy, in agreement with
observations. We predict most strong HI absorbers to be most closely associated
with extremely low-mass galaxies, M_star < 10^8 M_sun and star formation rate
<10^-1 M_sun/yr. We also find a correlation between the column density of
absorbers and the mass of the nearest galaxy. This correlation is most
pronounced for DLAs with N_HI > 10^21 /cm^2 which are typically close to
galaxies with M_star >~ 10^9 M_sun. Similar correlations exist between column
density and other properties of the associated galaxies such as their star
formation rates, halo masses and HI content. The galaxies nearest to HI
absorbers are typically far too faint to be detectable with current
instrumentation, which is consistent with the high rate of (often unpublished)
non-detections in observational searches for the galaxy counterparts of strong
HI absorbers. Moreover, we predict that the detected nearby galaxies are
typically not the galaxies that are most closely associated with the absorbers,
thus causing the impact parameters, star formation rates and stellar masses of
the observed counterparts to be biased high.Comment: 21 pages, 14 figures; Accepted for publication in MNRA
How Did the IGM Become Enriched?
The enrichment of the intergalactic medium with heavy elements is a process
that lies at the nexus of poorly-understood aspects of physical cosmology. We
review current understanding of the processes that may remove metals from
galaxies, the basic predictions of these models, the key observational
constraints on enrichment, and how intergalactic enrichment may be used to test
cosmological simulations.Comment: 12 pages; To appear in the proceedings of the CRAL-Conference Series
I "Chemodynamics: from first stars to local galaxies", Lyon 10-14 July 2006,
France, Eds. Emsellem, Wozniak, Massacrier, Gonzalez, Devriendt, Champavert,
EAS Publications Serie
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