2,618 research outputs found
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
Arabidopsis PEN3/PDR8, an ATP binding cassette transporter, contributes to nonhost resistance to inappropriate pathogens that enter by direct penetration
Moonlighting function of Phytochelatin synthase1 in extracellular defense against fungal pathogens
The merger rate of massive galaxies
We calculate the projected two point correlation function for samples of
luminous and massive galaxies in the COMBO-17 photometric redshift survey,
focusing particularly on the amplitude of the correlation function at small
projected radii and exploring the constraints such measurements can place on
the galaxy merger rate. For nearly volume-limited samples with 0.4<z<0.8, we
find that 4+/-1% of luminous M_B<-20 galaxies are in close physical pairs (with
real space separation of <30 proper kpc). The corresponding fraction for
massive galaxies with M_*>2.5e10 M_sun is 5+/-1%. Incorporating close pair
fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid
evolution of the merger fraction of massive galaxies between z=0.8 and the
present day. Assuming that the major merger timescale is of order the dynamical
timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%)
of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers
between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since
z=0.8(1): major mergers between massive galaxies are a significant driver of
galaxy evolution over the last eight billion years.Comment: ApJ, in press. 8 pages, 3 figures. Expanded discussion section with
explicit discussion of merger fraction vs. close pair fraction. Change of
typical close pair timescale results in increased inferred merger rat
A Mutation at the fad8 Locus of Arabidopsis Identifies a Second Chloroplast [omega]-3 Desaturase
The difference that tenure makes
This paper argues that housing tenures cannot be reduced to either production relations or consumption relations. Instead, they need to be understood as modes of housing distribution, and as having complex and dynamic relations with social classes. Building on a critique of both the productionist and the consumptionist literature, as well as of formalist accounts of the relations between tenure and class, the paper attempts to lay the foundations for a new theory of housing tenure. In order to do this, a new theory of class is articulated, which is then used to throw new light on the nature of class-tenure relations
ATTENUATION CHANGES DURING OILFLOOD EXPERIMENTS
ABSTRACT Reservoir saturation changes can be detected by repeat 3-D seismic surveys. These rely on some seismic attribute such as travel time, amplitude or velocity changing with the change in saturation of either gas, oil or water. Rock physics measurements show that rocks with a high compliance or low stiffness have the greatest sensitivity of Vp to saturation changes. Stiff rocks may only exhibit small changes in Vp with saturation, which may be undetectable on repeat surveys. However other seismic attributes, such as signal amplitude may show large variations. In the experiments described here, the velocity and signal amplitude was measured in a stiff sandstone core during the replacement of brine with oil. During the experiment Vp changed by 1%, but the signal amplitude varied by over 200%. A 1% change in velocity is unlikely to be detectable in a repeat seismic survey since it causes a reservoir base shift of only 0.3ms for a 100m thick reservoir. However a 200% increase in amplitude may be detectable. The material used in the experiments was an aeolean sandstone known as Clashach sandstone, commonly used in rock physics as a North Sea reservoir analogue. The sandstone was from a surface deposit from a quarry in the NE of Scotland. Clashach sandstone has some variation in its properties but the samples tested had porosity of 13.5-14.5%, a permeability of 100-350mD and were strongly water wet with an Amott-Harvey index of 0.65-0.98. The material was stiff with a modulus of elasticity of 40-50GPa. The samples were initially vacuum saturated with brine, of density 1.067g/cc, then saturated under a high pore pressure of 13.8MPa (2000psi) to achieve 100% saturation. The cores were weighed prior to the experiment to confirm 100% saturation with brine. METHOD The experiments were performed in a Hoek cell, which applied realistic levels of insitu effective stress to the core. The platens of the Hoek cell were equipped with ultrasonic transducers, which were used to measure compression and shear wave velocity through the core during the experiments. The velocities were measured by the time taken for a pulse of ultrasonic waves to travel across the sample. The frequency of the waves was approximately 600kHz. Fluids can be flowed through the core under high stress and at high pore pressure to perform fluid substitutions. In these experiments a light oil with a density of ρ oil =0.76g/cc and a viscosity of 1.3cp was used as a flooding fluid. The brine filled cores were flooded with oil, the average saturation S w was calculated from the expelled fluids, which were collected in a separator vessel. The experiments were back to content
Ultrastructural Distinctions Between Treatment Responders and Non-Responders in Schizophrenia: Postmortem Studies of the Striatum
The Evolution of the Optical and Near-Infrared Galaxy Luminosity Functions and Luminosity Densities to z~2
Using Hubble Space Telescope and ground-based U through K- band photometry
from the Great Observatories Origins Deep Survey (GOODS), we measure the
evolution of the luminosity function and luminosity density in the rest-frame
optical (UBR) to z ~ 2, bridging the poorly explored ``redshift desert''
between z~1 and z~2. We also use deep near-infrared observations to measure the
evolution in the rest-frame J-band to z~1. Compared to local measurements from
the SDSS, we find a brightening of the characteristic magnitude, (M*), by ~2.1,
\~0.8 and ~0.7 mag between z=0.1 and z=1.9, in U, B, and R bands, respectively.
The evolution of M* in the J-band is in the opposite sense, showing a dimming
between redshifts z=0.4 and z=0.9. This is consistent with a scenario in which
the mean star formation rate in galaxies was higher in the past, while the mean
stellar mass was lower, in qualitative agreement with hierarchical galaxy
formation models. We find that the shape of the luminosity function is strongly
dependent on spectral type and that there is strong evolution with redshift in
the relative contribution from the different spectral types to the luminosity
density.
We find good agreement in the luminosity function derived from an R-selected
and a K-selected sample at z~1, suggesting that optically selected surveys of
similar depth (R < 24) are not missing a significant fraction of objects at
this redshift relative to a near-infrared-selected sample. We compare the
rest-frame B-band luminosity functions from z~0--2 with the predictions of a
semi-analytic hierarchical model of galaxy formation, and find qualitatively
good agreement. In particular, the model predicts at least as many optically
luminous galaxies at z~1--2 as are implied by our observations.Comment: 43 pages; 15 Figures; 5 Tables, Accepted for publication in Ap.
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