1,970 research outputs found
Microrheological Characterisation of Anisotropic Materials
We describe the measurement of anisotropic viscoelastic moduli in complex
soft materials, such as biopolymer gels, via video particle tracking
microrheology of colloid tracer particles. The use of a correlation tensor to
find the axes of maximum anisotropy, and hence the mechanical director, is
described. The moduli of an aligned DNA gel are reported, as a test of the
technique; this may have implications for high DNA concentrations in vivo. We
also discuss the errors in microrheological measurement, and describe the use
of frequency space filtering to improve displacement resolution, and hence
probe these typically high modulus materials.Comment: 5 pages, 5 figures. Replaced after refereeing/ improvement. Main
results are the same. The final, published version of the paper is here
http://link.aps.org/abstract/PRE/v73/e03190
Evolution of the Gas Mass Fraction of Progenitors to Today's Massive Galaxies: ALMA Observations in the CANDELS GOODS-S Field
We present an ALMA survey of dust continuum emission in a sample of 70
galaxies in the redshift range z=2-5 selected from the CANDELS GOODS-S field.
Multi-Epoch Abundance Matching (MEAM) is used to define potential progenitors
of a z = 0 galaxy of stellar mass 1.5 10^11 M_sun. Gas masses are derived from
the 850um luminosity. Ancillary data from the CANDELS GOODS-S survey are used
to derive the gas mass fractions. The results at z<=3 are mostly in accord with
expectations: The detection rates are 75% for the z=2 redshift bin, 50% for the
z=3 bin and 0% for z>=4. The average gas mass fraction for the detected z=2
galaxies is f_gas = 0.55+/-0.12 and f_gas = 0.62+/-0.15 for the z=3 sample.
This agrees with expectations for galaxies on the star-forming main sequence,
and shows that gas fractions have decreased at a roughly constant rate from z=3
to z=0. Stacked images of the galaxies not detected with ALMA give upper limits
to f_gas of <0.08 and <0.15, for the z=2 and z=3 redshift bins. None of our
galaxies in the z=4 and z=5 sample are detected and the upper limit from
stacked images, corrected for low metallicity, is f_gas<0.66. We do not think
that lower gas-phase metallicities can entirely explain the lower dust
luminosities. We briefly consider the possibility of accretion of very
low-metallicity gas to explain the absence of detectable dust emission in our
galaxies at z>4.Comment: Accepted for publication in the Astrophysical Journal. 33 pages; 11
figure
Dark Matter Fraction in Disk-Like Galaxies Over the Past 10 Gyr
We present an observational study of the dark matter fraction in star-forming
disk-like galaxies up to redshift , selected from publicly
available integral field spectroscropic surveys, namely KMOS3D}, KGES, and
KROSS. We provide novel observational evidence, showing that at a fixed
redshift, the dark matter fraction gradually increases with radius, indicating
that the outskirts of galaxies are dark matter dominated, similarly to local
star-forming disk galaxies. This observed dark matter fraction exhibits a
decreasing trend with increasing redshift. However, on average, the fraction
within the effective radius (upto outskirts) remains above 50\%, similar to
locals. Furthermore, we investigated the relationships between the dark matter,
baryon surface density, and circular velocity of galaxies. We observe a
decreasing trend in the dark matter fraction as baryon surface densities
increase, which is consistent across all stellar masses, redshift ranges, and
radii, with a scatter of 0.13 dex. On the other hand, the correlation between
the circular velocity at the outermost radius and the dark matter fraction
within this radius has a relatively low scatter (0.11 dex), but its slope
varies with stellar mass and with redshift, providing observational evidence of
the dynamical evolution of the interplay between the baryonic and dark matter
distributions with cosmic time. We observe that low stellar mass galaxies
() undergo a higher degree of
evolution, which may be attributed to the hierarchical merging of galaxies.Comment: Comments are welcom
Towards a resolved Kennicutt-Schmidt law at high redshift
Massive galaxies in the distant Universe form stars at much higher rates than
today. Although direct resolution of the star forming regions of these galaxies
is still a challenge, recent molecular gas observations at the IRAM Plateau de
Bure interferometer enable us to study the star formation efficiency on
subgalactic scales around redshift z = 1.2. We present a method for obtaining
the gas and star formation rate (SFR) surface densities of ensembles of clumps
composing galaxies at this redshift, even though the corresponding scales are
not resolved. This method is based on identifying these structures in
position-velocity diagrams corresponding to slices within the galaxies. We use
unique IRAM observations of the CO(3-2) rotational line and DEEP2 spectra of
four massive star forming distant galaxies - EGS13003805, EGS13004291,
EGS12007881, and EGS13019128 in the AEGIS terminology - to determine the gas
and SFR surface densities of the identifiable ensembles of clumps that
constitute them. The integrated CO line luminosity is assumed to be directly
proportional to the total gas mass, and the SFR is deduced from the [OII] line.
We identify the ensembles of clumps with the angular resolution available in
both CO and [OII] spectroscopy; i.e., 1-1.5". SFR and gas surface densities are
averaged in areas of this size, which is also the thickness of the DEEP2 slits
and of the extracted IRAM slices, and we derive a spatially resolved
Kennicutt-Schmidt (KS) relation on a scale of ~8 kpc. The data generally
indicates an average depletion time of 1.9 Gyr, but with significant variations
from point to point within the galaxies.Comment: 6 pages, 4 figures, 2 tables, accepted by Astronomy and Astrophysic
Combined CO & Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star Formation Rate and Stellar Mass
We combine molecular gas masses inferred from CO emission in 500 star forming
galaxies (SFGs) between z=0 and 3, from the IRAM-COLDGASS, PHIBSS1/2 and other
surveys, with gas masses derived from Herschel far-IR dust measurements in 512
galaxy stacks over the same stellar mass/redshift range. We constrain the
scaling relations of molecular gas depletion time scale (tdepl) and gas to
stellar mass ratio (Mmolgas/M*) of SFGs near the star formation main-sequence
with redshift, specific star formation rate (sSFR) and stellar mass (M*). The
CO- and dust-based scaling relations agree remarkably well. This suggests that
the CO-H2 mass conversion factor varies little within 0.6dex of the main
sequence (sSFR(ms,z,M*)), and less than 0.3dex throughout this redshift range.
This study builds on and strengthens the results of earlier work. We find that
tdepl scales as (1+z)^-0.3 *(sSFR/sSFR(ms,z,M*))^-0.5, with little dependence
on M*. The resulting steep redshift dependence of Mmolgas/M* ~(1+z)^3 mirrors
that of the sSFR and probably reflects the gas supply rate. The decreasing gas
fractions at high M* are driven by the flattening of the SFR-M* relation.
Throughout the redshift range probed a larger sSFR at constant M* is due to a
combination of an increasing gas fraction and a decreasing depletion time
scale. As a result galaxy integrated samples of the Mmolgas-SFR rate relation
exhibit a super-linear slope, which increases with the range of sSFR. With
these new relations it is now possible to determine Mmolgas with an accuracy of
0.1dex in relative terms, and 0.2dex including systematic uncertainties.Comment: ApJ accepte
Aluminium content of spanish infant formula
Aluminium toxicity has been relatively well documented in infants with impaired renal
function and premature neonates.
The aims of this study were to analyse the concentration of aluminium in the majority of
infant formulae sold commercially in Spain, to determine the influence of aluminium
content in the tap water in reconstituted powder formulae and to estimate the theoretical
toxic aluminium intake in comparison with the PTWI, and lastly, to discuss the possible
interactions of certain essential trace elements added to formulation with aluminium
according to type or main protein based infant formula.
A total of 82 different infant formulae from 9 different manufacturers were studied.
Sample digestion was simulated in a closed acid-decomposition microwave system.
Aluminium concentration was determined by atomic absorption spectrophotometry with
graphite furnace.
In general, the infant formulae studied provide an aluminium level higher than that found
in human milk, especially in the case of soya, preterm or hydrolysed casein-based
formulae.
Standard formulae provide lower aluminium intakes amounting to about 4 % PTWI.
Specialised and preterm formulae result in moderate intake (11 – 12 % and 8 – 10 %
PTWI, respectively). Soya formulae contribute the highest intake (15 % PTWI).
Aluminium exposure from drinking water used for powder formula reconstitution is not
considered a clear potential risk.
In accordance with the present state of knowledge about aluminium toxicity, it seems
prudent to call for continued efforts to standardise routine quality control and reduce
aluminium levels in infant formula as well as to keep the aluminium concentration under
300 g l-1 for all infant formulae, most specifically those formulae for premature and low
birth neonates
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