241 research outputs found
The Futility of Exoplanet Biosignatures
The ultimate goal of astrobiology is to determine the distribution and
diversity of life in the universe. But as the word "biosignature" suggests,
what will be detected is not life itself, but an observation implicating a
particular process associated with living systems. Technical constraints and
our limited access to other worlds suggest we are more likely to detect an
out-of-equilibrium suite of gasses than a writhing octopus. Yet, anything short
of a writhing octopus will raise skepticism among astrobiologists about what
has been detected. Resolving that skepticism requires a theory to delineate
processes due to life and those due solely to abiotic mechanisms. This poses an
existential question for the endeavor of life detection: How do astrobiologists
plan to detect life via features shared between non-living and living systems?
We argue that you cannot without an underlying theory of life. We illustrate
this by analyzing the hypothetical detection of an "Earth 2.0" exoplanet. In
the absence of a theory of life, we argue the community should focus on
identifying unambiguous features of life via four areas of active research:
understanding the principles of life on Earth, building life in the lab,
detecting life in the solar system and searching for technosignatures.
Ultimately, we ask, what exactly do astrobiologists hope to learn by searching
for life?Comment: 15 pages, 2 figures, 1 bo
Spontaneous formation of autocatalytic sets with self-replicating inorganic metal oxide clusters
Here we show how a simple inorganic salt can spontaneously form autocatalytic sets of replicating inorganic molecules that work via molecular recognition based on the {PMo12} ≡ [PMo12O40]3– Keggin ion, and {Mo36} ≡ [H3Mo57M6(NO)6O183(H2O)18]22– cluster. These small clusters are able to catalyze their own formation via an autocatalytic network, which subsequently template the assembly of gigantic molybdenum-blue wheel {Mo154} ≡ [Mo154O462H14(H2O)70]14–, {Mo132} ≡ [MoVI72MoV60O372(CH3COO)30(H2O)72]42– ball-shaped species containing 154 and 132 molybdenum atoms, and a {PMo12}⊂{Mo124Ce4} ≡ [H16MoVI100MoV24Ce4O376(H2O)56 (PMoVI10MoV2O40)(C6H12N2O4S2)4]5– nanostructure. Kinetic investigations revealed key traits of autocatalytic systems including molecular recognition and kinetic saturation. A stochastic model confirms the presence of an autocatalytic network involving molecular recognition and assembly processes, where the larger clusters are the only products stabilized by the cycle, isolated due to a critical transition in the network
GALICS -- VI. Modelling Hierarchical Galaxy Formation in Clusters
High-resolution N-body re-simulations of 15 massive (10^{14}-10^{15} Msun)
dark matter haloes have been combined with the hybrid galaxy formation model
GalICS (Hatton et al. 2003), to study the formation and evolution of galaxies
in clusters, within the framework of the hierarchical merging scenario. New
features in GalICS include a better description of galaxy positioning within
dark matter haloes, a more reliable computation of the temperature of the
inter-galactic medium as a function of redshift, and a description of the ram
pressure stripping process. We focus on the luminosity functions, morphological
fractions and colour distributions of galaxies in clusters and in cluster
outskirts, at z=0. No systematic dependency on cluster richness is found either
for the galaxy luminosity functions, morphological mixes, or colour
distributions. Moving from higher density (cluster cores), to lower density
environments (cluster outskirts), we detect a progressive flattening of the
luminosity functions, an increase of the fraction of spirals and a decrease of
that of ellipticals and S0s, and the progressive emergence of a bluer tail in
the distributions of galaxy colours, especially for spirals. As compared to
cluster spirals, early-type galaxies show a flatter luminosity function, and
more homogeneous and redder colours. An overall good agreement is found between
our results and the observations, particularly in terms of the cluster
luminosity functions and morphological mixes. However, some discrepancies are
also apparent, with too faint magnitudes of the brightest cluster members,
especially in the B band, and galaxy colours tendentially too red (or not blue
enough) in the model, with respect to the observations. Finally, ram pressure
stripping appears to affect very little our results.Comment: Accepted for publication in MNRAS. 17 pages, 11 figures.
High-resolution Figure 1 available in the on-line version of the pape
Exploring and mapping chemical space with molecular assembly trees
The rule-based search of chemical space can generate an almost infinite number of molecules, but exploration of known molecules as a function of the minimum number of steps needed to build up the target graphs promises to uncover new motifs and transformations. Assembly theory is an approach to compare the intrinsic complexity and properties of molecules by the minimum number of steps needed to build up the target graphs. Here, we apply this approach to prebiotic chemistry, gene sequences, plasticizers, and opiates. This allows us to explore molecules connected to the assembly tree, rather than the entire space of molecules possible. Last, by developing a reassembly method, based on assembly trees, we found that in the case of the opiates, a new set of drug candidates could be generated that would not be accessible via conventional fragment-based drug design, thereby demonstrating how this approach might find application in drug discovery
Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey
Kinematic distances to 750 molecular clouds identified in the 13CO J=1-0
Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
(BU-FCRAO GRS) are derived assuming the Clemens rotation curve of the Galaxy.
The kinematic distance ambiguity is resolved by examining the presence of HI
self-absorption toward the 13CO emission peak of each cloud using the Very
Large Array Galactic Plane Survey (VGPS). We also identify 21 cm continuum
sources embedded in the GRS clouds in order to use absorption features in the
HI 21 cm continuum to distinguish between near and far kinematic distances. The
Galactic distribution of GRS clouds is consistent with a four-arm model of the
Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS
clouds match star count data from the Galactic Legacy Infrared Mid-Plane Survey
Extraordinaire (GLIMPSE) star-count data. We conclude that molecular clouds
must form in spiral arms and be short-lived (lifetimes < 10 Myr) in order to
explain the absence of massive, 13CO bright molecular clouds in the inter-arm
space
Simulating the Formation of the Local Galaxy Population
We simulate the formation and evolution of the local galaxy population
starting from initial conditions with a smoothed linear density field which
matches that derived from the IRAS 1.2 Jy galaxy survey. Our simulations track
the formation and evolution of all dark matter haloes more massive than 10e+11
solar masses out to a distance of 8000 km/s from the Milky Way. We implement
prescriptions similar to those of Kauffmann et al. (1999) to follow the
assembly and evolution of the galaxies within these haloes. We focus on two
variants of the CDM cosmology: an LCDM and a tCDM model. Galaxy formation in
each is adjusted to reproduce the I-band Tully-Fisher relation of Giovanelli et
al. (1997). We compare the present-day luminosity functions, colours,
morphology and spatial distribution of our simulated galaxies with those of the
real local population, in particular with the Updated Zwicky Catalog, with the
IRAS PSCz redshift survey, and with individual local clusters such as Coma,
Virgo and Perseus. We also use the simulations to study the clustering bias
between the dark matter and galaxies of differing type. Although some
significant discrepancies remain, our simulations recover the observed
intrinsic properties and the observed spatial distribution of local galaxies
reasonably well. They can thus be used to calibrate methods which use the
observed local galaxy population to estimate the cosmic density parameter or to
draw conclusions about the mechanisms of galaxy formation. To facilitate such
work, we publically release our z=0 galaxy catalogues, together with the
underlying mass distribution.Comment: 25 pages, 20 figures, submitted to MNRAS. High resolution copies of
figures 1 and 3, halo and galaxy catalogues can be found at
http://www.mpa-garching.mpg.de/NumCos/CR/index.htm
Environmental control programs the emergence of distinct functional ensembles from unconstrained chemical reactions
Many approaches to the origin of life focus on how the molecules found in biology might be made in the absence of biological processes, from the simplest plausible starting materials. Another approach could be to view the emergence of the chemistry of biology as process whereby the environment effectively directs “primordial soups” toward structure, function, and genetic systems over time. This does not require the molecules found in biology today to be made initially, and leads to the hypothesis that environment can direct chemical soups toward order, and eventually living systems. Herein, we show how unconstrained condensation reactions can be steered by changes in the reaction environment, such as order of reactant addition, and addition of salts or minerals. Using omics techniques to survey the resulting chemical ensembles we demonstrate there are distinct, significant, and reproducible differences between the product mixtures. Furthermore, we observe that these differences in composition have consequences, manifested in clearly different structural and functional properties. We demonstrate that simple variations in environmental parameters lead to differentiation of distinct chemical ensembles from both amino acid mixtures and a primordial soup model. We show that the synthetic complexity emerging from such unconstrained reactions is not as intractable as often suggested, when viewed through a chemically agnostic lens. An open approach to complexity can generate compositional, structural, and functional diversity from fixed sets of simple starting materials, suggesting that differentiation of chemical ensembles can occur in the wider environment without the need for biological machinery
VLT/NACO near-infrared imaging and spectroscopy of N88A in the SMC
We present near-infrared imaging and spectroscopic high spatial resolution
observations of the SMC region N88 containing the bright, excited, extincted
and compact H II region N88A of size ~ 1 pc. To investigate its stellar content
and reddening, N88 was observed using spectroscopy and imagery in the JHKs- and
L'-band at a spatial resolution of ~ 0.1-0.3", using the VLT UT4 equipped with
the NAOS adaptive optics system. In order to attempt to establish if the origin
of the infra-red (IR) excess is due to bright nebulosity, circumstellar
material and/or local dust, we used Ks vs J-K colour-magnitude (CM) and JHK
colour-colour (CC) diagrams, as well as L' imagery.Our IR-data reveal in the
N88 area an IR-excess fraction of geq 30 per cent of the detected stars,as well
as an unprecedently detailed morphology of N88A. It consists of an embedded
cluster of ~3.5" (~ 1 pc) in diameter, of at least thirteen resolved stars
superposed with an unusual bright continuum centered on a very bright star. The
four brightest stars in this cluster lie red-ward of H-K geq 0.45 mag, and
could be classified as young stellar object (YSO) candidates. Four other
probable YSO candidates are also detected in N88 along a south-north bow-shaped
thin H2 filament at ~ 7" east of the young central bright star. At 0.2" east of
this star, a heavily embedded core is detected that could be a massive class I
protostar candidate. The 2.12 mu H2 image of N88A resembles a shell of diameter
~ 3" ~ 0.9 pc) centered on the bright star. The line ratios of H2 2-1 S(1) and
1-0 S(0) relative to 1-0 S(1), as well as the presence of high v lines, are
indicative of photodissociation regions, rather than shocks.Comment: 15 pages, 14 figures, accepted by Astronomy and Astrophysics, uses
pdflatex, aa.cl
The K20 survey. VI. The Distribution of the Stellar Masses in Galaxies up to z~2
We present a detailed analysis of the stellar mass content of galaxies up to
z=2.5 in the K20 galaxy sample, that has a 92% spectroscopic completeness and a
complete multicolor coverage. We find that the M/L ratio decreases
with redshift: in particular, the average M/L ratio of early type galaxies
decreases with , with a scatter that is indicative of a range of
star--formation time-scales and redshift of formation. More important, the
typical M/L of massive early type galaxies is larger than that of less massive
ones, suggesting that their stellar population formed at higher z. The final
K20 galaxy sample spans a range of stellar masses from M*=10^9Msun to
M*=10^12Msun, with massive galaxies ($M*>10^11Msun) detected up to z~2. We
compute the Galaxy Stellar Mass Function at various z, of which we observe only
a mild evolution (i.e. by 20-30%) up to z~1. At z>1, the evolution of the GSMF
appears to be much faster: at z~2, about 35% of the present day stellar mass in
objects with M*~10^11Msun appear to have assembled. We also detect a change in
the physical nature of the most massive galaxies, since at z>1 a population of
massive star--forming galaxies progressively appears. We finally analyze our
results in the framework of Lambda-CDM hierarchical models. First, we show that
the large number of massive galaxies detected at high z does not violate any
fundamental Lambda-CDM constraint based on the number of massive DM halos.
Then, we compare our results with the predictions of renditions of both
semianalytic and hydro-dynamical models, that range from severe underestimates
to slight overestimates of the observed mass density at z<~2. We discuss how
the differences among these models are due to the different implementation of
the main physical processes. (Abridged)Comment: Accepted for publication on Astronomy & Astrophysic
Spitzer Observations of IC 2118
IC 2118, also known as the Witch Head Nebula, is a wispy, roughly cometary,
~5 degree long reflection nebula, and is thought to be a site of triggered star
formation. In order to search for new young stellar objects (YSOs), we have
observed this region in 7 mid- and far-infrared bands using the Spitzer Space
Telescope and in 4 bands in the optical using the U. S. Naval Observatory
40-inch telescope. We find infrared excesses in 4 of the 6 previously-known T
Tauri stars in our combined infrared maps, and we find 6 entirely new candidate
YSOs, one of which may be an edge-on disk. Most of the YSOs seen in the
infrared are Class II objects, and they are all in the "head" of the nebula,
within the most massive molecular cloud of the region.Comment: Accepted to Ap
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