2,363 research outputs found
The most ancient spiral galaxy: a 2.6-Gyr-old disk with a tranquil velocity field
We report an integral-field spectroscopic (IFS) observation of a
gravitationally lensed spiral galaxy A1689B11 at redshift . It is the
most ancient spiral galaxy discovered to date and the second kinematically
confirmed spiral at . Thanks to gravitational lensing, this is also
by far the deepest IFS observation with the highest spatial resolution (
400 pc) on a spiral galaxy at a cosmic time when the Hubble sequence is about
to emerge. After correcting for a lensing magnification of 7.2 0.8, this
primitive spiral disk has an intrinsic star formation rate of 22 2
yr, a stellar mass of 10 and a
half-light radius of kpc, typical of a main-sequence
star-forming (SF) galaxy at . However, the H\alpha\ kinematics show a
surprisingly tranquil velocity field with an ordered rotation ( =
200 12 km/s) and uniformly small velocity dispersions ( = 23 4 km/s and = 15 2 km/s).
The low gas velocity dispersion is similar to local spiral galaxies and is
consistent with the classic density wave theory where spiral arms form in
dynamically cold and thin disks. We speculate that A1689B11 belongs to a
population of rare spiral galaxies at that mark the formation epoch
of thin disks. Future observations with JWST will greatly increase the sample
of these rare galaxies and unveil the earliest onset of spiral arms.Comment: 18 pages, 13 figures, 1 table; accepted for publication in Ap
How do dwarf galaxies acquire their mass & when do they form their stars?
We apply a simple, one-equation, galaxy formation model on top of the halos
and subhalos of a high-resolution dark matter cosmological simulation to study
how dwarf galaxies acquire their mass and, for better mass resolution, on over
10^5 halo merger trees, to predict when they form their stars. With the first
approach, we show that the large majority of galaxies within group- and
cluster-mass halos have acquired the bulk of their stellar mass through gas
accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are
not built up by galaxy mergers. With the second approach, we constrain the star
formation histories of dwarfs by requiring that star formation must occur
within halos of a minimum circular velocity set by the evolution of the
temperature of the IGM, starting before the epoch of reionization. We
qualitatively reproduce the downsizing trend of greater ages at greater masses
and predict an upsizing trend of greater ages as one proceeds to masses lower
than m_crit. We find that the fraction of galaxies with very young stellar
populations (more than half the mass formed within the last 1.5 Gyr) is a
function of present-day mass in stars and cold gas, which peaks at 0.5% at
m_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. We
predict that the baryonic mass function of galaxies should not show a maximum
at masses above 10^5.5, M_Sun, and we speculate on the nature of the lowest
mass galaxies.Comment: 6 pages, to appear in "A Universe of Dwarf Galaxies: Observations,
Theories, Simulations", ed. M. Koleva, P. Prugniel & I. Vauglin, EAS Series
(Paris: EDP
Gravity-driven Lyman-alpha blobs from cold streams into galaxies
We use high-resolution cosmological hydrodynamical AMR simulations to predict
the characteristics of La emission from the cold gas streams that fed galaxies
in massive haloes at high redshift. The La luminosity in our simulations is
powered by the release of gravitational energy as gas flows from the
intergalactic medium into the halo potential wells. The UV background
contributes only <20% to the gas heating. The La emissivity is due primarily to
electron-impact excitation cooling radiation in gas ~2x10^4K. We calculate the
La emissivities assuming collisional ionisation equilibrium (CIE) at all gas
temperatures. The simulated streams are self-shielded against the UV
background, so photoionisation and recombination contribute negligibly to the
La line formation. We produce theoretical maps of the La surface brightnesses,
assuming that ~85% of the La photons are directly observable. We find that
typical haloes of mass Mv~10^12-13 Msun at z~3 emit as La blobs (LABs) with
luminosities 10^43-44 erg/s. Most of the La comes from the extended narrow,
partly clumpy, inflowing, cold streams that feed the growing galaxies. The
predicted LAB morphology is therefore irregular, with dense clumps and
elongated extensions. The linewidth is expected to range from 10^2 to more than
10^3 km/s with a large variance. The typical La surface brightness profile is
proportional to r^-1.2 where r is the distance from the halo centre. Our
simulated LABs are similar in luminosity, morphology and extent to the observed
LABs, with distinct kinematic features. The predicted La luminosity function is
consistent with observations, and the predicted areas and linewidths roughly
recover the observed scaling relations. This mechanism for producing LABs
appears inevitable in many high-z galaxies. Some of the LABs may thus be
regarded as direct detections of the cold streams that drove galaxy evolution
at high z.Comment: 21 pages, 20 figures, final version accepted for publication in MNRA
Analytic and numerical realisations of a disk galaxy
Recent focus on the importance of cold, unshocked gas accretion in galaxy
formation -- not explicitly included in semi-analytic studies -- motivates the
following detailed comparison between two inherently different modelling
techniques: direct hydrodynamical simulation and semi-analytic modelling. By
analysing the physical assumptions built into the Gasoline simulation, formulae
for the emergent behaviour are derived which allow immediate and accurate
translation of these assumptions to the Galform semi-analytic model. The
simulated halo merger history is then extracted and evolved using these
equivalent equations, predicting a strikingly similar galactic system. This
exercise demonstrates that it is the initial conditions and physical
assumptions which are responsible for the predicted evolution, not the choice
of modelling technique. On this level playing field, a previously published
Galform model is applied (including additional physics such as chemical
enrichment and feedback from active galactic nuclei) which leads to starkly
different predictions.Comment: 15 pages, 15 figure
Cold streams in early massive hot haloes as the main mode of galaxy formation
The massive galaxies in the young universe, ten billion years ago, formed
stars at surprising intensities. Although this is commonly attributed to
violent mergers, the properties of many of these galaxies are incompatible with
such events, showing gas-rich, clumpy, extended rotating disks not dominated by
spheroids (Genzel et al. 2006, 2008). Cosmological simulations and clustering
theory are used to explore how these galaxies acquired their gas. Here we
report that they are stream-fed galaxies, formed from steady, narrow, cold gas
streams that penetrate the shock-heated media of massive dark matter haloes
(Dekel & Birnboim 2006; Keres et al. 2005). A comparison with the observed
abundance of star-forming galaxies implies that most of the input gas must
rapidly convert to stars. One-third of the stream mass is in gas clumps leading
to mergers of mass ratio greater than 1:10, and the rest is in smoother flows.
With a merger duy cycle of 0.1, three-quarters of the galaxies forming stars at
a given rate are fed by smooth streams. The rarer, submillimetre galaxies that
form stars even more intensely are largely merger-induced starbursts. Unlike
destructive mergers, the streams are likely to keep the rotating disk
configuration intact, although turbulent and broken into giant star-forming
clumps that merge into a central spheroid (Noguchi 1999; Genzel et al. 2008,
Elmegreen, Bournaud & Elmegreen 2008, Dekel, Sari & Ceverino 2009). This
stream-driven scenario for the formation of disks and spheroids is an
alternative to the merger picture.Comment: Improved version, 25 pages, 13 figures, Letter to Nature with
Supplementary Informatio
On the Origin of the Galaxy Star-Formation-Rate Sequence: Evolution and Scatter
We use a semi-analytic model for disk galaxies to explore the origin of the
time evolution and small scatter of the galaxy SFR sequence -- the tight
correlation between star-formation rate (SFR) and stellar mass (M_star). The
steep decline of SFR from z~2 to the present, at fixed M_star, is a consequence
of the following: First, disk galaxies are in a steady state with the SFR
following the net (i.e., inflow minus outflow) gas accretion rate. The
evolution of the SFR sequence is determined by evolution in the cosmological
specific accretion rates, \propto (1+z)^{2.25}, but is found to be independent
of feedback. Although feedback determines the outflow rates, it shifts galaxies
along the SFR sequence, leaving its zero point invariant. Second, the
conversion of accretion rate to SFR is materialized through gas density, not
gas mass. Although the model SFR is an increasing function of both gas mass
fraction and gas density, only the gas densities are predicted to evolve
significantly with redshift. Third, star formation is fueled by molecular gas.
Since the molecular gas fraction increases monotonically with increasing gas
density, the model predicts strong evolution in the molecular gas fractions,
increasing by an order of magnitude from z=0 to z~2. On the other hand, the
model predicts that the effective surface density of atomic gas is ~10 M_sun
pc^{-2}, independent of redshift, stellar mass or feedback. Our model suggests
that the scatter in the SFR sequence reflects variations in the gas accretion
history, and thus is insensitive to stellar mass, redshift or feedback. The
large scatter in halo spin contributes negligibly, because it scatters galaxies
along the SFR sequence. An observational consequence of this is that the
scatter in the SFR sequence is independent of the size (both stellar and
gaseous) of galaxy disks.Comment: 24 pages, 19 figures, accepted to MNRAS, minor changes to previous
versio
Feedback and Recycled Wind Accretion: Assembling the z=0 Galaxy Mass Function
We analyse cosmological hydrodynamic simulations that include
observationally-constrained prescriptions for galactic outflows. If these
simulated winds accurately represent winds in the real Universe, then material
previously ejected in winds provides the dominant source of gas infall for new
star formation at redshifts z<1. This recycled wind accretion, or wind mode,
provides a third physically distinct accretion channel in addition to the "hot"
and "cold" modes emphasised in recent theoretical studies. Because of the
interaction between outflows and gas in and around halos, the recycling
timescale of wind material (t_rec) is shorter in higher-mass systems, which
reside in denser gaseous environments. In these simulations, this differential
recycling plays a central role in shaping the present-day galaxy stellar mass
function (GSMF). If we remove all particles that were ever ejected in a wind,
then the predicted GSMFs are much steeper than observed; galaxy masses are
suppressed both by the direct removal of gas and by the hydrodynamic heating of
their surroundings, which reduces subsequent infall. With wind recycling
included, the simulation that incorporates our favoured momentum-driven wind
scalings reproduces the observed GSMF for stellar masses 10^9 < M < 5x10^10
Msolar. At higher masses, wind recycling leads to excessive galaxy masses and
excessive star formation rates relative to observations. In these massive
systems, some quenching mechanism must suppress the re-accretion of gas ejected
from star-forming galaxies. In short, as has long been anticipated, the form of
the GSMF is governed by outflows; the unexpected twist here for our simulated
winds is that it is not primarily the ejection of material but how the ejected
material is re-accreted that governs the GSMF.Comment: 16 pages, 7 figures, accepted by MNRA
Reporting interventions in trials evaluating cognitive rehabilitation in people with Multiple Sclerosis: a systematic review
Objective: To determine the quantity and quality of description of cognitive rehabilitation for cognitive deficits in people with Multiple Sclerosis, using a variety of published checklists, and suggest ways of improving the reporting of these interventions.
Data sources: Ten electronic databases were searched, including MEDLINE, EMBASE, CINAHL and PsycINFO, from inception to May 2017. Grey literature databases, trials registers, reference lists and author citations were also searched.
Review methods: Papers were included if participants were people with multiple sclerosis aged 18 years and over, and if the effectiveness of cognitive rehabilitation in improving functional ability for memory, attention or executive dysfunction, with or without a control group, was being evaluated.
Results: Fifty-four studies were included in this review. The reporting of a number of key aspects of cognitive rehabilitation was poor. This was particularly in relation to content of interventions (reported completely in 26 of the 54 studies), intervention procedures (reported completely in 16 of the 54 studies), delivery mode (reported completely in 24 of the 54 studies) and intervention mechanism of action (reported completely in 21 of the 54 studies).
Conclusion: The quality of reporting of cognitive rehabilitation for memory, attention and executive function for multiple sclerosis, across a range of study designs, is poor. Existing reporting checklists do not adequately cover aspects relevant to cognitive rehabilitation, such as the approaches used to address cognitive deficits. Future checklists could consider these aspects we have identified in this review
Tomato protoplast DNA transformation: physical linkage and recombination of exogenous DNA sequences
Tomato protoplasts have been transformed with plasmid DNA's, containing a chimeric kanamycin resistance gene and putative tomato origins of replication. A calcium phosphate-DNA mediated transformation procedure was employed in combination with either polyethylene glycol or polyvinyl alcohol. There were no indications that the tomato DNA inserts conferred autonomous replication on the plasmids. Instead, Southern blot hybridization analysis of seven kanamycin resistant calli revealed the presence of at least one kanamycin resistance locus per transformant integrated in the tomato nuclear DNA. Generally one to three truncated plasmid copies were found integrated into the tomato nuclear DNA, often physically linked to each other. For one transformant we have been able to use the bacterial ampicillin resistance marker of the vector plasmid pUC9 to 'rescue' a recombinant plasmid from the tomato genome. Analysis of the foreign sequences included in the rescued plasmid showed that integration had occurred in a non-repetitive DNA region. Calf-thymus DNA, used as a carrier in transformation procedure, was found to be covalently linked to plasmid DNA sequences in the genomic DNA of one transformant. A model is presented describing the fate of exogenously added DNA during the transformation of a plant cell. The results are discussed in reference to the possibility of isolating DNA sequences responsible for autonomous replication in tomato.
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