133 research outputs found
Critical spin-flip scattering at the helimagnetic transition of MnSi
We report spherical neutron polarimetry (SNP) and discuss the spin-flip
scattering cross sections as well as the chiral fraction close to the
helimagnetic transition in MnSi. For our study, we have developed a
miniaturised SNP device that allows fast data collection when used in small
angle scattering geometry with an area detector. Critical spin-flip scattering
is found to be governed by chiral paramagnons that soften on a sphere in
momentum space. Carefully accounting for the incoherent spin-flip background,
we find that the resulting chiral fraction decreases gradually above the
helimagnetic transition reflecting a strongly renormalised chiral correlation
length with a temperature dependence in excellent quantitative agreement with
the Brazovskii theory for a fluctuation-induced first order transition.Comment: 5 pages, 3 figure
Large Scales - Long Times: Adding High Energy Resolution to SANS
The Neutron Spin Echo (NSE) variant MIEZE (Modulation of IntEnsity by Zero
Effort), where all beam manipulations are performed before the sample position,
offers the possibility to perform low background SANS measurements in strong
magnetic fields and depolarising samples. However, MIEZE is sensitive to
differences \DeltaL in the length of neutron flight paths through the
instrument and the sample. In this article, we discuss the major influence of
\DeltaL on contrast reduction of MIEZE measurements and its minimisation.
Finally we present a design case for enhancing a small-angle neutron scattering
(SANS) instrument at the planned European Spallation Source (ESS) in Lund,
Sweden, using a combination of MIEZE and other TOF options, such as TISANE
offering time windows from ns to minutes. The proposed instrument allows
studying fluctuations in depolarizing samples, samples exposed to strong
magnetic fields, and spin-incoherently scattering samples in a straightforward
way up to time scales of \mus at momentum transfers up to 0.01 {\AA}-1, while
keeping the instrumental effort and costs low.Comment: 5 pages, 8 figure
Galaxy and mass assembly (GAMA) : The wavelength-dependent sizes and profiles of galaxies revealed by MegaMorph
We investigate the relationship between colour and structure within galaxies using a large, volume-limited sample of bright, low-redshift galaxies with optical-near-infrared imaging from the Galaxy AndMass Assembly survey.We fit single-component,wavelength-dependent, elliptical SĂ©rsic models to all passbands simultaneously, using software developed by the MegaMorph project. Dividing our sample by n and colour, the recovered wavelength variations in effective radius (Re) and SĂ©rsic index (n) reveal the internal structure, and hence formation history, of different types of galaxies. All these trends depend on n; some have an additional dependence on galaxy colour. Late-type galaxies (nr 2.5), even though they maintain constant n with wavelength, revealing that ellipticals are a superimposition of different stellar populations associated with multiple collapse and merging events. Processes leading to structures with larger Re must be associated with lower metallicity or younger stellar populations. This appears to rule out the formation of young cores through dissipative gas accretion as an important mechanism in the recent lives of luminous elliptical galaxies.Peer reviewe
Phase Diagram for the Winfree Model of Coupled Nonlinear Oscillators
In 1967 Winfree proposed a mean-field model for the spontaneous
synchronization of chorusing crickets, flashing fireflies, circadian pacemaker
cells, or other large populations of biological oscillators. Here we give the
first bifurcation analysis of the model, for a tractable special case. The
system displays rich collective dynamics as a function of the coupling strength
and the spread of natural frequencies. Besides incoherence, frequency locking,
and oscillator death, there exist novel hybrid solutions that combine two or
more of these states. We present the phase diagram and derive several of the
stability boundaries analytically.Comment: 4 pages, 4 figure
Galaxy And Mass Assembly (GAMA): understanding the wavelength dependence of galaxy structure with bulge-disc decompositions
With a large sample of bright, low-redshift galaxies with opticalânear-IR imaging from the GAMA survey we use bulge-disc decompositions to understand the wavelength-dependent behaviour of single-Sersic structural measurements. We denote the variation in single-Sersic index with wavelength as N, likewise for effective radius we use R. We find that most galaxies with a substantial disc, even those with no discernable bulge, display a high value of N. The increase in Sersic index to longer wavelengths is therefore intrinsic to discs, apparently resulting from radial variations in stellar population and/or dust reddening. Similarly, low values of R (< 1) are found to be ubiquitous, implying an element of universality in galaxy colour gradients. We also study how bulge and disc colour distributions vary with galaxy type. We find that, rather than all bulges being red and all discs being blue in absolute terms, both components become redder for galaxies with redder total colours. We even observe that bulges in bluer galaxies are typically bluer than discs in red galaxies, and that bulges and discs are closer in colour for fainter galaxies. Trends in total colour are therefore not solely due to the colour or flux dominance of the bulge or disc
Galaxy And Mass Assembly (GAMA): bulge-disc decomposition of KiDS data in the nearby Universe
We derive single SĂ©rsic fits and bulge-disc decompositions for 13â096 galaxies at redshifts z \u3c 0.08 in the GAMA II equatorial survey regions in the Kilo-Degree Survey (KiDS) g, r, and i bands. The surface brightness fitting is performed using the Bayesian two-dimensional profile fitting code PROFIT. We fit three models to each galaxy in each band independently with a fully automated Markov chain Monte Carlo analysis: a single SĂ©rsic model, a SĂ©rsic plus exponential and a point source plus exponential. After fitting the galaxies, we perform model selection and flag galaxies for which none of our models are appropriate (mainly mergers/Irregular galaxies). The fit quality is assessed by visual inspections, comparison to previous works, comparison of independent fits of galaxies in the overlap regions between KiDS tiles and bespoke simulations. The latter two are also used for a detailed investigation of systematic error sources. We find that our fit results are robust across various galaxy types and image qualities with minimal biases. Errors given by the MCMC underestimate the true errors typically by factors 2â3. Automated model selection criteria are accurate to \u3e90 per cent role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-variant: inherit; font-stretch: inherit; line-height: normal; font-family: inherit; vertical-align: baseline; display: inline; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative; \u3e\u3e90 per cent\u3e90 per cent as calibrated by visual inspection of a subsample of galaxies. We also present gâr component colours and the corresponding colourâmagnitude diagram, consistent with previous works despite our increased fit flexibility. Such reliable structural parameters for the components of a diverse sample of galaxies across multiple bands will be integral to various studies of galaxy properties and evolution. All results are integrated into the GAMA database
Observational constraints on the co-evolution of supermassive black holes and galaxies
The star formation rate (SFR) and black hole accretion rate (BHAR) functions
are measured to be proportional to each other at z < ~3. This close
correspondence between SF and BHA would naturally yield a BH mass-galaxy mass
correlation, whereas a BH mass-bulge mass correlation is observed. To explore
this apparent contradiction we study the SF in spheroid-dominated galaxies
between z=1 and the present day. We use 903 galaxies from the COMBO-17 survey
with M* >2x10^10M_sun, ultraviolet and infrared-derived SFRs from Spitzer and
GALEX, and morphologies from GEMS HST/ACS imaging. Using stacking techniques,
we find that <25% of all SF occurs in spheroid-dominated galaxies (Sersic index
n>2.5), while the BHAR that we would expect if the global scalings held is
three times higher. This rules out the simplest picture of co-evolution, in
which SF and BHA trace each other at all times. These results could be
explained if SF and BHA occur in the same events, but offset in time, for
example at different stages of a merger event. However, one would then expect
to see the corresponding star formation activity in early-stage mergers, in
conflict with observations. We conclude that the major episodes of SF and BHA
occur in different events, with the bulk of SF happening in isolated disks and
most BHA occurring in major mergers. The apparent global co-evolution results
from the regulation of the BH growth by the potential well of the galactic
spheroid, which includes a major contribution from disrupted disk stars.Comment: 14 pages, 5 figures, accepted for publication in Ap
History of Galaxy Interactions and their Impact on Star Formation over the Last 7 Gyr from GEMS
We perform a comprehensive estimate of the frequency of galaxy mergers and
their impact on star formation over z~0.24--0.80 (lookback time T_b~3--7 Gyr)
using 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data.
Our results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, the
visually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%.
Lower limits on the major and minor merger fractions over this interval range
from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first
approximate empirical estimate of the frequency of minor mergers at z<1. For a
visibility timescale of ~0.5 Gyr, it follows that over T_b~3--7 Gyr, ~68% of
high mass systems have undergone a merger of mass ratio >1/10, with ~16%, 45%,
and 7% of these corresponding respectively to major, minor, and ambiguous
`major or minor' mergers. The mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2)
We compare the empirical merger fraction and rate for high mass galaxies to a
suite of Lambda CDM-based models: halo occupation distribution models,
semi-analytic models, and hydrodynamic SPH simulations. We find qualitative
agreement between observations and models such that the (major+minor) merger
fraction or rate from different models bracket the observations, and show a
factor of five dispersion. Near-future improvements can now start to rule out
certain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find that
the mean SFR of visibly merging systems is only modestly enhanced compared to
non-interacting galaxies over z~0.24--0.80. Visibly merging systems only
account for less than 30% of the cosmic SFR density over T_b~3--7 Gyr. This
suggests that the behavior of the cosmic SFR density over the last 7 Gyr is
predominantly shaped by non-interacting galaxies.Comment: Accepted for Publication in the Astrophysical Journal. 17 pages of
text, 21 figures, 3 tables. Uses emulateapj5.st
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