230 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
MegaMorph: classifying galaxy morphology using multi-wavelength S\'ersic profile fits
Aims. This work investigates the potential of using the wavelength-dependence
of galaxy structural parameters (S\'ersic index, n, and effective radius, Re)
to separate galaxies into distinct types. Methods. A sample of nearby galaxies
with reliable visual morphologies is considered, for which we measure
structural parameters by fitting multi-wavelength single-S\'ersic models.
Additionally, we use a set of artificially redshifted galaxies to test how
these classifiers behave when the signal-to-noise decreases. Results. We show
that the wavelength-dependence of n may be employed to separate
visually-classified early- and late-type galaxies, in a manner similar to the
use of colour and n. Furthermore, we find that the wavelength variation of n
can recover galaxies that are misclassified by these other morphological
proxies. Roughly half of the spiral galaxies that contaminate an early-type
sample selected using (u-r) versus n can be correctly identified as late-types
by N, the ratio of n measured in two different bands. Using a set of
artificially-redshifted images, we show that this technique remains effective
up to z ~ 0.1. N can therefore be used to achieve purer samples of early-types
and more complete samples of late-types than using a colour-n cut alone. We
also study the suitability of R, the ratio of Re in two different bands, as a
morphological classifier, but find that the average sizes of both early- and
late-type galaxies do not change substantially over optical wavelengths.Comment: 6 pages, 2 figures, 2 tables, Accepted for publication in A&
FERENGI: Redshifting galaxies from SDSS to GEMS, STAGES and COSMOS
We describe the creation of a set of artificially "redshifted" galaxies in
the range 0.1<z<1.1 using a set of ~100 SDSS low redshift (v<7000 km/s) images
as input. The intention is to generate a training set of realistic images of
galaxies of diverse morphologies and a large range of redshifts for the GEMS
and COSMOS galaxy evolution projects. This training set allows other studies to
investigate and quantify the effects of cosmological redshift on the
determination of galaxy morphologies, distortions and other galaxy properties
that are potentially sensitive to resolution, surface brightness and bandpass
issues. We use galaxy images from the SDSS in the u, g, r, i, z filter bands as
input, and computed new galaxy images from these data, resembling the same
galaxies as located at redshifts 0.1<z<1.1 and viewed with the Hubble Space
Telescope Advanced Camera for Surveys (HST ACS). In this process we take into
account angular size change, cosmological surface brightness dimming, and
spectral change. The latter is achieved by interpolating a spectral energy
distribution that is fit to the input images on a pixel-to-pixel basis. The
output images are created for the specific HST ACS point spread function and
the filters used for GEMS (F606W and F850LP) and COSMOS (F814W). All images are
binned onto the desired pixel grids (0.03" for GEMS and 0.05" for COSMOS) and
corrected to an appropriate point spread function. Noise is added corresponding
to the data quality of the two projects and the images are added onto empty sky
pieces of real data images. We make these datasets available from our website,
as well as the code - FERENGI: "Full and Efficient Redshifting of Ensembles of
Nearby Galaxy Images" - to produce datasets for other redshifts and/or
instruments.Comment: 11 pages, 10 figures, 3 table
Electron spin resonance study on the 4f honeycomb quantum magnet YbCl3
The local magnetic properties of Yb in the layered honeycomb material
YbCl were investigated by electron spin resonance on single crystals. For
in-plane and out-of-plane field orientations the -factor shows a clear
anisotropy ( and ), whereas the low temperature
exchange coupling and the spin relaxation display a rather isotropic character.
At elevated temperatures the contribution of the first excited crystal field
level (~meV) dominates the spin relaxation.Comment: 10 pages, 5 figure
The rapid transition from star-formation to AGN dominated rest-frame UV light at z ~ 4
With the advent of deep optical-to-near-infrared extragalactic imaging on the
degree scale, samples of high-redshift sources are being selected that contain
both bright star-forming (SF) galaxies and faint active galactic nuclei (AGN).
In this study we investigate the transition between SF and AGN-dominated
systems at in the rest-frame UV. We find a rapid transition to
AGN-dominated sources bright-ward of . The effect is
observed in the rest-frame UV morphology and size-luminosity relation, where
extended clumpy systems become point-source dominated, and also in the
available spectra for the sample. These results allow us to derive the
rest-frame UV luminosity function for the SF and AGN-dominated sub-samples. We
find the SF-dominated LF is best fit with a double-power law, with a lensed
Schechter function being unable to explain the existence of extremely luminous
SF galaxies at . If we identify AGN-dominated sources
according to a point-source morphology criterion we recover the relatively flat
faint-end slope of the AGN LF determined in previous studies. If we instead
separate the LF according to the current spectroscopic AGN fraction, we find a
steeper faint-end slope of . Using a simple model to
predict the rest-frame AGN LF from the galaxy LF we find that the
increasing impact of host galaxy light on the measured morphology of faint AGN
can explain our observations.Comment: 17 pages, 11 figures, 2 tables. Accepted to MNRA
The bright end of the galaxy luminosity function at from the VISTA VIDEO survey
We have conducted a search for Lyman break galaxies over 8.2
square degrees of near-infrared imaging from the VISTA Deep Extragalactic
Observations (VIDEO) survey in the XMM-Newton - Large Scale Structure (XMM-LSS)
and the Extended Chandra Deep Field South (ECDF-S) fields. Candidate galaxies
were selected from a full photometric redshift analysis down to a depth
of 25.3 (), utilizing deep auxiliary optical and Spitzer/IRAC data to
remove brown dwarf and red interloper galaxy contaminants. Our final sample
consists of 28 candidate galaxies at with . We derive stellar masses of for the sample, suggesting that these
candidates represent some of the most massive galaxies known at this epoch. We
measure the rest-frame UV luminosity function (LF) at , confirming
previous findings of a gradual decline in number density at the bright-end
() that is well described by a double-power law (DPL).
We show that quasar contamination in this magnitude range is expected to be
minimal, in contrast to conclusions from recent pure-parallel Hubble studies.
Our results are up to a factor of ten lower than previous determinations from
optical-only ground-based studies at . We find that
the inclusion of photometry is vital for removing brown-dwarf
contaminants, and samples based on red-optical data alone could be
highly contaminated ( per cent). In comparison with other robust samples, our results further support little evolution in the very
bright-end of the rest-frame UV LF from , potentially signalling a
lack of mass quenching and/or dust obscuration in the most massive galaxies in
the first Gyr.Comment: 15 pages, 6 figures, 5 tables (plus additional figures/tables in
Appendix). Submitted to MNRA
The total rest-frame UV luminosity function from : A simultaneous study of AGN and galaxies from
We present measurements of the rest-frame ultraviolet luminosity function (UV
LF) at redshifts , and , using 96894, 38655 and 7571 sources
respectively to map the transition between AGN and galaxy-dominated ultraviolet
emission shortly after the epoch of reionization. Sources are selected using a
comprehensive photometric redshift approach, using \ds\, of deep
extragalactic legacy fields covered by both HSC and VISTA. The use of template
fitting spanning a wavelength range of achieves
per cent completeness, much higher than classical colour-colour
cut methodology. The measured LF encompasses at
. This is further extended to using
complementary results from other studies, allowing for the simultaneous fitting
of the combined AGN and galaxy LF. We find that there are fewer UV luminous
galaxies () at than , indicative of an onset
of widespread quenching alongside dust obscuration, and that the evolution of
the AGN LF is much more rapid than the galaxy LF, with their number density
rising by around 2 orders of magnitude from . We also find that it
remains difficult to determine if a double power law (DPL) functional form is
preferred over the Schechter function to describe the galaxy UV LF with
photometric data alone. Estimating the Hydrogen ionizing photon budget from our
UV LFs, we find that AGN can contribute to, but cannot solely maintain, the
reionization of the Universe at . However, the rapidly evolving AGN LF
strongly disfavours a significant contribution within the EoR.Comment: 20 pages, 5 Tables, 12 Figures, Submitted to MNRA
Multi-wavelength structure analysis of local cluster galaxies: The WINGS project
© ESO 2020. We present a multi-wavelength analysis of the galaxies in nine clusters selected from the WINGS dataset, examining how galaxy structure varies as a function of wavelength and environment using the state of the art software GALAPAGOS-2. We simultaneously fit single-Sérsic functions on three optical (u, B and V) and two near-infrared (J and K) bands thus creating a wavelength-dependent model of each galaxy. We measure the magnitudes, effective radius (Re), the Sérsic index (n), axis ratio, and position angle in each band. The sample contains 790 cluster members (located close to the cluster centre < 0.64 R200) and 254 non-member galaxies that we further separate based on their morphology into ellipticals, lenticulars, and spirals. We find that the Sérsic index of all galaxies inside clusters remains nearly constant with wavelength while Re decreases as wavelength increases for all morphological types. We do not observe a significant variation on n and Re as a function of projected local density and distance from the clusters centre. Comparing the n and Re of bright cluster galaxies with a subsample of non-member galaxies we find that bright cluster galaxies are more concentrated (display high n values) and are more compact (low Re). Moreover, the light profile and size of bright cluster galaxies does not change as a function of wavelength in the same manner as non-member galaxies
Structural Evolution of Early-type Galaxies to z=2.5 in CANDELS
Projected axis ratio measurements of 880 early-type galaxies at redshifts
1<z<2.5 selected from CANDELS are used to reconstruct and model their intrinsic
shapes. The sample is selected on the basis of multiple rest-frame colors to
reflect low star-formation activity. We demonstrate that these galaxies as an
ensemble are dust-poor and transparent and therefore likely have smooth light
profiles, similar to visually classified early-type galaxies. Similar to their
present-day counterparts, the z>1 early-type galaxies show a variety of
intrinsic shapes; even at a fixed mass, the projected axis ratio distributions
cannot be explained by the random projection of a set of galaxies with very
similar intrinsic shapes. However, a two-population model for the intrinsic
shapes, consisting of a triaxial, fairly round population, combined with a flat
(c/a~0.3) oblate population, adequately describes the projected axis ratio
distributions of both present-day and z>1 early-type galaxies. We find that the
proportion of oblate versus triaxial galaxies depends both on the galaxies'
stellar mass, and - at a given mass - on redshift. For present-day and z<1
early-type galaxies the oblate fraction strongly depends on galaxy mass. At z>1
this trend is much weaker over the mass range explored here
(10^10<M*/M_sun<10^11), because the oblate fraction among massive (M*~10^11
M_sun) was much higher in the past: 0.59+-0.10 at z>1, compared to 0.20+-0.02
at z~0.1. In contrast, the oblate fraction among low-mass early-type galaxies
(log(M*/M_sun)1 to
0.72+-0.06 at z=0. [Abridged]Comment: accepted for publication in ApJ; 14 pages; 10 figures; 4 table
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