584 research outputs found
Major mergers are not significant drivers of star formation or morphological transformation around the epoch of peak cosmic star formation
We investigate the contribution of major mergers (mass ratios > 1:5) to stellar mass growth and morphological transformations around the epoch of peak cosmic star formation (z ~ 2). We visually classify a complete sample of massive (M > 1010M_) galaxies at this epoch, drawn from the CANDELS survey, into late-type galaxies, major mergers, spheroids and disturbed spheroids which show morphological disturbances. Given recent simulation work, which indicates that recent (<0.3-0.4 Gyr) major-merger remnants exhibit clear tidal features in such images, we use the fraction of disturbed spheroids to probe the role of major mergers in driving morphological transformations. The percentage of blue spheroids (i.e. with ongoing star formation) that show morphological disturbances is only 21 +- 4 per cent, indicating that major mergers are not the dominant mechanism for spheroid creation at z ~ 2 - other processes, such as minor mergers or cold accretion are likely to be the main drivers of this process. We also use the rest-frame U-band luminosity as a proxy for star formation to show that only a small fraction of the star formation budget (~3 per cent) is triggered by major mergers. Taken together, our results show that major mergers are not significant drivers of galaxy evolution at z ~ 2
Limits on non-Gaussianities from WMAP data
We develop a method to constrain the level of non-Gaussianity of density
perturbations when the 3-point function is of the "equilateral" type.
Departures from Gaussianity of this form are produced by single field models
such as ghost or DBI inflation and in general by the presence of higher order
derivative operators in the effective Lagrangian of the inflaton. We show that
the induced shape of the 3-point function can be very well approximated by a
factorizable form, making the analysis practical. We also show that, unless one
has a full sky map with uniform noise, in order to saturate the Cramer-Rao
bound for the error on the amplitude of the 3-point function, the estimator
must contain a piece that is linear in the data. We apply our technique to the
WMAP data obtaining a constraint on the amplitude f_NL^equil of "equilateral"
non-Gaussianity: -366 < f_NL^equil < 238 at 95% C.L. We also apply our
technique to constrain the so-called "local" shape, which is predicted for
example by the curvaton and variable decay width models. We show that the
inclusion of the linear piece in the estimator improves the constraint over
those obtained by the WMAP team, to -27 < f_NL^local < 121 at 95% C.L.Comment: 20 pages, 12 eps figure
PKS 0537-441: extended [O II] emission and a binary QSO?
We present high-resolution imaging and low-resolution spectroscopy of the BL
Lac object PKS 0537-441 (z = 0.893) and its environment. The observations were
designed to clarify, whether the properties of PKS 0537-441 are affected by
gravitational microlensing, or whether PKS 0537-441 and its environment act as
a lensing system itself. Our observations show that neither case seems to be
likely. We did not find a galaxy along the line-of-sight to the BL Lac as
claimed previously, our spectroscopy shows that none of the four closest
companion galaxies is at high redshift. Two of the four nearby companion
galaxies to PKS 0537-441 are within 200 km/s of the systemic velocity of the BL
Lac (z = 0.892 and 0.895, respectively). The third companion galaxy is at
higher redshift (z = 0.947). The fourth companion galaxy shows evidence of Mg
II absorption redwards of its systemic velocity and is perhaps a mini low
ionization BAL QSO at z = 0.885. If the latter can be confirmed, PKS 0537-441
is the first BL Lacertae object being a member of a binary Quasar. We also
detected extended [O II] emission in the off-nuclear spectrum of PKS 05371-441,
which is most likely due to photoionization from the active nucleus.
Alternatively, the extended [O II] emission is due to jet-cloud interaction
with the counterjet of PKS 0537-441. Our clustering analysis indicates that PKS
0537-441 is located in a cluster environment as rich as Abell type 0-1. This is
supported by the detection of four galaxies in the field with similar redshifts
as the BL Lac (Delta z < 0.002). We found serendipitously even more galaxies at
somewhat higher redshifts (z = 0.9-1). Thus, PKS0537-441 might be located in
front of a galaxy cluster at somewhat higher redshift or even be part of a
large-scale structure with an extension towards the BL Lac.Comment: 13 pages, 9 figures, accepted for publication in A&
Freshwater distributions and water mass structure in the Amundsen Sea Polynya region, Antarctica
We present the first densely-sampled hydrographic survey of the Amundsen Sea Polynya (ASP) region, including a detailed characterization of its freshwater distributions. Multiple components contribute to the freshwater budget, including precipitation, sea ice melt, basal ice shelf melt, and iceberg melt, from local and non-local sources. We used stable oxygen isotope ratios in seawater (ÎŽ18O) to distinguish quantitatively the contributions from sea ice and meteoric-derived sources. Meteoric fractions were high throughout the winter mixed layer (WML), with maximum values of 2â3% (±0.5%). Because the ASP region is characterized by deep WMLs, column inventories of total meteoric water were also high, ranging from 10â13 m (±2 m) adjacent to the Dotson Ice Shelf (DIS) and in the deep trough to 7â9 m (±2 m) in shallower areas. These inventories are at least twice those reported for continental shelf waters near the western Antarctic Peninsula. Sea ice melt fractions were mostly negative, indicating net (annual) sea ice formation, consistent with this area being an active polynya. Independently determined fractions of subsurface glacial meltwater (as one component of the total meteoric inventory) had maximum values of 1â2% (±0.5%), with highest and shallowest maximum values at the DIS outflow (80â90 m) and in iceberg-stirred waters (150â200 m). In addition to these upwelling sites, contributions of subsurface glacial meltwater could be traced at depth along the ~ 27.6 isopycnal, from which it mixes into the WML through various processes. Our results suggest a quasi-continuous supply of melt-laden iron-enriched seawater to the euphotic zone of the ASP and help to explain why the ASP is Antarcticaâs most biologically productive polynya per unit area
The scientific potential of space-based gravitational wave detectors
The millihertz gravitational wave band can only be accessed with a
space-based interferometer, but it is one of the richest in potential sources.
Observations in this band have amazing scientific potential. The mergers
between massive black holes with mass in the range 10 thousand to 10 million
solar masses, which are expected to occur following the mergers of their host
galaxies, produce strong millihertz gravitational radiation. Observations of
these systems will trace the hierarchical assembly of structure in the Universe
in a mass range that is very difficult to probe electromagnetically. Stellar
mass compact objects falling into such black holes in the centres of galaxies
generate detectable gravitational radiation for several years prior to the
final plunge and merger with the central black hole. Measurements of these
systems offer an unprecedented opportunity to probe the predictions of general
relativity in the strong-field and dynamical regime. Millihertz gravitational
waves are also generated by millions of ultra-compact binaries in the Milky
Way, providing a new way to probe galactic stellar populations. ESA has
recognised this great scientific potential by selecting The Gravitational
Universe as its theme for the L3 large satellite mission, scheduled for launch
in ~2034. In this article we will review the likely sources for millihertz
gravitational wave detectors and describe the wide applications that
observations of these sources could have for astrophysics, cosmology and
fundamental physics.Comment: 18 pages, 2 figures, contribution to Gravitational Wave Astrophysics,
the proceedings of the 2014 Sant Cugat Forum on Astrophysics; v2 includes one
additional referenc
Conceptualizing the adventure-sports coach
As a comparatively recent development, the adventure-sports coach struggles for a clear and distinct identity. The generic term âinstructorâ no longer characterizes the role and function of this subgroup of outdoor professionals. Indeed, although the fields of adventure/outdoor education and leadership are comparatively well researched, the arrival of this ânew kid on the blockâ appears to challenge both the adventure-sports old guard and traditional views of sports coaching. In an attempt to offer clarity and stimulate debate, this paper attempts to conceptualize the adventure-sports coach in the context of the existing roles in the field and current motivations for activity in the outdoors. We identify issues that are specific to the adventure-sports coach while also recognizing those skills and competencies shared with other professionals, both in the adventure sports profession and traditional sports coaching fields. Based on this review, we offer a conceptual model which may be used to focus debate, stimulate research and, at a possible later stage, to underpin accreditation, training and professional development
Quantifying biomarkers of axonal degeneration in early glaucoma to find the disc at risk
Abstract: To evaluate regional axonal-related parameters as a function of disease stage in primary open angle glaucoma (POAG) and visual field (VF) sensitivity. Spectral domain optical coherence tomography was used to acquire 20° scans of POAG (n = 117) or healthy control (n = 52) human optic nerve heads (ONHs). Region specific and mean nerve fibre layer (NFL) thicknesses, border NFL and peripapillary NFL, minimum rim width (MRW)/ area (MRA) and prelamina thickness; and volume were compared across POAG disease stages and with visual field sensitivity. Differences identified between early glaucoma (EG), preperimetric glaucoma (PG) and control (C) ONHs included thinner PG prelamina regions than in controls (p < 0.05). Mean border NFL was thinner in EG (p < 0.001) and PG (p = 0.049) compared to control eyes; and EG mean, and inferior and ST, border NFL was thinner than in PG (p < 0.01). Mean, superior and inferior PG peripapillary NFL were thinner than in controls (p < 0.05), and EG ST peripapillary NFL was thinner than in PG (p = 0.023). MRW differences included: PG SN and inferior less than in controls (p < 0.05); thinner EG mean regional, inferior, nasal, and ST MRW versus PG MRW (p < 0.05). Regional border NFL, peripapillary NFL, MRW, MRA, prelamina thickness (except centre, p = 0.127) and prelamina volume (p < 0.05) were significantly associated with VF mean deviation (MD). Novel axon-derived indices hold potential as biomarkers to detect early glaucoma and identify ONHs at risk
The inflationary trispectrum
We calculate the trispectrum of the primordial curvature perturbation
generated by an epoch of slow-roll inflation in the early universe, and
demonstrate that the non-gaussian signature imprinted at horizon crossing is
unobservably small, of order tau_NL < r/50, where r < 1 is the tensor-to-scalar
ratio. Therefore any primordial non-gaussianity observed in future microwave
background experiments is likely to have been synthesized by gravitational
effects on superhorizon scales. We discuss the application of Maldacena's
consistency condition to the trispectrum.Comment: 23 pages, 2 diagrams drawn with feynmp.sty, uses iopart.cls. v2,
replaced with version accepted by JCAP. Estimate of maximal tau_NL refined in
Section 5, resulting in smaller numerical value. Sign errors in Eq. (44) and
Eq. (48) corrected. Some minor notational change
The evolving slope of the stellar mass function at 0.6 <= z < 4.5 from deep WFC3 data
We used Early Release Science (ERS) observations taken with the Wide Field
Camera 3 (WFC3) in the GOODS-S field to study the galaxy stellar mass function
(GSMF) at 0.6<=z<4.5. Deep WFC3 near-IR data (for Y as faint as 27.3, J and H
as faint as 27.4 AB mag at 5 sigma), as well as deep Ks (as faint as 25.5 at 5
sigma) Hawk-I band data, provide an exquisite data set with which determine in
an unprecedented way the low-mass end of the GSMF, allowing an accurate probe
of masses as low as M~7.6 10^9 Msun at z~3. Although the area used is
relatively small (~33 arcmin^2), we found generally good agreement with
previous studies on the entire mass range. Our results show that the slope of
the faint-end increases with redshift, from alpha=-1.44+/-0.03 at z~0.8 to
alpha=-1.86+/-0.16 at z~3, although indications exist that it does not steepen
further between z~3 and z~4. This result is insensitive to any uncertainty in
the M* parameter. The steepness of the GSMF faint-end solves the well-known
disagreement between the stellar mass density (SMD) and the integrated star
formation history at z>2. However, we confirm the that there appears to be an
excess of integrated star formation with respect to the SMD at z<2, by a factor
of ~2-3. Our comparison of the observations with theoretical predictions shows
that the models forecast a greater abundance of low mass galaxies, at least up
to z~3, as well as a dearth of massive galaxies at z~4 with respect to the
data, and that the predicted SMD is generally overestimated at z<~2.Comment: Accepted for publication in Astronomy & Astrophysics. Minor language
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