2,391 research outputs found
The distribution of local star formation activity as a function of galaxy stellar mass, environment and morphology
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present a detailed inventory of star formation in the local Universe, dissecting the cosmic star formation budget as a function of key variables that influence the star formation rate (SFR) of galaxies: stellar mass, local environment and morphology. We use a large homogeneous dataset from the SDSS to first study how the star-formation budget in galaxies with stellar masses greater than log(M/MSun) = 10 splits as a function of each parameter separately. We then explore how the budget behaves as a simultaneous function of these three parameters. We show that the bulk of the star formation at z < 0.075 (~65 per cent) takes place in spiral galaxies, that reside in the field, and have stellar masses between 10 < log(M/MSun) < 10.9. The ratio of the cosmic star formation budget hosted by galaxies in the field, groups and clusters is 21:3:1. Morphological ellipticals are minority contributors to local star formation. They make a measurable contribution to the star formation budget only at intermediate to high stellar masses, 10.3 < log(M/MSun) < 11.2 (where they begin to dominate by number), and typically in the field, where they contribute up to ~13 per cent of the total star-formation budget. This inventory of local star formation serves as a z~0 baseline which, when combined with similar work at high redshift, will enable us to understand the changes in SFR that have occurred over cosmic time and offers a strong constraint on models of galaxy formation.Peer reviewe
A Chandra study of particle acceleration in the multiple hotspots of nearby radio galaxies
We present Chandra observations of a small sample of nearby classical double
radio galaxies which have more than one radio hotspot in at least one of their
lobes. The X-ray emission from the hotspots of these comparatively low-power
objects is expected to be synchrotron in origin, and therefore to provide
information about the locations of high-energy particle acceleration. In some
models of the relationship between the jet and hotspot the hotspots that are
not the current jet termination point should be detached from the energy supply
from the active nucleus and therefore not capable of accelerating particles to
high energies. We find that in fact some secondary hotspots are X-ray sources,
and thus probably locations for high-energy particle acceleration after the
initial jet termination shock. In detail, though, we show that the spatial
structures seen in X-ray are not consistent with naive expectations from a
simple shock model: the current locations of the acceleration of the
highest-energy observable particles in powerful radio galaxies need not be
coincident with the peaks of radio or even optical emission.Comment: Accepted for ApJ. 33 pages, 8 figures inc. 2 in colo
Mobile metal adatoms on single layer, bilayer and trilayer graphene: an ab initio study correlated with experimental electron microscopy data
The plane-wave density functional theory code CASTEP was used with the Tkatchenko-Scheffler van der Waals correction scheme and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA PBE) to calculate the binding energy of Au, Cr, and Al atoms on the armchair and zigzag edge binding sites of monolayer graphene, and at the high-symmetry adsorption sites of single layer, bilayer, and trilayer graphene. All edge site binding energies were found to be substantially higher than the adsorption energies for all metals. The adatom migration activation barriers for the lowest energy migration paths on pristine monolayer, bilayer, and trilayer graphene were then calculated and found to be smaller than or within an order of magnitude of kBT at room temperature, implying very high mobility for all adatoms studied. This suggests that metal atoms evaporated onto graphene samples quickly migrate across the lattice and bind to the energetically favorable edge sites before being characterized in the microscope. We then prove this notion for Al and Au on graphene with scanning transmission electron microscopy (STEM) images showing that these atoms are observed exclusively at edge sites, and also hydrocarbon-contaminated regions, where the pristine regions of the lattice are completely devoid of adatoms. Additionally, we review the issue of fixing selected atomic positions during geometry optimization calculations for graphene/adatom systems and suggest a guiding principle for future studies
Chandra detection of the radio and optical double hot spot of 3C 351
In this letter we report a Chandra X-ray detection of the double northern hot
spot of the radio quasar 3C 351. The hot spot has also been observed in the
optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio
Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are
interpreted as the results of the synchrotron and synchrotron-self-Compton
(SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times
smaller than the equipartition values. In the framework of shock acceleration
theory, we show that the requirement for such a relatively small field strength
is in agreement with the fitted synchrotron spectral models and with the sizes
of the hot spots. Finally, we show that the combination of a lower magnetic
field strength with the high frequencies of the synchrotron cut-off in the
fitted synchrotron spectra provides strong evidence for electron acceleration
in the hot spots.Comment: 16 pag. + 2 .PS figures (fig.2 color), ApJ Letter in pres
Clear detection of dusty torus signatures in a Weak-Line Radio Galaxy: the case of PKS 0043-42
We report the clearest detection to date of dusty torus signatures in a
Weak-Line Radio Galaxy (WLRG). The deep Spitzer InfraRed Spectrograph (IRS)
rest-frame mid-infrared (MIR) spectrum of the WLRG PKS 0043-42 (z=0.116) shows
a clear spectral turnover at wavelengths longer than ~20 micron suggestive of
warm dust, as well as a 9.7 micron silicate absorption feature. In addition,
the hard X-ray results, based on Chandra data, strongly support a picture in
which PKS 0043-42 has a torus and accretion disc more typical of Strong-Line
Radio Galaxies (SLRGs). The MIR and X-ray spectra are markedly different from
those of other WLRGs at similar redshifts, and here we show that the former can
be successfully fitted with clumpy torus models with parameters characteristic
of Type-2 AGN tori: close to edge-on (i=74 deg) and relatively broad (torus
angular width=60 deg), with an outer radius of 2 pc, hydrogen column density
~1.6x10^(23) cm^(-2), and AGN bolometric luminosity ~1.6x10^(44) erg s^(-1).
The presence of a compact torus in PKS 0043-42 provides evidence that this WLRG
is fuelled by cold, rather than hot, gas accretion. We suggest that WLRGs are a
diverse population, and PKS 0043-42 may represent a type of radio galaxy in
which the AGN activity has been recently re-triggered as a consequence of
intermittent gas supply, or in which the covering factor of the Narrow-Line
Region (NLR) clouds is relatively low.Comment: 7 pages, 6 figures, 1 table. Accepted by MNRA
Knot in Cen A: Stochastic Magnetic Field for Diffusive Synchrotron Radiation?
The emission of relativistic electrons moving in the random and small-scale
magnetic field is presented by diffusive synchrotron radiation (DSR). In this
Letter, we revisit the perturbative treatment of DSR. We propose that random
and small-scale magnetic field might be generated by the turbulence. As an
example, multi-band radiation of the knot in Cen A comes from the electrons
with energy in the magnetic field of . The
multi-band spectrum of DSR is well determined by the feature of stochastic
magnetic field. These results put strong constraint to the models of particle
acceleration.Comment: accepted by ApJL, comments are welcom
Intensive monitoring of the strongly variable BL Lac S5 0716+714
The BL Lac object S5 0716+714 was monitored during a multifrequency campaign
in 1996. Preliminary analysis of the optical, ROSAT and RXTE data are
presented. Strong variability on short time scales was observed. The data
suggest an interpretation within a multi-component model.Comment: To appear in The Active X-ray Sky: Results from BeppoSAX and
Rossi-XTE, Rome, Italy, 21-24 October, 1997. Eds.: L. Scarsi, Bradt, P.
Giommi and F. Fiore. PS-file avialable at
http://www.lsw.uni-heidelberg.de/projects/extragalactic/bl_lac.htm
Evidence of non-thermal X-ray emission from radio lobes of Cygnus A
Using deep Chandra ACIS observation data for Cygnus A, we report evidence of
non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster
medium (ICM). The diffuse X-ray emission, which are associated with the eastern
and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The
lobe spectra are reproduced with not only a single-temperature Mekal model,
such as that of the surrounding ICM component, but also an additional power-law
(PL) model. The X-ray flux densities of PL components for the eastern and
western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and
52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are
1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal
component is considered to be produced via the inverse Compton (IC) process, as
is often seen in the X-ray emission from radio lobes. From a re-analysis of
radio observation data, the multiwavelength spectra strongly suggest that the
seed photon source of the IC X-rays includes both cosmic microwave background
radiation and synchrotron radiation from the lobes. The derived parameters
indicate significant dominance of the electron energy density over the magnetic
field energy density in the Cygnus A lobes under the rich ICM environment.Comment: 8 pages, 5 figures, accepted for publication in Ap
Sub-Arcsecond Imaging of 3C123:108-GHz Continuum Observations of the Radio Hotspots
We present the results of sub-arcsecond 108 GHz continuum interferometric
observations toward the radio luminous galaxy 3C123. Using multi-array
observations, we utilize the high u,v dynamic range of the BIMA millimeter
array to sample fully spatial scales ranging from 0.5" to 50". This allows us
to make one-to-one comparisons of millimeter-wavelength emission in the radio
lobes and hotspots to VLA centimeter observations at 1.4, 4.9, 8.4, and 15 GHz.
At 108 GHz, the bright, eastern double hotspot in the southern lobe is
resolved. This is only the second time that a multiple hotspot region has been
resolved in the millimeter regime. We model the synchrotron spectra of the
hotspots and radio lobes using simple broken power-law models with high energy
cutoffs, and discuss the hotspot spectra and their implications for models of
multiple hotspot formation.Comment: 16 pages, 3 Figures, ApJ Accepte
Ventilation-on-Demand: Quantity or Quality — A Pilot Trial at Barrick Gold\u27s Bousquet Mine
Common to many established mines in Canada, is the problem that their ventilation systems are continually being stretched to meet new needs that were not anticipated in the original design. This can be the result of a variety of different factors, such as the discovery of new reserves at depth, accelerated production, new worker exposure regulations or a change of mining method. At Bousquet, the ventilation of a new multilevel mining block over 1 km away from the existing bottom of the mine presents a challenge. The airflow available for this region is limited to 57 m3/s by the size of a ventilation drift and raise, and by the total volume the mine can supply. This volume is sufficient only if it can be directed to the desired locations, namely where the diesel equipment is operating and with minimal losses (\u3c10%). Due to the mobile nature of the mining equipment and changing production areas, such tight management of air can only be achieved with Ventilation on Demand . This paper describes a pilot trial, within the existing mine, of an automated demand based ventilation management system. Vehicle tracking and identification determine ventilation demand, location and duration. Specific air volumes are achieved with remote controlled regulators, and auxiliary fan systems and verified with airflow monitors. Air quality is monitored with gas sensors. Based upon this trial the mine will be able to determine the viability of an air quality driven system over an engine based air quantity dictated system. This would then, with the approval of the local regulators, allow the mine to operate its ventilation more efficiently and gain the maximum benefit of cleanengine technology and exhaust treatment devices
- …