Key Uncertainties in the Recent Air‐Sea Flux of CO2 Woolf, D.K.; Shutler, J.D.; Goddijn‐Murphy, L. ...
Global biogeochemical cycles,
December 2019, 20191201, 2019-12, Letnik:
33, Številka:
12
Journal Article
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The contemporary air‐sea flux of CO2 is investigated by the use of an air‐sea flux equation, with particular attention to the uncertainties in global values and their origin with respect to that ...equation. In particular, uncertainties deriving from the transfer velocity and from sparse upper ocean sampling are investigated. Eight formulations of air‐sea gas transfer velocity are used to evaluate the combined standard uncertainty resulting from several sources of error. Depending on expert opinion, a standard uncertainty in transfer velocity of either ~5% or ~10% can be argued and that will contribute a proportional error in air‐sea flux. The limited sampling of upper ocean fCO2 is readily apparent in the Surface Ocean CO2 Atlas databases. The effect of sparse sampling on the calculated fluxes was investigated by a bootstrap method, that is, treating each ship cruise to an oceanic region as a random episode and creating 10 synthetic data sets by randomly selecting episodes with replacement. Convincing values of global net air‐sea flux can only be achieved using upper ocean data collected over several decades but referenced to a standard year. The global annual referenced values are robust to sparse sampling, but seasonal and regional values exhibit more sampling uncertainty. Additional uncertainties are related to thermal and haline effects and to aspects of air‐sea gas exchange not captured by standard models. An estimate of global net CO2 exchange referenced to 2010 of −3.0 ± 0.6 Pg C/year is proposed, where the uncertainty derives primarily from uncertainty in the transfer velocity.
Plain Language Summary
The oceanic carbon sink reduces the rate of accumulation of CO2 in the atmosphere but is also responsible for the acidification of the ocean. One method of estimating the size of the oceanic carbon sink depends on a calculation of upward and downward flows of CO2 at the sea surface. This study revisits this calculation using updated knowledge of the transfer processes at the sea surface and the results of a large international collaborative effort (Surface Ocean CO2 Atlas) to collect and compile measurements of CO2 in the upper ocean. Greater sampling of the oceans improves estimates, but direct calculation in each year is not practical. Instead, we calculate fluxes in a recent year (2010) using upper ocean measurements of CO2 over many years. The remaining uncertainty is dominated by limited knowledge of the efficiency of stirring of gas across the sea surface, the air‐sea transfer velocity. The study suggests a relatively large downward flow of CO2 into the ocean compared to previous applications of this method and other methods to estimate the oceanic carbon sink. Increased knowledge is rewarded by reduced uncertainty in the net global flux; that flux is estimated at −3.0 ± 0.6 Pg C/year. Further understanding of transfer velocities and better sampling may reduce the uncertainty in the future.
Key Points
Increased understanding of air‐sea gas transfer processes and better sampling of the upper ocean enables higher confidence in calculations of air‐sea CO2 fluxes
The calculations imply a relatively large global net air‐to‐sea flux of −3.0 Pg C/year (referenced to 2010)
This flux is known within 0.6 Pg C/year, where uncertainty in air‐sea transfer velocity is the largest contribution to the combined uncertainty
The NSF's Karl G. Jansky Very Large Array (VLA) was used at 3 cm to search for accretion signatures from intermediate-mass black holes (IMBHs) in 19 globular star clusters (GCs) in NGC 3115, an ...early-type galaxy at a distance of 9.4 Mpc. The 19 GCs have stellar masses M ∼ (1.1-2.7) × 106 M , with a mean . None were detected. An IMBH accretion model was applied to the individual GCs and their radio stack. The radio-stacked GCs have an IMBH mass and mass fraction , with each limit being uncertain by a factor of about 2.5. The latter limit contrasts with the extremes of some stripped nuclei, suggesting that the set of stacked GCs in NGC 3115 is not a set of such nuclei. The radio luminosities of the individual GCs correspond to X-ray luminosities LX < (3.3-10) × 1038 erg s−1, with a factor of about 2.5 uncertainty. These limits predicted for putative IMBHs in the GCs are consistent with extant Chandra observations. Finally, a simulated observation with a next-generation VLA demonstrates that accretion signatures from IMBHs in GCs can be detected in a radio-only search, yet they elude detection in an X-ray-only search due to confusion from X-ray binaries in the GCs.
Since its commissioning in 1980, the Very Large Array (VLA) has consistently demonstrated its scientific productivity. However, its fundamental capabilities have changed little since 1980, ...particularly in the key areas of sensitivity, frequency coverage, and velocity resolution. These limitations have been addressed by a major upgrade of the array, which began in 2001 and will be completed at the end of 2012. When completed, the Expanded VLA--the EVLA--will provide complete frequency coverage from 1 to 50 GHz, a continuum sensitivity of typically 1 Delta *mJy beam--1 (in 9 hr with full bandwidth), and a modern correlator with vastly greater capabilities and flexibility than the VLA's. In this Letter, we describe the goals of the EVLA project, its current status, and the anticipated expansion of capabilities over the next few years. User access to the array through the Open Shared Risk Observing and Resident Shared Risk Observing programs is described. The following papers in this special issue, derived from observations in its early science period, demonstrate the astonishing breadth of this most flexible and powerful general-purpose telescope.
Abstract
We test the merger-induced dual active galactic nuclei (dAGNs) paradigm using a sample of 35 radio galaxy pairs from the Sloan Digital Sky Survey Stripe 82 field. Using Keck optical ...spectroscopy, we confirm 21 pairs have consistent redshifts, constituting kinematic pairs; the remaining 14 pairs are line-of-sight projections. We classify the optical spectral signatures via emission line ratios, equivalent widths, and excess of radio power above star formation predicted outputs. We find six galaxies are classified as LINERs and seven are AGN/starburst composites. Most of the LINERs are retired galaxies, while the composites likely have AGN contribution. All of the kinematic pairs exhibit radio power more than 10× above the level expected from just star formation, suggestive of a radio AGN contribution. We also analyze high-resolution (0.″3) imaging at 6 GHz from the NSF’s Karl G. Jansky Very Large Array for 17 of the kinematic pairs. We find six pairs (two new, four previously known) host two separate radio cores, confirming their status as dAGNs. The remaining 11 pairs contain single AGNs, with most exhibiting prominent jets/lobes overlapping their companion. Our final census indicates a dAGN duty cycle slightly higher than predictions of purely stochastic fueling, although a larger sample (potentially culled from VLASS) is needed to fully address the dAGN fraction. We conclude that while dAGNs in the Stripe 82 field are rare, the merger process plays some role in their triggering and it facilitates low to moderate levels of accretion.
Abstract
The merger of two galaxies, each hosting a supermassive black hole (SMBH) of mass 10
6
M
⊙
or more, could yield a bound SMBH binary. For the early-type galaxy NGC 4472, we study how ...astrometry with a next-generation Very Large Array could be used to monitor the reflex motion of the primary SMBH of mass
M
pri
, as it is tugged on by the secondary SMBH of mass
M
sec
. Casting the orbit of the putative SMBH binary in terms of its period
P
, semimajor axis
a
bin
, and mass ratio
q
=
M
sec
/
M
pri
≤
1
, we find the following: (1) Orbits with fiducial periods of
P
= 4 yr and 40 yr could be spatially resolved and monitored. (2) For a 95% accuracy of 2
μ
as per monitoring epoch, subparsec values of
a
bin
could be accessed over a range of mass ratios notionally encompassing major
q
>
1
4
and minor
q
<
1
4
galaxy mergers. (3) If no reflex motion is detected for
M
pri
after 1 (10) yr of monitoring, an SMBH binary with period
P
= 4 (40) yr and mass ratio
q
> 0.01 (0.003) could be excluded. This would suggest no present-day evidence for a past major merger like that recently simulated, where scouring by a
q
∼ 1 SMBH binary formed a stellar core with kinematic traits like those of NGC 4472. (4) Astrometric monitoring could independently check the upper limits on
q
from searches for continuous gravitational waves from NGC 4472.
ABSTRACT Nantais et al. used the Hubble Space Telescope to localize probable globular clusters (GCs) in M81, a spiral galaxy at a distance of 3.63 Mpc. Theory predicts that GCs can host ...intermediate-mass black holes (IMBHs) with masses . Finding IMBHs in GCs could validate a formation channel for seed BHs in the early universe, bolster gravitational-wave predictions for space missions, and test scaling relations between stellar systems and the central BHs they host. We used the NRAO Karl G. Jansky Very Large Array to search for the radiative signatures of IMBH accretion from 206 probable GCs in a mosaic of M81. The observing wavelength was 5.5 cm, and the spatial resolution was 1 5 (26.4 pc). None of the individual GCs are detected, nor are weighted-mean image stacks of the 206 GCs and the 49 massive GCs with stellar masses . We apply a semiempirical model to predict the mass of an IMBH that, if undergoing accretion in the long-lived, hard X-ray state, is consistent with a given radio luminosity. The 3 radio-luminosity upper limits correspond to IMBH masses of for the all-cluster stack and for the massive-cluster stack. We also apply the empirical fundamental-plane relation to two X-ray-detected clusters, finding that their individual IMBH masses at 95% confidence are MBH < 99,000 M and . Finally, no analog of HLX-1, a strong IMBH candidate in an extragalactic star cluster, occurs in any individual GC in M81. This underscores the uniqueness or rarity of the HLX-1 phenomenon.
Merger simulations predict that tidally induced gas inflows can trigger kiloparsec-scale dual active galactic nuclei (dAGN) in heavily obscured environments. Previously, with the Very Large Array, we ...have confirmed four dAGN with redshifts between 0.04 < z < 0.22 and projected separations between 4.3 and 9.2 kpc in the Sloan Digital Sky Survey Stripe 82 field. Here, we present Chandra X-ray observations that spatially resolve these dAGN and compare their multiwavelength properties to those of single AGN from the literature. We detect X-ray emission from six of the individual merger components and obtain upper limits for the remaining two. Combined with previous radio and optical observations, we find that our dAGN have properties similar to nearby low-luminosity AGN, and they agree with the black hole fundamental plane relation well. There are three AGN-dominated X-ray sources, whose X-ray hardness-ratio derived column densities show that two are unobscured and one is obscured. The low obscured fraction suggests these dAGN are no more obscured than single AGN, in contrast to the predictions from simulations. These three sources show an apparent X-ray deficit compared to their mid-infrared continuum and optical O iii line luminosities, suggesting higher levels of obscuration, in tension with the hardness-ratio derived column densities. Enhanced mid-infrared and O iii luminosities from star formation may explain this deficit. There is ambiguity in the level of obscuration for the remaining five components because their hardness ratios may be affected by nonnuclear X-ray emissions, or are undetected altogether. They require further observations to be fully characterized.
Galaxy mergers play an important role in the growth of galaxies and their supermassive black holes. Simulations suggest that tidal interactions could enhance black hole accretion, which can be tested ...by the fraction of binary active galactic nuclei (AGNs) among galaxy mergers. However, determining the fraction requires a statistical sample of binaries. We have identified kiloparsec-scale binary AGNs directly from high-resolution radio imaging. Inside the 92 deg super(2) covered by the high-resolution Very Large Array survey of the Sloan Digital Sky Survey (SDSS) Stripe 82 field, we identified 22 grade A and 30 grade B candidates of binary radio AGNs with angular separations less than 5" (10 kpc at z = 0.1). Eight of the candidates have optical spectra for both components from the SDSS spectroscopic surveys and our Keck program. Two grade B candidates are projected pairs, but the remaining six candidates are all compelling cases of binary AGNs based on either emission line ratios or the excess in radio power compared to the H alpha -traced star formation rate. Only two of the six binaries were previously discovered by an optical spectroscopic search. Based on these results, we estimate that ~60% of our binary candidates would be confirmed once we obtain complete spectroscopic information. We conclude that wide-area high-resolution radio surveys offer an efficient method to identify large samples of binary AGNs. These radio-selected binary AGNs complement binaries identified at other wavelengths and are useful for understanding the triggering mechanisms of black hole accretion.
We present follow-up radio observations of ESO 243-49 HLX-1 from 2012 using the Australia Telescope Compact Array (ATCA) and the Karl G. Jansky Very Large Array (VLA). We report the detection of ...radio emission at the location of HLX-1 during its hard X-ray state using the ATCA. Assuming that the 'Fundamental Plane' of accreting black holes is applicable, we provide an independent estimate of the black hole mass of M sub(BH) less than or equal to 2.8 super(+7.5) sub(-2.1) x10 super(6) M. at 90 per cent confidence. However, we argue that the detected radio emission is likely to be Doppler-boosted and our mass estimate is an upper limit. We discuss other possible origins of the radio emission such as being due to a radio nebula, star formation, or later interaction of the flares with the large-scale environment. None of these were found adequate. The VLA observations were carried out during the X-ray outburst. However, no new radio flare was detected, possibly due to a sparse time sampling. The deepest, combined VLA data suggest a variable radio source and we briefly discuss the properties of the previously detected flares and compare them with microquasars and active galactic nuclei.
Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters ...and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be *qth(1)=0.16 J/cm. The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy.
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