49 research outputs found
The AGN-starburst connection, Galactic superwinds, and M_BH - sigma
Recent observations of young galaxies at redshifts z ~ 3 have revealed
simultaneous AGN and starburst activity, as well as galaxy-wide superwinds. I
show that there is probably a close connection between these phenomena by
extending an earlier treatment of the M_BH - sigma relation (King, 2003). As
the black hole grows, an outflow drives a shell into the surrounding gas. This
stalls after a dynamical time at a size determined by the hole's current mass
and thereafter grows on the Salpeter timescale. The gas trapped inside this
bubble cools and forms stars and is recycled as accretion and outflow. The
consequent high metallicity agrees with that commonly observed in AGN
accretion. Once the hole reaches a critical mass this region attains a size
such that the gas can no longer cool efficiently. The resulting energy-driven
flow expels the remaining gas as a superwind, fixing both the M_BH - sigma
relation and the total stellar bulge mass at values in good agreement with
observation. Black hole growth thus produces starbursts and ultimately a
superwind.Comment: ApJ, in press, 4 page
Black Holes, Galaxy Formation, and the M_BH-sigma Relation
Recent X-ray observations of intense high-speed outflows in quasars suggest
that supercritical accretion on to the central black hole may have an important
effect on a host galaxy. I revisit some ideas of Silk and Rees, and assume such
flows occur in the final stages of building up the black hole mass. It is now
possible to model explicitly the interaction between the outflow and the host
galaxy. This is found to resemble a momentum-driven stellar wind bubble,
implying a relation M_BH = (f_g kappa/2 pi G^2) sigma^4 = 1.5 10^8 sigma_200^4
Msun between black hole mass and bulge velocity dispersion (f_g = gas fraction
of total matter density, kappa = electron scattering opacity), without free
parameters. This is remarkably close to the observed relation in both slope and
normalization.
This result suggests that the central black holes in galaxies gain most of
their mass in phases of super-Eddington accretion, which are presumably
obscured or at high redshift. Observed super-Eddington quasars are apparently
late in growing their black hole masses.Comment: 8 pages, no figures Accepted for publication in ApJ Letters; typos
and references correcte
Mitigating machine learning bias between high income and low–middle income countries for enhanced model fairness and generalizability
Collaborative efforts in artificial intelligence (AI) are increasingly common between high-income countries (HICs) and low- to middle-income countries (LMICs). Given the resource limitations often encountered by LMICs, collaboration becomes crucial for pooling resources, expertise, and knowledge. Despite the apparent advantages, ensuring the fairness and equity of these collaborative models is essential, especially considering the distinct differences between LMIC and HIC hospitals. In this study, we show that collaborative AI approaches can lead to divergent performance outcomes across HIC and LMIC settings, particularly in the presence of data imbalances. Through a real-world COVID-19 screening case study, we demonstrate that implementing algorithmic-level bias mitigation methods significantly improves outcome fairness between HIC and LMIC sites while maintaining high diagnostic sensitivity. We compare our results against previous benchmarks, utilizing datasets from four independent United Kingdom Hospitals and one Vietnamese hospital, representing HIC and LMIC settings, respectively
Efficient photoelectrochemical Kolbe C-C coupling at BiVO4 electrodes under visible light irradiation
Electrochemical Kolbe C-C coupling of carboxylic acids at Pt electrodes has been studied for over 150 years and remains relevant today because renewable electricity is envisaged to make an increasing contribution to clean chemical processes and carboxylic acids are readily available precursors for chemical synthesis. Traditional electrochemical Kolbe occurs typically at very high potential (>10 V) which is required to achieve high selectivity for C-C coupling. Here we describe porous BiVO4 photoelectrodes that mediate C-C Kolbe coupling with near quantitative faradaic efficiency under visible light irradiation at <2 V. High substrate concentrations are also found to stabilise the double layer avoiding the need for additional supporting electrolyte. Comparison with related literature describing photocatalytic Kolbe C-C coupling shows that the apparent quantum yield can be raised from <1% to 12% demonstrating the distinct advantage of using photoelectrochemistry in this system
Measuring Gas Accretion and Angular Momentum near Simulated Supermassive Black Holes
Using cosmological simulations with a dynamic range in excess of 10 million,
we study the transport of gas mass and angular momentum through the
circumnuclear region of a disk galaxy containing a supermassive black hole
(SMBH). The simulations follow fueling over relatively quiescent phases of the
galaxy's evolution (no mergers) and without feedback from active galactic
nuclei (AGNs), as part of the first stage of using state-of-the-art,
high-resolution cosmological simulations to model galaxy and black hole
co-evolution. We present results from simulations at different redshifts (z=6,
4, and 3) and three different black hole masses (30 million, 90 million, and
300 million solar masses; at z=4), as well as a simulation including a
prescription that approximates optically thick cooling in the densest regions.
The interior gas mass throughout the circumnuclear disk shows transient and
chaotic behavior as a function of time. The Fourier transform of the interior
gas mass follows a power law with slope -1 throughout the region, indicating
that, in the absence of the effects of galaxy mergers and AGN feedback, mass
fluctuations are stochastic with no preferred timescale for accretion over the
duration of each simulation (~ 1-2 Myr). The angular momentum of the gas disk
changes direction relative to the disk on kiloparsec scales over timescales
less than 1 Myr, reflecting the chaotic and transient gas dynamics of the
circumnuclear region. Infalling clumps of gas, which are driven inward as a
result of the dynamical state of the circumnuclear disk, may play an important
role in determining the spin evolution of an SMBH, as has been suggested in
stochastic accretion scenarios.Comment: 13 pages, 10 figures; accepted to ApJ; corrected minor typos and
reference error
Black Hole Masses and Eddington Ratios at 0.3<z<4
We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of
active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey
(AGES). We combine H-beta, MgII, and CIV line widths with continuum
luminosities to estimate black hole (BH) masses in 407 AGNs, covering the
redshift range z~0.3-4 and the bolometric luminosity range L_bol~10^45-10^47
erg/s. The sample consists of X-ray or mid-infrared (24 micron) point sources
with optical magnitude R<=21.5 mag and optical emission line spectra
characteristic of AGNs. For the range of luminosity and redshift probed by
AGES, the distribution of estimated Eddington ratios is well described as
log-normal with a peak at L_bol/L_edd ~ 1/4 and a dispersion of 0.3 dex. Since
additional sources of scatter are minimal, this dispersion must account for
contributions from the scatter between estimated and true BH mass and the
scatter between estimated and true bolometric luminosity. Therefore, we
conclude that: (1) neither of these sources of error can contribute more than
\~0.3 dex rms; and (2) the true Eddington ratios of optically luminous AGNs are
even more sharply peaked. Because the mass estimation errors must be smaller
than ~0.3 dex, we can also investigate the distribution of Eddington ratios at
fixed BH mass. We show for the first time that the distribution of Eddington
ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor ~10
below Eddington is real and not an artifact of sample selection. These results
provide strong evidence that supermassive BHs gain most of their mass while
radiating close to the Eddington limit, and they suggest that the fueling rates
in luminous AGNs are ultimately determined by BH self-regulation of the
accretion flow rather than galactic scale dynamical disturbances.Comment: 34 pages including 12 figures. Incorporates referee's comments.
Accepted for publication in Ap
Letting the Sun Shine in at the WTO: How Transparency Brings the Trading System to Life
The environment of radio galaxies: a signature of AGN feedback at high redshifts
We use the semi-analytical model of galaxy formation GALFORM to characterize an indirect signature of active galactic nucleus (AGN) feedback in the environment of radio galaxies at high redshifts. The predicted environment of radio galaxies is denser than that of radio-quiet galaxies with the same stellar mass. This is consistent with observational results from the CARLA survey. Our model shows that the differences in environment are due to radio galaxies being hosted by dark matter haloes that are ∼1.5 dex more massive than those hosting radio-quiet galaxies with the same stellar mass. By running a control simulation in which AGN feedback is switched off, we identify AGN feedback as the primary mechanism affecting the build up of the stellar component of radio galaxies, thus explaining the different environment in radio galaxies and their radio-quiet counterparts. The difference in host halo mass between radio-loud and radio-quiet galaxies translates into different galaxies populating each environment. We predict a higher fraction of passive galaxies around radio-loud galaxies compared to their radio-quiet counterparts. Furthermore, such a high fraction of passive galaxies shapes the predicted infrared luminosity function in the environment of radio galaxies in a way that is consistent with observational findings. Our results suggest that the impact of AGN feedback at high redshifts and environmental mechanisms affecting galaxies in high halo masses can be revealed by studying the environment of radio galaxies, thus providing new constraints on galaxy formation physics at high redshifts
Impact of opioid-free analgesia on pain severity and patient satisfaction after discharge from surgery: multispecialty, prospective cohort study in 25 countries
Background: Balancing opioid stewardship and the need for adequate analgesia following discharge after surgery is challenging. This study aimed to compare the outcomes for patients discharged with opioid versus opioid-free analgesia after common surgical procedures.Methods: This international, multicentre, prospective cohort study collected data from patients undergoing common acute and elective general surgical, urological, gynaecological, and orthopaedic procedures. The primary outcomes were patient-reported time in severe pain measured on a numerical analogue scale from 0 to 100% and patient-reported satisfaction with pain relief during the first week following discharge. Data were collected by in-hospital chart review and patient telephone interview 1 week after discharge.Results: The study recruited 4273 patients from 144 centres in 25 countries; 1311 patients (30.7%) were prescribed opioid analgesia at discharge. Patients reported being in severe pain for 10 (i.q.r. 1-30)% of the first week after discharge and rated satisfaction with analgesia as 90 (i.q.r. 80-100) of 100. After adjustment for confounders, opioid analgesia on discharge was independently associated with increased pain severity (risk ratio 1.52, 95% c.i. 1.31 to 1.76; P < 0.001) and re-presentation to healthcare providers owing to side-effects of medication (OR 2.38, 95% c.i. 1.36 to 4.17; P = 0.004), but not with satisfaction with analgesia (beta coefficient 0.92, 95% c.i. -1.52 to 3.36; P = 0.468) compared with opioid-free analgesia. Although opioid prescribing varied greatly between high-income and low- and middle-income countries, patient-reported outcomes did not.Conclusion: Opioid analgesia prescription on surgical discharge is associated with a higher risk of re-presentation owing to side-effects of medication and increased patient-reported pain, but not with changes in patient-reported satisfaction. Opioid-free discharge analgesia should be adopted routinely