Distant Sources Research Articles

Ants (Lasius niger) deposit more pheromone close to food sources and further from the nest but do not attempt to update erroneous pheromone trails

Many social insects use pheromone trails to recruit workers to resources. Pheromone trails have some limitations, however. Two major limitations are that they evaporate over time, meaning that more distant locations may be more difficult to recruit precisely to, and that they cannot be removed if laid erroneously, or if they become outdated. Here, we ask whether Lasius niger ants can adaptively modulate pheromone deposition to overcome these limitations. Specifically, we first ask whether ants which have followed a pheromone trail to a wrong (unrewarded) location upregulate pheromone deposition to the correct food location to compete with the erroneous trail. They do not. Then, we examine how food distance and proximity to food influence pheromone deposition. We find that ants deposit up to 22 times more pheromone within 10 cm of a food source compared to when they are about to reach the nest. Moreover, they deposit up to four times more pheromone next to a food source placed 100 cm away from the nest as compared to one placed 20 cm away from the nest. Lasius niger ants thus do not display a mechanism for overcoming outdated or erroneously-laid trails by strengthening correct alternatives in our experiment, but do strongly upregulate recruitment to more distant food sources, which are more difficult to locate and recruit to.

New understanding on the interplay of longitudinal and transverse sediment dispersal: Combination of 3D seismic profile and u-pb ages of detrital zircons of Paleogene sedimentary rocks in the northern South China Sea

The northern South China Sea (SCS) experienced a significant transition from a lacustrine to marine environment in the Paleogene, and its deep-water basin of Zhu-II Depression is in particular extensively studied due to its richness in oil and gas resources. However, limited number of boreholes in the deep-water area has long constrained a better understanding on the provenance of the Paleogene strata and the associated sediment transport processes. In this study, detrital zircon U-Pb ages and three-dimensional (3D) seismic-reflection data were systematically employed to investigate the “source-to-sink” pathways and interplay of longitudinal and transverse sediment dispersal. Our results indicate that the Zhu-II Depression sediments of the northern SCS were predominantly derived from the surrounding nearby paleo-uplifts in the early and middle Eocene. A significant provenance shift took place in the late Eocene, when the local paleo-uplift source was replaced by a distant source from the western SCS. Sediments were transported from west to east by the “Kontum-Ying-Qiong River” as a longitudinal dispersal. In the Oligocene, the “Kontum-Ying-Qiong River” delivered large amounts of sediments from Central Vietnam to the eastern part of the northern SCS. Meanwhile, the Pearl River gradually evolved through regional tectonic processes and influenced the deep-water area of Zhu-II Depression as a transverse dispersal. Sediments from both “Kontum-Ying-Qiong” and Pearl Rivers converged and deposited as deep-water deltas in the Zhu-II Depression. This dual provenance system in the northern SCS deep-water area was featured by the interplay between longitudinal and transverse sediment dispersal. It was largely controlled by the tectonic-palaeogeographic pattern inherited from the Mesozoic arc system.

Merging hierarchical triple black hole systems with intermediate-mass black holes in population III star clusters

ABSTRACT Theoretical predictions suggest that very massive stars have the potential to form through multiple collisions and eventually evolve into intermediate-mass black holes (IMBHs) within Population III star clusters embedded in mini dark matter haloes. In this study, we investigate the long-term evolution of Population III star clusters, including models with a primordial binary fraction of $f_{\rm b}=0$ and 1, using the N-body simulation code petar. We comprehensively examine the phenomenon of hierarchical triple black holes in the clusters, specifically focusing on their merging inner binary black holes (BBHs), with post-Newtonian correction, by using the tsunami code. Our findings suggest a high likelihood of the inner BBHs containing IMBHs with masses on the order of $\mathcal {O}(100)\,{\rm M}_{\odot }$, and as a result, their merger rate could be up to $0.1{\rm Gpc}^{-3}{\rm yr}^{-3}$. The orbital eccentricities of some merging inner BBHs oscillate over time periodically, known as the Kozai–Lidov oscillation, due to dynamical perturbations. Detectable merging inner BBHs for mHz GW detectors LISA/TianQin/Taiji concentrate within $z\lt 3$. More distant sources would be detectable for CE/ET/LIGO/KAGRA/DECIGO, which are sensitive from $\mathcal {O}(0.1)$Hz to $\mathcal {O}(100)$ Hz. Furthermore, compared with merging isolated BBHs, merging inner BBHs affected by dynamical perturbations from tertiary BHs tend to have higher eccentricities, with a significant fraction of sources with eccentricities closing to 1 at mHz bands. GW observations would help constrain the formation channels of merging BBHs, whether through isolated evolution or dynamical interaction, by examining eccentricities.

Constraint on the Cosmic Curvature in a Model with the Schwarzschild–de Sitter Metric from Supernovae and Gamma-Ray Burst Observational Data

In developing his cosmological model of 1917, de Sitter theoretically predicted the phenomenon of cosmological redshift (the de Sitter effect), which he did long before the discovery of this phenomenon in observations. The de Sitter effect is gravitational by its nature, as it is due to differences between the coordinate systems of the observer and the distant source. However, the relationship between the redshift and distance derived from the de Sitter metric is at odds with observations, since this relationship is nonlinear (quadratic) for small redshifts, while the observed relationship between the same quantities is strictly linear. This paper discusses the possibility that cosmological redshift is gravitational by its nature, as in de Sitter’s 1917 model. At the same time, here, as in de Sitter’s model, an elliptical space is used, the main characteristic of which is the identification of its antipodal points. But, unlike de Sitter’s model, here, in order to ensure strict linear dependence of the redshift on distance, the origin of the reference system is transferred to the observer’s antipodal point. The Schwarzschild–de Sitter metric used in this model allows you to estimate the curvature of space from observational data. To achieve this, a theoretical Hubble diagram is built within the framework of the model with the Schwarzschild–de Sitter metric, which is compared with observations from the Pantheon+ catalogue of type Ia supernovae and the Amati catalogue of gamma-ray bursts in the redshift range of 0<z<8. As a result of this comparison, we found that the lower estimate of the radius of curvature of space was quite large: 2.4×1015 Mpc. This means that the observational data indicate a negligible curvature of space.

Адаптивный алгоритм локализации шумовых подводных источников в мелководных акваториях

An adaptive noise-resistant algorithm for holographic processing of hydroacoustic signals is presented, which provides restoration of the distance and radial velocity of underwater noise sources in shallow waters in the absence of information about the characteristics of the propagation medium. The algorithm is based on the measured values of the source hologram. The results of numerical simulation are presented. An analysis of the stability of the algorithm for estimating the source parameters with respect to intense internal waves that determine the horizontal refraction of sound field modes is performed.

Proof of principle X-ray reflection mass measurement of the black hole in H1743−322

ABSTRACT The black hole X-ray binary H1743−322 lies in a region of the Galaxy with high extinction, and therefore it has not been possible to make a dynamical mass measurement. In this paper, we make use of a recent model which uses the X-ray reflection spectrum to constrain the ratio of the black hole mass to the source distance. By folding in a reported distance measurement, we are able to estimate the mass of the black hole to be $12\pm 2~\text{M}_\odot {}$ ($1\sigma$ credible interval). We are then able to revise a previous disc continuum fitting estimate of black hole spin $a_*$ (previously relying on a population mass distribution) using our new mass constraint, finding $a_*=0.47\pm 0.10$. This work is a proof of principle demonstration of the method, showing it can be used to find the mass of black holes in X-ray binaries.

Distance-based indicators for evaluating environmental multi-contamination and related exposure: how far should we go?

Assessing environmental exposure to pollution is a challenging task, and scientists often use distance-based or proximity indicators when field or modeled data are unavailable. Although buffers are commonly used to represent the impact of a pollution source on neighboring populations, they can result in high-exposure misclassification. Euclidean distance-based indicators offer a promising alternative, but practices vary significantly in the literature. In this study, we aimed to compare several distance-based indicators for multiple environmental contaminants in an industrial and urban area. At the population's grid cell resolution of 200 × 200m, we compared the distance to the closest source, the average or median distance to all sources, or a restricted number of nearby sources for six types of sources (industries, railways, rail areas, roadways, road crossings, and agricultural patches) against environmental contamination data (PM10, NO2, and multimetallic contamination in lichens). Our findings revealed that the representativeness of contamination by indicators is significantly affected by the type and number of nearby sources considered. Specifically, we found that considering the distance to the nearest source or the average distance to all sources can lead to exposure misclassifications. The optimal correlation between distance indicators and pollutant levels was observed when considering 10-14 of the closest industrial sources, located within a 4.9- to 5.5-km radius. For rail areas, the optimal number was two to three sources within a 5.4- to 7.4-km radius. For main roads, intersections, and railways, the optimal number of sources varied depending on the pollutant, generally falling within a 3- to 9.4-km radius. Environmental contamination is influenced by the diversity of nearby sources, and considering only one source increases the risk of misclassification. Our results suggest that proximity models are still appropriate for study areas where the etiology of existing health effects is unclear, providing an exploratory analysis before more sophisticated research.

Wound healing: Surprising support from distant sources

The resurfacing of cutaneous wounds in mammals takes up to several weeks, but in zebrafish complete coverage is achieved within hours. New work uncovers that the rapid wound healing on zebrafish body surfaces involves the mobilization of fin-resident epithelial cells.

The Effect of Pollution Source Distance, Construction, and Behavior of Dug Well Users on Total Coliform Content

Dug well is a water source that is located relatively close to the ground surface so it is prone to pollution. This research aims to analyze the effect pollution source distance, construction and behavior of dug well users on total coliform content in Karangturi Village. Data collection was carried out by examining samples of dug well water in the laboratory, observing and measuring well construction, and using questionnaires to measure the behavior of well users. The research results show that 23 (95.8%) wells have a total coliform >0 MPN/100ml and 1 (4.2%) well have a total coliform content of 0 MPN/100ml. Simultaneously (p value=0.106) the variable distance to the pollutant source and partially (p value= 0.998) each livestock pen and septic tank variable do not have a significant influence on the total coliform content. Simultaneous well construction (p value= 0.489) and partially (p value= 0.999) of each well construction variable did not have a significant effect on the total coliform content. The behavioral variable simultaneously (p value= 0.346) and partially the knowledge variable (p value= 1.000), the attitude variable (p value= 0.999), and the attitude variable (p value= 0.999) did not have a significant effect on the total coliform content.

Implications of gamma ray burst GRB221009A for extra dimensions

Anomalous high-energy photons from GRB221009A, with energies of 18 TeV and 251 TeV, were observed by LHAASO and Carpet-2 recently. Such observation of high energy gamma-ray bursts from distant source causes a mystery since high energy photons suffer severe attenuation before reaching the earth. One possibility is the existence of axion-like particles (ALP), and high energy photons at the source can convert to these ALPs which travel intergalactically. In this paper, we study the effects of extra dimensions on the conversion probability between photon and ALPs. The conversion probability saturates and may reach almost 100% for high energy photons. We show that the size of extra dimension affects the energy at which saturation occurs. The observations of high-energy photons may support the possibility of smaller extra dimension.

Transfer functions for QA /QB international regulatory limits for the safe transport of radioactive materials

This paper presents a proposed revision of the International Atomic Energy Agency transport regulations, related to the A 1 and A 2 limit values used to determine the radioactive transport classification. Based on the ‘Q system’, a novel methodology was introduced to derive QA and QB values related to scenarios involving external exposure from a distant source. These values are key parameters that respectively represent the total effective dose and total equivalent dose to the skin, from all primary and secondary particles contributing to radiation exposure. The International Working Group (WG A 1/A 2) is established and associated with the TRANSSC Technical Expert Group on Radiation Protection. A review of the A 1 and A 2 values is performed in response to identified limitations within the existing Q system. The followed approach is based on Monte Carlo simulations that enabled the development of transfer functions aimed at reducing computational time and increasing the flexibility of dose evaluations for any radionuclide with known particle emission spectra. This method allows updating the QA and QB values to account for future data evolutions (decay data, fluence-to-dose conversion coefficients) and standardizing the calculation of regulation limits across all referenced radionuclides and scenarios related to external exposure. The transfer functions are established using three Monte Carlo simulation codes—FLUKA, Geant4, and MCNP—and address the previous limitations of the ‘Q system’, reflecting the latest International Commission for Radiation Protection recommendations and improvements in calculation techniques. The results of the WG show consistent agreement across the codes, with minor discrepancies observed at low primary energies due to statistical uncertainties and different handling of stopping power for electrons/positrons in the codes. This revised approach aligns with current standards and recommendations, ensuring that the radiological consequences of transport accidents are acceptable for the new A 1 and A 2 limits from a radiological protection perspective.

Calibrating the global network of gravitational wave observatories via laser power calibration at NIST and PTB

Current gravitational wave (GW) observatories rely on photon calibrators that use laser radiation pressure to generate displacement fiducials used to calibrate detector output signals. Reducing calibration uncertainty enables optimal extraction of astrophysical information such as source distance and sky position from detected signals. For the ongoing O4 observing run that started on 24 May 2023, the global GW detector network is employing a new calibration scheme with transfer standards calibrated at both the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB). These transfer standards will circulate between the observatories and the metrology institutes to provide laser power calibration traceable to the International System of Units (SI) and enable assessment and reduction of relative calibration errors for the observatory network. The Laser Interferometer Gravitational-Wave Observatory (LIGO) project and the Virgo project are currently participating in the new calibration scheme. The Large-scale Cryogenic Gravitational-wave Telescope project (KAGRA) is expected to join in 2024, with the LIGO Aundha Observatory in India joining later. Before implementing this new scheme, a NIST-PTB bilateral comparison was conducted. It validated the scale representation by both laboratories, with a degree of equivalence of −0.2% and an associated expanded uncertainty of 0.32% (k = 2) which is significantly lower than previous studies. We describe the transfer of power sensor calibration, including detailed uncertainty estimates, from the transfer standards calibrated by NIST and PTB to the sensors operating continuously at the interferometer end stations. Finally, we discuss the ongoing calibration of Pcal-induced displacement fiducials for the O4 observing run. Achieved combined standard uncertainty levels as low as 0.3% facilitate calibrating the interferometer output signals with sub-percent accuracy.

Assessing PM2.5 Dynamics and Source Contributions in Southwestern China: Insights from Winter Haze Analysis

Despite enhancements in pollution control measures in southwestern China, detailed assessments of PM2.5 dynamics following the implementation of the Clean Air Action remain limited. This study explores the PM2.5 concentrations and their chemical compositions during the winter haze period of 2017 across four major urban centers—Chengdu, Chongqing, Guiyang, and Kunming. Significant variability in mean PM2.5 concentrations was observed: Chengdu (71.8 μg m−3) and Chongqing (53.3 μg m−3) recorded the highest levels, substantially exceeding national air quality standards, while Guiyang and Kunming reported lower concentrations, suggestive of comparatively milder pollution. The analysis revealed that sulfate, nitrate, and ammonium (collectively referred to as SNA) constituted a substantial portion of the PM2.5 mass—47.2% in Chengdu, 62.2% in Chongqing, 59.9% in Guiyang, and 32.0% in Kunming—highlighting the critical role of secondary aerosol formation. The ratio of NO3−/SO42− and nitrogen oxidation ratio to sulfur oxidation ratio (NOR/SOR) indicate a significant transformation of NO2 under conditions of heavy pollution, with nitrate formation playing an increasingly central role in the haze dynamics, particularly in Chengdu and Chongqing. Utilizing PMF for source apportionment, in Chengdu, vehicle emissions were the predominant contributor, accounting for 33.1%. Chongqing showed a similar profile, with secondary aerosols constituting 36%, followed closely by vehicle emissions. In contrast, Guiyang’s PM2.5 burden was heavily influenced by coal combustion, which contributed 46.3%, reflecting the city’s strong industrial base. Kunming presented a more balanced source distribution. Back trajectory analysis further confirmed the regional transport of pollutants, illustrating the complex interplay between local and distant sources. These insights underscore the need for tailored, region-specific air quality management strategies in southwestern China, thereby enhancing our understanding of the multifaceted sources and dynamics of PM2.5 pollution amidst ongoing urban and industrial development.

A CRISPR-Cas9-Mediated Large-Fragment Assembly Method for Cloning Genomes and Biosynthetic Gene Cluster.

The ability to clone large DNA fragments from genomes is valuable for both basic and applied research, such as the construction of synthetic genomes, and the expression of biosynthetic gene clusters (BGCs) for natural product discovery. Here, we report a fast and efficient platform for the direct capture of genome DNAs, by combining CRISPR and Gibson assembly. We demonstrate this method with the ability of cloning large DNA fragments ranging from 30 to 77 kb from various host genomes, achieving a near 100% cloning fidelity for DNA fragments below 50 kb. We next demonstrate this method by the cloning of a 40 kb fragment from Streptomyces ceruleus A3(2), which is rich in BGCs for natural products; and used this method cloning the 40 kb fengycin synthetic gene cluster from B. subtilis 168, encoding for a class of peptides with bioactivity. This method provides efficient and simple opportunities for assembling large DNA constructs from distant sources.

Audio Pre-Processing and Beamforming Implementation on Embedded Systems

Since the invention of the microphone by Barina in 1876, there have been numerous applications of audio processing, such as phonographs, broadcasting stations, and public address systems, which merely capture and amplify sound and play it back. Nowadays, audio processing involves analysis and noise-filtering techniques. There are various methods for noise filtering, each employing unique algorithms, but they all require two or more microphones for signal processing and analysis. For instance, on mobile phones, two microphones located in different positions are utilized for active noise cancellation (one for primary audio capture and the other for capturing ambient noise). However, a drawback is that when the sound source is distant, it may lead to poor audio capture. To capture sound from distant sources, alternative methods, like blind signal separation and beamforming, are necessary. This paper proposes employing a beamforming algorithm with two microphones to enhance speech and implementing this algorithm on an embedded system. However, prior to beamforming, it is imperative to accurately detect the direction of the sound source to process and analyze the audio from that direction.

Grain Size Distribution and Provenance of Holocene Sand from the Sava River (Zagreb, Croatia)

This study involves an investigation into the grain size distribution and provenance of the sand deposited near Zagreb (Croatia) in the riverbed of the regionally important, almost 1000 km long Sava River, which connects several SE European countries. Recent research in the study area has mainly focused on the deposits forming the Zagreb alluvial aquifer system, rather on the Sava River sediment deposited in its riverbed, which is the focus of this study. The grain size distribution results obtained by dry sieving and laser granulometry showed a predominately fine and medium sand deposition at riverbanks and sand point bars. Medium sand increased downstream towards the east, within the artificially more channelized riverbed in the urban area. Fine sand prevailed 50 km further downstream in a more meandering low-relief area, near the city of Sisak and Lonjsko Polje Nature Park. Provenance analysis showed predominately carbonate sand in the western part of the city of Zagreb, originating from distant (Alpine) and local (Medvednica Mt. and Samobor Hills) sources. More siliciclastic sand was deposited in the Sava riverbed in the middle and eastern parts of Zagreb, originating mainly from the Medvednica Mt. The prevailing siliciclastic sand further downstream of the Sava River is probably sourced from the Kupa River tributary. Although various studies of the Zagreb alluvial aquifer system have been conducted so far, this study represents a novelty in its investigation into the grain size distribution of the Sava riverbed sand itself, setting the foundations for investigations in the future.

Tsunami Waves off the Southeastern Coast of Sakhalin Island Caused by the Eruption of the Hunga–Tonga–Hunga–Ha’apai Volcano on 15 January 2022 According to Observations by Bottom Devices of Waves

The observation of waves (sea level oscillations) in the coastal water area of the southeastern coast of Sakhalin Island was carried out from November 2021 to May 2022 using bottom-mounted high-precision ARW 14 K devices with one second discreteness. The analysis of the obtained time series revealed the presence of anomalous waves in the range of tsunami waves in January 2022. It is shown that the waves detected on 15 January 2022 with periods of 15 minutes–4 hours, significantly standing out above the background, are associated with volcanic tsunami waves. Spectral analysis of the time series showed that the maximum in the spectra of sea level oscillations found after the volcanic eruption is close to the period of the first mode of the shelf seiche. This indicates that incoming tsunami waves excite shelf seiches in the coastal zone and contribute to the generation of longshore and transverse seiches into Mordvinov Bay. Time series analysis made it possible to establish that the spectral density of sea level oscillations for the time interval of the beginning of the eruption of the Hunga–Tonga–Hunga–Ha’apai volcano from 21 December 2021 to 11 January 2022 significantly differs from the spectral density for the time of the Plinian volcanic eruption. An analysis of the statistical characteristics of sea level oscillations performed on two–day series in the range of periods 1 min–4 h for background waves and tsunamis showed an increase in the rms by 3.5 times for a time series containing tsunami waves. It has been established that for background time series containing tsunami waves, the coefficients of asymmetry and kurtosis differ little from“Gaussia” values. This indicates that the arrival of tsunami waves from distant sources does not significantly disrupt the stationarity of the wave field in the observation area.

A low cost and simple webcam spectrometer for exploring the visible wavelengths of light

This paper presents a method for constructing a simple, cost-effective, and versatile spectrometer using readily available materials such as a basic webcam, a DVD, tin foil, a cardboard tube, and a microphone boom arm. Building upon previous designs (Widiatmoko et al 2011 Phys. Educ. 46 332, Lorenz 2014 Am. J. Phys. 82 169–73, Rodrigues et al 2016 Phys. Educ. 51 014002, Likith et al 2021 J. Opt. 50 489–94), this spectrometer is mounted on a microphone boom arm, enhancing flexibility in capturing spectra from various angles and distant light sources. The spectrometer, coupled with Theremino Spectrometer V3.1 software, achieves sufficient resolution to measure solar Fraunhofer lines and closely spaced Mercury double lines at 436 nm and 577 nm. The procedure for assembling the spectrometer is detailed, including calibration using a Mercury CFL lamp or phone screens. Results demonstrate the spectrometer’s capability to capture high-resolution spectra of the blue sky and light sources, identifying significant spectral lines. This DIY spectrometer offers an accessible tool for amateur scientists and educators to explore visible light spectra, facilitating both qualitative and quantitative analyses. Also, through measuring the light from phone screens work, perception of colour can be explored.

Bioenergetic Shifts in Humpback Whale Fibroblasts Upon Chemical Exposure.

Southern Hemisphere humpback whales accumulate persistent and toxic chemicals, which are transported to Antarctica through distant sources and in situ usage. The extreme seasonal migration-associated fast of humpback whales results in the remobilization of persistent and lipophilic environmental contaminants from liberated fat stores. Mitochondria play a key role in lipid metabolism, and any disruption to mitochondrial function is expected to influence whole-organism bioenergetics. It is therefore of interest to advance understanding of the impact of known contaminants of the Antarctic sea-ice ecosystem upon humpback whale cellular bioenergetics. Using cell line-based in vitro testing, this study employed the Seahorse Extracellular Flux Analyzer to study cellular metabolic activity in live humpback whale fibroblast cells. The assay, based on oxygen consumption rate, provides insights into the cause of cellular bioenergetic disruption. Immortalized skin fibroblasts were exposed to four priority environmental chemicals found in the Antarctic sea-ice ecosystem. Our findings reveal chemical-dependent functional alterations and varying bioenergetic profile responses. Chlorpyrifos was observed to decrease mitochondrial basal oxygen consumption; dieldrin increased basal oxygen consumption; trifluralin's impact was dose-specific, and endosulfan displayed no effect. Our results provide unique insights into environmental chemical mechanisms of action on cellular bioenergetics, generating much-needed taxa-specific chemical effect data in support of evidence-based conservation policy and management.

The radio properties of z>3.5 quasars: Are most high-redshift radio-loud active galactic nuclei obscured?

We explore the radio properties of powerful (rest-frame luminosity $ at 500 MHz) high-redshift ($z 3.5$) quasars. The aim of this study is to gain a better understanding of radio-loud sources at the epoch when they reach the highest space density. We selected 29 radio-loud quasars at low radio frequencies (76 MHz). Their radio spectra, covering the range from 76 MHz to 5 GHz, are generally well reproduced by a single power law. We created samples that were matched in radio luminosity at lower redshift (from $z to $z to investigate any spectral evolution. We find that the fraction of flat-spectrum radio quasars (FSRQs) strongly increases with redshift (from $ 8<!PCT!>$ at z=1.2 to $ 45<!PCT!>$ at z$>$3.5). This effect is also observed in quasars with lower luminosities (down to $ The increase in the fraction of FSRQs with redshift corresponds to a decrease in the steep- spectrum radio quasars. This result can be explained when we assume that the beaming factor and the slope of the luminosity function do not change with redshift and if high-redshift radio-loud sources can be recognized as quasars only when they are seen at a small viewing angle ($ but most of them, about 90<!PCT!> are obscured in the UV and optical bands. We also found a trend for the size of radio sources to decrease with increasing redshift. Because projection effects are insufficient to cause this trend, we suggest that the large amount of gas causing the nuclear obscuration also hampers the growth of the more distant sources.