We present a novel integrated wavelet-domain based framework (w-ICA) for 3-D denoising functional magnetic resonance imaging (fMRI) data followed by source separation analysis using independent ...component analysis (ICA) in the wavelet domain. We propose the idea of a 3-D wavelet-based multi-directional denoising scheme where each volume in a 4-D fMRI data set is sub-sampled using the axial, sagittal and coronal geometries to obtain three different slice-by-slice representations of the same data. The filtered intensity value of an arbitrary voxel is computed as an expected value of the denoised wavelet coefficients corresponding to the three viewing geometries for each sub-band. This results in a robust set of denoised wavelet coefficients for each voxel. Given the de-correlated nature of these denoised wavelet coefficients, it is possible to obtain more accurate source estimates using ICA in the wavelet domain. The contributions of this work can be realized as two modules: First, in the analysis module we combine a new 3-D wavelet denoising approach with signal separation properties of ICA in the wavelet domain. This step helps obtain an activation component that corresponds closely to the true underlying signal, which is maximally independent with respect to other components. Second, we propose and describe two novel shape metrics for post-ICA comparisons between activation regions obtained through different frameworks. We verified our method using simulated as well as real fMRI data and compared our results against the conventional scheme (Gaussian smoothing+spatial ICA: s-ICA). The results show significant improvements based on two important features: (1) preservation of shape of the activation region (shape metrics) and (2) receiver operating characteristic curves. It was observed that the proposed framework was able to preserve the actual activation shape in a consistent manner even for very high noise levels in addition to significant reduction in false positive voxels.
►A novel 3-D denoising algorithm that utilizes 3-D wavelet transform and signal estimation theory to compute the denoised estimates from noisy task-related simulated fMRI data (SNR ~11–14dB). ►Utilize these denoised estimates for estimating the activation maps using independent component analysis in the wavelet domain. Performing ICA in the wavelet domain utilizes the de-correlated nature of wavelet coefficients to extract maximally independent components. ►Introduce two new metrics for validation of the shape (and spatial extent) of the activation regions by utilizing their geometric properties such as perimeter and Centroids. ►The above framework was tested on large of signal and noise levels (for simulated data) in addition to a real motor-tapping task data set. Results show that the proposed methodology and wavelets in general are able to achieve superior accuracy as compared to the conventionally used framework. ►The main advantage of our framework is its capability to accurately extract the shape of an activation region after large amount of processing. This feature is expected to help researchers performing studies on large number of subjects in control or patient groups to obtain more accurate activation maps.
We present sensitive, multifrequency Very Long Baseline Array (VLBA) images of the nuclear radio sources of NGC 1068. At 5 and 8.4 GHz, the radio continuum source S1, argued to mark the location of ...the hidden active nucleus, resolves into an elongated, similar to 0.8 pc source oriented nearly at right angles to the radio jet axis but more closely aligned to the distribution of the nuclear H sub(2)O maser spots. S1 is detected at 5 GHz but not at 1.4 GHz, indicating strong free-free absorption below 5 GHz, and it has a flat spectrum between 5 and 8.4 GHz. A 5-8.4 GHz spectral index map reveals an unresolved, inverted spectrum source at the center of the S1 structure that may mark the AGN proper. The average brightness temperature is too low for synchrotron self-absorption to impact the integrated spectrum significantly. In addition, a careful registration with the nuclear H sub(2)O masers argues that the S1 continuum source arises from the inner regions of the maser disk rather than a radio jet. The emission mechanism may be direct, thermal free-free emission from an X-ray-heated corona or wind arising from the molecular disk. We demonstrate that the hidden active nucleus is sufficiently luminous, to within the current estimates, to provide the requisite heating. The radio jet components C and S2 both show evidence for free-free absorption of a compact, steep-spectrum source. The free-free absorption might arise from a shock cocoon enveloping the compact radio sources. The presence of H sub(2)O masers specifically at component C supports the interpretation for the presence of a jet-ISM interaction. Component NE remains a steep-spectrum source on VLBA baselines and appears to be a local enhancement of the synchrotron emissivity of the radio jet. The reason for the enhancement is not clear; the region surrounding component NE is virtually devoid of narrow-line region filaments, and so there is no clear evidence for interaction with the surrounding ISM. Component NE might instead arise in an internal shock or perhaps in denser jet plasma that broke away from an earlier interaction with the circumnuclear ISM.
We are conducting a large observing program with the Spitzer Space Telescope to determine the mid- to far-IR spectral energy distributions of a well-defined sample of 87 nearby, 12 mm-selected ...Seyfert galaxies. In this paper we present the results of Spitzer IRS low-resolution spectroscopy of a statistically representative subsample of 51 of the galaxies (59%), with an analysis of the continuum shapes and a comparison of the Seyfert types. We find that the spectra clearly divide into groups based on their continuum shapes and spectral features. The largest group (47% of the sample of 51) shows a very red continuum suggestive of cool dust and strong emission features attributed to polycyclic aromatic hydrocarbons. Sixteen objects (31%) have a power-law continuum with spectral indices of a5-20 mm = -2.3 to -0.9 that flatten to a20-35 mm = -1.1 to 0.0 at ~20 mm. Clear silicate emission features at 10 and 18 mm are found in two of these objects (Mrk 6 and Mrk 335). A further 16% of the sample show power-law continua with unchanging slopes of a5-35 mm = -1.7 to -1.1. Two objects are dominated by a broad silicate absorption feature. One object in the sample shows an unusual spectrum dominated by emission features, which is unlike any of the other spectra. Some spectral features are clearly related to a starburst contribution to the IR spectrum, while the mechanisms producing observed power-law continuum shapes, attributed to an active galactic nucleus (AGN) component, may be dust or nonthermal emission. The IR spectral types appear to be related to the Seyfert types. Principal component analysis results suggest that the relative contribution of starburst emission may be the dominant cause of variance in the observed spectra. The derived starburst component of each spectrum, however, contributes <40% of the total flux density. We compare the IR emission with the optically thin radio emission associated with the AGN and find that Seyfert 1 galaxies have higher ratios of IR to radio emission than Seyfert 2 galaxies, as predicted by the unified model if the torus is optically thick in the mid-IR. However, smooth-density torus models predict a much larger difference between Seyfert types 1 and 2 than the factor of 2 difference observed in our sample; the observed factor of ~2 difference between the type 1 and type 2 galaxies in their IR-to-radio ratios above 15 mm requires the standard smooth-density torus models to be optically thin at these wavelengths. However, the resulting low torus opacity requires that the high observed columns detected in X-ray absorption be produced in gas with a very low dust-to-gas ratio (perhaps within the dust sublimation region). On the other hand, our observations may be consistent with clumpy torus models containing a steep radial distribution of optically thick dense clumps. The selection of our sample at 12 mm, where the torus may be optically thick, implies that there may be orientation-dependent biases in the sample; however, we do not find that the sample is biased toward Seyfert 2 galaxies with more luminous central engines, as would be expected. We find that the Seyfert 2 galaxies typically show stronger starburst contributions than the Seyfert 1 galaxies in the sample, contrary to what is expected based on the unified scheme for AGNs. This may be due to the selection effect that only those Seyfert 2 galaxies with strong starburst contributions had high enough integrated 12 mm flux densities to fall above the flux limit of the sample.
We present Hubble Space Telescope Advanced Camera for Surveys and STIS FUV/NUV/optical imaging of the radio galaxy 3C 236, whose relic {approx}4 Mpc radio jet lobes and inner 2 kpc compact steep ...spectrum (CSS) radio source are evidence of multiple epochs of active galactic nucleus (AGN) activity. Consistent with previous results, our data confirm the presence of four bright knots of FUV emission in an arc along the edge of the inner circumnuclear dust disk in the galaxy's nucleus, as well as FUV emission cospatial with the nucleus itself. We interpret these to be sites of recent or ongoing star formation. We present photometry of these knots, as well as an estimate for the internal extinction in the source using line ratios from archival ground-based spectroscopy. We estimate the ages of the knots by comparing our extinction-corrected photometry with stellar population synthesis models. We find the four knots cospatial with the dusty disk to be young, of order {approx}10{sup 7} yr old. The FUV emission in the nucleus, to which we do not expect scattered light from the AGN to contribute significantly, is likely due to an episode of star formation triggered {approx}10{sup 9} yr ago. We argue that the young {approx}10{sup 7} yr old knots stem from an episode of star formation that was roughly coeval with the event resulting in reignition of radio activity, creating the CSS source. The {approx}10{sup 9} yr old stars in the nucleus may be associated with the previous epoch of radio activity that generated the 4 Mpc relic source, before being cut off by exhaustion or interruption. The ages of the knots, considered in the context of both the disturbed morphology of the nuclear dust and the double-double morphology of the 'old' and 'young' radio sources, present evidence for an AGN/starburst connection that is possibly episodic in nature. We suggest that the AGN fuel supply was interrupted for {approx}10{sup 7} yr due to a minor merger event and has now been restored. The resultant nonsteady flow of gas in the disk is likely responsible for both the new episode of infall-induced star formation and also the multiple epochs of radio activity.
Identification of functionally connected regions while at rest has been at the forefront of research focusing on understanding interactions between different brain regions. Studies have utilized a ...variety of approaches including seed based as well as data-driven approaches to identifying such networks. Most such techniques involve differentiating groups based on group mean measures. There has been little work focused on differences in spatial characteristics of resting fMRI data. We present a method to identify between group differences in the variability in the cluster characteristics of network regions within components estimated via independent vector analysis (IVA). IVA is a blind source separation approach shown to perform well in capturing individual subject variability within a group model. We evaluate performance of the approach using simulations and then apply to a relatively large schizophrenia data set (82 schizophrenia patients and 89 healthy controls). We postulate, that group differences in the intra-network distributional characteristics of resting state network voxel intensities might indirectly capture important distinctions between the brain function of healthy and clinical populations. Results demonstrate that specific areas of the brain, superior, and middle temporal gyrus that are involved in language and recognition of emotions, show greater component level variance in amplitude weights for schizophrenia patients than healthy controls. Statistically significant correlation between component level spatial variance and component volume was observed in 19 of the 27 non-artifactual components implying an evident relationship between the two parameters. Additionally, the greater spread in the distance of the cluster peak of a component from the centroid in schizophrenia patients compared to healthy controls was observed for seven components. These results indicate that there is hidden potential in exploring variance and possibly higher-order measures in resting state networks to better understand diseases such as schizophrenia. It furthers comprehension of how spatial characteristics can highlight previously unexplored differences between populations such as schizophrenia patients and healthy controls.
When both structural magnetic resonance imaging (sMRI) and functional MRI (fMRI) data are collected they are typically analyzed separately and the joint information is not examined. Techniques that ...examine joint information can help to find hidden traits in complex disorders such as schizophrenia. The brain is vastly interconnected, and local brain morphology may influence functional activity at distant regions. In this paper we introduce three methods to identify inter-correlations among sMRI and fMRI voxels within the whole brain. We apply these methods to examine sMRI gray matter data and fMRI data derived from an auditory sensorimotor task from a large study of schizophrenia. In Method 1 the sMRI–fMRI cross-correlation matrix is reduced to a histogram and results show that healthy controls (HC) have stronger correlations than do patients with schizophrenia (SZ). In Method 2 the spatial information of sMRI–fMRI correlations is retained. Structural regions in the cerebellum and frontal regions show more positive and more negative correlations, respectively, with functional regions in HC than in SZ. In Method 3 significant sMRI–fMRI inter-regional links are detected, with regions in the cerebellum showing more significant positive correlations with functional regions in HC relative to SZ. Results from all three methods indicate that the linkage between gray matter and functional activation is stronger in HC than SZ. The methods introduced can be easily extended to comprehensively correlate large data sets.
Although the energetic feedback from active galactic nuclei (AGNs) is believed to have a profound effect on the evolution of galaxies and clusters of galaxies, details of AGN heating remain elusive. ...Here, we study NGC 193-a nearby lenticular galaxy-based on X-ray (Chandra) and radio (Very Large Array and Giant Meter-wave Radio Telescope) observations. These data reveal the complex AGN outburst history of the galaxy: we detect a pair of inner X-ray cavities, an outer X-ray cavity, a shock front, and radio lobes extending beyond the inner cavities. We suggest that the inner cavities were produced ~78 Myr ago by a weaker AGN outburst, while the outer cavity, the radio lobes, and the shock front are due to a younger (13-26 Myr) and 4-8 times more powerful outburst. Combining this with the observed morphology of NGC 193, we conclude that NGC 193 likely represents the first example of a second, more powerful, AGN outburst overrunning an older, weaker outburst. These results help us to understand how the outburst energy is dissipated uniformly in the core of galaxies, and therefore may play a crucial role in resolving how AGN outbursts suppress the formation of large cooling flows at cluster centers.
The host galaxies of luminous quasars Floyd, David J. E.; Kukula, Marek J.; Dunlop, James S. ...
Monthly Notices of the Royal Astronomical Society,
November 2004, Letnik:
355, Številka:
1
Journal Article
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ABSTRACT
We present the results of a deep Hubble Space Telescope (HST)/Wide Field and Planetary Camera 2 imaging study of 17 quasars at z≃ 0.4, designed to determine the properties of their host ...galaxies. The sample consists of quasars with absolute magnitudes in the range −24 ≥MV≥− 28, allowing us to investigate host galaxy properties across a decade in quasar luminosity, but at a single redshift. Our previous imaging studies of active galactic nuclei hosts have focused primarily on quasars of moderate luminosity, but the most powerful objects in the current sample have powers comparable to the most luminous quasars found at high redshifts.
We find that the host galaxies of all the radio‐loud quasars, and all the radio‐quiet quasars in our sample with nuclear luminosities MV < −24, are massive bulge‐dominated galaxies, confirming and extending the trends deduced from our previous studies. From the best‐fitting model host galaxies we have estimated spheroid and hence black hole (BH) masses, and the efficiency (with respect to the Eddington luminosity) with which each quasar emits radiation. The largest inferred black hole mass in our sample is MBH≃; 3 × 109 M⊙, comparable to the mass of the black holes at the centres of M87 and Cygnus A. We find no evidence for super‐Eddington accretion rates in even the most luminous objects (0.05 < L/LEdd < 1.0).
We investigate the role of scatter in the black hole–spheroid mass relation in determining the ratio of quasar to host‐galaxy luminosity, by generating simulated populations of quasars lying in hosts with a Schechter mass function. Within the subsample of the highest‐luminosity quasars, the observed variation in nuclear‐host luminosity ratio is consistent with being the result of the scatter in the black hole–spheroid relation. Quasars with high nuclear‐host luminosity ratios can be explained in terms of sub‐Eddington accretion rates on to black holes in the high‐mass tail of the black hole–spheroid relation. Our results imply that, owing to the Schechter function cut‐off, host mass should not continue to increase linearly with quasar luminosity, at the very highest luminosities. Any quasars more luminous than MV=−27 should be found in massive elliptical hosts, which at the present day would have MV≃−24.5.
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