arXiv subjects

James M. Moran

Publications and source records attributed to James M. Moran.

At least 37 records · Page 2Linked to original sources

arXiv2010

The High-Density Ionized Gas in the Central Parsec of the Galaxy

We report a study of the H30$α$ line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2\arcsec\ over a field of 60\arcsec\ (2 parsec) and a velocity range of -360 to +345 \kms. This field encompasses most of the Galactic center's "minispiral". With an isothermal homogeneous HII model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the H30$α$ line data along with Very Large Array data from the H92$α$ line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are 3-21$\times10^4$ cm$^{-3}$ and 5,000-13,000 K, respectively. The H30$α$ and H92$α$ line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a 3D model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the "Bar" region, which is located 0.1-0.2 parsec south of and behind Sgr A*. Finally, a total Lyman continuum flux of $3\times10^{50}$ photons s$^{-1}$ is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to $\sim250$ O9-type zero-age-main-sequence stars.

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arXiv2010

The High-Density Ionized Gas in the Central Parsecs of the Galaxy

We report the results from observations of H30$α$ line emission in Sgr A West with the Submillimeter Array at a resolution of 2\arcsec and a field of view of about 40\arcsec. The H30$α$ line is sensitive to the high-density ionized gas in the minispiral structure. We compare the velocity field obtained from H30$α$ line emission to a Keplerian model, and our results suggest that the supermassive black hole at Sgr A* dominates the dynamics of the ionized gas. However, we also detect significant deviations from the Keplerian motion, which show that the impact of strong stellar winds from the massive stars along the ionized flows and the interaction between Northern and Eastern arms play significant roles in the local gas dynamics.

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arXiv2009

Discovery of Candidate H$_2$O Disk Masers in AGN and Estimations of Centripetal Accelerations

Based on spectroscopic signatures, about one-third of known H$_2$O maser sources in active galactic nuclei (AGN) are believed to arise in highly inclined accretion disks around central engines. These "disk maser candidates" are of interest primarily because angular structure and rotation curves can be resolved with interferometers, enabling dynamical study. We identify five new disk maser candidates in studies with the Green Bank Telescope, bringing the total number published to 30. We discovered two (NGC1320, NGC17) in a survey of 40 inclined active galaxies (v_{sys}< 20000 kms^{-1}). The remaining three disk maser candidates were identified in monitoring of known sources: NGC449, NGC2979, NGC3735. We also confirm a previously marginal case in UGC4203. For the disk maser candidates reported here, inferred rotation speeds are 130-500 kms^{-1}. Monitoring of three more rapidly rotating candidate disks (CG211, NGC6264, VV340A) has enabled measurement of likely orbital centripetal acceleration, and estimation of central masses (2-7x10^7 M_\odot) and mean disk radii (0.2-0.4pc). Accelerations may ultimately permit estimation of distances when combined with interferometer data. This is notable because the three AGN are relatively distant (10000<v_{sys}<15000 kms^{-1}). As signposts of highly inclined geometries at galactocentric radii of \sim0.1-1pc, disk masers also provide robust orientation references that allow analysis of (mis)alignment between AGN and surrounding galactic stellar disks, even without interferometric mapping. We find no preference among published disk maser candidates to lie in high-inclination galaxies, providing independent support for conclusions that central engines and galactic plane orientations are not correlated. (ABRIDGED)

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arXiv2009

The Rotating Molecular Structures and the Ionized Outflow Associated with IRAS 16547-4247

We present VLA 1.3 cm radio continuum and water maser observations as well as SMA SO$_2$ (226.300 GHz) and 1.3 mm dust continuum observations toward the massive star formation region IRAS 16547-4247. We find evidence of multiple sources in the central part of the region. There is evidence of a rotating structure associated with the most massive of these sources, traced at small scales (~50 AU) by the water masers. At large scales (~1000 AU) we find a velocity gradient in the SO2 molecular emission with a barely resolved structure that can be modeled as a rotating ring or two separate objects. The velocity gradients of the masers and of the molecular emission have the same sense and may trace the same structure at different size scales. The position angles of the structures associated with the velocity gradients are roughly perpendicular to the outflow axis observed in radio continuum and several molecular tracers. We estimate the mass of the most massive central source to be around 30 solar masses from the velocity gradient in the water maser emission. The main source of error in this estimate is the radius of the rotating structure. We also find water masers that are associated with the large scale molecular outflow of the system, as well as water masers that are associated with other sources in the region. Our results suggest that the formation of this source, one of the most luminous protostars or protostellar clusters known, is taking place with the presence of ionized jets and disk-like structures.

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arXiv2008

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation (ref 1). Sagittarius A*, the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4 million times that of the Sun (refs. 2,3). A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A* where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering (refs. 4-7). Here we report observations at a wavelength of 1.3 mm that set a size of 37 (+16, -10; 3-sigma) microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of SgrA* emission may not be not centred on the black hole, but arises in the surrounding accretion flow.

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arXiv2008

The Black Hole Accretion Disk in NGC 4258: One of Nature's Most Beautiful Dynamical Systems

In this talk I will summarize some of the work that the CfA group has done to study the structure of the water masers in the accretion disk of NGC 4258. A series of 18 epochs of VLBA data taken from 1997.3 to 2000.8 were used for this study. The vertical distribution of maser features in the systemic group was found to have a Gaussian distribution, as expected for hydrostatic equilibrium, with a $σ$-width of 5.1 $μ$as. If the disk is in hydrostatic equilibrium, its temperature is about 600K. The systemic features exhibit a small, but persistent, gradient in acceleration versus impact parameter. This characteristic may indicate the presence of a spiral density wave rotating at sub-Keplerian speed. A more precise understanding of the dynamical properties of the disk is expected to lead to a more refined estimate of the distance to the galaxy.

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arXiv2008

The Collimated Jet Source in IRAS 16547-4247: Time Variation, Possible Precession, and Upper Limits to the Proper Motions Along the Jet Axis

The triple radio source detected in association with the luminous infrared source IRAS 16547-4247 has previously been studied with high angular resolution and high sensitivity with the Very Large Array (VLA) at 3.6-cm wavelength. In this paper, we present new 3.6 cm observations taken 2.68 years after the first epoch that allow a search for variability and proper motions, as well as the detection of additional faint sources in the region. We do not detect proper motions along the axis of the outflow in the outer lobes of this source at a 4-$σ$ upper limit of $\sim$160 km s$^{-1}$. This suggests that these lobes are probably working surfaces where the jet is interacting with a denser medium. However, the brightest components of the lobes show evidence of precession, at a rate of $0\rlap.^\circ08$ yr$^{-1}$ clockwise in the plane of the sky. It may be possible to understand the distribution of almost all the identified sources as the result of ejecta from a precessing jet. The core of the thermal jet shows significant variations in flux density and morphology. We compare this source with other jets in low and high mass young stars and suggest that the former can be understood as a scaled-up version of the latter.

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arXiv2008

The Parsec-scale Accretion Disk in NGC 3393

We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC3393. The maser emission has a linear distribution oriented at a position angle of $\sim -34\degr$, perpendicular to both the kpc-scale radio jet and the axis of the narrow line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical center, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of $(3.1\pm 0.2) \times 10^7 M_{\sun}$ within $0.36\pm 0.02$ pc ($1.48\pm 0.06$ mas), which corresponds to a mean mass density of $\sim10^{8.2} M_{\sun}$ pc$^{-3}$. We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of $5\pm 1$ km s $^{-1}$ yr$^{-1}$ in the $\sim3880$ km s$^{-1}$ maser feature, which is most likely located along the line of sight to the dynamical center of the system. From the acceleration of this feature, we estimate a disk radius of $0.17\pm 0.02$ pc, which is smaller than the inner disk radius ($0.36\pm 0.02$ pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical center evidently occurs much closer to the center than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC4258 and NGC1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.

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arXiv2008

SMA Imaging of the Maser Emission from the H30$α$ Radio Recombination Line in MWC349A

We used the Submillimeter Array to map the angular distribution of the H30$α$ recombination line (231.9 GHz) in the circumstellar region of the peculiar star MWC349A. The resolution was $1\farcs2$, but because of high signal-to-noise ratio we measured the positions of all maser components to accuracies better than $0\farcs01$, at a velocity resolution of $1 kms$. The two strongest maser components (called high velocity components) at velocities near -14 and $32 kms$ are separated by $0\farcs048 \pm 0\farcs001$ (60 AU) along a position angle of $102 \pm 1\arcdeg$. The distribution of maser emission at velocities between and beyond these two strongest components were also provided. The continuum emission lies at the center of the maser distribution to within 10 mas. The masers appear to trace a nearly edge-on rotating disk structure, reminiscent of the water masers in Keplerian rotation in the nuclear accretion disk of the galaxy NGC4258. However, the maser components in MWC349A do not follow a simple Keplerian kinematic prescription with $v \sim r^{-1/2}$, but have a larger power law index. We explore the possibility that the high velocity masers trace spiral density or shock waves. We also emphasize caution in the interpretation of relative centroid maser positions where the maser is not clearly resolved in position or velocity, and we present simulations that illustrate the range of applicability of the centroiding method.

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arXiv2008

Infall and Outflow of Molecular Gas in Sgr B2

Observations of two H$_2$CO ($3_{03}-2_{02}$ and $3_{21}-2_{20}$) lines and continuum emission at 1.3 mm towards Sgr B2(N) and Sgr B2(M) have been carried out with the SMA. The mosaic maps of Sgr B2(N) and Sgr B2(M) in both continuum and lines show a complex distribution of dust and molecular gas in both clumps and filaments surrounding the compact star formation cores. We have observed a decelerating outflow originated from the Sgr B2(M) core, showing that both the red-shifted and blue-shifted outflow components have a common terminal velocity. This terminal velocity is 58$\pm$2 km s$^{-1}$. It provides an excellent method in determination of the systematic velocity of the molecular cloud. The SMA observations have also shown that a large fraction of absorption against the two continuum cores is red-shifted with respect to the systematic velocities of Sgr B2(N) and Sgr B2(M), respectively, suggesting that the majority of the dense molecular gas is flowing into the two major cores where massive stars have been formed. We have solved the radiative transfer in a multi-level system with LVG approximation. The observed H$_2$CO line intensities and their ratios can be adequately fitted with this model for the most of the gas components. However, the line intensities between the higher energy level transition H$_2$CO ($3_{21}-2_{20}$) and the lower energy level transition H$_2$CO ($3_{03}-2_{02}$) is reversed in the red-shifted outflow region of Sgr B2(M), suggesting the presence of inversion in population between the ground levels in the two K ladders (K$_{-1}$= 0 and 2). The possibility of weak maser processes for the H$_2$CO emission in Sgr B2(M) is discussed.

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arXiv2006

Discovery of Water Maser Emission in Five AGN and a Possible Correlation Between Water Maser and Nuclear 2-10 keV Luminosities

We report the discovery of water maser emission in five active galactic nuclei (AGN) with the 100-m Green Bank Telescope (GBT). The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1 sigma (0.3 arcsec radio and 1.3 arcsec optical) and most likely mark the locations of the embedded central engines. The spectra of three sources, 2MASX J08362280+3327383, NGC 6264, and UGC 09618 NED02, display the characteristic spectral signature of emission from an edge-on accretion disk with maximum orbital velocity of ~700, ~800, and ~1300 km s^-1, respectively. We also present a GBT spectrum of a previously known source MRK 0034 and interpret the narrow Doppler components reported here as indirect evidence that the emission originates in an edge-on accretion disk with orbital velocity of ~500 km s^-1. We obtained a detection rate of 12 percent (5 out of 41) among Seyfert 2 and LINER systems with 10000 km s^-1 < v_sys < 15000 km s^-1. For the 30 nuclear water masers with available hard X-ray data, we report a possible relationship between unabsorbed X-ray luminosity (2-10 keV) and total isotropic water maser luminosity, L_{2-10} proportional to L_{H2O}^{0.5+-0.1}, consistent with the model proposed by Neufeld and Maloney in which X-ray irradiation and heating of molecular accretion disk gas by the central engine excites the maser emission.

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arXiv2006

Infall, Fragmentation and Outflow in Sgr B2

Observations of H$_{2}$CO lines and continuum at 1.3 mm towards Sgr B2(N) and Sgr B2(M) cores were carried out with the SMA. We imaged H$_{2}$CO line absorption against the continuum cores and the surrounding line emission clumps. The results show that the majority of the dense gas is falling into the major cores where massive stars have been formed. The filaments and clumps of the continuum and gas are detected outside of Sgr B2(N) and Sgr B2(M) cores. Both the spectra and moment analysis show the presence of outflows from Sgr B2(M) cores. The H$_{2}$CO gas in the red-shifted outflow of Sgr B2(M) appears to be excited by a non-LTE process which might be related to the shocks in the outflow.

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arXiv2006

The Submillimeter Polarization of Sgr A*

We report on the submillimeter properties of Sagittarius A* derived from observations with the Submillimeter Array and its polarimeter. We find that the spectrum of Sgr A* between 230 and 690 GHz is slightly decreasing when measured simultaneously, indicating a transition to optically thin emission around 300-400 GHz. We also present very sensitive and well calibrated measurements of the polarization of Sgr A* at 230 and 345 GHz. With these data we are able to show for the first time that the polarization of Sgr A* varies on hour timescales, as has been observed for the total intensity. On one night we find variability that may arise from a polarized "blob" orbiting the black hole. Finally, we use the ensemble of observations to determine the rotation measure. This represents the first statistically significant rotation measure determination and the only one made without resorting to comparing position angles measured at separate epochs. We find a rotation measure of (-5.6+/-0.7)x10^5 rad/m^2, with no evidence for variability on inter-day timescales at the level of the measurement error. The stability constrains interday fluctuations in the accretion rate to 8%. The mean intrinsic polarization position angle is 167+/-7 degrees and we detect variations of 31+18/-9 degrees. This separation of intrinsic polarization changes and possible rotation measure fluctuations is now possible because of the frequency coverage and sensitivity of our data. The observable rotation measure restricts the accretion rate to the range 2x10^{-7} Msun/yr to 2x10^{-9} Msun/yr, if the magnetic field is near equipartition and ordered.

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arXiv2005

A disk of dust and molecular gas around a high-mass protostar

The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (> 8 x Sun's mass) stars has heretofore remained poorly understood. Recent observational studies suggest that high-mass stars may form in essentially the same way as low-mass stars, namely via an accretion process, instead of via merging of several low-mass (< 8 Msun) stars. However, there is as yet no conclusive evidence. Here, we report the discovery of a flattened disk-like structure observed at submillimeter wavelengths, centered on a massive 15 Msun protostar in the Cepheus-A region. The disk, with a radius of about 330 astronomical units (AU) and a mass of 1 to 8 Msun, is detected in dust continuum as well as in molecular line emission. Its perpendicular orientation to, and spatial coincidence with the central embedded powerful bipolar radio jet, provides the best evidence yet that massive stars form via disk accretion in direct analogy to the formation of low-mass stars.

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arXiv2005

Probing the Magnetic Field at Sub-Parsec Radii in the Accretion Disk of NGC 4258

We present an analysis of polarimetric observations at 22 GHz of the water vapor masers in NGC 4258 obtained with the VLA and the GBT. We do not detect any circular polarization in the spectrum indicative of Zeeman-induced splitting of the maser lines of water, a non-paramagnetic molecule. We have improved the 1-sigma upper limit estimate on the toroidal component of the magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from 300 mG to 90 mG. We have developed a new method for the analysis of spectra with blended features and derive a 1-sigma upper limit of 30 mG on the radial component of the magnetic field at a radius of 0.14 pc. Assuming thermal and magnetic pressure balance, we estimate an upper limit on the mass accretion rate of ~10^(-3.7) M_sun/yr for a total magnetic field of less than 130 mG. We discuss the ramifications of our results on current maser models proposed to explain the observed maser emission structure and the consequences for current accretion theories. We find from our magnetic field limits that the thin-disk model and the jet-disk model are better candidates for accounting for the extremely low-luminosity nature of NGC 4258, than models that include advection-dominated accretion flows.

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arXiv2005

X-ray Luminosity and Absorption Column Fluctuations in the H2O Maser Galaxy NGC 4258 from Weeks to Years

We report monitoring of the 0.3-10 keV spectrum of NGC4258 with XMM over 1.5 years.We als o report reprocessing of an overlapping series of archival Chandra observations. By including earlier ASCA and SAX observations, we present a new, nine-year time series of models fit to the X-ray spectrum of NGC4258. Over the nine years, the photoelectric absorbing column (~10^23 cm^-2) did not vary detectably, except for a ~40% drop between two ASCA epochs separated by 3 years and a ~60% rise between two XMM epochs separated by just 5 months. In contrast, factor of 2-3 changes are seen in absorbed flux on the timescale of years. These are uncorrelated with changes in absorbing column and indicative of central engine variability. The most rapid change in luminosity (5-10 keV) that we detect is ~30% over 19 days. The warped disk, a known source of H2O maser emission in NGC4258, is believed to cross the line of sight to the central engine. We propose that the variations in absorbing column arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius where the disk crosses the line of sight. We estimate that the inhomogeneities are ~10^15 cm in size at the crossing radius of 0.29 pc, slightly smaller than the expected scale height of the disk. This result thus provides strong evidence that the warped accretion disk is the absorber. This is the first direct confirmation that obscuration in type-2 AGN may, in some cases, arise in thin, warped accretion disks, rather than in geometrically thick tori. We do not detect Fe Kalpha line emission in any of our XMM spectra. We do not observe evidence of absorption lines in the XMM or reprocessed Chandra data.

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arXiv2004

Evidence for a Geometrically Thick Self-Gravitating Accretion Disk in NGC 3079

We have mapped, for the first time, the full velocity extent of the water maser emission in NGC 3079. The largely north-south distribution of emission, aligned with a kpc-scale molecular disk, and the segregation of blue- and red-shifted emission on the sky are suggestive of a nearly edge-on molecular disk on pc-scales. Positions and line-of-sight velocities of blue- and red-shifted maser emission are consistent with a central mass of ~2 x 10^6 Msun enclosed within a radius of ~0.4 pc. The corresponding mean mass density of 10^6.8 Msun pc^-3 is suggestive of a central black hole, which is consistent with the detection of hard X-ray excess (20-100 keV) and an Fe Kalpha line from the nucleus. Because the rotation curve traced by the maser emission is flat, the mass of the pc-scale disk is significant with respect to the central mass. Since the velocity dispersion of the maser features does not decrease with radius and constitutes a large fraction of the orbital velocity, the disk is probably thick and flared. The rotation curve and the physical conditions necessary to support maser emission imply a Toomre Q-parameter that is << 1. Thus, the disk is most likely clumpy, and we argue that it is probably forming stars. Overall, the accretion disk in NGC 3079 stands in contrast to the compact, thin, warped, differentially rotating disk in the archetypal maser galaxy NGC 4258 (abridged).

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arXiv2004

Interferometric Observation of the Highly Polarized SiO Maser Emission from the $v=1, J=5-4$ Transition Associated with VY Canis Majoris

We used the Submillimeter Array to image the SiO maser emission in the $v=1$, \$J=5-4$ transition associated with the peculiar red supergiant VY Canis Majoris. We identified seven maser components and measured their relative positions and linear polarization properties. Five of the maser components are coincident to within about 150 mas ($\sim$ 200 AU at the distance of 1.5 kpc); most of them may originate in the circumstellar envelope at a radius of about 50 mas from the star along with the SiO masers in the lowest rotational transitions. Our measurements show that two of the maser components may be offset from the inner stellar envelope (at the 3$σ$ level of significance) and may be part of a larger bipolar outflow associated with VY CMa identified by Shinnaga et al. The strongest maser feature at a velocity of 35.9 kms$^{-1}$ has a 60 percent linear polarization, and its polarization direction is aligned with the bipolar axis. Such a high degree of polarization suggests that maser inversion is due to radiative pumping. Five of the other maser features have significant linear polarization.

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