Person:

Greenhill, Lincoln

We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is the determination of a new geometric maser distance to NGC 4258, accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range (–0.7 to +0.7 km s^{−1} yr^{−1}), indicating an origin within 13° of the disk midline (the perpendicular to the line of sight [LOS] to the black hole). Comparison of the projected positions of high-velocity emission in VLBI images with those derived from acceleration data provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range (7.7 to 8.9 km s^{−1} yr^{−1}), consistent with emission originating from a concavity where the thin, warped disk is tangent to the LOS. A trend in accelerations of low-velocity emission, as a function of Doppler velocity, may be associated with disk geometry and orientation or with the presence of spiral structure.

We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC 3393. The maser emission has a linear distribution oriented at a position angle of ~–34°, perpendicular to both the kiloparsec-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 ± 0.2) × 10^7 M☉) within 0.36 ± 0.02 pc (1.48 ± 0.06 mas), which corresponds to a mean mass density of (\sim10^{8.2} M☉ 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 ± 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 ± 0.02 pc, which is smaller than the inner disk radius (0.36 ± 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 NGC 4258 and NGC 1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.

Based on spectroscopic signatures, about one-third of known H2O maser sources in active galactic nuclei (AGNs) 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 (NGC 1320, NGC 17) in a survey of 40 inclined active galaxies (vsys < 20,000 km s−1). The remaining three disk maser candidates were identified in monitoring of known sources: NGC 449, NGC 2979, and NGC 3735. We also confirm a previously marginal case in UGC 4203. For the disk maser candidates reported here, inferred rotation speeds are 130–500 km s−1. Monitoring of three more rapidly rotating candidate disks (CG 211, NGC 6264, VV 340A) has enabled measurement of likely orbital centripetal acceleration, and estimation of central masses ((2–7) ×107 M) and mean disk radii (0.2–0.4 pc). Accelerations may ultimately permit estimation of distances when combined with interferometer data. This is notable because the three AGNs are relatively distant (10,000 km s−1 < vsys < 15,000 km s−1), and fractional error in a derived Hubble constant, due to peculiar motion of the galaxies, would be small. As signposts of highly inclined geometries at galactocentric radii of ∼0.1–1 pc, disk masers also provide robust orientation references that allow analysis of (mis)alignment between AGNs and surrounding galactic stellar disks, even without extensive interferometric mapping. We find no preference among published disk maser candidates to lie in high-inclination galaxies. This provides independent support for conclusions that in late-type galaxies, central engine accretion disks and galactic plane orientations are not correlated.

We report a new geometric maser distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight (LOS) velocities and sky positions from 18 epochs of very long baseline interferometry observations, and LOS accelerations measured from a 10 yr monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is (7.60 \pm 0.17 \pm 0.15 Mpc), a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is (H_0 = 72.0 \pm 3.0 km s^{–1} Mpc^{–1}).

A wealth of new information about the structure of the maser disk in NGC 4258 has been obtained from a series of 18 VLBA observations spanning three years, as well as from 32 additional epochs of spectral monitoring data from 1994 to the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk has been defined precisely. The thickness of the maser disk has been measured to be 12 microarcseconds (FWHM), which is slightly smaller than previously quoted upper limits. Under the assumption that the masers trace the true vertical distribution of material in the disk, from the condition of hydrostatic equilibrium the sound speed is 1.5 km s−1 , corresponding to a thermal temperature of 600K. The accelerations of the high velocity maser components have been accurately measured for many features on both the blue and red side of the spectrum. The azimuthal offsets of these masers from the midline (the line through the disk in the plane of the sky) and derived projected offsets from the midline based on the warp model correspond well with the measured offsets. This result suggests that the masers are well described as discrete clumps of masing gas, which accurately trace the Keplerian motion of the disk. However, we have continued to search for evidence of apparent motions caused by “phase effects.” This work provides the foundation for refining the estimate of the distance to NGC 4258 through measurements of feature acceleration and proper motion. The refined estimate of this distance is expected to be announced in the near future.

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 1σ (0.′′3 radio and 1.′′3 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 km s−1, ∼ 800 km s−1, 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% (5 out of 41) among Seyfert 2 and LINER systems with 10000 km s−1 < vsys < 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, L2−10 ∝ L 0.5±0.1 H2O, 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.

We report the discovery of water maser emission in eight active galactic nuclei (AGN) with the 70-m NASA Deep Space Network (DSN) antennas at Tidbinbilla, Australia and Robledo, Spain. The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1 σ (0.′′3 radio and 1.′′3 optical) and most likely mark the locations of the embedded central engines. The spectra of two sources, NGC 3393 and NGC 5495, display the characteristic spectral signature of emission from an edge-on accretion disk, with orbital velocities of ∼ 600 and ∼ 400 km s−1, respectively. In a survey with DSN facilities of 630 AGN selected from the NASA Extragalactic Database, we have discovered a total of 15 water maser sources. The resulting incidence rate of maser emission among nearby ( vsys < 7000 km s − 1 ) Seyfert 1 . 8 − 2 .0 and LINER systems is ∼ 10% for a typical rms noise level of ∼ 14 mJy over 1 .3 km s − 1 spectral channels. As a result of this work, the number of nearby AGN (vsys < 7000 km s − 1) observed with < 20 mJy rms noise has increased from 130 to 449.

A maximum likelihood analysis of the NGC 4258 maser positions and velocities reveals a ∼ 2 σ deviation from Keplerian motion in the projected rotation curve of the high-velocity features, corresponding to a ∼ 9 km s−1, or 0.8%, flattening of the LOS velocities with respect to Keplerian motion over the range of the high-velocity masers. While there are a number of potential explanations for this flattening, we argue for pure Keplerian rotation in an inclination-warped disk based on the ability of this model to explain a number of otherwise puzzling features of the system. A radial gradient in the disk inclination of 0.034 mas − 1 is not only consistent with the observed rotation curve, but it generates a bowl along the near edge of the disk that naturally explains the otherwise puzzling narrow spread in the declinations of the systemic masers. It also explains the existence and location of an apparently recurring flare amongst the systemic masers. There is no significant evidence for non-Keplerian rotation in the inclinationwarped disk. An additional implication of the inclination warp is that the disk rises in front of and obscures the central engine at a disk radius of about 8.3 mas, or 0.29 pc. By comparing the observed X-ray column to conditions in the disk at this radius, we argue the disk must be atomic at 0.29 pc. Hence we conclude that the molecular-to-atomic transition occurs just beyond the outermost maser at 0.28 pc and from this we infer an accretion rate of ∼ 10 − 4 α M ⊙ yr − 1, where α (. 1) is the standard dimensionless parameterization of the kinematic viscosity. Our model suggests that most of the observed X-ray column arises in the warped accretion disk at 0.29 pc, and that the maser emission is truncated at large radii predominantly as a result of the molecular-to-atomic phase transition originally proposed by Neufeld & Maloney (1995). The inferred accretion rate is consistent with the jet-dominated accretion models of Yuan et al. (2002).

Using the Green Bank Telescope, we have conducted a survey for 1.3 cm water maser emission toward the nuclei of nearby active galaxies, the most sensitive large survey for H2O masers to date. Among 145 galaxies observed, maser emission was newly detected in eleven sources and confirmed in one other. Our survey targeted nearby (v < 12,000 km s−1), mainly type 2 AGNs north of δ = –20◦, and includes a few additional sources as well. We find that more than a third of Seyfert 2 galaxies have strong maser emission, though the detection rate declines beyond v ∼ 5000 km s−1 due to sensitivity limits. Two of the masers discovered during this survey are found in unexpected hosts: NGC 4151 (Seyfert 1.5) and NGC 2782 (starburst). We discuss the possible relations between the large X-ray column to NGC 4151 and a possible hidden AGN in NGC 2782 to the detected masers. Four of the masers discovered here, NGC 591, NGC 4388, NGC 5728 and NGC 6323, have high-velocity lines symmetrically spaced about the systemic velocity, a likely signature of molecular gas in a nuclear accretion disk. The maser source in NGC 6323, in particular, reveals the classic spectrum of a “disk maser” represented by three distinct groups of Doppler components. Future single-dish and VLBI observations of these four galaxies could provide a measurement of the distance to each galaxy, and of the Hubble constant, independent of standard candle calibrations.

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 lineof-sight velocities of blue- and red-shifted maser emission are consistent with a central mass of ∼ 2 × 106 M⊙ enclosed within a radius of ∼ 0.4 pc. The corresponding mean mass density of 106.8 M⊙ 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 Kα 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 Qparameter 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. We have also mapped radio continuum emission in the vicinity of the disk and identify a new, time-variable, non-thermal component (E) that is not collinear with the previously imaged putative jet. Based on the large luminosity and the unusually steep spectrum (α < −2.1), we exclude a radio supernova as the progenitor of E. However, because its spectrum is consistent with an aging electron energy distribution, E might be a rapidly cooling remnant, which may indicate that the jet axis wobbles. Alternatively, considering its location, the component might mark a shock in a wide-angle outflow that is interacting with a dense ambient medium. In this context, masers at high latitudes above the disk, mapped in this and previous studies, may be tracing an inward extension of the kpc-scale bipolar wide-angle outflow previously observed along the galactic minor axis.