Person:

Reid, Mark

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.

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.

Submillimeter Array observations of Orion-KL at ∼ 1′′ resolution in the 440 µm/690 GHz band reveal new insights about the continuum and line emission of the region. The 440 µm continuum flux density measurement from source I allows us to differentiate among the various proposed physical models: Source I can be well modeled by a “normal” protostellar SED consisting of a proton-electron free-free emission component at low frequencies and a strong dust component in the submillimeter bands. Furthermore, we find that the protostellar object SMA1 is clearly distinct from the hot core. The non-detection of SMA1 at cm and infrared wavelengths suggests that it may be one of the youngest sources in the entire Orion-KL region. The molecular line maps show emission mainly from the sources I, SMA1 and the hot core peak position. An analysis of the CH 3CN(37 K − 36 K) K-ladder ( K = 0...3) indicates a warm gas component of the order 600 ± 200K. In addition, we detect a large fraction ( ∼ 58%) of unidentified lines and discuss the difficulties of line identifications at these frequencies.

We present the first submm (865 µm) imaging spectral line survey at one arcsecond resolution conducted with the Submillimeter Array toward Orion-KL. Within the two × two GHz bandpasses (lower and upper sidebands, 337.2–339.2GHz and 347.2–349.2GHz), we find about 145 spectral lines from 13 species, 6 isotopologues, and 5 vibrational excited states. Most nitrogen-bearing molecules are strong toward the hot core, whereas the oxygen-bearing molecules peak toward the south-west in the so-called compact ridge. Imaging of spectral lines is shown to be an additional tool to improve the identifications of molecular lines. Arcsecond spatial resolution allows us to distinguish the molecular line emission of the sources I and n from that of the hot core. The only molecular species detected strongly toward source I is SiO, delineating mainly the collimated north-east south-west low-velocity outflow. The two positions close to source I, which have previously been reported to show maser emission in the v=0 28SiO(1–0) and (2–1) lines, show no detectable maser emission in the v=0 28SiO(8–7) line at our spatial resolution. SiO is weak toward source n, and thus source n may not currently be driving a molecular outflow. CH 3OH is the molecule with the highest number of identified lines (46) in this spectral window. This “line forest” allows us to estimate temperatures in the region, and we find temperatures between 50 and 350 K, with the peak temperatures occurring toward the hot core. The detection of strong vibrational excited line emission from the submm continuum peak SMA1 supports the interpretation that the source SMA1 is likely of protostellar nature.

We report on the results of a 14 year long VLBI study of proper motions in the IC 133 H2O maser source in the galaxy M33. The method of Ordered Motion Parallax was used to model the 3-dimensional structure and dynamics of IC 133 and obtain a distance estimate, 800 ± 180 kpc. Our technique for determining the distance to M33 is independent of calibrations common to other distance indicators, such as Cepheid Period-Luminosity relations, and therefore provides an important check for previous distance determinations.

We present the first 865 µm continuum image with sub-arcsecond resolution obtained with the Submillimeter Array. These data resolve the Orion-KL region into the hot core, the nearby radio source I, the sub-mm counterpart to the infrared source n (radio source L), and new sub-mm continuum sources. The radio to submillimeter emission from source I may be modeled as either the result of proton-electron free-free emission that is optically thick to ∼ 100 GHz plus dust emission that accounts for the majority of the submillimeter flux, or H− free-free emission that gives rise to a power-law spectrum with power-law index of ∼ 1.6. The latter model would indicate similar physical conditions as found in the inner circumstellar environment of Mira variable stars. Future sub-arcsecond observations at shorter sub-mm wavelengths should easily discriminate between these two possibilities. The sub-mm continuum emission toward source n can be interpreted in the framework of emission from an accretion disk.

We report a three year, 18 epoch, VLBI monitoring study of H 2O masers in the sub-parsec, warped, accretion disk within the NGC 4258 AGN. Our immediate goals are to trace the geometry of the underlying disk, track rotation via measurement of proper motion, and ascertain the radii of masers for which centripetal acceleration may be measured separately. The monitoring includes ∼ 4× as many epochs, ∼ 3× denser sampling, and tighter control over sources of systematic error than earlier VLBI investigations. Coverage of a ∼ 2400 km s − 1 bandwidth has also enabled mapping of molecular material ∼ 30% closer to the black hole than accomplished previously, which will strengthen geometric and dynamical disk models. Through repeated observation we have also measured for the first time a 5 µas (1 σ) thickness of the maser medium. Assuming this corresponds to the thickness of the accretion disk, hydrostatic equilibrium requires a disk plane temperature of ≈ 600 K. Our long-term goal is a geometric distance to NGC 4258 that is accurate to ∼ 3%, a ∼ 2 × improvement over the current best estimate. A geometric estimate of distance can be compared to distances obtained from analysis of Cepheid light curves, with the intent to recalibrate the extragalactic distance scale with reduced systematic uncertainties. This is the first paper in a series. We present here VLBI observations, data reduction, and temporal and spatial characteristics of the maser emission. Later papers will report estimation of orbital acceleration and proper motion, modeling of disk 3-D geometry and dynamics, and estimation of a “ maser distance.” Estimation of a “Cepheid distance” is presented in a parallel paper series.