Person: Argon, Alice
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Argon
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Alice
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Argon, Alice
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Publication Toward a New Geometric Distance to the Active Galaxy NGC 4258. II. Centripetal Accelerations and Investigation of Spiral Structure(American Astronomical Society, 2008) Humphreys, E. M. L.; Reid, Mark; Greenhill, Lincoln; Moran, James; Argon, AliceWe 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.Publication Toward a New Geometric Distance To the Active Galaxy NGC 4258. III. Final Results and The Hubble Constant(American Astronomical Society, 2013) Humphreys, E. M. L.; Reid, Mark; Moran, James; Greenhill, Lincoln; Argon, AliceWe 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 \(H_0 = 72.0 \pm 3.0 km s^{–1} Mpc^{–1}\).Publication The structure of the accretion disk in NGC 4258(Cambridge University Press (CUP), 2007) Moran, James; Humphreys, Elizabeth; Greenhill, Lincoln; Reid, Mark; Argon, AliceA 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.Publication Toward a New Geometric Distance to the Active Galaxy NGC 4258. I. VLBI Monitoring of Water Maser Emission(IOP Publishing, 2007) Argon, Alice; Greenhill, Lincoln; Reid, Mark; Moran, James; Humphreys, E. M. L.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.Publication The IC 133 Water Vapor Maser in the Galaxy M33: A Geometric Distance(IOP Publishing, 2004) Argon, Alice; Greenhill, Lincoln; Moran, James; Reid, Mark; Menten, K. M.; Inoue, M.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.