J/ApJ/659/1040 VLBI monitoring of NGC 4258 (Argon+, 2007)
Toward a new geometric distance to the active galaxy NGC 4258.
I. VLBI monitoring of water maser emission.
Argon A.L., Greenhill L.J., Reid M.J., Moran J.M., Humphreys E.M.L.
<Astrophys. J., 659, 1040-1062 (2007)>
=2007ApJ...659.1040A 2007ApJ...659.1040A
ADC_Keywords: Active gal. nuclei ; VLBI ; Masers ; Radio sources
Keywords: galaxies: active - galaxies: individual (NGC 4258) -
galaxies: kinematics and dynamics - galaxies: nuclei - masers -
techniques: interferometric
Abstract:
We report a 3yr, 18 epoch, VLBI monitoring study of H2O masers in
the subparsec, 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 times as many epochs, 3 times
denser sampling, and tighter control over sources of systematic error
than earlier VLBI investigations. Coverage of a 2400km/s 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 5uas (1sigma) thickness of the maser
medium.
Description:
We observed NGC 4258 18 times between 1997 March 6 and 2000 August 12,
using the VLBA, the Very Large Array2 (VLA), and the Effelsberg 100m
telescope (EFLS) of the Max-Planck-Institut fur Radioastronomie
Objects:
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RA (2000) DE Designation(s)
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12 18 58 +47 18.2 NGC 4258 = M 106
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 83 20 The observations
table5.dat 121 14291 Channel data for 18 epochs of observation
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See also:
J/A+A/268/419 : HII Regions in NGC 4258 (Courtes+ 1993)
J/ApJ/652/1133 : BVI photometry of NGC 4258 Cepheids (Macri+, 2006)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1 A1 --- Obs Observation code
3- 8 A6 --- ExpCode Experiment code
10- 19 A10 "YYYY/MM/DD" ObsDate Observation date
21- 27 F7.2 yr ObsYear ? Observation date in decimal year
29- 43 A15 --- Ant Antennas used for the observation (1)
45- 48 F4.2 mas Beam1 ? Size beam, average restoring beam
full-width half-power
49 A1 --- --- [x]
50- 53 F4.2 mas Beam2 ? Size beam, average restoring beam
full-width half-power
55- 59 F5.1 deg PA [-32/39]? Beam position angle,
measured east of north
61- 63 F3.1 mJy Sen ? Sensivity (2)
65 A1 --- CL [x-] Coverage in velocity range
L = low-velocity emission
67 A1 --- CR [x-] Coverage in velocity range
R = redshift emission
69 A1 --- CB [x-] Coverage in velocity range
B = blue-shifted emission
71- 83 A13 --- Com Comments
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Note (1): Antennas as follows:
VLBA = Very Long Baseline Array
VLA = 27x25m Very Large Array in Socorro, New Mexico
EFLS = Max-Planck-Institut fur Radioastronomie 100m antenna in
Effelesberg, Germany
Note (2): The unusually large synthesized beam in BM112M is due to the loss
of two antennas, HN and NL.
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
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1 A1 --- Obs Observation code, as defined in table1.dat
3- 5 I3 -- Chan Channel number
7- 15 F9.3 km/s cz Nonrelativistic LSR velocity (1)
17- 25 F9.3 km/s RelVel Relativistic LSR velocity (1)
27- 33 F7.3 Jy Flux Flux density (2)
35- 43 F9.3 mJy e_Flux The rms error in Flux
45- 51 F7.3 mas EWoff East-West offset (3)
53- 59 F7.3 mas e_EWoff Error (fitted random) in EWoff
61- 67 F7.3 mas NSoff North-South offset (3)
69- 75 F7.3 mas e_NSoff Error (fitted random) in NSoff
77- 82 F6.2 mas Majaxis Major axis of spot
84- 89 F6.2 mas e_Majaxis Error in Majaxis
91- 96 F6.2 mas Minaxis Minor axis of spot
98-103 F6.2 mas e_Minaxis Error in Minaxis
105-108 I4 deg PA Position angle of spot (3)
110-112 I3 deg e_PA Error in PA
114-121 F8.2 km/s RefVel Band identifier for reference velocity (4)
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Note (1): Velocities according to the classical and relativistic
definitions of the Doppler shift in the LSR frame. See Section 2.6 for
a detailed discussion of conversion from classical to relativistic LSR
velocity. Classical Doppler tracking was implemented using the AIPS
task CVEL, which is accurate to <0.004km/s (Bragg et al.,
2000ApJ...535...73B 2000ApJ...535...73B).
Note (2): Fitted peak intensity (Jy/beam) is reported in this table. Note
that F(Jy)=Fp(Jy/beam)*(θmeas2/θbeam2), where F is
the flux density, Fp is the fitted peak intensity, θmeas2 is
the measured maser spot area and θbeam2 is the area of the
restoring beam. Since maser spots are assumed to be unresolved, these
values can be quoted in Jy.
Note (3): Measured with respect to the emission at 510.0km/s
(relativistic). Position uncertainties reflect fitted random error
only. The true random error is estimated to be the maximum of the
fitted random error and the random error computed according to
Table 4, footnote "d".
Note (4): Band identifier refers to the nonrelativistic velocity in
channel 256.5.
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History:
From electronic version of the journal.
Table5 from erratum published in 2008, ApJ, 673, 1249.
(End) Greg Schwarz [AAS], Patricia Vannier [CDS] 25-Jan-2009