Towards a 3% Geometric Distance to NGC 4258

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Towards a 3 Geometric Distance to NGC 4258 A.
Argon, L. Greenhill, M. Reid, J. Moran E.
Humphreys (Harvard-Smithsonian CfA) 20 April
2007 Adam Trotter (UNC-Chapel Hill)
J. Kagaya (Hoshi No Techou)
Palomar Observatory
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The Host Galaxy

• NGC 4258 (M106)
• Barred spiral SABbc
• Seyfert 1.9, D7.2 Mpc
• Anomalous spiral arms detected in Ha and radio
  continuum
• Recent Chandra, XMM, Spitzer and other images
  highlight the dual nature of spiral arms normal
  jet impinging on ISM

Palomar Observatory
Wilson, Yang Cecil 2001
http//chandra.harvard.edu/photo/2007/ngc4258/
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VLBA
Angular resolution 200 ?as (0.006 pc at 7.2
Mpc) Spectral resolution lt 1 km s-1
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H2O Masers

• Conditions required
• Dense (108 - 1010 cm-3)
• Warm (300 - 800 K)
• Quiescent (?v ?vD)

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The Nuclear Megamaser

• Discovered by Claussen, Heiligman Lo (1984)
• Centered on galactic systemic vsys 470 km s-1
• Isotropic Luminosity 120 LA
• Nakai et al. (1993) discovered emission at ?1000
  km s-1

• Greenhill et al. 1995 VLBI Observations of
  systemic emission show linear structure.
  Secular velocity drift implies centripetal
  acceleration in a flat disk.

Owens Valley Radio Observatory, 40-m antenna
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The Big Picture

• Miyoshi et al. (1995) present VLBI images of full
  velocity range of maser emission
• Systemic emission in narrow band S of radio
  continuum (jet)
• High-velocity emission distributed ?0.3 pc to
  either side
• High-velocity emission exhibits nearly perfect
  Keplerian rotation curve

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Global Model Fits

• Assume warped disk (position angle and
  inclination change as functions of radius)
• Systemic velocity v0 479?3 km s-1
• Central Mass M03.78?0.01x107 MA
• Position of disk center determined to ?30µas
• Plummer model of central cluster would require
  central density ?c gt 5.7x1011 MA pc-3 unstable
  to collapse/disruption in lifetime of galaxy
• Massive disk model would imply densities 10-1000x
  too great to support water maser emission
• Must be a supermassive black hole!

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0.1pc
Observers view

• Inclination warp (flattening of high velocity
  rotation curve) 8 across reds
• Position angle warp (declinations) 9 across
  reds
• Disk obscures much of central region
• Systemic masers in bowl on front side of disk

NGC4258
Top View
0.1pc
Redshifted masers
High
Blueshifted masers
Bowl
TO OBSERVER
Systemic Masers
Courtesy CfA Maser Group, Humphreys, et al. (in
prep)
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• Natural explanation for
• Faint blue high velocity feature (free-free
  absorption along the line-of-sight)
• Strongest red high velocity feature 1306 km s-1
  (where i90º)
• Strongest systemic maser flares at 490 km s-1
  (optimum coherence path)
• X-ray absorbing layer (nH 1023 cm-2) radius
  0.29 pc where disk crosses line-of-sight
  (Fruscione et al. 2005)

Warp is Well-Characterized
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How to Get a Distance? Measure accelerations
proper motions
or
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Herrnstein et al. 1999
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The Result

• Acceleration Distance

• Proper Motion Distance

Including all systematic model uncertainties, a
5 geometric distance measurement
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Comparison to Other MeasurementsD 7.2 ? 0.3 Mpc

• H-band Tully Fisher (Aaronson et al. 1982)
• D 7.1 ? 1.1 Mpc
• Blue Tully Fisher (Richter Huchtmeier 1984)
• D 7.9 ? 1.8 Mpc
• Luminosity Class Estimate (Rowan-Robinson 1985)
• D 8.4 ? 2.2 Mpc
• Cepheid Variables
• D 8.1 ? 0.4 Mpc (Maoz et al. 1999 - 15
  Cepheids)
• D 7.8 ? 0.3 ? 0.5 Mpc (Newman et al. 2001 -
  same 15 Cepheids)
• D 7.4 Mpc (Macri et al 2006 - 300 Cepheids)
• All these depend on Cepheid calibrations

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Towards a 3 Distance MeasurementArgon et al.
2007 (in preparation)

• 18 epochs of VLBI observations, 1997 March 2000
  August, using VLBA, VLA Phased Array and
  Effelsberg 100-m antenna
• Wider bandwidth ? Detection of new high-velocity
  features 30 closer to disk center
• Quantification of maser disk thickness, h5 µas,
  will lead to more accurate models and estimates
  of mass accretion rate ADAF?
• More robust genetic modeling algorithm to refine
  warped disk shape and reduce systematic
  uncertainties
• Inclusion of possible spiral structure in
  high-velocity masers, and ellipticity in systemic
  maser orbits
• HST observations (in progress) of Cepheids in
  NGC4258 to provide independent calibration of
  Cepheid distance scale
• Ultimately, a new measurement of H0, though at a
  distance of 7.2 Mpc, unmodeled local velocity
  field perturbations may limit accuracy of the
  result.

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Mai Les Tres Riches Heures du Duc de Berry, c.
1410
The May jaunt, a pageant celebrating the "joli
mois de Mai" in which one had to wear green
garments known as livree de mai. The riders are
young noblemen and women, with princes and
princesses.