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Recent Progress on a New Geometric Distance to NGC 4258

Recent Progress on a New Geometric Distance to NGC 4258. Liz Humphreys (CfA) Alice Argon, Lincoln Greenhill, Jim Moran & Mark Reid. Astronomy Picture of the Day, April 17 2003. NGC 4258. 10 000 ly. Credit: Slotnick, Slotnick & Block. Claussen & Lo (1986) VLA H 2 O maser emission compact

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Recent Progress on a New Geometric Distance to NGC 4258

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  1. Recent Progress on a New Geometric Distance to NGC 4258 Liz Humphreys (CfA) Alice Argon, Lincoln Greenhill, Jim Moran & Mark Reid

  2. Astronomy Picture of the Day, April 17 2003 NGC4258 10 000 ly Credit: Slotnick, Slotnick & Block

  3. Claussen & Lo (1986) VLA H2O maser emission compact High-velocity emission discovered (Nakai et al. 1993) Velocity drifts systemic features ~ 9 kms-1yr-1, ~ no drift high v features. Centripetal acceleration disk @ 7 Mpc (Greenhill et al. 94,95; Haschick et al. 94; Watson & Wallin 94) Miyoshi et al. (1995): VLBA reveals Keplerian disk Multi-epoch VLBA data measures systemic proper motions ~30asyr-1(Herrnstein et al. 1999) Current Picture

  4. This Work • Herrnstein et al. (1999) distance of 7.2 +/- 0.5 Mpc (7%) based on 4 VLBA epochs over 3 yrs (acceleration & proper motion distances same) • Aim: drive down systematic (disk modeling) and random experimental errors by a factor of 2 - 3 • Random error • So far 36 epochs (17 VLBA, 14 VLA, 5 Effelsberg) over 6 years

  5. Velocity (km/s) Blue = max flux Red = min flux Time (days) Trotter Spectra

  6. New feature @ 1562 kms-1 b c e f g j Offset Relative Dec (mas) k l m Herrnstein (1997) n o p Relative RA (mas) Sky Positions

  7. Velocity (kms-1) Distance along major axis (mas) Rotation Curve

  8. highv highv highv Acceleration Distance sys vlos2  M/Dhighv M/D dvlos/d [M/Dsys3]1/2 sys V D alos  v2/Dsys M . Proper Motion Distance  ~ vt/D  (M/D)1/2/Dsys1/2 D M Measure x,y, vlos, alos,& proper motions or

  9. Previous method: Bayesian analysis Now: non-linear, least squares Gaussian fitting routine Spectra over 6 years are fit simultaneously: alos, vlos, jerklos, amplitudes,linewidths are all parameters Green=dataBlue=modelRed=residuals Accelerations& Proper Motions Flux (Jy) Velocity (km/s)

  10. Green=dataBlue=modelRed=residuals Red Features Flux (Jy) Velocity (km/s)

  11. 9 parameter 2 fit: x0, y0, vsys, M/D, i, di/dr, , d/dr, d2/dr2 Genetic Algorithm  global minimum Warped Disk Model

  12. Systemic maser emission originates from “bowl” on front side of disk

  13. Results & Summary • A very accurate, geometric distance to NGC 4258  calibrating the Extragalactic Distance Scale • Work in progress addresses the reduction of both the random and systematic errors in this experiment • To date, we have driven down the random component of the error to a third of its previous value. The total error in the distance to NGC 4258 has decreased from 7% to 5.5%. This work is funded by NASA grant NAG5-10311

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