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

NGC 4258 Astronomy Picture of the Day, April 17 2003 Add credit, remove red line round picture, add scale 10 000 ly Credit: Slotnick, Slotnick & Block

Current Picture 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)

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

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

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

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

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

Accelerations & Proper Motions Green=data Blue=model Red=residuals Accelerations & Proper Motions vlos (kms-1) vlos alos (kms-1yr-1) alos 536.91 0.037 9.074 0.015 540.88 0.075 9.130 0.028 543.96 0.239 9.110 0.078 545.61 0.085 9.003 0.043 549.07 0.082 0.033 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 Flux (Jy) Velocity (km/s)

Red Features vlos (kms-1) vlos alos (kms-1yr-1) alos 1305.79 0.021 Green=data Blue=model Red=residuals Red Features vlos (kms-1) vlos alos (kms-1yr-1) alos 1305.79 0.021 0.055 0.014 1306.49 0.018 0.080 0.010 1308.46 0.273 1.127 0.110 Flux (Jy) Velocity (km/s)

Warped Disk Model 9 parameter 2 fit: x0, y0, vsys, M/D, i, di/dr, , d/dr, d2/dr2 Genetic Algorithm  global minimum (x0,y0) (-0.132,0.548) mas vsys 474.1 kms-1 M/D 0.526 107 M/Mpc i (r) 71.6° + 2.4°r ( r ) 65.3° + 2.3°r – 0.23°r2

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

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