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

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

3. 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 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)

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

6. 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 1562 kms-1
Herrnstein (1997)

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

9.  ~ 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

10. 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)

11. 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
0.021
0.055
0.014
0.018
0.080
0.010
0.273
1.127
0.110
Flux (Jy)
Velocity (km/s)

12. 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
kms-1
M/D
M/Mpc
i (r)
71.6° °r
( r )
65.3° + 2.3°r – 0.23°r2

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

14. Results & Summary
A very accurate, geometric distance to NGC  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 NAG