Recent Progress on a New Geometric Distance to NGC
4258
Liz Humphreys (CfA)
Alice Argon, Lincoln Greenhill, Jim Moran & Mark Reid
NGC4258
Astronomy Picture of the Day, April 17 2003
Credit: Slotnick, Slotnick & Block
10 000 ly
Current PictureClaussen & 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 ~30asyr-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
121 TNN epochsepochsD
Trotter
Spectra
Time (days)
Blue = max flux Red = min flux
Vel
oci
ty (
km/s
)
Herrnstein (1997)
b
ec
f
jg
kl
op
mn
Relative RA (mas)
Off
set
Rel
ativ
e D
ec (
mas
) Sky Positions
New feature @ 1562 kms-1
Rotation Curve
Distance along major axis (mas)
Vel
ocity
(km
s-1)
highv highv
V
highv
sys
Measure x,y, vlos, alos,& proper motions
yx ,
vlos2 M/Dhighv M/D
dvlos/d [M/Dsys3]1/2
alos v2/ Dsys D M
sys
Acceleration Distance
or
~ vt/D (M/D)1/2/Dsys1/2
.D M
Proper Motion Distance
Accelerations&
Proper MotionsPrevious 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=data Blue=model Red=residuals
vlos
(kms-
1)
vlos
(kms-1)
alos
(kms-1yr-
1)
alos
(kms-1yr-
1)
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 9.074 0.033 Velocity (km/s)
Flu
x (J
y)
vlos
(kms-1)
vlos
(kms-
1)
alos
(kms-1yr-1)
alos
(kms-1yr-1)
1305.79 0.021 0.055 0.014
1306.49 0.018 0.080 0.010
1308.46 0.273 1.127 0.110
Red Features
Green=data Blue=model Red=residuals
Velocity (km/s)
Flu
x (J
y)
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