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