Towards a 3% Geometric Distance to NGC 4258A. Argon, L. Greenhill, M. Reid, J. Moran & E. Humphreys (Harvard-Smithsonian CfA)

20 April 2007

Adam Trotter (UNC-Chapel Hill)

Palomar Observatory

J. Kagaya (Hoshi No Techou)

The Host Galaxy• NGC 4258 (M106)

– Barred spiral SABbc– Seyfert 1.9, D=7.2 Mpc

• “Anomalous spiral arms” detected in Hα and radio continuum

• Recent Chandra, XMM, Spitzer and other images highlight the dual nature of spiral arms – normal + jet impinging on ISMPalomar ObservatoryWilson, Yang & Cecil 2001

http://chandra.harvard.edu/photo/2007/ngc4258/

Angular resolution = 200 Angular resolution = 200 as (0.006 pc at 7.2 Mpc)as (0.006 pc at 7.2 Mpc)

Spectral resolution < 1 km sSpectral resolution < 1 km s-1-1

VLBAVLBA

H2O Masers

• Conditions required– Dense (108 - 1010 cm-3)– Warm (300 - 800 K)

– Quiescent (Δv ~ ΔvD)

• Masers trace “hotspots” in underlying structures– Compact (θ < 1 mas)

– High surface brightness (TB ~ 107 - 1015 K)

– Yield θx, θy, vlos, dθx/dt, dθy/dt, dvlos/dt

yx , yx , yx ,

Greenhill et al. 1995, A&A 304, 21

Nobeyana 40-m Telescope

Owens Valley Radio Observatory, 40-m antenna

The Nuclear Megamaser

• Discovered by Claussen, Heiligman & Lo (1984)– Centered on galactic

systemic vsys = 470 km s-1

– Isotropic Luminosity ~120 LA

• Nakai et al. (1993) discovered emission at 1000 km s-1

• Greenhill et al. 1995: VLBI Observations of systemic emission show linear structure. Secular velocity drift implies centripetal acceleration in a flat disk.

The Big Picture

• Miyoshi et al. (1995) present VLBI images of full velocity range of maser emission– Systemic emission in

narrow band S of radio continuum (jet)

– High-velocity emission distributed 0.3 pc to either side

– High-velocity emission exhibits nearly perfect Keplerian rotation curve

Global Model Fits

• Assume warped disk (position angle and inclination change as functions of radius)

• Systemic velocity v0 = 4793 km s-1

• Central Mass M0=3.780.01x107 MA

• Position of disk center determined to 30μas• Plummer model of central cluster would require central

density ρc > 5.7x1011 MA pc-3 : unstable to collapse/disruption in lifetime of galaxy

• Massive disk model would imply densities 10-1000x too great to support water maser emission

• Must be a supermassive black hole!

Herrnstein, Moran, Greenhill & Trotter, ApJ 2005

TO OBSERVER

0.1pcObserver’s view

Top View

High

Bowl

Redshiftedmasers

Blueshiftedmasers

SystemicMasers Courtesy CfA Maser Group, Humphreys, et al. (in prep)

NGC4258

• Inclination warp (flattening of high velocity rotation curve): 8° across reds

• Position angle warp (declinations): 9° across reds

• Disk obscures much of central region

• Systemic masers in bowl on front side of disk

0.1pc

Warp is Well-Characterized

Natural explanation for:• Faint blue high velocity

feature (free-free absorption along the line-of-sight)

• Strongest red high velocity feature ~1306 km s-1 (where i=90º)

• Strongest systemic maser flares at ~490 km s-1 (optimum coherence path)

• X-ray absorbing layer (nH ~ 1023 cm-2) radius = 0.29 pc where disk crosses line-of-sight (Fruscione et al. 2005)

Top View

High

Bowl

0.1pc

How to Get a Distance? Measure accelerations & proper motions

vlos2 M/Dhi 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

11sys yr s km 0.3 9.3 v

-1x yr μas 1 31.5

Herrnstein et al. 1999

The Result• Acceleration Distance:

Mpc2.02.7acc D

• Proper Motion Distance:

Mpc2.01.7pm D

Including all systematic model uncertainties, a 5% geometric distance measurement:

Mpc3.02.7 D

Comparison to Other MeasurementsD = 7.2 0.3 Mpc

• H-band Tully Fisher (Aaronson et al. 1982)D = 7.1 1.1 Mpc• Blue Tully Fisher (Richter & Huchtmeier 1984)D = 7.9 1.8 Mpc• Luminosity Class Estimate (Rowan-Robinson 1985)D = 8.4 2.2 Mpc• Cepheid VariablesD = 8.1 0.4 Mpc (Maoz et al. 1999 - 15 Cepheids)

D = 7.8 0.3 0.5 Mpc (Newman et al. 2001 - same 15 Cepheids)

D = 7.4 Mpc (Macri et al 2006 - 300 Cepheids)

All these depend on Cepheid calibrations

• 18 epochs of VLBI observations, 1997 March – 2000 August, using VLBA, VLA Phased Array and Effelsberg 100-m antenna

• Wider bandwidth → Detection of new high-velocity features 30% closer to disk center

• Quantification of maser disk thickness, h~5 μas, will lead to more accurate models and estimates of mass accretion rate – ADAF?

• More robust genetic modeling algorithm to refine warped disk shape and reduce systematic uncertainties

• Inclusion of possible spiral structure in high-velocity masers, and ellipticity in systemic maser orbits

• HST observations (in progress) of Cepheids in NGC4258 to provide independent calibration of Cepheid distance scale

• Ultimately, a new measurement of H0, though at a distance of 7.2 Mpc, unmodeled local velocity field perturbations may limit accuracy of the result.

Towards a 3% Distance MeasurementArgon et al. 2007 (in preparation)

Mai: Les Tres Riches Heures du Duc de Berry, c. 1410

The May jaunt, a pageant celebrating the "joli mois de Mai" in which one had to wear green garments known as livree de mai. The riders are young noblemen and women, with princes and princesses.