High Resolution Observations of OH Megamaser Galaxies

Ylva Pihlström (NRAO)J. Conway (OSO)

W. Baan (ASTRON)A. Polatidis (MPIfR)

H.-R. Klöckner (Univ. of Groningen)

MERLIN EVN

Early radio interferometry results

• Standard model:

Starburst heats dust grains FIR radiation FIR pumps a diffuse OH screen and seed photons stimulate unsaturated, low-gain maser emission

• Continuum and maser emission scales

• Early results confirmed this picture

Arp 220 (Norris et al 1985, Lonsdale et al 1994)

III Zw 35 (Montgomery & Cohen 1992)

MERLIN 0.28 arcsec resolution(Montgomery & Cohen 1992)

Maser peaks Continuumpeak

Global VLBI – III Zw 35

Single dish

VLBA

•Failed to detect continuum high amplification factors and standard model questioned (Diamond et al. 1998; Lonsdale et al 1999)

• Missing flux is a potential problem

III Zw 35

Diamond et al (1998) Trotter et al (1997)

IIIZw35 1667 MHz

• EVN+MERLIN to pick up a larger fraction of the total flux density• A smooth distribution of emission, with a velocity gradient• Enclosed mass within 22pc radius is 7x10^6 solar masses

Pihlström et al. 2001

50pc

Geometrical model

A 22pc radius torusInclination 30 deg.

Path length and brightness maximizedat tangent points

Pihlström et al. 2001

Pathlength

Brightness

EVN Global VLBI

Brightness

Monte Carlo models

Parra et al. 2003 in prep

Tilted torus with pc-scale clouds in Keplerian orbits

Velocity field

Parra et al. 2003 in prep

ModelData

Brightness distribution

Parra et al. 2003 in prep

Model Data

Radio continuum

The continuum distribution follows the OH maser emission

Pihlström et al. 2001

•Diffuse continuum luminosity agrees with radio luminosity estimated from the FIR luminosity => Starburst dominated

• Compact components are either individual RSN or super star clusters

Mrk231Klöckner et al. 2003

•Seyfert 1 galaxy with OHMM consistent with 65pc radius torus•Also a possible outflow component in the West

IRAS 12032+1707

Redshift z = 0.217Very broad velocity extent: 1500 km/s

•Observed with the VLBA in July 2002•100% of the flux density is recovered in the VLBA observations.•The emission is compact (100 x 100 pc)

100 pc

Darling & Giovanelli 2001

IRAS 12032+1707 VLBA results

Velocity gradients are apparent, but a disturbed field

•possibly two disks

•1667/1665MHz blending?

Moment 1

IRAS 14070+2505

Redshift z = 0.265Velocity extent 2000 km/s

•Observed with an 8 element VLBA array•Only 20% of the flux density is recovered

Darling & Giovanelli 2000

VLBA Arecibo

IRAS 14070+2505 VLBA results

Signs of a rotating disk with a gradient 8 km/s/pc.

Estimated enclosed mass within 25 pc 10^8 solar masses.

Summary

• VLBI observations demonstrate that the OHMM emission is confined to sub-kpc scales

• The velocity fields are ordered, with gradients

• Best explained by rotating torus structures

• Geometry can explain much of the emission characteristics

• Probing the OH gas at all angular resolutions is important to get a complete picture

• Our data supports the standard model (Baan 1985)