Issue |
A&A
Volume 377, Number 2, October II 2001
|
|
---|---|---|
Page(s) | 413 - 424 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20011107 | |
Published online | 15 October 2001 |
EVN and MERLIN observations of III Zw 35
A starburst continuum and an OH maser ring
1
Onsala Space Observatory, 439 92 Onsala, Sweden
2
Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire, SK11 9DL, UK
Corresponding author: Y. Pihlström, ylva@oso.chalmers.se
Received:
14
May
2001
Accepted:
31
July
2001
EVN and MERLIN observations of the 18 cm continuum and OH
maser emission in the luminous infrared galaxy III Zw 35
are presented. The continuum consists of two components. The first
component is made up of a number of compact sources, resembling the
types of objects observed in Arp 220 and
Mrk 273. In III Zw 35 the origin of these
compact components is unclear, and we discuss the possibility they
are individual young radio supernovae. Alternatively they could be
clusters or clumps of young supernova remnants. The second
continuum component is diffuse and has an angular size of
~ (230 pc) and
. We argue that this component is also powered by star
formation. From the FIR flux we estimate a star formation rate of
, which in turn implies a supernova
rate of
, sufficient to power the diffuse radio
emission. Turning to the OH maser emission we find, in agreement
with earlier VLBI observations of III Zw 35, that the
bright OH maser emission is concentrated in two compact
clumps. However, in addition we observe diffuse maser emission
which lies in a clear ring structure of radius
. The two compact maser features occur at the tangent
points of the ring. This geometry and their similar 1667/1665 MHz
line ratios suggests that the compact and diffuse masers appear
different because of path length effects and not pumping or other
physical conditions. A simple model with many clouds within a ring
seems able to fit the main properties of both the diffuse and
compact maser emission. We find a velocity gradient across the
diffuse maser emitting region which confirms rotation. From this
gradient we estimate a dynamical mass of
within
. OH absorption is also present just
outside the ring of maser emission. Both the OH maser emission and
the continuum is weaker on the Eastern side of the source. We argue
that this could be due to free-free absorption effects, which
implies gas with an emission measure of
.
Key words: galaxies: active / galaxies: individual: III Zw 35 / galaxies: ISM / masers / radio lines: galaxies / radio continuum: galaxies
© ESO, 2001
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