The Very Small Array (VSA) has been used to survey the l ~ 27° to
region of the Galactic plane at a resolution of 13arcmin. This l-range
covers a section through the Local, Sagittarius and the Cetus spiral
arms. The survey consists of 44 pointings of the VSA, each with an rms
sensitivity of ~90mJybeam-1. These data are combined in a
mosaic to produce a map of the area. The majority of the sources within
the map are HII regions.
The main aim of the programme was to investigate the anomalous radio
emission from the warm dust in individual HII regions of the survey.
This programme required making a spectrum extending from GHz frequencies
to the far-infrared (FIR) IRAS frequencies for each of nine strong
sources selected to lie in unconfused areas. It was necessary to process
each of the frequency maps with the same u, v coverage as was used for
the VSA 33GHz observations. The additional radio data were at 1.4, 2.7,
4.85, 8.35, 10.55, 14.35 and 94GHz in addition to the 100, 60, 25 and
12μm IRAS bands. From each spectrum the free-free, thermal dust and
anomalous dust emission were determined for each HII region. The mean
ratio of 33GHz anomalous flux density to FIR 100μm flux density for
the nine selected HII regions was ΔS(33GHz)/S(100μm) = 1.10 +/-
0.21 × 10-4. When combined with six HII regions
previously observed with the VSA and the Cosmic Background Imager, the
anomalous emission from warm dust in HII regions is detected with a
33GHz emissivity of 4.65 +/- 0.40μK (MJysr-1)-1
(11.5σ). This level of anomalous emission is 0.3 to 0.5 of that
detected in cool dust clouds.
A radio spectrum of the HII region anomalous emission covering GHz
frequencies is constructed. It has the shape expected for spinning dust
composed of very small grains. The anomalous radio emission in HII
regions is on average 41 +/- 10 per cent of the radio continuum at
33GHz. Another result is that the excess (i.e. non-free-free) emission
from HII regions at 94GHz correlates strongly with the 100μm
emission; it is also inversely correlated with the dust temperature.
Both these latter results are as expected for very large grain dust
emission. The anomalous emission on the other hand is expected to
originate in very small spinning grains and correlates more closely with
the 25μm emission.