Download Into the sub-mm

Meeting report
W
ith the recent introduction of
SCUBA (Submillimetre Common
User Bolometer Array) on the James
Clerk Maxwell Telescope, ground-based submillimetre astronomy has finally come of age.
With an increase in data collection rate of
almost 10 000 compared to its single-pixel predecessor, SCUBA represents a bigger leap forward than was seen almost 15 years ago with
the revolution brought about by the first
infrared arrays. SCUBA is both a camera and a
highly sensitive photometer and the audience
of well over 100 at the RAS Specialist Discussion Meeting on 12 December was treated to
an overview of the exciting new results bursting onto the astronomical scene. Thirteen
speakers covered a range of topics, and while
space limitations prevent a discussion of all
talks in this article, a more complete report of
the meeting will be found in The Observatory.
Derek Ward-Thompson (Royal Observatory
Edinburgh) began the day with a review of the
earliest phases of star formation, the Class 0
sources. Sub-mm observations over recent years
have enabled significant progress to be made in
the understanding of low-mass star formation.
Starless cores
Much of the current interest centres around the
pre-stellar stage, the phase in which a gravitationally bound core has formed in a molecular
cloud, but no central hydrostatic protostar
exists yet within the core. The significance of
pre-stellar cores is that they constrain the initial
conditions of protostellar collapse; previous
work has shown that some have flat inner radial density profiles which may be consistent with
a mechanism for magnetic support, with evolution by ambipolar diffusion.
Forty-four of the starless cores from the list of
Myers’ ammonia cores have already been studied using SCUBA and the result is that more
cores are detected than would be predicted by
simple ambipolar diffusion models. Hence basic
assumptions of the models may have to be
revised, such as departures from cylindrical
symmetry, or significant turbulent velocity
fields within the pre-collapse cores. This theme
was expanded by Anja Visser (Mullard Radio
Astronomy Observatory) who reported on a
pilot study of a SCUBA survey of Lynds Class 6
Clouds. The goals of their unbiased survey is to
constrain star formation models by identifying
complete samples of protostellar condensations
for further study so as to measure star formation efficiencies, determine mass accretion rates
and the minimum size/mass of a cloud needed
for star formation to take place, and to establish statistics and hence lifetimes of the various
star formation phases. The results of the pilot
study already show the validity of this approach
and the sample will be extended to achieve a
statistically significant unbiased measure.
February 1998 Vol 39
Into
the
sub-mm
Ian Robson reports on the first
results from a revolutionary
new instrument, presented in
December 1997.
Glenn White (Queen Mary and Westfield)
showed spectacular images from a large collaboration working on the Eagle Nebula (M16).
The emission from dust seen in all the SCUBA
images matches closely the dust obscuration in
the famous HST picture, apart from bright
points of emission at the tips of the fingers.
Although it is too early to say conclusively
what the emission signifies, it is possibile that
these mark the earliest phase of star formation.
Modelling of the chemistry and dynamics of
the finger-tip condensations has begun.
The final presentation of the morning was by
Ben Zuckerman (University of California Los
Angeles) who showed the results of very deep
SCUBA imaging of four nearby main sequence
stars. The results for all four (Beta Pictoris,
Vega, Fomalhaut and Epsilon Eridani) were all
spectacular and are almost certainly the highlight of the SCUBA work to date. The most
spectacular image is that of Fomalhaut, where
a dust torus is clearly seen at the radius of the
Kuiper Belt in the Solar System. This suggests
that the inner zone may have been swept clear
of dust by planets. If correct, it will be a major
discovery: planet-sweeping is the best-favoured
of the limited possibilities for the observations.
The other images are also intriguing in terms of
planetary formation and the results have been
submitted to Nature (Holland et al. 1998).
The afternoon session was devoted to extragalactic submillimetre astronomy; James Dunlop (Edinburgh) gave an overview of the significance that SCUBA will hold for cosmology.
Recent advances in optical cosmology have
strongly enhanced the importance of performing observational cosmology at sub-millimetre
wavelengths. The sub-millimetre range is
uniquely suited to detecting major starburst
activity, and, due to the way the k-correction
kicks in and the thermal nature of the dust
emitting spectrum, a star-forming galaxy is as
likely to be detected at a redshift of 2 as it is at
10. Therefore the early universe is transparent
to SCUBA as far as massive galaxies undergoing major star formation are concerned.
SCUBA has already detected a number of previously-known high-redshift objects and the
most recent results (obtained the night before
the talk) of a large sample of high-redshift
radio galaxies show that extensive star formation is probably much more widespread than
previously suspected. The second approach to
the study of galaxy evolution is to undertake
unbiased deep sub-millimetre surveys of the
sky: the first will commence in the New Year.
Unknown galaxies
Andrew Blain (MRAO) followed with a discussion of the results of a SCUBA pilot survey for
previously unknown galaxies. The results and
implications of Smail, Ivison and Blain’s observations of distant lensing clusters, revealing the
first sub-millimetre selected objects (including a
redshift 2 galaxy), is that there are far more than
predicted from other wavelengths. A further factor, often overlooked, was discussed. This is
source confusion, a major aspect for deep submillimetre surveys. Andrew showed that this will
be a major problem for facilities with limited
angular resolution such as FIRST and ISO. Its
sensitivity and angular resolution make SCUBA
the premier instrument for cosmology for the
next decade, only superseded by the eventual
introduction of the millimetre array(s).
Coming closer to home, Paul Alton (Cardiff)
showed some brilliant images of 450 µm and
850 µm dust emission of the nearby edge-on
galaxy NGC 891. The data are sufficiently sensitive to trace dust emission from 2⁄3 of the optical disk and also to detect dust chimneys escaping from the main absorption layer up to
z-heights of 2 kpc. At both 450 µm and
850 µm, peak emission is associated with the
nucleus but there are also two bright secondary
maxima, on either side of the nucleus, which
probably constitute the limb of a ring with
radius 4 kpc. The sub-millimetre emission
resembles the distribution of molecular rather
than atomic hydrogen. A comparison between
the major axis profile at 850 µm with the corresponding profile at 60 µm (IRAS HIRES
data) implies that cold dust, rather than warm
grains, predominates at larger radii. Steve
Eales (Cardiff) showed the great promise of
SCUBA for investigating the general question
of the dust content of spiral galaxies, long
plagued by problems in determination of optical extinction measurements. Even in the pilot
project, 17 galaxies have been observed, with
dramatic improvements over previous work. ●
Prof. E I Robson is Director of the Joint Astronomy
Centre and the JCMT, University Park, Hilo,
Hawaii. Further information about SCUBA and the
observing programmes underway can be found at
“http://www.jach.hawaii.edu”.
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