We have observed continuum emission at \lambda = 850 \mu m over \sim 4 square degrees of the Ophiuchus star-forming cloud using SCUBA on the JCMT , producing a submillimetre continuum map twenty times larger than previous Ophiuchus surveys .
Our sensitivity is 40 mJy beam ^ { -1 } , a factor of \sim 2 less sensitive than earlier maps .
Using an automated identification algorithm , we detect 100 candidate objects .
Only two new objects are detected outside the boundary of previous maps , despite the much wider area surveyed .
We compare the submillimetre continuum map with a map of visual extinction across the Ophiuchus cloud derived using a combination of 2MASS and R -band data .
The total mass in submillimetre objects is \approx 50 M _ { \odot } compared with \approx 2000 M _ { \odot } in observed cloud mass estimated from the extinction .
The submillimetre objects represent only 2.5 % of the cloud mass .
A clear association is seen between the locations of detected submillimetre objects and high visual extinction , with no objects detected at A _ { V } < 7 magnitudes .
Using the extinction map , we estimate pressures within the cloud from P / k \approx 2 \times 10 ^ { 5 } { cm } ^ { -3 } { K } in the less-extincted regions to P / k \approx 2 \times 10 ^ { 6 } { cm } ^ { -3 } { K } at the cloud centre .
Given our sensitivities , cold ( T _ { d } \approx 15 K ) clumps supported by thermal pressure , had they existed , should have been detected throughout the majority of the map .
Such objects may not be present at low A _ { V } because they may form only where A _ { V } > 15 , by some mechanism ( e.g. , loss of non-thermal support ) .