We have used data taken as part of the Herschel infrared Galactic Plane survey ( Hi-GAL ) to study 3171 infrared-dark cloud ( IRDC ) candidates that were identified in the mid-infrared ( 8 \mu m ) by Spitzer ( we refer to these as ‘ Spitzer -dark ’ regions ) .
They all lie in the range l = 300 - 330 \degr and |b| \leq 1 ° .
Of these , only 1205 were seen in emission in the far-infrared ( 250–500 \mu m ) by Herschel ( we call these ‘ Herschel -bright ’ clouds ) .
It is predicted that a dense cloud will not only be seen in absorption in the mid-infrared , but will also be seen in emission in the far-infrared at the longest Herschel wavebands ( 250–500 \mu m ) .
If a region is dark at all wavelengths throughout the mid-infrared and far-infrared , then it is most likely to be simply a region of lower background infrared emission ( a ‘ hole in the sky ’ ) .
Hence , it appears that previous surveys , based on Spitzer and other mid-infrared data alone , may have over-estimated the total IRDC population by a factor \sim 2 .
This has implications for estimates of the star formation rate in IRDCs in the Galaxy .
We studied the 1205 Herschel -bright IRDCs at 250 \mu m , and found that 972 of them had at least one clearly defined 250- \mu m peak , indicating that they contained one or more dense cores .
Of these , 653 ( 67 per cent ) contained an 8- \mu m point source somewhere within the cloud , 149 ( 15 per cent ) contained a 24- \mu m point source but no 8- \mu m source , and 170 ( 18 per cent ) contained no 24- \mu m or 8- \mu m point sources .
We use these statistics to make inferences about the lifetimes of the various evolutionary stages of IRDCs .