Context :

Aims : Characterizing the gas and dust properties prior to and in the neighborhood of active intermediate- to high-mass star formation .

Methods : Two Infrared Dark Clouds ( IRDCs ) – IRDC 19175-4 and IRDC 19175-5 – that are located in the vicinity of the luminous massive star-forming region IRAS 19175+1357 , but that remain absorption features up to 70 \mu m wavelength , were observed with the Plateau de Bure Interferometer in the 3.23 mm dust continuum as well as the N _ { 2 } H ^ { + } ( 1–0 ) and ^ { 13 } CS ( 2–1 ) line emission .

Results : While IRDC 19175-4 is clearly detected in the 3.23 mm continuum , the second source in the field , IRDC 19175-5 , is only barely observable above the 3 \sigma continuum detection threshold .
However , the N _ { 2 } H ^ { + } ( 1–0 ) observations reveal 17 separate sub-sources in the vicinity of the two IRDCs .
Most of them exhibit low levels of turbulence ( \Delta v \leq 1 km s ^ { -1 } ) , indicating that the fragmentation process in these cores may be dominated by the interplay of thermal pressure and gravity , but not so much by turbulence .
Combining the small line widths with the non-detection up to 70 \mu m and the absence of other signs of star formation activity , most of these 17 cores with masses between sub-solar to \sim 10 M _ { \odot } are likely still in a starless phase .
The N _ { 2 } H ^ { + } column density analysis indicates significant abundance variations between the cores .
Furthermore , we find a large CS depletion factor of the order 100 .
Although the strongest line and continuum peak is close to virial equilibrium , its slightly broader line width compared to the other cores is consistent with it being in a contraction phase potentially at the verge of star formation .
Based on the 3.23 mm upper limits , the other cores may be gravitationally stable or even transient structures .
The relative peak velocities between neighboring cores are usually below 1 km s ^ { -1 } , and we do not identify streaming motions along the filamentary structures .
Average densities are between 10 ^ { 5 } and 10 ^ { 6 } cm ^ { -3 } ( one to two orders of magnitude larger than for example in the Pipe nebula ) implying relatively small Jeans-lengths that are consistent with the observed core separations of the order 5000 AU .
Environmental reasons potentially determining these values are discussed .

Conclusions : These observations show that multiple low- to intermediate-mass low-turbulence starless cores can exist in the proximity of more turbulent active intermediate- to high-mass star-forming regions .
While masses and levels of turbulence are consistent with low-mass starless core regions , other parameters like the densities or Jeans-lengths differ considerably .
This may be due to environmental effects .
The quest for high-mass starless cores prior to any star formation activity remains open .