Context : The discovery of several clusters of red supergiants towards l = 24 \degr – 30 \degr has triggered interest in this area of the Galactic plane , where lines of sight are very complex and previous explorations of the stellar content were very preliminary .

Aims : We attempt to characterise the stellar population associated with the H ii region RCW 173 ( =Sh2-60 ) , located at l = 25 \aas@@fstack { \circ } 3 , as previous studies have suggested that this population could be beyond the Sagittarius arm .

Methods : We obtained UBV photometry of a stellar field to the south of the brightest part of RCW 173 , as well as spectroscopy of about twenty stars in the area .
We combined our new data with archival 2MASS near-infrared photometry and Spitzer /GLIMPSE imaging and photometry , to achieve more accurate characterisation of the stellar sources and the associated cloud .

Results : We find a significant population of early-type stars located at d = 3.0 kpc , in good agreement with the “ near ” dynamical distance to the H ii region .
This population should be located at the near intersection of the Scutum-Crux arm .
A luminous O7 II star is likely to be the main source of ionisation .
Many stars are concentrated around the bright nebulosity , where GLIMPSE images in the mid infrared show the presence of a bubble of excited material surrounding a cavity that coincides spatially with a number of B0-1 V stars .
We interpret this as an emerging cluster , perhaps triggered by the nearby O7 II star .
We also find a number of B-type giants .
Some of them are located at approximately the same distance , and may be part of an older population in the same area , characterised by much lower reddening .
A few have shorter distance moduli and are likely to be located in the Sagittarius arm .

Conclusions : The line of sight in this direction is very complex .
Optically visible tracers delineate two spiral arms , but seem to be absent beyond d \approx 3 kpc .
Several H ii regions in this area suggest that the Scutum-Crux arm contains thick clouds actively forming stars .
All these populations are projected on top of the major stellar complex signposted by the clusters of red supergiants .