Astron. Astrophys. 363, 451-454 (2000)

4. Discussion and conclusions

With regard to the optical results, the present images show no strong evidence for a stellar association in either source or offset field. This is unsurprising as the observed region lies at the extreme southern edge of the Bridge. It is possible, however, that a few of the bluer objects observed in Fig. 1 with =19-22 mag. are stars associated with the young general field population of the Bridge (Demers & Battinelli 1998). Alternatively, such objects have the correct visual magnitude for blue horizontal branch stars at the distance of the Bridge. However, given the relatively young age of the Bridge (less than 200 Myr; Fujimoto & Murai 1984), combined with the fact that our observed position lies outside the haloes of both the SMC and LMC (Irwin et al. 1990) we believe this to be unlikely. A final alternative is that the blue stars may be foreground Galactic evolved blue horizontal branch stars. To distinguish between these various possibilities would require optical spectroscopy in order to determine stellar spectral types/luminosity classes and radial velocities.

Although disappointing, the lack of ¹²CO detection is perhaps not surprising. Towards 0311-7651, and only considering Bridge velocities from 100-350 km s^-1, KD99 determined an HI column density of 1.210²⁰ cm^-2, a peak temperature of 1.37 K and a brightness temperature integral of 65 K km s^-1. For the current data, assuming a SMC-like conversion factor from CO-H₂ of 610²¹ cm^-2 (K km s^-1)^-1, the limiting H₂ detectable column density is 3.610²⁰ V, where V is the width of the molecular line in km s^-1, which is typically 5 km s^-1 for clouds within the SMC (Israel et al. 1993). Hence, for there to have been a detection at the current position, the H₂/HI ratio would have to have exceeded 3V. Given that the H₂/HI ratio in the SMC is of the order 0.07 (Rubio et al. 1991), and assuming a similar ratio for the Bridge, it is clear that the current observations were of insufficient sensitivity to test this limit.

Future work in this area should concentrate on CO observations towards regions of higher HI column density within the Bridge. KD99 only sampled a few ICM regions for cold HI , and it may be that hydrogen in this phase, with associated CO, is present and detectable in other regions of the Bridge.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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