We report the relative abundances of the three stable isotopes of silicon,
$^{28}$Si, $^{29}$Si and $^{30}$Si, across the Galaxy using the $v = 0, J = 1
\to 0$ transition of silicon monoxide. The chosen sources represent a range in
Galactocentric radii ($R_{\rm GC}$) from 0 to 9.8 kpc. The high spectral
resolution and sensitivity afforded by the GBT permit isotope ratios to be
corrected for optical depths. The optical-depth-corrected data indicate that
the secondary-to-primary silicon isotope ratios $^{29}{\rm Si}/^{28}{\rm Si}$
and $^{30}{\rm Si}/^{28}{\rm Si}$ vary much less than predicted on the basis of
other stable isotope ratio gradients across the Galaxy. Indeed, there is no
detectable variation in Si isotope ratios with $R_{\rm GC}$. This lack of an
isotope ratio gradient stands in stark contrast to the monotonically decreasing
trend with $R_{\rm GC}$ exhibited by published secondary-to-primary oxygen
isotope ratios. These results, when considered in the context of the
expectations for chemical evolution, suggest that the reported oxygen isotope
ratio trends, and perhaps that for carbon as well, require further
investigation. The methods developed in this study for SiO isotopologue ratio
measurements are equally applicable to Galactic oxygen, carbon and nitrogen
isotope ratio measurements, and should prove useful for future observations of
these isotope systems.
