Frequency coverage (300 MHz to 50 GHz) with the GBT
In Figures 2 and 3, we compare 1 GHz of passband at 230 GHz and 23 GHz
toward SgrB2(N), respectively. While these two passbands are NOT on
the same velocity scale, it can be seen that in Figure 2, the
high-frequency bands are so full of lines by comparison that it is
difficult, if not impossible, to reliably identify species simply due
to all the line confusion. On the other hand, in Figure 3, the low
frequency bands are relatively uncluttered. The prominent lines in
this band are mainly recombination lines of hydrogen and helium and
ammonia complexes.
Figure 4 presents a graph that represents the number of interstellar
molecules as a function of constituent atoms. There are 141 total
molecules. Notice that there is a high degree of isomerism (isomer
pairs are illustrated in orange and yellow, triads in blue),
suggesting that the formation of molecules is a quasi-random process
that gets modified by ensuing gas or grain chemistry. Low energy
transitions of large molecules fall in the range of 300 MHz to 50 GHz.
Such molecules are organic and are spatially widespread. Low energy
transitions of light molecules (e.g., H2D+) fall at much higher frequencies.
