The first spectral line surveys searching for signals from the dark ages

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Table 14:

Estimated largest possible absorption intensity $T_{\rm b}$ at different observing frequencies for *optically thick absorption lines* assuming that the background temperature is the CMB.
Obs. Freq.	Max intensity $\mid T_{\rm b} \mid$
	$T_{\rm ex}$ = 0	$T_{\rm ex}$ = 0.90 $\times$ $T_{\rm CMB}$	$T_{\rm ex}$ = 0.98 $\times$ $T_{\rm CMB}$
[GHz]	[mK]	[mK]	[mK]
1	2700	270	54
50	1700	250	51
100	1000	210	42
200	290	97	21
300	73	33	7.5
400	17	9.1	2.2
500	3.6	2.2	0.6
600	0.74	0.5	0.1

Notes. We have calculated this using Eq. (23) and $T_{\rm ex}$ = 0, $T_{\rm ex}$ = 0.90 $\times$ $T_{\rm CMB}$ and $T_{\rm ex}$ = 0.98 $\times$ $T_{\rm CMB}$ . Note that these values are redshift independent.

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