We present a measurement of the mean intensity of the hydrogen-ionizing background radiation field at low redshift using 906 Ly \alpha absorption lines in 151 quasar spectra from the archives of the Faint Object Spectrograph ( FOS ) on the Hubble Space Telescope ( HST ) .
Using a maximum likelihood technique and the best estimates possible for each QSO ’ s Lyman limit flux and systemic redshift , we find J ( \nu _ { 0 } ) = 7.6 ^ { +9.4 } _ { -3.0 } \times 10 ^ { -23 } ergs s ^ { -1 } cm ^ { -2 } Hz ^ { -1 } sr ^ { -1 } at 0.03 < z < 1.67 .
This is in good agreement with the mean intensity expected from models of the background which incorporate only the known quasar population .
When the sample is divided into two subsamples , consisting of lines with z < 1 and z > 1 , the values of J ( \nu _ { 0 } ) found are 6.5 ^ { +38. } _ { -1.6 } \times 10 ^ { -23 } ergs s ^ { -1 } cm ^ { -2 } Hz ^ { -1 } sr ^ { -1 } , and 1.0 ^ { +3.8 } _ { -0.2 } \times 10 ^ { -22 } ergs s ^ { -1 } cm ^ { -2 } Hz ^ { -1 } sr ^ { -1 } , respectively , indicating that the mean intensity of the background is evolving over the redshift range of this data set .
Relaxing the assumption that the spectral shapes of the sample spectra and the background are identical , the best fit HI photoionization rates are found to be 6.7 \times 10 ^ { -13 } s ^ { -1 } for all redshifts , and 1.9 \times 10 ^ { -13 } s ^ { -1 } and 1.3 \times 10 ^ { -12 } s ^ { -1 } for z < 1 and z > 1 , respectively .