74 MHz discrete HII absorption regions toward the inner Galaxy

At low radio frequencies (v less than or similar to 100 MHz), classical H II regions may become optically thick (optical depth tau >= 1) and can be observed as discrete absorption regions against the Galactic nonthermal background emission created by Galactic cosmic-ray electrons spiraling around magnetic fields. However, the historically poor angular resolution (> 30') of previous low-frequency surveys has limited such observations to the largest and nearest H II regions. The significantly enhanced resolution and surface brightness sensitivity of the 74 MHz system on the Very Large Array now allow for the detection of absorption regions on scale sizes of just a few arcminutes that can be readily identified with many more H II regions previously cataloged in emission at higher frequencies. These absorption measurements directly constrain the brightness temperature of the cosmic-ray synchrotron emission emanating from behind the H II regions based on reasonable physical assumptions. Many such observations could be used to map out the three-dimensional cosmic-ray emissivity in the Galaxy without resorting to a priori assumptions about Galactic structure. This measurement is unique to low-frequency radio astronomy. In this work we present 74 MHz observations in the region 26 degrees > l > -15 degrees, -5 degrees < b < 5 degrees; report the detection of 92 absorption features associated with known H II regions; and derive the brightness temperature of the Galactic cosmic-ray electron synchrotron emission emanating from the column behind these regions. For the 42 H II regions with known distances, the average emissivity of the column behind the H II region is derived. The 74 MHz emissivity values range between 0.3 and 1.0 K pc(-1) for a model assuming uniform distribution of emissivity. Methods for using this type of data to model the three-dimensional distribution of cosmic-ray emissivity and the possibility of using this method to break the H II region kinematic distance degeneracy are discussed.