A Search for OH 18-cm Emission from Intermediate-Velocity Gas at High Galactic Latitudes

TL;DR

This study conducted a sensitive search for OH 18-cm emission in intermediate-velocity molecular clouds at high galactic latitudes but found no detections, suggesting these clouds have lower molecular content and density than previously predicted.

Contribution

The paper provides the first sensitive OH emission search in IVMCs, setting upper limits on molecular content and challenging existing models of their structure and composition.

Findings

No OH emission detected in 22 sightlines.

H2 column densities are over an order of magnitude lower than predictions.

Cloud densities are less than approximately 25 cm^-3.

Abstract

We present search results of 22 high latitude (b > 25 deg.) sightlines for OH 18-cm emission using the 305-m radio telescope at the Arecibo Observatory. These sightlines appear in neutral hydrogen emission at intermediate velocities (V_lsr values ranging from -90 to -20 km/s) and are predicted to have a sufficient molecular composition so as to be detectable in molecular emission. Such objects, known as Intermediate-Velocity Molecular Clouds (IVMCs), have historically been detected through 12CO emission. Recent studies indicate that IVMCs may be widespread in the Galaxy and have important implications for models of the interstellar medium and star formation. However, we report non-detections of OH emission toward the 22 sightlines and provide stringent upper limits on the OH column density. Using available HI and Av data in combination with existing state-of-the-art PDR models, we estimate H2 column densities and find that they are more than an order of magnitude lower than the predicted values. We also find that the hydrogen volume density of these clouds is less than roughly 25 per cubic centimeter. In addition, we discuss the known IVMCs with previous 12CO detections in the context of the PDR models. Our analysis of these clouds indicates that the structure of molecular material in IVMCs is morphologically clumpy. These results motivate the need for future sensitive, on-the-fly searches (rather than targeted searches) for CO emission from IVMCs with of order roughly 1' resolution. High angular resolution (1') HI and Av data will also be helpful to better constrain the structure and composition of IVMCs.