DI in the outer Galaxy

TL;DR

This study conducted a deep radio search for deuterium in the outer Galaxy, detecting a tentative signal that could inform understanding of deuterium abundance and galactic chemical evolution.

Contribution

It presents the first sensitive search for DI at 327 MHz in the outer Galaxy, with implications for primordial deuterium levels and galactic chemical processes.

Findings

Detected a low significance DI feature consistent with previous reports.

Implied D/H ratio comparable to pre-solar deuterium abundance.

Results support low D/H measurements in high-redshift systems.

Abstract

We report on a deep search with the Westerbork Synthesis Radio Telescope towards the galactic anticenter for the 327 MHz hyperfine transition of DI. This is a favorable direction for a search because: (i) the HI optical depth is high due to velocity crowding; (ii) the observed molecular column density is low (implying that most of the deuterium would probably be in atomic form, rather than in HD); and (iii) the stellar reprocessing should be minimal. Our observations are about a factor of two more sensitive than previous searches for DI in this direction. We detect a low significance (about 4 sigma) feature, consistent in both amplitude and center frequency with an emission feature reported previously (Blitz & Heiles 1987). If this is the DI line, then the implied N_D/N_H of 3.9+/-1.0 x 10^-5 is comparable to the inferred pre-solar deuterium abundance. Our observation is consistent with the recent low measurements of D/H towards high-redshift Lyman-limit systems. On the other hand, if the reports of high DI abundance (about 24 x 10^-5) in such systems are confirmed, then our observations imply that even in regions of reduced star formation within the outer Galaxy, the DI abundance has been reduced by a factor of about 6 from the primordial abundance.