A pilot search for extragalactic OH absorption with FAST

Author:

Zheng Zheng1,Li Di123ORCID,Sadler Elaine M456ORCID,Allison James R67ORCID,Tang Ningyu1

Affiliation:

1. National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, China

2. University of Chinese Academy of Sciences, Beijing 100049, China

3. NAOC-UKZN Computational Astrophysics Centre, University of KwaZulu-Natal, Durban 4000, South Africa

4. Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia

5. ATNF, CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia

6. ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)

7. Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Rd., Oxford OX1 3RH, UK

Abstract

ABSTRACT OH absorption is currently the only viable way to detect OH molecules in non-masing galaxies at cosmological distances. There have been only six such detections at z > 0.05 to date and so it is hard to put a statistically robust constraint on OH column densities in distant galaxies. We carried out a pilot OH absorption survey towards eight associated and one intervening H i 21-cm absorbers using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We were able to constrain the OH abundance relative to H i ([OH]/[H i]) to be lower than 10−6 ∼ 10−8 for redshifts z∈ [0.1919, 0.2241]. Although no individual detection was made, stacking three associated absorbers free of RFI provides a sensitive OH column density 3σ upper-limit $\sim 1.57 \times 10^{14} (T_x^{\rm OH}/10\,\mathrm{ K})(1/f_\mathrm{ c}^{\rm OH})\mathrm{ cm}^{-2}$, which corresponds to a [OH]/[H i] < 5.45 × 10−8. Combining with archival data, we show that associated absorbers have a slightly lower OH abundance than intervening absorbers. Our results are consistent with a trend of decreasing OH abundance with decreasing redshift.

Funder

National Natural Science Foundation of China

Chinese Academy of Sciences

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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