On the scarcity of redshifted OH and millimetre-band molecular absorption

Abstract

Despite much searching, redshifted decimetre and millimetre-band absorption by molecular gas remains very rare, limited to just six systems at z > 0.05. Detection of these transitions can yield precise diagnostics of the conditions of the star forming gas in the earlier Universe, the hydroxyl (OH) radical being of particular interest as in the 18-cm ground state there are four different transitions located close to HI 21-cm and thus detectable with the Square Kilometre Array and its pathfinders. The four transitions of OH have very different dependences on the fundamental constants, thus having much potential in testing for any evolution in these over large look-back times. By collating the photometry in a uniform manner, we confirm our previous hypothesis that the normalised OH absorption strength is correlated with the optical--near-infrared red colour of the sight-line. Applying this to the published searches, we find that all, but one (J0414+054), have simply not been searched sufficiently deeply. We suggest that this is due to the standard selection of sources with reliable optical redshifts introducing a bias against those with enough dust with which to shield the molecular gas. For the single source searched to sufficient depth, we have reason to suspect that the high degree of reddening arises from another system along the sight-line, thus not being inconsistent with our hypothesis. We also show that the same optical redshift bias can account for the scarcity of millimetre-band absorption.

MNRAS in press

References in corpus (23)

• Where is the Molecular Hydrogen in Damped Lyman-Alpha Absorbers?
• Extinction Curves of Lensing Galaxies out to z=1
• A survey for redshifted molecular and atomic absorption lines I
• VLT/UVES observations of extremely strong intervening damped Lyman-alpha systems: Molecular hydrogen and excited carbon, oxygen and silicon at log N(HI)=22.4
• Molecular hydrogen absorption systems in Sloan Digital Sky Survey
• Illuminating the past 8 billion years of cold gas towards two gravitationally lensed quasars
• OH+ and H2O+ absorption toward PKS1830-211
• HI Absorption in High-Frequency Peaker Galaxies
• FLASH Early Science -- Discovery of an intervening HI 21-cm absorber from an ASKAP survey of the GAMA 23 field
• A Search for Intrinsic HI 21 cm and OH 18 cm Absorption Toward Compact Radio Sources
• Blind HI and OH absorption line search: first results with MALS and uGMRT processed using ARTIP
• HI 21-cm absorption survey of quasar-galaxy pairs: Distribution of cold gas around z<0.4 galaxies
• New molecular species at redshift z=0.89
• HI and OH absorption in the lensing galaxy of MG J0414+0534
• Detection of CH+, SH+, and their 13C- and 34S- isotopologues toward PKS1830-211
• The evolution of neutral hydrogen over the past 11 Gyr via HI 21 cm absorption
• Ionisation of the atomic gas in redshifted radio sources
• QSO photometric redshifts using machine learning and neural networks
• All good things come in threes: the third image of the lensed quasar PKS1830-211
• Atomic and Molecular Absorption in Redshifted Radio Sources
• A pilot search for extragalactic OH absorption with FAST
• uGMRT search for cold gas at z~1-1.4 towards red quasars
• Intervening or associated? Machine learning classification of redshifted H I 21-cm absorption

Cited by in corpus (2)

• Redshifts of radio sources in the Million Quasars Catalogue from machine learning
• New constraints on the values of the fundamental constants at a look-back time of 7.3 Gyr