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.

arXiv commentMNRAS in press

References in corpus (64)

• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III
• Indications of a spatial variation of the fine structure constant
• Molecules at z=0.89: A 4-mm-rest-frame absorption line survey toward PKS 1830-211
• Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts
• A precise and accurate determination of the cosmic microwave background temperature at z=0.89
• Constraints on changes in fundamental constants from a cosmologically distant OH absorber/emitter
• Redshifted Neutral Hydrogen 21cm Absorption toward Red Quasars
• New Limits on the Possible Variation of Physical Constants
• Redshifts of the Gravitational Lenses MG0414+0534 and MG0751+2716
• The complex molecular absorption line system at z=0.886 towards PKS1830-211
• Kinematics and physical conditions of HI in nearby radio sources. The last survey of the old Westerbork Synthesis Radio Telescope
• Where is the Molecular Hydrogen in Damped Lyman-Alpha Absorbers?
• Methods for Constraining Fine Structure Constant Evolution with OH Microwave Transitions
• Extinction Curves of Lensing Galaxies out to z=1
• A survey for redshifted molecular and atomic absorption lines - II. Associated HI, OH and millimetre lines in the z >~ 3 Parkes quarter-Jansky flat-spectrum sample
• A survey for redshifted molecular and atomic absorption lines I
• HST Observations and Models of The Gravitational Lens System MG 0414+0534
• The kinetic temperature of a molecular cloud at redshift 0.7: Ammonia in the gravitational lens B0218+357
• Does the fine structure constant vary? A detailed investigation into systematic effects
• 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
• A complete sample of 21-cm absorbers at z~1.3: Giant Metrewave Radio Telescope Survey Using MgII Systems
• Molecular hydrogen absorption systems in Sloan Digital Sky Survey
• Molecular gas at intermediate redshifts
• Using 21cm Absorption in Small Impact Parameter Galaxy-QSO Pairs to Probe Low-Redshift Damped and Sub-Damped Lyman-alpha System
• On the HI column density - radio source size anti-correlation in compact radio sources
• Illuminating the past 8 billion years of cold gas towards two gravitationally lensed quasars
• Detection of OH and wide HI absorption toward B0218+357
• OH+ and H2O+ absorption toward PKS1830-211
• GMRT mini-survey to search for 21-cm absorption in Quasar-Galaxy Pairs at z~0.1
• HI Absorption in High-Frequency Peaker Galaxies
• Redshifted HI and OH absorption in radio galaxies and quasars
• FLASH Early Science -- Discovery of an intervening HI 21-cm absorber from an ASKAP survey of the GAMA 23 field
• HI absorption in a gravitational lens at $z\sim 0.7645$
• 21-cm absorption from galaxies at z ~ 0.3
• Blind HI and OH absorption line search: first results with MALS and uGMRT processed using ARTIP
• A Search for Intrinsic HI 21 cm and OH 18 cm Absorption Toward Compact Radio Sources
• HI 21-cm absorption survey of quasar-galaxy pairs: Distribution of cold gas around z<0.4 galaxies
• The geometry effects of an expanding Universe on the detection of cool neutral gas at high redshift
• New molecular species at redshift z=0.89
• Detection of CH+, SH+, and their 13C- and 34S- isotopologues toward PKS1830-211
• HI and OH absorption in the lensing galaxy of MG J0414+0534
• The evolution of neutral hydrogen over the past 11 Gyr via HI 21 cm absorption
• Circumnuclear and infalling HI gas in a merging galaxy pair at z=0.123
• On the absence of molecular absorption in high redshift millimetre-band searches
• Revealing HI gas in emission and absorption on pc to kpc scales in a galaxy at z ~ 0.017
• Chlorine-bearing molecules in molecular absorbers at intermediate redshifts
• Ionisation of the atomic gas in redshifted radio sources
• Measuring Changes in the Fundamental Constants with Redshifted Radio Absorption Lines
• QSO photometric redshifts using machine learning and neural networks
• Discovery of OH absorption from a galaxy at z~0.05: implications for large surveys with SKA pathfinders
• All good things come in threes: the third image of the lensed quasar PKS1830-211
• Atomic and Molecular Absorption in Redshifted Radio Sources
• PKSB1740-517: An ALMA view of the cold gas feeding a distant interacting young radio galaxy
• A pilot search for extragalactic OH absorption with FAST
• A search for HI and OH absorption in z > 3 CO emitters
• ALMA Observations of Molecular Absorption in the Gravitational Lens PMN 0134-0931
• A Search for Molecular Gas in GHz Peaked Spectrum Radio Sources
• A search for high redshift molecular absorption lines toward millimetre-loud, optically faint quasars
• uGMRT search for cold gas at z~1-1.4 towards red quasars
• Broad, weak 21 cm absorption in an early type galaxy: spectral-line finding and parametrization for future surveys
• Intervening or associated? Machine learning classification of redshifted H I 21-cm absorption
• QSO redshift estimates from optical, near-infrared and ultraviolet colours
• A fourth HI 21-cm absorption system in the sight-line of MG J0414+0534: a record for intervening absorbers

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• Redshifts of radio sources in the Million Quasars Catalogue from machine learning
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