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ALMACAL V : absorption-selected galaxies with evidence for excited ISMs.

Klitsch, A. and Zwaan, M. A. and Péroux, C. and Smail, I. and Oteo, I. and Popping, G. and Swinbank, A. M. and Ivison, R. J. and Biggs, A. D. (2018) 'ALMACAL V : absorption-selected galaxies with evidence for excited ISMs.', Monthly notices of the Royal Astronomical Society : letters., 482 (1). L65-L69.


Gas-rich galaxies are selected efficiently via quasar absorption lines. Recently, a new perspective on such absorption-selected systems has opened up by studying the molecular gas content of absorber host galaxies using ALMA CO emission line observations. Here, we present an analysis of multiple CO transitions (L CO ∼ 109 K km s−1) in two z ∼ 0.5 galaxies associated with one Ly α absorber towards J0238+1636. The CO spectral line energy distributions (CO SLEDs) of these galaxies appear distinct from that of typical star-forming galaxies at similar redshifts and is comparable with that of luminous infrared galaxies or active galactic nucleus (AGN). Indeed, these galaxies are associated with optically identified AGN activity. We infer that the CO line ratios and the αCO conversion factor differ from the Galactic values. Our findings suggest that at least a fraction of absorption-selected systems shows interstellar medium conditions deviating from those of normal star-forming galaxies. For a robust molecular gas mass calculation, it is therefore important to construct the CO SLED. Absorption line selection identifies systems with widely distributed gas, which may preferentially select interacting galaxies, which in turn will have more excited CO SLEDs than isolated galaxies. Furthermore, we raise the question whether quasar absorbers preferentially trace galaxy overdensities.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society: Letters © 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:24 September 2018
Date deposited:06 December 2018
Date of first online publication:09 October 2018
Date first made open access:06 December 2018

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