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We explore indirect methods to detect Polycyclic Aromatic Hydrocarbons (PAHs)in gas-rich, absorption-selected galaxies at high redshift. We look at theoptical VLT/X-shooter observations of an intervening, extremely strong dampedLyman-alpha absorber (or ESDLA, with log(N(HI))>~21.7)) towards QSO SDSSJ1143+1420 at redshift, z(ESDLA)=2.323. Literature studies have shown that thisESDLA contains signatures of dust and diffuse molecular hydrogen and it wasspecifically chosen for our study due to its close spatial proximity (impactparameter, rho=0.6+/-0.3 kpc) with its associated galaxy. There is no directdetection of PAHs emission in the limited observations of infrared(IR)-spectraalong this sight-line. Hence, we use CLOUDY numerical simulation modelling toindirectly probe the presence of PAH in the ESDLA. We note that PAHs need to beincluded in the models to reproduce the observed column densities of warm H2and CI. Thus, we infer the presence of PAHs indirectly in our ESDLA, with anabundance of PAH/H = 10^(-7.046). We also measure a low 2175 A bump strength(E(bump)~0.03-0.19 mag) relative to star-forming galaxies by modellingextinction of QSO spectra by dust at the absorber rest-frame. This isconsistent with the low PAH abundance obtained indirectly using CLOUDYmodelling. Our study highlights the usage of CLOUDY modelling to indirectlydetect PAH in high-redshift gas-rich absorption-selected galaxies.

monthly notices of the royal astronomical society

Detecting PAHs in high-z galaxies in proxy: Modelling physical  conditions in an extremely strong damped Lyman-alpha absorber towards QSO  SDSS J1143+1420 at z=2.323