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
  We explore indirect methods to detect Polycyclic Aromatic Hydrocarbons (PAHs)
in gas-rich, absorption-selected galaxies at high redshift. We look at the
optical VLT/X-shooter observations of an intervening, extremely strong damped
Lyman-alpha absorber (or ESDLA, with log(N(HI))>~21.7)) towards QSO SDSS
J1143+1420 at redshift, z(ESDLA)=2.323. Literature studies have shown that this
ESDLA contains signatures of dust and diffuse molecular hydrogen and it was
specifically chosen for our study due to its close spatial proximity (impact
parameter, rho=0.6+/-0.3 kpc) with its associated galaxy. There is no direct
detection of PAHs emission in the limited observations of infrared(IR)-spectra
along this sight-line. Hence, we use CLOUDY numerical simulation modelling to
indirectly probe the presence of PAH in the ESDLA. We note that PAHs need to be
included in the models to reproduce the observed column densities of warm H2
and CI. Thus, we infer the presence of PAHs indirectly in our ESDLA, with an
abundance 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 modelling
extinction of QSO spectra by dust at the absorber rest-frame. This is
consistent with the low PAH abundance obtained indirectly using CLOUDY
modelling. Our study highlights the usage of CLOUDY modelling to indirectly
detect PAH in high-redshift gas-rich absorption-selected galaxies.
