Relation between polycyclic aromatic hydrocarbon, Br$α$ and infrared luminosity of local galaxies observed with AKARI

Abstract

We produce a catalogue of polycyclic aromatic hydrocarbon (PAH) 3.3 $μ$m, Br$α$ and infrared luminosity ($L$(IR)) of 412 local galaxies, and investigate a relation between these physical parameters. We measure the PAH 3.3 $μ$m and Br$α$ flux using AKARI 2-5 $μ$m spectra and the $L$(IR) using the AKARI-all-sky-survey data. The $L$(IR) and redshift ranges of our sample are $L$(IR)=$10^{9.7-12.8}$L$_\odot$ and $z_{\rm spec}=0.002-0.3$, respectively. We found that the ratio of $L$(PAH 3.3 $μ$m) to $L$(IR) is constant at $L$(IR) $<$ $10^{11} \rm L_\odot$ whereas it decreases with the $L$(IR) at higher $L$(IR). Also, the ratio of $L$(Br$α$) to $L$(IR) decreases with the $L$(IR). The both $L$(PAH)/$L$(IR) and $L$(Br$α$)/$L$(IR) ratios are not strongly dependent on galaxy type and dust temperature. The relative weakness of the two ratios could be attributed to destruction of PAH, a lack of UV photons exciting PAH molecules or ionising hydrogen gas, extremely high dust attenuation, or active galactic nucleus contribution to the $L$(IR). Although we cannot determine the cause of the decreases of the luminosity ratios, a clear correlation between them implies that they are related with each other. The catalogue presented in our work will be available at the AKARI archive web page.