Hubble Space Telescope observations of nearby type 1 quasars. I. Characterization of the extended [O III] 5007 Å emission

Abstract

We use the Hubble Space Telescope to analyse the extended [O III] 5007 Å emission in seven bright radio-quiet type 1 quasars (QSO1s), focusing on the morphology and physical conditions of their extended Narrow-Line Regions (NLRs). We find NLRs extending 3-9 kpc, with four quasars showing roughly symmetrical structures ($b/a$=1.2-1.5) and three displaying asymmetric NLRs ($b/a$=2.4-5.6). When included with type 1 and type 2 AGNs from previous studies, the sizes of the extended [O III] regions scale with luminosity as $R_{\rm [O\,{\rm {\small III}}]}\sim L_{\rm [O\,{\rm {\small III}}]}^{0.5}$, consistent with photoionization. However, when analysed separately, type 1s exhibit a steeper slope ($\gamma _{1}$ = 0.57 $\pm$ 0.05) compared to type 2 AGNs ($\gamma _{2}$ = 0.48 $\pm$ 0.02). We use photoionization modelling to estimate the maximum NLRs sizes, assuming a minimum ionization parameter of log$(U)=-3$, an ionizing luminosity based on the $L_{\rm [O\,{\rm {\small III}}]}$-derived bolometric luminosity, and a minimum gas number density $n_{\rm H}\sim 100$ cm$^{-3}$, assuming that molecular clouds provide a reservoir for the ionized gas. The derived sizes agree well with direct measurements for a sample of type 2 quasars, but are underestimated for the current sample of QSO1s. A better agreement is obtained for the QSO1s using bolometric luminosities derived from the 5100 Å continuum luminosity. Radial mass profiles for the QSO1s show significant extended mass in all cases, but with less [O III]-emitting gas near the central AGN compared to QSO2s. This may suggest that the QSO1s are in a later evolutionary stage than QSO2s, further past the blow-out stage.