The Intrinsically X-ray Weak Quasar PHL 1811. II. Optical and UV Spectra and Analysis

TL;DR

This study presents optical and UV spectral analysis of the X-ray weak quasar PHL 1811, revealing unusual emission lines and a soft spectral energy distribution that affects line emission and cooling processes.

Contribution

It provides detailed optical and UV spectra of PHL 1811 and explains its unusual emission features through photoionization modeling considering its soft SED.

Findings

Weak high-ionization lines like CIV are observed.

Strong FeII and FeIII emission dominate the UV spectrum.

Unusual low-ionization lines and absence of forbidden lines are detected.

Abstract

This is the second of two papers reporting observations and analysis of the unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line quasar PHL 1811. The first paper reported that PHL 1811 is intrinsically X-ray weak, and presented a spectral energy distribution (SED). Here we present HST STIS optical and UV spectra, and ground-based optical spectra. The optical and UV line emission is very unusual. There is no evidence for forbidden or semiforbidden lines. The near-UV spectrum is dominated by very strong FeII and FeIII, and unusual low-ionization lines such as NaID and CaII H&K are observed. High-ionization lines are very weak; CIV has an equivalent width of 6.6A, a factor of ~5 smaller than measured from quasar composite spectra. An unusual feature near 1200A can be deblended in terms of Ly\alpha, NV, SiII, and CIII* using the blueshifted CIV profile as a template. Photoionization modeling shows that the unusual line emission can be explained qualitatively by the unusually soft SED. Principally, a low gas temperature results in inefficient emission of collisionally excited lines, including the semiforbidden lines generally used as density diagnostics. The emission resembles that of high-density gas; in both cases this is a consequence of inefficient cooling. PHL 1811 is very unusual, but we note that quasar surveys are generally biased against finding similar objects.