Spectral principal component analysis of mid-infrared spectra of a sample of PG QSOs

TL;DR

This study applies spectral principal component analysis to mid-infrared spectra of PG QSOs, revealing key spectral features and their correlations with accretion rate, Eddington ratio, and star formation rate, enhancing understanding of quasar properties.

Contribution

First application of spectral PCA to mid-infrared spectra of PG QSOs, identifying eigenspectra and their links to physical parameters like accretion and star formation.

Findings

Eigenspectra account for 85.2% of spectral variation.

Mid-IR forbidden lines are suppressed at higher accretion rates.

Strong correlation between silicate feature ratio and accretion rate.

Abstract

A spectral principal component analysis (SPCA) of a sample of 87 PG QSOs at $z < 0.5$ is presented for their mid-infrared spectra from Spitzer Space Telescope. We have derived the first five eigenspectra, which account for 85.2\% of the mid-infrared spectral variation. It is found that the first eigenspectrum represents the mid-infrared slope, forbidden emission line strength and $9.7~\mu m$ silicate feature, the 3rd and 4th eigenspectra represent the silicate features at $18~ \mu m$ and $9.7~\mu m$, respectively. With the principal components (PC) from optical PCA, we find that there is a medium strong correlation between spectral SPC1 and PC2 (accretion rate). It suggests that more nuclear contribution to the near-IR spectrum leads to the change of mid-IR slope. We find mid-IR forbidden lines are suppressed with higher accretion rate. A medium strong correlation between SPC3 and PC1 (Eddington ratio) suggests a connection between the silicate feature at $18~\mu m$ and the Eddington ratio. For the ratio of the silicate strength at 9.7 $\mu m$ to that at 18 $\mu m$, we find a strong correlation with PC2 (accretion rate or QSO luminosity). We also find that there is a medium strong correlation between the star formation rate (SFR) and PC2. It implies a correlation between star formation rate and the central accretion rate in PG QSOs.