LeMMINGs IV: The X-ray properties of a statistically-complete sample of the nuclei in active and inactive galaxies from the Palomar sample

TL;DR

This study presents Chandra X-ray observations of 213 nearby galaxies from the Palomar sample, revealing the X-ray properties of nuclei across various galaxy types and constructing an X-ray luminosity function to understand their activity levels.

Contribution

It provides the first comprehensive X-ray analysis of a statistically complete sample of nearby galaxies, including both active and inactive types, and explores their nuclear activity and luminosity functions.

Findings

X-ray emission detected in 150/213 galaxies, including most Seyferts and LINERs.

The galaxy XLF follows a single power-law with slope -0.54 ± 0.06.

Eddington ratios are higher in Seyferts compared to LINERs, ALGs, and HII galaxies.

Abstract

All 280 of the statistically-complete Palomar sample of nearby (<120 Mpc) galaxies dec > 20 degrees have been observed at 1.5 GHz as part of the LeMMINGs e-MERLIN legacy survey. Here, we present Chandra X-ray observations of the nuclei of 213 of these galaxies, including a statistically-complete sub-set of 113 galaxies in the declination range 40 degrees to 65 degrees. We observed galaxies of all optical spectral types, including 'active' galaxies (e.g., LINERs and Seyferts) and 'inactive' galaxies like HII galaxies and absorption line galaxies (ALG). The X-ray flux limit of our survey is 1.65$\times$10$^{-14}$~erg s$^{-1}$ cm$^{-2}$ (0.3$-$10 keV). We detect X-ray emission coincident within 2-arcsec of the nucleus in 150/213 galaxies, including 13/14 Seyferts, 68/77 LINERs, 13/22 ALGs and 56/100 HII galaxies, but cannot completely rule out contamination from non-AGN processes in sources with nuclear luminosities <10$^{39}$ erg s$^{-1}$. We construct an X-ray Luminosity function (XLF) and find that the local galaxy XLF, when including all AGN types, can be represented as a single power-law of slope $-0.54 \pm 0.06$. The Eddington ratio of the Seyferts is usually 2-4 decades higher than that of the LINERs, ALGs and HII galaxies, which are mostly detected with Eddington ratios <10$^{-3}$. Using [O III] line measurements and BH masses from the literature, we show that LINERs, HII galaxies and ALGs follow similar correlations to low luminosities, suggesting that some 'inactive' galaxies may harbour AGN.