A New Compton-thick AGN in our Cosmic Backyard: Unveiling the Buried Nucleus in NGC 1448 with NuSTAR

TL;DR

This study uncovers a heavily obscured, low-luminosity Compton-thick AGN in NGC 1448 using multiwavelength data, including NuSTAR X-ray observations, optical spectroscopy, and MIR imaging, revealing its buried nucleus and confirming its active galactic nucleus nature.

Contribution

First broadband X-ray spectral analysis of NGC 1448 revealing its Compton-thick AGN nature and multiwavelength confirmation of its active nucleus.

Findings

NGC 1448 hosts a Compton-thick AGN with N_H > 2.5 x 10^24 cm^-2.

The AGN has a low intrinsic 2-10 keV luminosity (~3.5-7.6 x 10^40 erg/s).

Optical and MIR observations support the AGN classification and obscured nature.

Abstract

NGC 1448 is one of the nearest luminous galaxies ($L_{8-1000\mu m} >$ 10$^{9} L_{\odot}$) to ours ($z$ $=$ 0.00390), and yet the active galactic nucleus (AGN) it hosts was only recently discovered, in 2009. In this paper, we present an analysis of the nuclear source across three wavebands: mid-infrared (MIR) continuum, optical, and X-rays. We observed the source with the Nuclear Spectroscopic Telescope Array (NuSTAR), and combined this data with archival Chandra data to perform broadband X-ray spectral fitting ($\approx$0.5-40 keV) of the AGN for the first time. Our X-ray spectral analysis reveals that the AGN is buried under a Compton-thick (CT) column of obscuring gas along our line-of-sight, with a column density of $N_{\rm H}$(los) $\gtrsim$ 2.5 $\times$ 10$^{24}$ cm$^{-2}$. The best-fitting torus models measured an intrinsic 2-10 keV luminosity of $L_{2-10\rm{,int}}$ $=$ (3.5-7.6) $\times$ 10$^{40}$ erg s$^{-1}$, making NGC 1448 one of the lowest luminosity CTAGNs known. In addition to the NuSTAR observation, we also performed optical spectroscopy for the nucleus in this edge-on galaxy using the European Southern Observatory New Technology Telescope. We re-classify the optical nuclear spectrum as a Seyfert on the basis of the Baldwin-Philips-Terlevich diagnostic diagrams, thus identifying the AGN at optical wavelengths for the first time. We also present high spatial resolution MIR observations of NGC 1448 with Gemini/T-ReCS, in which a compact nucleus is clearly detected. The absorption-corrected 2-10 keV luminosity measured from our X-ray spectral analysis agrees with that predicted from the optical [OIII]$\lambda$5007\AA\ emission line and the MIR 12$\mu$m continuum, further supporting the CT nature of the AGN.