Obscured AGN at z < 1.5: X-ray to Far-Infrared SEDs and Host Galaxy Morphologies in the GOODS Fields

TL;DR

This study analyzes 194 X-ray luminous AGN at $z<1.5$ in the GOODS fields by constructing $1 ext{ }\mu$m-normalized SEDs and performing detailed host-galaxy morphologies via galight, complemented by visual classifications. It finds that 94% show obscured UV/MIR SEDs, and the host population is diverse, with disks dominating ~52% of hosts while bulges account for ~31%, and a notable subset (Shape5) of heavily obscured, bulge-dominated, post-merger-like systems. Importantly, only ~34% show interaction signatures, indicating secular processes play a major role in fueling AGN activity across this luminosity range; mergers contribute mainly to the most luminous sources. The Shape5 population suggests a late evolutionary stage with nuclear obscuration, potentially linked to early-type hosts and reduced star formation, highlighting the complex, multi-stage coevolution of AGN and their hosts. Overall, the work demonstrates that AGN fueling at moderate luminosities occurs across diverse host morphologies and is not solely merger-driven, with implications for interpreting SMBH growth over cosmic time.

Abstract

We present an analysis of spectral energy distributions (SEDs), galaxy light profiles, and visual morphological classifications for 194 X-ray luminous AGN (intrinsic absorption-corrected log10 LX(0.5 to 7 keV) less than 42.5, with a maximum of 45.2 ergs per second) at redshift z less than 1.5 in the GOODS fields. We generate X-ray to far-infrared SEDs normalized at 1 micron for all AGN and sort them according to their emission slopes in the ultraviolet and infrared. We visually classify their host galaxy morphologies and compute their bulge-to-total light ratios using the software Galaxy Shapes of Light (galight). Most (94 percent) GOODS AGN exhibit obscured SEDs, defined by diminished ultraviolet and/or mid-infrared emission, while only 6 percent show unobscured, quasar-like SEDs. Secular processes appear to play a large role in stimulating AGN emission, as only around one-third of galaxies are undergoing interactions. We also describe the morphological identification of a population of suspected post-merger spheroid galaxies with obscured ultraviolet and infrared SEDs, and distinguish them from the host galaxies of AGN with less obscuration in the ultraviolet or infrared.

Figures (18)

• Figure 1: Histogram of intrinsic 0.5-7 keV X-ray luminosities in units of log [$\mathrm{erg \ s^{-1}}$]. GOODS is unique in that its deep Chandra observations allow us to reliably detect faint, obscured AGN that are selected against when studying fields such as Stripe82X or COSMOS. In studies such as Auge2023, these low luminosity ($10^{42.5} < \mathrm{L_{\rm X}} < 10^{43} \ \mathrm{erg \ s^{-1}}$) are removed due to potential contamination from luminous starburst galaxies present in larger area surveys. However, our sample constitutes entirely AGN, as verified through the construction of spectral energy distributions later in this paper.
• Figure 2: SEDs for the 194 GOODS-N and GOODS-S sources in our sample. Fluxes are normalized at $1\mu$m and are colored based on intrinsic X-ray luminosity. Note the break between $10^{-3} \ \mathrm{\mu m}$ and $0.5 \times 10^{-2} \ \mathrm{\mu m}$ where there is no data.
• Figure 3: Four shapes of SEDs sorted according to the criteria in Table \ref{['tab: panel_lims']}. The SEDs are normalized to 1 $\mathrm{\mu m}$ to emphasize the emission of the AGN. Note the break between $10^{-3} \ \mathrm{\mu m}$ and $0.5 \times 10^{-2} \ \mathrm{\mu m}$ where there is no data. For each shape, the bold black line shows the median SED. Almost half the AGN ($94/194$) are in Shape 3, as in Auge2023 (note: Auge2023 breaks the SEDs into five panels. Our Shape 1-2 is a combination of their Shape 1 and Shape 2 due to the dearth of unobscured AGN in our sample). 51% of Shape 3 galaxies have far-infrared (100 $\mathrm{\mu m}$ detections, higher than both Shape 4 (45%) and Shape 5 (12%).
• Figure 4: Flow chart showing the process for iteratively choosing the best fit for a given galaxy. Through this process we test multiple PSFs, choose the best PSF for a given source, and decide whether it is necessary to subtract a point source based on the ratio of light from the point source to light from the host galaxy. Colors correspond to different steps in the process. Blue denotes fitting steps, purple denotes selection steps, and beige computational steps.
• Figure 5: Galaxy-only bulge/total (B/T) ratio versus galaxy minus point source B/T ratio for 187 GOODS X-ray AGN colored by the ratio of light in the point source to the host galaxy (PS Ratio). Note that the majority of fits with $\mathrm{PS \ Ratio} > 1$ show large diversions between the fit results. In some of these cases, the two-component only fit performs comparably to those with a point source subtracted, but for many sources with a large point source contribution, the B/T ratio appears to be overestimated if a point source is not subtracted. Sources above the $+1\sigma$ line show, except for three galaxies, no improvement and even some evidence for over-fitting if a point source is included.