The X-ray Link Between High Eddington Ratio Dust-Obscured Galaxies (DOGs) and Hot DOGs

Fan Zou, W. N. Brandt, Elena Gallo, Fabio Vito, Zhibo Yu

First listed 2025-07-29 | Last updated 2025-07-29

Abstract

Dust-obscured galaxies (DOGs) with extremely red optical-to-infrared colors are often associated with intense starburst and AGN activity. Studying DOGs can provide insights into the processes that drive the growth of galaxies and their central supermassive black holes. However, the general DOG population is heterogeneous, spanning a wide range of evolutionary stages, and has X-ray obscuring column densities ($N_\mathrm{H}$) covering low-to-high levels. In this work, we focus on seven high Eddington ratio DOGs ($\log λ_\mathrm{Edd} \gtrsim -0.5$) to examine their X-ray obscuration properties using new and archival X-ray observations. We confirm that these systems are generally heavily obscured, with 6/7 having $N_\mathrm{H}\gtrsim10^{23}~\mathrm{cm^{-2}}$ and 3/7 having $N_\mathrm{H}\gtrsim10^{24}~\mathrm{cm^{-2}}$. Based on the observed similarity with the rare Hot DOG population, we argue that both high-$λ_\mathrm{Edd}$ DOGs and Hot DOGs likely trace the post-merger phase during which AGNs are enshrouded by large columns of dust-rich material.

Short digest

Targets seven high-λEdd DOGs and combines new, deeper Chandra exposures with archival X-ray data to measure line-of-sight obscuration. Finds these systems are typically buried: 6/7 have N_H ≳ 10^23 cm^-2 and 3/7 are Compton-thick (N_H ≳ 10^24 cm^-2), with observed X-rays strongly suppressed relative to the mid-IR relation. Placing the sources on the λEdd–N_H plane shows they overlap the regime occupied by Hot DOGs. Authors argue both populations trace a post-merger phase where rapidly accreting SMBHs are enshrouded by dust-rich gas.

Key figures to inspect

• Figure 1: Compare LX,obs versus mid-IR (Stern 2015 relation) for the seven DOGs; the vertical offsets between LX,obs and LX,int for four sources visualize heavy suppression expected from high N_H, reinforcing the Compton-thick candidates.
• Figure 2: Inspect the posterior PDFs of N_H for the four newly observed DOGs—note the 95% lower limits for J1042+2451 and J1513+1451, the 95% upper limit for J1235+4827, and the median/68% interval for J1210+6105—to see which are securely CT versus only lower-limited.
• Figure 3: Position the sample on the λEdd–N_H plane against comparison sets (red type 1 quasars, DOGs, Hot DOGs) to confirm that high-λEdd DOGs lie in the same heavily obscured locus as Hot DOGs.
• Figure 4: Use the λEdd–N_H diagram with radiation-pressure boundaries (single scattering vs. trapping), the long-lived-cloud region, and the host-obscuration band to judge whether the inferred columns require nuclear-scale enshrouding or can be explained by galaxy-scale dust; identify sources plausibly in the outflow/…
• Cross-check specific cases: J1324+4501 (XMM; CT) and J0825+3002 (XMM+NuSTAR; CT) as anchors for the sample’s high-end N_H.