The X-ray statistical properties of dust-obscured galaxies detected by eROSITA

TL;DR

This study leverages wide-area eROSITA eFEDS X-ray data together with optical/NIR/MIR surveys to construct 65 robust X-ray detected DOGs and 74 eFEDS-detected DOGs in a 60 deg^2 footprint. Through MIR/optical SED classification and CIGALE fitting, it shows that eFEDS-DOGs span NH from gas-rich to near-unobscured regimes, revealing a population of gas+dust unobscured DOGs likely in a transition toward lower obscuration. The sample mostly follows established L6μm–LX relations, but a subset deviates by up to ~1 dex, implying high or even super-Eddington accretion in some sources, while others may be intrinsically X-ray weak or affected by lensing/jet contributions. The results support a picture where DOGs occupy a short-lived transitional phase in the co-evolution of SMBHs and their hosts, with eROSITA uncovering a new, less-obscured segment of this population and providing constraints on AGN feedback and lifetimes.

Abstract

Dust-obscured galaxies (DOGs) are considered to be in a co-evolution phase, with the associated active galactic nuclei (AGN) obscured by dust and gas. Although the DOGs are thought to harbor rapidly growing SMBHs, their X-ray statistical properties, crucial for understanding the properties of obscuring gas as well as the accretion disk state and the hot electron corona around the SMBHs, remain unexplored due to the combination of the low number density of DOGs and the lack of X-ray surveys achieving both of the wide-area and uniformly high-sensitivity observations. We construct a sample of X-ray-detected DOGs in the eROSITA Final Equatorial Depth Survey (eFEDS) field and examine their X-ray statistical properties. By using Subaru/HSC SSP, VIKING, and WISE all-sky surveys, our results reveal the discovery of 5738 IR-bright DOGs in the footprint covered by both of the eFEDS and VIKING surveys (60 deg^2), with 65 objects identified as X-ray-detected DOGs (eFEDS-DOGs). Among them, 41 eFEDS-DOGs show a power-law slope in the near to mid-IR bands (power-law DOGs), indicating dust-obscured AGN. The hydrogen column density (N_H) suggests that eFEDS-DOGs cover even unobscured AGN, spanning 10^20 < N_H <= 10^23. On the other hand, the majority of IR-bright DOGs are not detected by eROSITA, suggesting that most IR-bright DOGs are heavily obscured by dust and gas with N_H > 10^23. Therefore, eFEDS-DOGs, discovered thanks to the wide-area survey by eROSITA, are newly found populations showing less obscured phases among the lifetime of DOGs. Additionally, some eFEDS-DOGs exhibit deviations, down to nearly 1.0 dex below the monochromatic luminosity at 6 micron versus absorption-corrected intrinsic X-ray luminosity between 0.5-2 keV relation, suggesting that it may signal high Eddington ratios reaching the Eddington limit.

Figures (17)

• Figure 1: Survey area comparison between previous studies and this work. The orange filled circle represents the estimated survey area of this work, while the star plot denotes the final release status of HSC-VIKING-eRASS. The green, purple, and pink triangles denote the survey areas of previous X-ray detected DOG studies in the Chandra Deep Field South (CDFS; 2016AA...592A.109C), the COSMOS field 2019AJ....157..233R, and XMM-LSS field 2024MNRAS.531..830K, respectively. The red line outlines the extent of the all-sky area.
• Figure 2: Flow chart of the sample selection for the "eFEDS-detected DOGs" (74 objects) and "eFEDS-DOGs" (65 objects).
• Figure 3: Example of the SED fitting result. The blue, green, and red points represent the HSC, VIKING, and WISE photometric data, respectively. The black, blue, green, red, and orange lines correspond to the best-fit model, stellar component, AGN component, star formation (SF) component, and nebular component, respectively
• Figure 4: $L_{\mathrm{6 \mu m}}$ estimated by CIGALE ($L_{\mathrm{6 \mu m}}^{\mathrm{CIGALE}}$) versus the rest-frame 6 $\mu$m luminosity ($L_{\mathrm{6 \mu m}}$) interpolated from the nearby WISE W1, W2, and W3 bands ($L_{\mathrm{6 \mu m}}^{\mathrm{MIR}}$). Red stars represent the eFEDS-DOGs, while blue squares indicate the eFEDS-DOGs with BluDOG-like features. The black solid line corresponds to $\log L^{\mathrm{MIR}}{\mathrm{6\mu m}} = \log L^{\mathrm{CIGALE}}{\mathrm{6\mu m}}$.
• Figure 5: MIR band versus optical band magnitudes of eFEDS-DOGs as well as previous X-ray detected DOGs. Red and orange stars denote eFEDS-DOGs and eFEDS-detected DOGs with NHclass$<$2, respectively. Blue squares show eFEDS-DOGs with the BluDOG-like feature. Lime, Green, purple, magenta, cyan, and pink plots denote X-ray detected DOGs from 2009AA...498...67L, 2016AA...592A.109C, 2019AJ....157..233R, 2020ApJ...888....8T, 2020MNRAS.499.1823Z, and 2024MNRAS.531..830K, respectively. For the sample fainter than $m_\mathrm{MIR}>15$ mag 2009AA...498...67L2016AA...592A.109C2019AJ....157..233R, we utilized Spitzer/MIPS 24 $\mu$m bands as $m_\mathrm{MIR}$. The optical bands were obtained from the Hubble/ACS F775-band 2016AA...592A.109C, Suprime-Cam $i_\mathrm{AB}'$ bands 2019AJ....157..233R and heterogeneous optical magnitudes by Vizier with the nearest matching with $<3$ arcsec 2009AA...498...67L. The red solid line represents the DOGs criterion, and three red dashed lines denote $(i-W4)_{\rm AB} = 8.0$, $9.0$, and $10.0$. The numbers in parentheses indicate the number of objects.