Investigating the Bulge Morphology of Dual AGN Host Galaxies from the GOTHIC survey

TL;DR

The study addresses how bulge morphology in dual AGN host galaxies relates to the formation of SMBH binaries. It employs 2D bulge–disk decompositions of SDSS DR18 gri images for 131 bulges in 61 galaxy pairs and 3 triplets from the GOTHIC survey, using GALFIT to derive Sérsic indices, luminosities, masses, and scale lengths. Results show that approximately $80\%$ of bulges are classical with Sérsic indices $n>2$, most reside in elliptical hosts ($64\%$ in ellipticals, $36\%$ in disks), and about $20\%$ are pseudobulges; major mergers with mass ratios $\leq 3$ dominate, with roughly two-thirds of systems hosting at least one elliptical. Together, these findings support a merger-driven evolutionary pathway for dual AGN and SMBH binaries in massive, evolved, red hosts, with implications for identifying such systems in future surveys.

Abstract

We present a structural analysis of bulges in dual active galactic nuclei (AGN) host galaxies. Dual AGN arise in galaxy mergers where both supermassive black holes (SMBHs) are actively accreting. The AGN are typically embedded in compact bulges, which appear as luminous nuclei in optical images. Galaxy mergers can result in bulge growth, often via star formation. The bulges can be disky (pseudobulges), classical bulges, or belong to elliptical galaxies. Using SDSS DR18 gri images and GALFIT modelling, we performed 2D decomposition for 131 dual AGN bulges (comprising 61 galaxy pairs and 3 galaxy triplets) identified in the GOTHIC survey. We derived sérsic indices, luminosities, masses, and scalelengths of the bulges. Most bulges (105/131) are classical, with sérsic indices lying between $n=2$ and $n=8$. Among these, 64% are elliptical galaxies, while the remainder are classical bulges in disc galaxies. Only $\sim$20% of the sample exhibit pseudobulges. Bulge masses span $1.5\times10^9$ to $1.4\times10^{12}\,M_\odot$, with the most massive systems being ellipticals. Galaxy type matching shows that elliptical--elliptical (E--E) and elliptical--disc (E--D) mergers dominate over disc--disc (D--D) mergers. At least one galaxy in two-thirds of the dual AGN systems is elliptical and only $\sim$30% involve two disc galaxies. Although our sample is limited, our results suggest that dual AGN preferentially occur in evolved, red, quenched systems, that typically form via major mergers. They are predominantly hosted in classical bulges or elliptical galaxies rather than star-forming disc galaxies.

Figures (3)

• Figure 5: The 3 plots from left to right show, respectively, the pairing of Sérsic index in merging samples, where $n_1$ and $n_2$ correspond to the Sérsic index of the heavier and lighter bulge mass nuclei, respectively. The size of the scatter points depends inversely on the bulge mass ratio, where a lower bulge mass ratio ($1 < \frac{M_1}{M_2} < 3$) which is the major merger sample is indicated in red, and the higher bulge mass ratio ($\frac{M_1}{M_2} \geq 3$) which is the minor merger sample is represented as blue.
• Figure 6: Pie diagrams from left to right. (a) Extreme left shows the different types of bulge combinations. (b) Center shows the fraction of different merger types. (c) The extreme right shows the major vs minor merger numbers.
• Figure 7: From left to right. (a) The plot shows the distribution of bulge scale radius $R_e$ in disk galaxies or the galaxy scale radii of elliptical galaxies. (b) The plot shows the distribution of total galaxy mass ratios for the sample. Note that there are 2 outliers having mass ratios $\sim$10 and $\sim$30. (c) The ratio of bulge to total galaxy mass for the 58 disk galaxies in our sample, where the median value is shown with a dashed red vertical line.