The Relation Between AGN and Host Galaxy Properties in the JWST Era: II. The merger-driven evolution of Seyferts at Cosmic Noon

TL;DR

This work tackles whether galaxy mergers trigger sub-quasar AGN at Cosmic Noon by combining deep JWST/NIRCam morphology with four additional merger metrics, X-ray and mid-IR AGN samples, and new inactive control populations across $0.5<z<4$. The authors employ an LD A-based, multi-metric merger classification and X-ray stacking to connect CT mid-IR AGN with post-coalescent X-ray–bright AGN, revealing a merger-driven evolutionary sequence rather than distinct AGN classes. They find that obscured Seyferts show strong merger signatures, majority consistent with major mergers, while inactive controls remain predominantly disk-dominated; disk morphologies can re-form after major mergers, supporting a downsized quasar-evolution picture. Overall, the results argue that mergers play a significant role in triggering sub-quasar AGN during Cosmic Noon and that X-ray and mid-IR selected AGN occupy successive phases along a single merger-driven timeline, a conclusion enabled by JWST’s high-resolution imaging and multi-wavelength data.

Abstract

In Paper I, we exploited the unsurpassed resolution and depth of JWST/NIRCam imagery to investigate the relationship between AGN and host-galaxy properties in the JWST era, finding a correlation between the level of spatial disturbance (as measured by shape asymmetry, $A_S$) and obscuration ($N_H$). Here in Paper II, we report an expansion of our X-ray and infrared analysis of Seyfert-luminosity host galaxies with four additional metrics to the single-metric morphology analysis of Paper I, as well as new samples of inactive control galaxies. This expanded study of one of the largest and most complete, multi-wavelength samples of AGN detected at $0.6<z<2.4$ in the GOODS-South and North fields, confirms that mergers surprisingly play a significant role in obscured, sub-quasar AGN host galaxies. Additionally, the pattern of morphological disturbances observed amongst the X-ray- and mid-IR-selected AGN suggests that these represent different phases of AGN evolution tied to a major-merger timeline, as opposed to distinct populations of AGN. These results indicate that mergers are important in triggering sub-quasar AGN at these redshifts.

Figures (7)

• Figure 1: The distributions of the stellar masses and redshifts of the inactive control samples matched to the AGN sample. The top figure shows the near-IR-selected control sample from CANDELS with the full primary AGN sample in yellow. The middle figure shows the mid-IR AGN (red) and corresponding MIRI-selected control sample. The bottom figure shows the X-ray AGN (blue) compared to the X-ray-selected control sample.
• Figure 2: Examples of AGN host galaxies that were classified as major, minor, and non-mergers by the combined major/minor merger classifications shown in Table 1, used to diagnose the likely merger stage corresponding to the spatial asymmetries uncovered in Paper I. Additionally, we show examples with the full set of morphology indicators required for a further refinement of the classification to merger stage. These images were cut from the F150W NIRCam v1 mosaics used for morphology characterization in the current paper and Paper I, with pixel intensities colored with SAOImage 'aips0' and shown on a log scale.
• Figure 3: The major, minor, and total merger fractions resulting from the N23 LDA analysis of our primary AGN sample as a function of redshift ($x$-axis) and $N_H$ (point color, simulated for CT mid-IR AGN show in dark red). We observe a rising (decreasing) fraction of major (minor) merger candidates towards higher redshifts; and an increasing total merger fraction with redshift, given that the majority of the merger candidates are classified as major mergers. Here we can see that the majority of the AGN sample exhibit the morphologies characteristic of a merger. (Not shown in the figure are the $8\%$ of the sample classified as non-mergers, $6.8\%$ with degenerate major/minor classifications, and $8\%$ unidentifiable due to poor S$\acute{e}$rsic profile fits.)
• Figure 4: From Paper I Bonaventura2025, the trend in the shape asymmetry parameter $A_S$ with $N_H$ and redshift, where the primary AGN sample considered here is contained in the points for $0.6<z<3.8$. Here it can be seen that at Cosmic Noon, the majority of AGN are significantly spatially disturbed; and that at the highest levels of $N_H$ and redshift, the observation of spatial disorder (i.e., an irregular morphology type of a single galaxy or an obvious merger with multiple visible nuclei) in combination with high $A_S$ indicates the early stages of a chaotic merger.
• Figure 5: Median values of $L_{x,int}$ and mass-normalized star formation rate (specific star formation rate, sSFR) versus $N_H$ bin for the CT mid-IR/X-ray-faint AGN, X-ray/mid-IR-faint AGN, and small subset of AGN detected at both mid-IR and X-ray wavelengths, in multiple redshift bins (the $L_{x,int}$ values for the mid-IR AGN represent the stacked values calculated as described in Section 4.1.1). Symbol sizes increase with redshift, where the smallest to largest points represent the redshift bins 0.5-1.2, 1.2-2.4, and 2.4-3.8. Error bars represent the Median Absolute Deviation of values about the median value in each data bin. As discussed in Section 4.1.2, the trend plotted in the top figure, tracing the points of the two highest redshift bins, correlates with the trends of decreasing shape asymmetry and $N_H$, and therefore suggests a major-merger-driven Seyfert evolutionary sequence.