Color-Magnitude Relations of Active and Non-Active Galaxies in the Chandra Deep Fields: High-Redshift Constraints and Stellar-Mass Selection Effects

[Abridged] We extend color-magnitude relations for moderate-luminosity X-ray AGN hosts and non-AGN galaxies through the galaxy formation epoch in the Chandra Deep Fields. We utilized analyses of color-magnitude diagrams (CMDs) to assess the role of moderate-luminosity AGNs in galaxy evolution. First, we confirm some previous results and extend them to higher redshifts, e.g., there is no apparent color bimodality for AGN hosts from z0-2, but non-AGN galaxy color bimodality exists up to z3; most AGNs reside in massive hosts and the AGN fraction rises strongly toward higher stellar mass up to z2-3; and the colors of both AGN hosts and non-AGN galaxies become redder as the stellar mass increases up to z2-3. Second, we point out that it is critical to use mass-matched samples to examine color-magnitude relations of AGN hosts and non-AGN galaxies. We show that for mass-matched samples up to z~2-3, AGN hosts lie in the same CMD region as non-AGN galaxies; i.e., there is no specific clustering of AGN hosts around the red sequence, the top of the blue cloud, or the green valley in between. The AGN fraction (10%) is mostly independent of host-galaxy color, providing an indication of the duty cycle of SMBH growth in typical massive galaxies. These results are in contrast to those obtained with non-mass-matched samples where there is apparent AGN clustering in the CMD and the AGN fraction generally increases as the color becomes redder. We also find, for mass-matched samples, that the SFRs of AGN hosts are typically a factor of 2-3 larger than those of non-AGN galaxies at z0-1, whereas this difference diminishes at z1-3. Our results can be reasonably explained by two main ingredients, color-mass correlation and passive or secular evolution of galaxies; and thus tightly constrain any effects from moderate-luminosity AGN feedback upon color-magnitude properties over the ~80% of cosmic time.

💡 Research Summary

The authors exploit the deepest X‑ray observations available in the Chandra Deep Field‑North and South, combined with extensive multi‑wavelength photometry (optical, near‑IR, mid‑IR), to investigate the colour–magnitude diagram (CMD) properties of moderate‑luminosity X‑ray AGN (L_X ≈ 10⁴²‑10⁴⁴ erg s⁻¹) and a control sample of non‑active galaxies over the redshift interval 0 ≤ z ≤ 3. The central methodological innovation is the construction of stellar‑mass‑matched samples: each AGN host is paired with non‑AGN galaxies that lie within Δlog M_* ≈ 0.2 dex, thereby removing the dominant mass‑driven colour bias that plagues earlier studies. Stellar masses are derived via SED fitting using BC03 templates and a Chabrier IMF; star‑formation rates (SFRs) are obtained from UV+IR indicators.

When the mass‑matched samples are plotted on the rest‑frame (U‑V) versus absolute magnitude plane, AGN hosts occupy exactly the same region as the non‑AGN galaxies. There is no excess of AGN in the “green valley”, at the top of the blue cloud, or on the red sequence. The AGN fraction among massive galaxies is roughly constant at ~10 % and shows no systematic dependence on colour. This implies that the duty cycle of SMBH growth in typical massive galaxies is roughly colour‑independent, contrary to the picture in which AGN preferentially appear during a transitional phase.

In contrast, if one ignores the mass matching, the apparent distribution changes dramatically: AGN seem to cluster toward redder colours and the AGN fraction rises with colour. The authors demonstrate that this effect is entirely driven by the underlying mass–colour correlation—more massive galaxies are both redder and more likely to host an AGN—rather than any intrinsic link between AGN activity and colour.

The study also compares SFRs of AGN hosts and non‑AGN galaxies. At low redshift (z ≈ 0‑1) AGN hosts have median SFRs that are 2‑3 times higher than those of mass‑matched non‑AGN galaxies, indicating that SMBH accretion and star formation can co‑exist in relatively blue, star‑forming systems. By z ≈ 1‑3 this SFR excess disappears, and the two populations have statistically indistinguishable SFR distributions, suggesting that the co‑evolution signal weakens at earlier cosmic times when the overall star‑formation activity of the galaxy population is higher.

The authors interpret these findings with two simple ingredients. First, the colour–mass relation (more massive galaxies are redder) dominates the CMD morphology. Second, the passive or secular evolution of galaxies—gradual quenching of star formation leading to reddening—applies equally to AGN hosts and non‑AGN galaxies. Consequently, moderate‑luminosity AGN feedback does not leave a strong imprint on the global colour or SFR properties of their hosts over ~80 % of cosmic history.

Overall, the paper delivers three key messages: (1) mass‑matched comparisons are essential to avoid spurious AGN‑colour correlations; (2) moderate‑luminosity AGN are not preferentially located in any specific CMD region, and their occurrence is roughly colour‑independent; (3) the impact of such AGN on host galaxy colours and star formation is modest, placing tight observational constraints on models that invoke strong AGN‑driven quenching at these epochs. These results refine our understanding of SMBH–galaxy co‑evolution and provide a robust benchmark for future semi‑analytic and hydrodynamic simulations.