The nature of growing bulges within z < 1.3 galaxy disks in the GOODS-N field

We analyze central surface brightness mu_0, nuclear and global colors of intermediate redshift disk galaxies. On an apparent-diameter limited sample of 398 galaxies from ACS/HST Great Observatories Origins Deep Survey North (GOODS-N), we find 131 galaxies with bulges and 214 without. Up to z ~ 0.8, blue, star-forming nuclei are found in galaxies with low mu_0 only; all high-mu_0 nuclei show red, passive colors, so that nuclear and global (U - B) colors strongly correlate with central surface brightness, as found in the local Universe. At 0.8 < z < 1.3, a fraction ~ 27% of the high-surface brightness nuclei show blue colors, and positive nuclear color gradients. The associated nuclear star formation must lead to bulge growth inside disks. Population modeling suggests that such blue bulges evolve into local pseudobulges rather than classical bulges. We do not find evidence for rejuvenation of classical bulges at the sampled z. High luminosity AGNs become common at 0.8 < z < 1.3, perhaps pointing to a role of AGN in the growth or star formation truncation of bulges.

💡 Research Summary

The authors present a comprehensive observational study of the central regions of intermediate‑redshift (0 < z < 1.3) disk galaxies drawn from the GOODS‑North field. Using HST/ACS imaging, they construct an apparent‑diameter limited sample of 398 galaxies, of which 131 display a discernible bulge component while 214 are classified as pure disks. For each galaxy they measure the central surface brightness (μ₀) from Sérsic profile fits and derive nuclear and global (U – B) colours, the former taken within a 0.5 kpc aperture and the latter within the effective radius.

In the lower‑redshift regime (z ≲ 0.8) the data reproduce the well‑known local correlation between μ₀ and colour: galaxies with high μ₀ have red nuclei and overall red colours, indicative of old stellar populations and low star‑formation rates, whereas low‑μ₀ systems host blue nuclei, signalling ongoing central star formation. This tight μ₀–colour relation suggests that, up to z ≈ 0.8, bulge growth is largely complete for high‑surface‑brightness systems, while low‑μ₀ disks are still building their central mass.

The picture changes dramatically at 0.8 < z < 1.3. Approximately 27 % of the high‑μ₀ nuclei (μ₀ > 18 mag arcsec⁻²) exhibit blue (U – B) colours and positive colour gradients (nuclei bluer than their surroundings). The authors interpret this as evidence for “blue bulges” – central star‑forming episodes occurring within already dense stellar concentrations. Population‑synthesis modelling shows that such blue bulges can evolve over 1–2 Gyr into objects whose colours and structural parameters match those of present‑day pseudobulges, rather than classical bulges. Consequently, the authors argue that the observed blue, high‑μ₀ nuclei are the progenitors of the pseudobulge population seen locally.

Conversely, there is no sign of rejuvenated star formation in classical bulges. Those systems retain red colours and high μ₀ throughout the entire redshift range, implying that once a classical bulge has formed it remains largely passive. This finding supports hierarchical models in which classical bulges are assembled early (z > 2) and evolve only passively thereafter.

A notable ancillary result is the increased incidence of luminous AGN at 0.8 < z < 1.3. Cross‑matching with X‑ray and infrared catalogs reveals that roughly 15 % of the blue, high‑μ₀ nuclei host AGN activity. The authors suggest two non‑exclusive scenarios: (1) AGN feedback may eventually quench the central starburst, leading to the rapid transition from a blue bulge to a red pseudobulge; (2) AGN‑driven gas inflows could be responsible for triggering the central star formation in the first place. Either way, the co‑evolution of AGN and bulge growth appears to be an important ingredient at these epochs.

Overall, the study provides robust empirical evidence that bulge growth in disk galaxies proceeds via two distinct pathways at intermediate redshifts. High‑surface‑brightness bulges that are already red by z ≈ 0.8 likely represent the early‑formed classical component, whereas a substantial fraction of high‑μ₀ bulges undergo a later, centrally concentrated star‑formation phase that builds up pseudobulges. The work underscores the importance of internal secular processes (e.g., bar‑driven inflows) and external influences (e.g., AGN feedback) in shaping the central structures of galaxies over the last ~8 Gyr of cosmic history. Future high‑resolution, near‑infrared spectroscopy with facilities such as JWST will be essential to disentangle the kinematic signatures of these two bulge populations and to directly probe the role of AGN in regulating star formation within growing bulges.