Blazars are among the most energetic objects in the observable Universe. They are active galaxies powered by matter spiralling into a supermassive black hole. As gas and dust fall inward, part of the gravitational energy released can drive narrow jets of plasma from the region around the black hole’s poles. When one of these jets happens to point almost directly toward Earth, the galaxy can appear as a brilliant, star-like point of light. Relativistic motion within the jet amplifies the radiation through a phenomenon known as Doppler boosting, making the source appear far brighter and more rapidly variable than it would from another viewing angle.

PKS 2155-304 lies in the southern sky, in the direction of the constellation Piscis Austrinus. It emits radiation across an enormous range of wavelengths, from radio waves and visible light to ultraviolet radiation, X-rays and gamma rays. The new study combines data collected by two NASA space observatories: the Neil Gehrels Swift Observatory, which monitors the optical, ultraviolet and X-ray bands, and the Fermi Gamma-ray Space Telescope, which observes the highest-energy gamma rays. Together, these observations provide a more complete picture of the blazar’s activity than measurements obtained in only one part of the spectrum.

The data set covers almost 20 years, a timespan that is unusually valuable for studying an object whose brightness can change dramatically. Blazars are often observed during brief campaigns lasting a few days or weeks, typically when they undergo an especially powerful flare. Such snapshots can reveal intense short-term changes, but they may miss slower variations or transitions between different states of activity. By analysing observations spread across two decades, the researchers were able to test whether relationships seen during individual flares remain valid over much longer periods.

Their results indicate that they do not. The most commonly used one-zone models assume that radiation is produced in a single region of the jet by one population of high-energy electrons. In these models, electrons accelerated to relativistic speeds radiate synchrotron emission as they spiral through magnetic fields, producing much of the light observed at lower energies. The same electrons may then transfer energy to photons through inverse Compton scattering, boosting those photons into the X-ray or gamma-ray range. This framework can reproduce some short-term changes in PKS 2155-304, but it fails to account for the complete long-term pattern.

One important missing relationship involves the optical and X-ray bands. If both forms of radiation were generated by the same electron population in the same region, a substantial change in one band would generally be expected to coincide with a change in the other, perhaps after a short delay caused by particle cooling or the travel time of disturbances through the jet. Yet the long-term observations showed no consistent correlation between optical and X-ray activity. The result suggests that separate regions, particle populations or emission mechanisms may contribute to the light detected at different energies.

The X-ray spectrum also revealed an unexpected departure from a familiar pattern. During many blazar flares, the increase in brightness is stronger at higher X-ray energies, meaning that the spectrum becomes harder as the source brightens. PKS 2155-304 displayed this behaviour during some shorter observing periods, but it was not sustained across the full 20-year record. The changing slopes of the spectral variations imply that different outbursts may be driven by different physical conditions, such as changes in the electron energy distribution, magnetic field strength, particle acceleration or the geometry of the emitting region.

The broad spectrum of a blazar usually contains two prominent peaks separated by a trough. The lower-energy peak is generally attributed to synchrotron radiation from relativistic electrons. The origin of the higher-energy peak is more uncertain. It may result from inverse Compton scattering, in which energetic electrons collide with lower-energy photons and transfer energy to them. Another possibility is that hadrons, including protons, participate in the process. In hadronic scenarios, high-energy protons can interact with photons or magnetic fields, producing secondary particles and potentially generating gamma rays, neutrinos and other radiation.

Particularly intriguing evidence appeared in two observations from 2012, when the spectrum of PKS 2155-304 contained an additional statistically significant dip despite the absence of a major outburst. Such an inflection could indicate that an extra process temporarily altered the balance between radiation components. The researchers say theoretical considerations make a hadronic contribution a plausible explanation, although the data do not yet establish it conclusively. If hadrons were indeed involved, the source could also be capable of producing high-energy neutrinos, offering a possible connection to one of the major unsolved questions in astrophysics: where cosmic neutrinos originate.

Neutrinos are electrically neutral, extremely light particles that pass through ordinary matter with remarkable ease. That same property makes them exceptionally difficult to detect. Some of the highest-energy neutrinos observed on Earth appear to come from deep space, but their sources remain uncertain. The identification of a neutrino arriving from the direction of another blazar during a powerful flare has already shown that blazar jets can be connected to neutrino production. The long-term behaviour of PKS 2155-304 now strengthens the case for more complex models in which electrons and hadrons share responsibility for the radiation. Continued monitoring across the electromagnetic spectrum, combined with future neutrino detections, may reveal whether this distant jet is also a cosmic particle accelerator producing messengers that cross the Universe to reach Earth.

Subject of Research: Long-term multiwavelength activity and emission mechanisms of the blazar PKS 2155-304

Article Title: 20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. I. The case of PKS 2155-304

News Publication Date: 6 August 2026

Web References: https://doi.org/10.1016/j.jheap.2026.100688

References: A. Wierzcholska and M. Zacharias, “20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. I. The case of PKS 2155-304,” Journal of High Energy Astrophysics, 2026, 54, 100688. DOI: 10.1016/j.jheap.2026.100688

Image Credits: NASA/JPL-Caltech

Keywords

Blazar, PKS 2155-304, active galaxy, supermassive black hole, relativistic jet, gamma rays, X-rays, Swift Observatory, Fermi Space Telescope, neutrinos, hadronic processes, astrophysics, cosmic particle acceleration

ALMA, situated high in the Chilean Andes, operates at millimeter and submillimeter wavelengths, probing cold gas and dust that are the raw materials for star formation. Meanwhile, JWST’s infrared capabilities enable it to peer through cosmic dust and reveal the stars themselves, as well as the morphologies and kinematics of distant galaxies. This complementary synergy transforms how astrophysicists can dissect the anatomy of galaxies residing in what is often termed “cosmic dawn.”

The early universe was a tumultuous era marked by rapid assembly of galaxies from primordial matter, yet understanding the physical processes that governed this growth remained elusive for decades. Traditional observatories struggled to capture the faint signatures of fledgling galaxies. However, the unprecedented sensitivity and spatial resolution of ALMA and JWST now illuminate the intricate interplay between gas inflows, star formation bursts, chemical enrichment, and feedback mechanisms driven by active galactic nuclei (AGN).

One of the core scientific breakthroughs enabled by ALMA’s millimeter/submillimeter observations lies in revealing the reservoirs of cold molecular gas, particularly carbon monoxide (CO) and ionized carbon ([CII]), which serve as key tracers of star-forming fuel in young galaxies. By mapping these components with exquisite spatial detail, astronomers can quantify gas masses, measure turbulence, and identify dynamic processes like inflows and outflows. Such observations have overturned simplistic models of galaxy growth, showing instead a highly heterogeneous and dynamic interstellar medium (ISM).

Simultaneously, JWST’s infrared imaging and spectroscopy unlock the secrets of stellar populations and dust obscuration. Its instruments can detect the rest-frame ultraviolet and optical emission lines from high-redshift galaxies, providing crucial insights into their chemical composition, ionization states, and star formation rates. The longer-wavelength sensitivity of JWST also captures thermal emission from dust, helping quantify how much starlight is absorbed and re-radiated, thereby revealing hidden star formation activity.

The synergy of JWST and ALMA observations has proved transformative not only for individual galaxies but also for understanding galaxy populations at early times. Deep field campaigns and gravitational lensing studies have identified large samples of star-forming galaxies at redshifts beyond 6, corresponding to when the universe was less than a billion years old. Importantly, resolved spectroscopy from the two observatories has highlighted a diversity of morphological features—ranging from clumpy, irregular star-forming regions to nascent disk-like structures—emphasizing the varied evolutionary pathways galaxies undertake.

Another fundamental aspect explored is the role of active galactic nuclei, powered by rapidly accreting supermassive black holes, in shaping galaxy evolution during the first billion years. ALMA observations can detect molecular outflows driven by AGN feedback, which can regulate or quench star formation by heating or expelling gas. JWST’s sensitivity to emission line diagnostics further refines our understanding of the co-evolution between black holes and their host galaxies, probing the early growth phases of these cosmic behemoths and their impact on the ISM.

Despite these advances, current observations are not without limitations. The angular resolution achievable is often just sufficient to resolve structures on kiloparsec scales but fails to probe smaller-scale star formation complexes or the detailed dynamics within galactic nuclei. Sensitivity constraints also limit the detection of extremely faint galaxies or diffuse gas components. These challenges highlight the urgent need for continued upgrades to existing observatories and the conception of next-generation facilities with enhanced capabilities.

State-of-the-art simulations and theoretical frameworks play a critical role in interpreting the massive influx of observational data. Cosmological hydrodynamical simulations are increasingly sophisticated in modeling the physics of gas cooling, star formation, feedback, and chemical enrichment in realistic scenarios. The interplay between simulated predictions and empirical data from ALMA and JWST constrains theories about gas accretion modes, the impact of environment, and the origin of galaxy scaling relations observed locally.

Future research directions sparked by the successes of JWST and ALMA focus on pushing the frontier deeper in redshift and resolution. Identifying and characterizing even earlier galaxy populations during the epoch of reionization holds the promise of answering how the first generations of stars and black holes influenced the ionization state of the universe. Higher angular resolution imaging combined with time-domain studies may also reveal the dynamics of star formation on sub-kiloparsec scales and the stochastic nature of feedback processes.

Collaborative, multi-wavelength survey programs that blend JWST’s IR prowess with ALMA’s millimeter/submillimeter insights are already setting new standards for comprehensive galaxy studies. Cross-correlating observational data with other probes, such as gravitational wave detections and 21-cm neutral hydrogen mapping, could holistically address galaxy assembly and evolution from multiple vantage points, reinforcing the multi-messenger astrophysics approach.

In addition to observational efforts, technology development remains paramount. Innovations in detector sensitivity, array design, and data analysis pipelines will enable both existing and future observatories to harness their full potential. For ALMA, expanding baseline lengths or integrating new receiver bands could improve resolution and spectral coverage, while JWST’s successors might aim at surpassing its infrared capabilities through increased aperture size or novel instrumentation.

The synergy between ALMA and JWST marks a paradigm shift in cosmic archaeology—transforming how astronomers trace the lineage of galaxies from diffuse gas clouds to mature systems. The holistic view these instruments provide is not only expanding the observable horizon but fundamentally deepening our understanding of the physics driving the earliest phases of galaxy formation. As this research frontier advances, it will undoubtedly rewrite textbooks and shape the next chapters of cosmic evolution science.

In sum, the incredible union of JWST’s infrared eye and ALMA’s submillimeter gaze is redefining our portrait of the universe’s formative years. Their combined observations unveil the complexity buried within the first billion years after the Big Bang by allowing scientists to probe the interplay between gas, stars, and black holes with unprecedented clarity and depth. While current achievements are breathtaking, the horizon promises even greater discoveries, urging continued investment and ingenuity in astronomical exploration.

Subject of Research:
The formation and evolution of galaxies in the early universe, especially within the first billion years after the Big Bang, leveraging observations from JWST and ALMA.

Article Title:
The early Universe with JWST and ALMA

Article References:
Herrera-Camus, R., Förster Schreiber, N.M., Vallini, L. et al. The early Universe with JWST and ALMA. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02726-0

Image Credits:
AI Generated

DOI:
https://doi.org/10.1038/s41550-025-02726-0