Science
My research focuses on understanding the formation and evolution of galaxies within their cosmic environment, particularly in the early Universe. By analyzing distant galaxy clusters and protoclusters at z ~ 2, I investigate the physical processes that drive galaxy growth, the role of large-scale structure, and the interplay between star formation, supermassive black holes, and gas dynamics.
I specialize in using multi-wavelength data, combining integral field spectroscopy (IFS) and high-resolution imaging to probe the interaction between galaxies and their surroundings. Through observations with instruments such as KCWI, HST, and MUSE, I analyze gas kinematics, feedback from active galactic nuclei (AGN), and the presence of diffuse intracluster light (ICL) as an indicator of early structure formation. My work also involves numerical simulations and cosmological models to interpret observational data and provide a theoretical framework for galaxy evolution studies.
A significant part of my research is dedicated to the circumgalactic and intergalactic medium (CGM/IGM), exploring how galaxies exchange gas with their environment. By studying Lyman-alpha emitting galaxies and their extended gaseous halos, I place constraints on the cosmic ultraviolet background (UVB) and the distribution of matter in the large-scale structure of the Universe.
My work is highly interdisciplinary, spanning astrophysics, cosmology, and data science. I actively collaborate with international teams to develop innovative observational strategies and analytical techniques. Additionally, I am committed to science communication and public outreach, integrating astronomy with broader cultural and educational initiatives to make complex scientific concepts accessible to diverse audiences.
See my list of publications here
Latest paper
- Caught in the act: diffuse stellar and Lyα emission around a halo-less quasar in the assembling cluster CARLA J0800+4029 at z = 2
S. G. Gallego, S. Mei, E. Daddi, G. Noirot, D. Wylezalek & R. M. Rich, submitted to Astronomy & Astrophysics, arXiv:2609.37445 (2026).
A quasar at the heart of a forming galaxy cluster shows almost no glowing gas around it. The gas lights up instead a little further away, along a tidal stream of stars and galaxies caught in the middle of merging.
Peer-Reviewed Publications
Here are the peer-reviewed papers I have contributed to, from galaxies and the cosmic web to planetary atmospheres.
- Ly-alpha emission reveals two satellite halos around massive groups at z ~ 3: The puzzling case of a quiescent central galaxy
S. Guo et al. (including S. G. Gallego), Astronomy & Astrophysics, 704, A340 (2025). - Coevolution of halo and quasar properties in dense environments: CARLA J1017+6116 at z = 2.8
S. G. Gallego et al., Astronomy & Astrophysics, 698, A243 (2025). - A giant disk galaxy two billion years after the Big Bang
W. Wang et al. (including S. G. Gallego), Nature Astronomy, 9, 710–719 (2025). - Characterizing the circumgalactic medium of quasars at z ~ 2.2 through Hα and Lyα emission
V. Langen et al. (including S. G. Gallego), Monthly Notices of the Royal Astronomical Society, 519, 5099–5113 (2023). - The MUSE eXtremely Deep Field: Individual detections of Lyα haloes around rest-frame UV-selected galaxies at z ≃ 2.9–4.4
H. Kusakabe et al. (including S. G. Gallego), Astronomy & Astrophysics, 660, A44 (2022). - Constraints on the structure and seasonal variations of Triton’s atmosphere from the 5 October 2017 stellar occultation and previous observations
J. Marques Oliveira et al. (including S. G. Gallego), Astronomy & Astrophysics, 659, A136 (2022). - Constraining the cosmic UV background at z > 3 with MUSE Lyman-α emission observations
S. G. Gallego et al., Monthly Notices of the Royal Astronomical Society, 504, 16–32 (2021). - A giant Lyα nebula and a small-scale clumpy outflow in the system of the exotic quasar J0952+0114 unveiled by MUSE
R. A. Marino et al. (including S. Gallego), The Astrophysical Journal, 880, 47 (2019). - The large- and small-scale properties of the intergalactic gas in the Slug Lyα nebula revealed by MUSE He II emission observations
S. Cantalupo et al. (including S. G. Gallego), Monthly Notices of the Royal Astronomical Society, 483, 5188–5204 (2019). - Stacking the Cosmic Web in fluorescent Lyα emission with MUSE
S. G. Gallego et al., Monthly Notices of the Royal Astronomical Society, 475, 3854–3869 (2018). - Dark Galaxy Candidates at Redshift ~3.5 Detected with MUSE
R. A. Marino et al. (including S. G. Gallego), The Astrophysical Journal, 859, 53 (2018). - Satellite infall and mass deposition on the Galactic Centre
S. G. Gallego & J. Cuadra, Monthly Notices of the Royal Astronomical Society: Letters, 467, L41–L45 (2017). - Ubiquitous giant Lyα nebulae around the brightest quasars at z ~ 3.5 revealed with MUSE
E. Borisova et al. (including S. G. Gallego), The Astrophysical Journal, 831, 39 (2016).
Current Projects
My current research explores the formation and evolution of large-scale structures in the early universe, focusing on galaxy clusters and protoclusters. Lately, I've been interested in understanding the connection between intracluster light (ICL) and cluster/protocluster environments.
Co-evolution of Galaxies and their Environment at Cosmic Noon
Investigating the interplay between galaxies and their large-scale environment at z~2, analyzing gas dynamics, AGN feedback, and intracluster light using KCWI and HST data to understand the early assembly of cosmic structures.
Latest paper →More details here
The Cosmic Ultraviolet Background
Constraining the Cosmic UVB intensity around Lyman-alpha emitting galaxies using MUSE data and cosmological simulations.
More details here
Cosmic Web Detection
Searching for Cosmic Web signals around Lyman-alpha emitting galaxies using MUSE data and constraints from cosmological simulations.
More details here
Galaxy Clusters Detection
Using the red-sequence technique to detect galaxy clusters in the CFHTLS/VIPERS surveys and estimate their optical richness.
More details here
Galactic Center Simulations
Using hydrodynamical simulations to understand gas inflows into the inner ~200 pc of the Milky Way due to a satellite galaxy merger.
More details hereAI safety research
Alongside astronomy, part of the work at CosmoVentures looks at how AI systems are overseen and contained. These reports come from Apart Research sprints: early results, shared openly.
AI Incident Response Sprint · Sep 2026
Different Reviewers, Same Blind Spot
Replaying a real sandbox escape from July 2026, the team compared one monitor, four identical monitors and four reviewers with distinct roles. Specialised reviewers held containment on more unsafe steps, yet every set-up shared the same blind spot.
With Muhammad Shoaib Altaf, Devesh Sawant and Xin Li.
Read the report →Digital Minds Research Sprint · Aug 2026
False Epistemic Redundancy
Does asking one model to reason as several personas widen what it considers? Across real cases, more varied answers did not mean the decisive explanation was found more often.
With Anya Habana.
Project page →Read the report →
AI Incident Response Sprint · Sep 2026
Playing Through an AI Incident
Little Agent Lab as a research piece: a browser game in which you build a team of AI agents and keep a secret from leaking, making oversight and verification tangible for younger players.
Play Little Agent Lab →Read the report →Research Journey
- Bachelor’s Research: My undergraduate research focused on the dynamics of gas inflow into the inner ~200 pc of the Milky Way, analyzing how a satellite galaxy merger perturbs the gas distribution in the Galactic Center. Using hydrodynamical simulations, I studied how gas is funneled toward the central region, leading to possible star formation and activity near the supermassive black hole, Sagittarius A*. This work contributed to understanding the role of minor mergers in fueling central black holes and shaping the Milky Way’s inner structure. I continued working on this topic during my Master’s research, refining the dynamical models and incorporating additional simulation constraints. This effort led to a published paper in MNRAS Letters, providing key insights into how satellite galaxy infall influences gas dynamics in the Galactic Center (MNRAS Paper).
- Master’s Research: My research thesis focused on detecting galaxy clusters using the red-sequence technique in the CFHTLS/VIPERS surveys, characterizing their optical properties and estimating cluster masses via optical richness (Master’s Thesis). This work refined photometric cluster detection methods and provided constraints on the mass distribution of galaxy clusters at intermediate redshifts. In addition to my thesis, I worked on a small project where I developed my own radiative transfer code to simulate supernova (SNe) polarization, investigating how asymmetries in the explosion mechanism and dust scattering affect observed polarization signals ([Link]). This allowed for a deeper understanding of how explosion geometry influences SNe light curves and polarization profiles. Additionally, I completed a semester project analyzing tidal tails in globular clusters, identifying stellar streams to help model the Galactic center potential. This work contributed to understanding how globular clusters dynamically interact with the Milky Way's gravitational field, providing insights into both the evolution of clusters and the underlying mass distribution of the Galactic center.
- Ph.D. Research (ETH Zurich): My main focus during my Ph.D. at ETH Zurich was unveiling the properties of the cosmic web through Lyman-alpha emission. To achieve this, I developed a specialized stacking technique aligning MUSE integral field spectroscopy observations of Lyman-alpha emitters to detect the faint filamentary structures of the cosmic web in emission for the first time. This method provided new observational evidence of large-scale intergalactic gas distribution, allowing for a direct connection between galaxies and their surrounding environment. Building on this detection, I then used the same data to constrain the intensity of the Cosmic Ultraviolet Background (UVB) in low-density regions of the cosmic web, placing new limits on how extragalactic ionizing radiation regulates intergalactic gas properties. My work bridged the gap between theoretical predictions and observational constraints on cosmic web emission, revealing key insights into how diffuse intergalactic gas is ionized, distributed, and connected to galaxy evolution in the early universe. (Ph.D. Thesis).
- Postdoctoral Research (Caltech): At Caltech, I studied a high-redshift (z=2.3) protocluster system using KCWI integral field spectroscopy, uncovering multiple extended Lyα halos surrounding galaxies in a dense environment. My research provided new evidence of AGN-driven outflows and enhanced star formation within overdense regions of the cosmic web. I integrated multiwavelength datasets (ALMA, HST, Spitzer, Chandra) to probe the interplay between gas kinematics, galaxy evolution, and AGN-driven processes. My findings contributed to understanding how feedback from supermassive black holes influences galaxy growth in the early universe, particularly in environments that will evolve into massive clusters by z=0. During my time at Caltech, I also collaborated with Prof. Shinsuke Shimojo's behavioral neuroscience group on exploring volition and sense of agency as a dimension for the flow state.
- Postdoctoral Research (Université Paris Cité): Currently, I am investigating several protoclusters from the CARLA survey, a sample of high-redshift structures centered on radio-loud AGNs. My research focuses on gas kinematics, AGN-driven outflows, and the emergence of intracluster light (ICL), key components in understanding cluster formation at Cosmic Noon (z~2). By combining HST, KCWI, and multiwavelength observations, I am examining how dense environments shape galaxy evolution and trigger enhanced AGN activity. My work contributes to characterizing early-stage cluster formation, identifying mechanisms that regulate star formation and gas accretion in the progenitors of today’s massive galaxy clusters.