Research profile of Dr. Roger Ianjamasimanana

1. Research interests

I am Dr. Roger Ianjamasimanana. I currently work as a postdoctoral researcher at the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Granada, Spain. I work within the AMIGA group (Analysis of the interstellar Medium of Isolated GAlaxies), led by Dr. Lourdes Verdes-Montenegro.

My research focuses on understanding how galaxies evolve through the study of their neutral atomic hydrogen (HI) content across two constrasting environments: dense Hickson Compact Groups and isolated galaxies. By comparing these two extremes, I aim to disentangle nature from nurture.

I obtained my PhD in Astronomy from the University of Cape Town in 2014. My thesis involves investigating the interplay between star formation and the interstellar medium in nearby galaxies under the supervision of Prof. W.J.G. de Blok.

During my PhD, I developed a stacking technique to study the phase structure of HI in nearby galaxies from The HI Nearby Galaxy Survey (THINGS). By co-adding individual HI velocity profiles, I constructed high signal-to-noise 'super profiles' that revealed two distinct Gaussian components: a narrow component with velocity dispersions of approximately 6.5 km/s and a broad component with dispersions around 16.8 km/s. I associated these with the Cold Neutral Medium (CNM) and the Warm Neutral Medium (WNM), two thermally stable phases of the atomic ISM that coexist in pressure equilibrium.

My analysis showed that the CNM fraction tends to be higher inside the optical radius where star formation is active, and that both components' velocity dispersions decline exponentially with galactocentric distance. I also found that the CNM fraction correlates with total gas column density and star formation rate surface density, supporting theoretical predictions that the transition from warm to cold atomic gas is a critical precursor to molecular cloud formation.

By comparing the observed velocity dispersions with models of supernova-driven turbulence, thermal broadening from ultraviolet photons, and magnetorotational instability, I concluded that supernova feedback can explain the gas kinematics within the star-forming disc, while thermal effects from the extragalactic UV background dominate in the outer regions.

Before joining IAA-CSIC, I held postdoctoral positions at the University of South Africa, at Rhodes University in South Africa, and at the Max Planck Institute for Astronomy in Germany as a Georg Forster Research Fellow of the Alexander von Humboldt Foundation.

2. Latest scientific contributions

2.1 MeerKAT observations of Hickson Compact Groups

I led the data reduction and analysis of a MeerKAT observing campaign targeting six Hickson Compact Groups selected to represent different evolutionary phases. Using the Containerized Automated Radio Astronomy Calibration (CARACal) pipeline, I reduced approximately 50 TB of raw data to produce the most sensitive HI maps of these systems to date. Our observations revealed significantly more extended tidal features in intermediate-phase groups compared to previous Very Large Array (VLA) observations, as well as new high surface brightness features in late-phase groups.

This work supports the evolutionary model proposing that compact groups transition abruptly from an intermediate phase (characterised by complex HI tidal structures) to a final phase where galaxies have lost most of their neutral gas.

2.2 Contribution to open science practice

I support open science practice. I consider it essential for the advancement of astronomy in the era of Big Data, where reproducibility and transparency is not just desirable but necessary for the global community to verify and build upon scientific results. As part of the AMIGA group's contribution to this effort, I have been involved in developing workflows that ensure our MeerKAT observations of Hickson Compact Groups can be fully reproduced.

To make my latest paper fully reproducible, I used Snakemake. It is a workflow management system originally developed in the bioinformatics community that has since found broad application across scientific disciplines. Snakemake allows to define analysis pipelines as a set of human-readable rules, where each step specifies its input files, output files, and the commands to execute. All HI data cubes, moment maps, and analysis scripts from our HCG observations have been made publicly available through Zenodo repositories, with links to GitHub repositories containing the Python scripts and Jupyter notebooks used in the analysis.

2.2 Extended HI halo of NGC 4945

Using MeerKAT, I discovered a large amount of halo gas around the nuclear starburst galaxy NGC 4945 that had been missed by previous HI observations. This extra-planar gas accounts for 6.8% of the galaxy's total HI mass and is most likely material blown into the halo by star formation activity.

I modelled the HI distribution using tilted-ring fitting techniques with TiRiFiC software, characterising the kinematic properties of both the disc and halo components. This work demonstrates how next-generation radio telescopes can uncover previously hidden gas reservoirs around galaxies.

2.3 Ram pressure stripping in isolated environments

My MeerKAT observations of the dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM) contributed to a discovery that the intergalactic medium may be denser than previously thought. The data I collected revealed four extended HI gas clouds trailing behind WLM, representing about 10% of the galaxy's total HI mass, along with a spatial offset between the gas and stellar components.

These findings indicate that WLM is undergoing ram pressure stripping despite being located over 900 kpc from both the Milky Way and M31. This was the first evidence of interaction with such a low-density intergalactic medium and challenges our understanding of dwarf galaxy evolution in supposedly isolated environments.

3. Technical expertise

I have extensive experience in radio interferometric data reduction, particularly with MeerKAT and VLA observations. I have extensive experience with the CARACal pipeline for calibration and imaging, as well as 3D kinematic modelling tools including TiRiFiC, FAT, and pyFAT.

I have extensively used different tools essential for data reduction and analysis of radio astronomical data cubes. I primarily use the CARACal pipeline and custom Stimela scripts for data reduction. More recently, I have begun exploring Bayesian decomposition methods through baygaud-PI to separate multiple kinematic components along individual lines of sight. For data visualization and analysis, I use CARTA, GIPSY, CASA, AIPS, Miriad, and Kvis.

Beyond research, I have developed skills in web development using the Django framework and PostgreSQL database management. I maintain a personal website and have contributed to initiatives promoting reproducible science in the Big Data era (https://arxiv.org/pdf/2601.07439), particularly in preparation for the Square Kilometre Array Observatory (SKAO).

4. Observing experience

I have been Principal Investigator on successful observing proposals with MeerKAT (totalling over 70 hours) and FAST (25 hours), and Co-Principal Investigator on additional MeerKAT projects (over 130 hours) and VLA proposals (168 hours). These programmes have targeted a range of science goals from mapping tidal interactions in galaxy groups to searching for diffuse HI emission in Local Group dwarf galaxies.

5. Supervision and outreach

I currently (remotely) supervise two MSc students at the University of Antananarivo in Madagascar, working on topics including galaxy parameterisation, star formation in nearby galaxies, and kinematic modelling techniques. I have also served as an external examiner for graduate theses at the University of Pretoria.

6. Contribution to the community

I am committed to promoting astronomy in Africa and Madagascar. I co-founded the Malagasy Astronomy and Space Science (MASS) organisation to advance astronomical education in my home country. Together with colleagues, I secured funding from the International Astronomical Union to organise workshops preparing Malagasy students for the 2024 IAU General Assembly in Cape Town.

I also contribute to the broader scientific community by refereeing a paper. I have also served as a proposal evaluator for the Five-hundred-meter Aperture Spherical radio Telescope (FAST), for which I received a certificate of honour.