Prachi Prajapati

Doctoral Researcher
International Max Planck Research School (IMPRS),
Bonn | Cologne

I work in multiwavelength observational cosmology, studying the formation and evolution of galaxies in the early universe. My PhD with Prof. Dominik Riechers (University of Cologne / MPIfR Bonn) has produced Vz-GAL, the far largest CO(1-0) cold gas catalog of dusty star-forming galaxies (DSFGs) across redshifts 1 to 6, a benchmark dataset until the advent of the ngVLA. Using CO(1-0), higher-J CO lines, [CI], and dust continuum observations, together with semi-empirical excitation models, my research constrains gas masses, depletion timescales, star formation efficiency, and gas-dust excitation in these distant galaxies.

Previously, I served as a Scientist/Engineer at the Physical Research Laboratory, Department of Space, Government of India, where I contributed to cutting-edge multiwavelength astronomy projects and the development of advanced ground-based optical and near-infrared instrumentation.

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About

Hello! I am Prachi, an astrophysicist of Indian origin. I graduated with a major in Astrophysics from the Indian Institute of Space Science and Technology (IIST), after which I worked at the Physical Research Laboratory (PRL) as a Scientist/Engineer for nearly four years.

My research delves into the evolution of galaxies and physics of star formation in the early universe. By leveraging multi-wavelength datasets, ranging from radio to far-infrared, I integrate cutting-edge observations with semi-empirical models to precisely constrain gas properties, excitation processes, and cold gas dynamics in these distant galaxies.

I am currently a graduate researcher at the University of Cologne and hold a guest researcher position at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. I collaborate with international teams across various leading observatories, and I am closely involved in projects such as the Vz-GAL and z-GAL, where I actively contribute to frontier research in the field. I am also a part of the SFB 1601, a collaborative research center by the German Science Foundation (DFG).

Research Interests

  • Multiwavelength Observational Cosmology
  • Large Surveys of High-z Galaxies
  • Galaxy Formation and Evolution
  • Cosmic Star Formation History
observations_galaxies
DSFG_early_universe

Research Projects

Cold Molecular Gas in Dusty Galaxies at Redshift z = 1 - 6

VLA Vz-GAL is a CO(1-0) large program (PI: Riechers, PhD Thesis: Prajapati), tracing cold gas — fuel for star formation — in 106 Herschel-selected high-redshift dusty star-forming galaxies (DSFGs). These objects have spectroscopic redshifts from the NOEMA z-GAL survey mainly using higher-J CO transitions. Vz-GAL robustly measures molecular gas masses, depletion timescales, star formation efficiencies, CO brightness temperature ratios, and gas-to-dust mass ratios — serving as the foundation for detailed studies of the interstellar medium (ISM) in DSFGs.

Observations Statistical Survey CO(1-0) Catalog VLA NOEMA Herschel see Prajapati+2025

Gas Excitation in Dusty Galaxies Across Cosmic Epochs

Using Vz-GAL and z-GAL results, we simultaneously analyze CO SLEDs and dust SEDs with the TUNER, a semi-empirical TUrbulent Non-Equilibrium Radiative transfer model. Relative to two-component (cold and warm) models, TUNER recovers physically tied gas and dust properties, gas-to-dust mass ratios, and the CO-H2 conversion factors with fewer ad-hoc assumptions. I am currently applying TUNER to a sub-sample of Vz-GAL galaxies with comprehensive CO SLEDs and comparing the results to local galaxies to lead the first direct, detailed comparison.

Semi-empirical Modeling Radiative Transfer ISM Properties Prajapati+(in prep.) TUNER: Harrington+2021; Boogaard+(in prep.)

Cold Molecular Gas Dynamics in DSFGs

Vz-GAL catalog includes numerous lensed DSFGs. These systems are gravitationally magnified, providing a key opportunity to probe their gas dynamics at sub-kiloparsec scales. To follow up one such DSFG for studying its resolved star formation, our high-resolution CO(1-0) observations are underway (VLA/25B-147, PI: Prajapati). This galaxy, HerS-3, is an exceptional Einstein cross with the central fifth image. Its detailed multiwavelength observations are presented in Cox+2025, including low-resolution CO(1-0) Vz-GAL results from Prajapati+2025.

Observations HST ALMA NOEMA VLA VLA/25B-147: ongoing observations

Alternative Cold Gas Tracer in High-z DSFGs

In Prajapati+2025, we establish [CI](1–0) as a viable cold gas tracer at high-z, showing consistent [CI]/CO ratios across redshift. This already suggests a common baseline state of ISM across various galaxy populations, upon which different star formation modes, efficiencies, and scales are built. In the future, I aim to statistically compare [CI](1–0) and CO(1–0) across cosmic time, incorporating gas excitation models and accounting for the impact of rising CMB temperature.

Semi-empirical Modeling Observations NOEMA VLA see Prajapati+2025

Near-infrared Astronomical Site Characterization (Past)

I analyzed near-infrared (NIR) data collected between 2010 and 2019 at the Mount Abu Infrared Observatory (MIRO) to assess variations in the NIR sky background. We identified potential contributors to the NIR background in our observations and explored the possible factors behind the observed seasonal fluctuations, with summer showing a brighter NIR sky. Overall, no systematic variation was detected throughout the observation period, supporting the reliability of the site for NIR astronomical studies.

Near-infrared Observations Sky Background Site Characterization see Prajapati+2023

Particle Acceleration in Wolf-Rayet Bubbles (Past; MS Thesis)

In numerous scenarios supersonic winds of massive stars have been proven to produce shocks in which relativistic particles are accelerated emitting nonthermal (NT) radiation. We reported the first detection of NT emission from a single Wolf-Rayet stellar bubble associated with a WO star. Our results using the upgraded Giant Meterwave Radio Telescope (uGMRT) radio continuum data showed that non-runaway isolated massive stars are capable of accelerating relativistic particles and are therefore confirmed as sources of Galactic cosmic rays.

Observations uGMRT Massive Stars see Prajapati+2019

Contact

Office

Room 211, I. Institute of Physics (PH1), University of Cologne, 50937 Germany


Email

prajapati [at] ph1.uni-koeln.de

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