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Vidit standing in front of a poster at a conference.
Vidit standing in front of a poster at a conference.
Vidit Bhandari is a junior at Denison University pursuing Physics & Data Analytics. This research was conducted at The Ohio State University under the Battelle Science Internship, supervised by Dr. Anil K. Pradhan and Dr. Sultana N. Nahar. The work combines spectroscopic analysis with computational methods to understand early universe galaxies. His main astronomy research interests are stellar evolution, galaxy evolution, and cosmology. When he is not doing astronomy, he loves playing cricket, reading/hearing about geopolitics, and enjoying the outdoors. He hopes to pursue more research experience (if possible!) before pursuing PhD in Physics/Astronomy after he graduates.
How did the first galaxies form and evolve? What were their chemical compositions? These questions have puzzled astronomers for decades. Now, with the revolutionary capabilities of the James Webb Space Telescope (JWST), we can peer back to when the universe was just 2.18 billion years old – less than 16% of its current age.
Our research focuses on the gas in star forming regions of galaxy Q2343-D40, nicknamed the “Cecilia Galaxy,” located at a redshift of z=2.96. At this distance, we’re observing the galaxy during a crucial period of cosmic history when galaxy formation was at its peak. Using the unprecedented infrared sensitivity of JWST’s Near Infrared Spectrograph (NIRSpec), we can now detect and analyze spectral lines that were previously invisible to us.
The key to understanding this ancient galaxy lies in its spectral fingerprints. Using our own collisional–radiative–recombination modelling code that characterizes various physical features using line ratios, called SPECTRA, we analyzed the emission lines from SII and OIII. Think of these emission lines as cosmic DNA; they reveal the galaxy’s physical conditions and chemical makeup. The ratio of SII lines (6717 Å/6731 Å) indicated that the gas temperature was between 10,000-20,000 Kelvin and the density was around 300 cm⁻³. To identify the exact temperature, we created a 3D ionization balancing model of temperature, density, and OIII line ratio (5007 Å/4363 Å). Given the known density and OIII line ratio of 2.5, we constrained the gas temperature to be approximately 13,000 Kelvin, as is demonstrated in Figure 1.
A graph of density vs. temperature, with a heatmap representing OIII line ratio. An ellipse marks the known line ratio and density, and derived temperature.
Figure 1: Temperature-density diagnostic plot for the Cecilia Galaxy (Q2343-D40) using OIII line ratios. The contours represent the logarithmic line ratio of OIII emission lines. The white ellipse marks our derived physical conditions, indicating a temperature of approximately 13,000 K and electron density of around 10² cm⁻³. This diagnostic plot demonstrates how emission line ratios can reveal the physical conditions in galaxies too distant to study by other means. Image credit: Vidit Bhandari
One of our most intriguing findings is the galaxy’s oxygen abundance, measured at 8.05 (in units expressed as 12 + log(O/H)). For comparison, the oxygen abundance of the Sun is 8.69. This lower-than-solar value isn’t just a number – it tells a story. Early universe galaxies, like Cecilia, haven’t had enough time to produce and accumulate heavy elements through multiple generations of stars. It’s like catching a glimpse of a cosmic teenager, still in its developmental phase.
An animated graph, of universe age vs. oxygen abundance, showing that oxygen abundances increases over time.
Figure 2: The evolution of oxygen abundance over the age of the universe, as derived from JWST observations and spectroscopic analysis. This plot highlights the relatively low oxygen abundance of galaxy Q2343-D40 at z=2.96 (shown at its corresponding age), illustrating its early stage of chemical evolution during the peak epoch of galaxy formation. Image credit: Vidit Bhandari
Oxygen is particularly fascinating because of its fundamental role in the universe. As the most abundant element after hydrogen and helium, oxygen is a cornerstone for building molecules essential to life, like water, and plays a key role in planetary habitability. Its abundance in galaxies provides a window into the chemical evolution of the universe. In galaxies like Cecilia, observing relatively low oxygen abundance allows us to study the earliest stages of this process, offering insights into how the building blocks of water and potentially life were seeded across the cosmos.
These findings don’t just validate our analysis methods; they open a window into understanding how galaxies evolve chemically over cosmic time. As we expand our analysis to more galaxies at even higher redshifts, we’re building a more complete picture of how the universe’s chemical complexity emerged from the simplicity of the early cosmos.
This research demonstrates the immense potential of JWST in revolutionizing our understanding of galaxy evolution. Each galaxy we study at these high redshifts adds another piece to the cosmic puzzle, helping us understand how we got from the pristine early universe to the chemically rich cosmos we inhabit today.
Astrobite edited by: Annelia Anderson
Featured image credit: Vidit Bhandari