Research
Current Projects
The connection between galaxy formation and cosmic web environment
Matter in our universe is woven into a large-scale distribution we call the cosmic web. While these structures are much larger than galaxies and their surrounding CGM, it has been known for decades that many internal galaxy properties are related to the external large-scale cosmic web environment, implying a cosmic connection that percolates from very large to much smaller scales. Observations of the cosmic web are quite limited as much of the intergalactic gas comprising the cosmic web cannot be detected by our instruments and galaxy surveys are also limited by the distance we can observe galaxies out to, resulting in limited samples for the vast majority of the universe. Thankfully, numerical simulations that include the necessary physics of the universe run on very powerful supercomputers have cooked up simulated universes which appear to agree remarkably well with our real universe in many aspects. This can tell us about the physics of structure formation from large to small scales.
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In my current work, I am analyzing the IllustrisTNG cosmological simulations to understand how the cosmic web environment, particularly the proximity to filaments and nodes of the cosmic web, affects the quenching, or cessation, of star formation and gas supply of galaxies. Our results suggest an important phase change in the cosmic web-dependence of star formation with time: cosmic web environment only seems to affect star formation quenching at redshifts <2, i.e. in the last 10 billion years, while at higher redshifts (in the first ~4 billion years) star formation is independent of cosmic web environment. Check out our paper for more: Hasan et al. (2023).
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I am involved in several follow-up projects that aim to understand these trends physically and also quantify the impact of cosmic web environment on galaxy formation - in both simulations and observations. One of these projects is a comparison between the galaxy formation-cosmic web connection in TNG and the CHILES survey. I have also been recently awarded a Hubble Space Telescope Archival Research grant for Cycle 31 to study gas properties in different cosmic web regions as traced by absorption lines in quasar spectra.
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The slime mold cosmic web
Reconstructing the cosmic web out of sparse inputs (typically galaxy positions) is a difficult and complicated task that has been approached in a variety of ways by different groups of researchers. Our group, in particular Oskar Elek (UCSC), has developed a novel method to produce a continuous 3D matter density field and therefore the filamentary structure of the cosmic web inspired by the feeding habits of the Physarum Polycephalum, or slime mold, micro-organism. This algorithm, which we call the Monte Carlo Physarum Machine (MCPM), has been applied to both observational and simulated datasets to reconstruct the cosmic web. We have found that when using the cosmic density field from MCPM, the widely used DisPerSE topological algorithm can identify filaments with much higher fidelity than with the sparse density field estimate from the traditional Delaunay Tessellation Field Estimator (DTFE). This analysis, in the TNG simulations, has yielded fresh new insights into the differences between less prominent (thin) and more prominent (thick) filaments and how they affect galaxy formation. Check out our recently-submitted paper introducing these filaments of the slime mold cosmic web in the TNG simulations
This work has also received some press after its release
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We are in the process of applying this method to produce the most detailed catalog of filaments ever in the Sloan Digital Sky Survey out to redshift ~0.5 (~5 Gyrs ago), and hope to apply it to other observational galaxy surveys!
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Check out below the talks focused on this research that I presented at multiple conferences:
Filaments (curves colored by their prominence) identified from the traditional DTFE density field (left) and the slime mold-inspired MCPM density field in DisPerSE in TNG (z=0). The underlying dark matter distribution (blue color-bar) and galaxies (grey circles) are traced far better with MCPM filaments.
MCPM-reconstructed cosmic web structure from Sloan Digital Sky Survey observations at low redshift; zoomed-in insets show the input data and weight (gold) and filamentary structures (magenta) in individual regions. [see Burchett et al. (2020)]
Metal enrichment history of the universe
Metals - elements heavier than helium - are crucial tracers of galaxy formation processes as virtually all of them were made by stars. By tracing metals in galaxy ecosystems, we can therefore probe cosmic star formation activity as well as related phenomena such as the intensity of the background radiation produced by galaxies.
In Hasan et al. (2020), we presented a survey of CIV - triply ionized carbon - absorbers obtained from high-resolution quasar spectra from the Keck Observatory (Mauna Kea, Hawaii) and the Very Large Telescope (Mt. Paranal, Chile).
We found that the incidence of CIV absorbers increases across the last ~12.5 Gyr (z=6 to 0) of cosmic time, with stronger absorbers (higher density gas) evolving more rapidly than weaker absorbers (lower density gas). However, there is a rapid rise in weak CIV in the last ~8 Gyr (z=1 to 0), which is unexpected since the universe became a lot less vigorously star-forming and the extragalactic background radiation became weaker in this epoch.
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In Hasan et al. (2022), we extended this analysis to include galaxies and the dark matter (DM) halos in which they live. We found that statistically, the weakest - i.e. very low-density - CIV absorbers likely live in the outskirts of galaxy halos and may even live in the diffuse IGM, very far from the stars they originated from. The dramatically different evolution and physical extent of high and low density absorbers likely means that they were produced by different physical mechanisms, possibly hinting at rapid star formation in low mass galaxies in the early universe.
The main result of Hasan et al. (2022) in cartoon form: high-mass galaxies have more extended metal-enriched gaseous envelopes than low-mass galaxies. Check out this post as well, which includes a brief video explanation.
The evolving cosmic incidence of CIV absorbers across ~12.5 Gyr; from Hasan et al. (2020)
Visualizing cosmological simulations with CosmoVis
I am the science development lead of the open-source software CosmoVis, developed at Creative Coding Lab at UC Santa Cruz, which allows for 3D data visualization and interactive real-time analysis of cosmological simulations such as IllustrisTNG and EAGLE.
Click here to open CosmoVis in your browser, ​Click here for the GitHub page.
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With CosmoVis, one can visualize the distribution of stars, gas, and dark matter (DM) particles in these simulations, identify galaxies, analyze cosmic web filaments and nodes, and even produce synthetic quasar absorption line spectra to make functional comparisons with observations. Some of my collaborators and I are using CosmoVis to understand the gaseous conditions around filaments and nodes and how this affects star formation and other important aspects of galaxy evolution. Ultimately, we will use our insight from simulations to better understand our interpretations of observations of galaxies and the cosmic web and to gain further insight into the physical mechanisms of structure formation in the universe.
A filamentary structure in stars and gas temperature (left) and gas entropy (right) in IllustrisTNG. Left: The distribution of reddish-orange hues imply that warm/hot (T>10^5 K) is ubiquitous. Right: The reddish-orange "shells" of high entropy surrounding bluer centers of filaments and nodes with low entropy suggests plausible signatures of shock heating from the formation of these structures. While this is very hard to observe in real life, it has important implications for how galaxies form!
Simulated background sight-lines running through a massive halo in EAGLE, with the neutral hydrogen Lyman alpha absorption line along one sightline (white) shown near the top
Past Projects
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Galaxies and supermassive black holes in the local universe: For my undergraduate thesis, I investigated the statistics of galaxies and supermassive black holes in the local universe (z<0.1, or within about a couple billion light years from us). Our work confirmed that the galaxy population is dominated by low-mass galaxies and that high-mass galaxies are exceedingly rare. We also showed that there could be significant differences in the observationally measured velocity dispersion of galaxies depending on the method of observation and apparent galaxy geometry. Further details can be found in my thesis.
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Star formation in NGC 4736: I measured the masses and ages of young star clusters (<1 Gyr old) in the nearby spiral galaxy NGC 4736 from photometric images in different wavelength bands. We used these properties to estimate the star formation rate near the center of this galaxy.
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Star formation in dusty galaxies: In my first research project as an undergraduate, I studied the star formation activity and dust content of galaxies at z~1. We utilized Hubble and Herschel Space Telescope data to measure dust excess and star formation rates from both nebular emission lines and infrared continuum luminosity.​​
Click on the posters below to find out more about these projects
AAS 230 (Austin, TX): Star formation in dusty z~1 galaxies
AAS 231 (National Harbor, MD): Star formation in NGC 4736
AAS 233 (Seattle, WA): Galaxies and massive black holes in the local universe