Research

We live in an elegant Universe that amply rewards observing and pondering. Trying to understand Nature stirs our intellectual appetite, raising more questions than we can answer and allowing us to reach profound realizations that stretch beyond the time and space we live in. Personally, this gives me intellectual satisfaction and makes life more meaningful.

During my career as an astrophysicist, I had the immense pleasure of contributing to investigations into various astrophysical problems, including the search for dark matter and the discovery and characterization of planets beyond our Solar System. I enjoy building statistical methods to test, select, or refine hypotheses in light of observations. Although I am easily drawn to the wonders of our Universe, I find dark matter and exoplanets exciting because they both relate to the cosmological origins of our planet and our species. And statistics excites me because inference is the essence of the scientific method, which is the quest to obtain consistent observations and models.

I serve as the PI of one of the Wide Field Science (WFS) programs of the Roman Space Telescope, currently scheduled for launch by the end of 2026. I also serve as one of the US PIs of the ULTRASAT Participation Program. My research program is predominantly computational, and my group maintains and uses a suite of high-performance computing resources.

A list of my research publications are available on NASA ADS or Google Scholar. The analysis and modeling pipelines I develop, myself or with group members, are available in my GitHub repositories or in the GitHub repositories of AstroMusers, respectively.

Exoplanets

Planets beyond our Solar System are intriguing objects that place our planet Earth in a cosmological context. They allow us to study the formation and evolution of planets and their atmospheres. The Transiting Exoplanet Survey Satellite (TESS) is a NASA space telescope that surveys the sky for transiting exoplanets. As a vetting lead for the TESS mission, I have worked on delivering more than 2100 exoplanet candidates and on detecting and characterizing hundreds of exoplanets using TESS data.

Exoplanets have a surprisingly broad range of radii, masses, equilibrium temperatures, and types of host stars. Some exoplanets are detected by the periodic dimming of the host star as they transit our line-of-sight, allowing the inference of the planet radius. Another class of detections is based on the periodic shift in the spectral features of the host star induced by orbital reflex motion, in which the planet's mass can be bounded from below. The former requires a geometrically rare alignment, whereas the latter requires the host star to be bright enough for high-resolution spectroscopy.

Hunting for transiting exoplanets in multiplanetary systems

Transiting planets hosted by bright stars are especially opportune, as they become amenable to precise radius and mass measurements, yielding insight into the planet's bulk composition. Furthermore, they potentially enable atmospheric characterization, with broad implications for probing atmospheric escape and biosignatures. An even more interesting situation is when multiple such planets transit the same bright star, as the resulting multiplanetary system enables comparative studies of the formation, dynamics, and atmospheric evolution of the planets.

In this context, a significant highlight from the TESS mission is the discovery of four small, transiting exoplanets hosted by the bright, Sun-like star HD 108236, also known as TOI 1233 (Daylan et al. 2021a). The system contains a hot, likely rocky super-Earth and three outer sub-Neptunes with gaseous envelopes.

Atmospheres of hot Jupiters

An interesting class of exoplanets is hot Jupiters, which are much hotter analogs of our Jupiter and are inflated by the intense irradiation from their host star. The very existence of hot Jupiters is intriguing since they likely migrated to their observed orbits long after their formation, farther away from their host stars. Their atmospheres are also of interest, as they provide an observationally accessible laboratory for studying atmospheric dynamics on exoplanets. Towards this end, we analyzed the TESS phase curve of WASP-121b, an ultra-hot Jupiter. We measured the nightside temperature and found that its redistribution of heat was inefficient (Daylan et al. 2021b).

Cosmology

Dark Matter

First inferred to explain the mass of the Coma cluster by Fritz Zwicky in 1933, dark matter is a non-luminous matter component of our Universe whose existence is inferred from its gravitational interactions and its effect on the structure of the Universe. It is gas-like, transparent, and does not interact with light.

Given the absence of consistent evidence of its particle nature, dark matter continues to elude us despite eight decades of research. There are three main ways to search for dark matter: direct detection, indirect detection, and production. Direct detection relies on searching for rare particle interactions in a low-background underground particle detector. Second, high-energy particle colliders search for collision products that escape the detector undetected. Finally, indirect detection consists of searching for astrophysical signals, such as cosmic-ray and high-energy photon fluxes, that can be explained by dark matter but not by the Standard Model of particle physics.

The gamma-ray excess in the inner Milky Way

An enigmatic feature of the gamma-ray sky, as surveyed by NASA's Fermi-LAT telescope, is anomalous emission from the center of our galaxy in excess of expectations based on its estimated cosmic-ray, gas, and dust content. The excess may be due to a population of millisecond pulsars in the galactic bulge, though this population would have to comprise a surprisingly large number of dim millisecond pulsars.

An intriguing feature of this emission is that it is also consistent with the simplest models of dark matter. In a highly cited work (Daylan et al. 2016), we characterized this excess emission. We showed that the spatial morphology, amplitude, and spectrum of the excess are consistent with dark matter, where weakly interacting massive particles annihilate with a thermal cross-section to high-energy photons that constitute the excess.

Cosmic Rays

The Alpha Magnetic Spectrometer 2 (AMS-02) is a particle detector on the International Space Station (ISS), which measures the cosmic-ray flux from outer space. It has a magnet and a silicon tracker to bend and measure the momentum of charged particles, an electromagnetic calorimeter to measure the particle energy, and other auxiliary subdetectors to identify incident particles. AMS-02 measures interesting observables, such as positrons and antihelium, whose production could be a signature of dark matter annihilation. I was an undergraduate research assistant at CERN between 2011 and 2013, where I implemented an improved event reconstruction algorithm for the detector.

Statistics

Astrostatistics

Transdimensional Bayesian inference

There are many problems in science, where the number of unknowns is unknown. Therefore, a robust inference framework that allows marginalization over the unknown degrees of freedom is desirable. The Probabilistic Cataloger (PCAT; Daylan et al. 2017) is a transdimensional and Bayesian sampler that allows fair sampling from the posterior probability distribution of a metamodel, i.e., a model that contains models with different numbers of parameters. This allows transdimensional uncertainties to be robustly propagated to the marginal posterior probability distribution of interest.

Probabilistic cataloging has been successfully used to model the isotropic gamma-ray emission in the northern galactic polar cap (Daylan et al. 2017), dark matter subhalos in strong gravitational lenses (Daylan et al. 2018), dim point sources in crowded SDSS images (Portillo et al. 2018), and measurement of thermal and relativistic Sunyaev–Zeldovich effect in submillimeter Herschel-SPIRE data (Butler et al. 2022, Feder et al. 2023).

Below is an illustration of probabilistic cataloging. The plot on the left emphasizes the multimodality in the catalog space. The image on the right shows fair samples drawn from the posterior of the catalog space, consistent, up to Poisson likelihood, with the gamma-ray sky towards the North Galactic Polar Cap, i.e., if you look towards the zenith, taking the galactic plane as datum.

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