I am Jiaxuan Li 李嘉轩 (pronounced as Lǐ Jiā Xuān ), a graduate student at Department of Astrophysical Sciences, Princeton University.

I'm an observer interested in a variety of topics in astronomy and astrophysics, such as galaxy formation and evolution, low surface brightness astrophysics, sky surveys, machine learning, and instrumentation. My current research primarily involves uncovering the formation and evolution of dwarf galaxies through both exquisite observations and numerical simulations.

I have been actively engaging with data from cutting-edge deep sky surveys such as the Hyper Suprime-Camera (HSC) Subaru Strategic Program (SSP). With my advisor Jenny Greene and her team at Princeton, we systematically studied the Ultra-Puffy Galaxies™ around galaxies similar to our Milky Way. These puffy galaxies are defined to be 1.5-sigma above the average mass-size relation. Compared with the commonly-used Ultra-Diffuse Galaxy concept, our definition for ultra-puffy galaxies, based on the mass-size relation, better represent the tail of the satellite size distribution. Surprisingly, the quenched fraction (the fraction of galaxies that are quiescent in star formation) of these ultra-puffy galaxies is very similar to normal-sized satellites of Milky Way-analogs, implying that quenching is not tied to being a mass-size outlier. These results will shed light on the formation and evolution mechanism of puffy satellites, and challenge the current theoretical models. Please check out Li et al. (2023a) and Li et al. (2023b) for details.

Recently, I am keen on harnessing advanced machine learning techniques to address astrophysical challenges. With Peter Melchior, Song Huang, ChangHoon Hahn, we developed a novel framework, "PopSED", for population-level inference of galaxy properties from photometric data. Unlike the traditional approach of first analyzing individual galaxies and then combining the results to determine the physical properties of the entire galaxy population, we directly make the population distribution the inference objective. PopSED reliably recovers the redshift and stellar mass distribution of 100K galaxies using broadband photometry within 1 GPU-hour, being 1 million times faster than the traditional SED modeling method. With the unprecedented number of galaxies in upcoming surveys, our method offers an efficient tool for studying galaxy evolution and deriving redshift distributions for cosmological analyses. Check out our paper here!

I worked on Hyper Suprime-Camera (HSC) data to study the stellar halo of massive galaxies. Together with Alexie Leauthaud, Song Huang, John Moustakas and the [Dragonfly](http://dragonflytelescope.org/) team, we explored the ability of different sky surveys (HSC, DECaLS, Dragonfly and SDSS) to extract the stellar halo light profiles of massive galaxies. We effectively addressed the major systematic error (sky background subtraction) in measuring the surface brightness profiles. We push the detection limit of HSC to 30 mag per square arcsec in r-band and find good agreement between DECaLS and HSC measurements (less than 0.05 dex difference on stellar mass measurements). Check out our paper: Reaching for the Edge I: Probing the Outskirts of Massive Galaxies with HSC, DECaLS, SDSS, and Dragonfly.

I helped Prof. Pieter van Dokkum (Yale) on developing and maintaining the "Multi-resolution filtering (MRF)" technique (van Dokkum et al. 2019), which is designed to isolate extended low surface brightness emissions in the Dragonfly imaging data. Using this technique, we are aiming to discover unseen low surface brightness objects including ultra-diffuse galaxies, tidal disruption debris and Galactic cirrus in the Dragonfly Wide Field Survey. I'm also working on the Dragonfly images of NGC 1052 field to constrain the total mass of NGC1052-DF2 and DF4, which are believed to be lacking dark matter.

Advised by Prof. Yingjie Peng (KIAA), I utilized MaNGA IFU data and discovered a population of galaxies which have H-alpha ring-like emission structures. We studied their properties and found high bar fraction, high AGN (Seyfert & LINER) fraction and high bulge-to-total ratio. This might support that it is the joint effect of bar, bulge and AGN that quenches the spiral galaxies in an inside-out mode.