My Research

Observations of Galaxy Formation and Evolution


Galaxies are fascinating objects in the universe. They are the interface between astrophysics and cosmology. Galaxies consist of stars, interstellar medium, black holes, and dark matter. To understand how galaxies form and evolve with time, we need to understand these celestial bodies as well as physics that governs their motion and their emission and absorption of radiation. On the other hand, galaxies exist in a dynamic universe whose expansion is accelerated by a mysterious form of energy, the so called "dark energy." Galaxies form in mass concentrations created by dark matter. Therefore the distribution of galaxies in the universe and how that changes with time is tightly related to the nature of dark energy and dark matter, the two most dominant components of the universe, which we embarrassingly understand very little.

The physics about galaxies is so rich that there can be numerous ways of studying them. I specialize in imaging observations of distant galaxies at various wavebands, from UV and optical, to millimeter/submillimeter and radio. A multi-wavelength approach is essential, not only because different wavelengths trace different physics, but also because certain types of galaxies may only be bright at certain wavebands and faint at others. Only by combining data from different wavelengths do we obtain complete understanding of the physical processes in galaxies and complete sampling of different types of galaxies.

Submillimeter Galaxies

Among all types of distant galaxies, I am most interested in galaxies detected in the submillimeter wavelengths, the so called "Submillimeter Galaxies (SMGs)." They are distant galaxies that are growing extremely rapidly. The large amount of dust and gas that fuel the formation of numerous young stars also block the light from the stars, making the galaxies very faint in the UV and optical. The UV/optical energy absorbed by the dust is re-radiated in the far-infrared, and hence the galaxies are very bright in the far-infrared and submillimeter. Dusty galaxies like the SMGs are responsible for large fractions of stars formed in the universe, but they are often missed by deep optical surveys conducted with large ground-based optical telescopes and the Hubble Space Telescope. Detecting them in the submillimeter and understanding their properties are therefore crucial for achieving a complete picture of galaxy formation and evolution.

Deep Imaging Surveys

I lead and participate in various imaging surveys of distant galaxies. By mapping the galaxies at different wavelengths, we can measure their stellar mass, mass growth rate, size, gas content, supermassive black hole accretion, distance, and spatial density. These are the key quantities for describing how galaxies form and evolve. Through measuring the spatial distribution of galaxies or carefully measuring the tiny distortion in their shapes caused by gravitational lensing, we can also infer the dark matter mass of the galaxies and the mass distribution in the universe. Making deep images of large numbers of distant galaxies is therefore the most fundamental and efficient way of studying galaxy formation and evolution. Making such deep images are also the essential first step for detailed follow-up observations using spectrographs and radio interferometers.

SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES)

I am leading a large program on the 15-m James Clerk Maxwell Telescope (JCMT) on Maunakea. It uses the SCUBA-2 camera on JCMT to get two extremely deep but narrow images of the 450 micron sky. The goal is to detect faint submillimeter galaxies that give rise to the bulk of the cosmic star formation. It is an ongoing program and is expected to finish in 2020.

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MegaCam Ultradeep Survey—U Band Imaging (MUSUBI)

I am leading a campaign to obtain extremely deep U-band images for the COSMOS and SXDS fields, using MegaCam on the Canada-France-Hawaii Telescope (CFHT). These two fields are also the Ultra Deep Survey fields for the Subaru HyperSuprimeCam SSP Survey. Our U-band images are complementary to the HSC SSP, providing measurements of unobscured star formation, estimates of distance via photo-z, and selections of high-redshift galaxies via the Lyman-Break technique.

CFHT Ultra Deep Ks-Band Imaging of the CDF-N

Our UH team (Prof. Len Cowie, Prof. Amy Barger, and myself) conducted extremely deep CFHT WIRCam Ks-band imaging of the Chandra Deep Field-North (CDF-N), which encloses the Great Observatories Origins Deep Survey-North (GOODS-N) and the Hubble Deep Field. We used the data to study various high-redshift galaxies. We publicly release the Ks data, along with high-quality Spitzer IRAC photometry.

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Taiwan ECDFS Near-Infrared Survey (TENIS)

Dr. Bau-Ching Hsieh and I conducted extremely deep CFHT WIRCam J and Ks imaging of the Extended Chandra Deep Field-South (ECDFS), which encloses the Great Observatories Origins Deep Survey-South (GOODS-S) and the Hubble Ultra Deep Field. We used the data to select luminous z > 7 galaxies. We publicly release the J and Ks data, along with high-quality Spitzer IRAC photometry.

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Other Research

In addition to the above, I am also interested in studying the host galaxies of Damped Lyman-alpha absorption systems at low and high redshifts. I used integral-field spectrographs on Gemini and CFHT to detect their host galaxies in various optical and UV emission lines to study their star formation properties. I am also interested in the host galaxies of gamma-ray bursts. I used ALMA to detect their host galaxies in the dust continuum emission, to measure their obscured star formation and to understand the nature of the extinction in optically dark gamma-ray bursts.

My Team

PhD Student

  • Zhen-Kai Gao