Astrophysics Colloquium schedule
Wednesdays, 3:30-4:30 pm
The Astronomy Colloquia Meetings are held in the P&A building, room 1-434. Zoom information sent in email.
The UCLA Department of Physics & Astronomy Astrophysics group established the Astrophysics Colloquium speaking series 20+ year ago. Throughout the academic year, distinguished speakers travel to UCLA to share their latest research. The department sponsors and organizes these events to encourage collaboration within the field. This exposure is critical for our graduate students particular. The content shared in these events broaden their academic horizons. Additionally, the colloquium provide opportunities for our students to network with other specialists and peers.
Help us continue this enriching speaking series by making a donation online here. Gifts to the Astronomy and Colloquium Fund offset event costs including those related to speaker travel. Support from our event attendees and donors is greatly appreciated. For check instructions or other giving related questions, please contact Madeleine Martin at mmartin@support.ucla.edu or (310) 882-3633.
Spring 2025
April 2, 2025
Gail Schaeferm (The CHARA Array of Georgia State University)
Imaging Stellar Surfaces with the CHARA Array
Abstract: The CHARA Array combines the light of six 1-meter telescopes at optical and near-infrared wavelengths with baselines ranging from 34 to 331 meters to achieve milliarcsecond resolution. The Array is used to measure the sizes of stars, image stellar surfaces, detect close binary companions, and resolve the inner structure of circumstellar disks. I will give an overview of the CHARA Array interferometer and highlight recent science results. I will discuss opportunities to apply for open access time and report on recent developments to add a seventh mobile telescope to expand the maximum baseline of the array.
April 9, 2025
Anna-Christina Eilers (MIT)
The Formation and Growth of Supermassive Black Holes at Cosmic Dawn
Abstract: The existence of luminous quasars powered by supermassive black holes (SMBHs) within the first billion years of cosmic history challenges our understanding of black hole growth. An important piece to the puzzle is the duty cycle and lifetime of the quasars, the fraction of cosmic time that galaxies shine as active quasars and during which the bulk of the black hole growth occurs. In this talk, I will present new avenues towards understanding the early growth phases of SMBHs: I will present the first measurement of the clustering strength between luminous quasars and their surrounding galaxies at z>6 using recent observations from the James Webb Space Telescope (JWST) that allow us to estimate properties of the quasars' host dark matter halos and their duty cycle. Secondly, I will show new results leveraging some of the deepest JWST observations of galaxies in the background and environment of luminous quasars at z>6 to tomographically map the quasars’ ionized bubbles, constraining the extent and geometry of the ionized region, as well as the quasar's lifetime. We find that SMBHs seem to grow on much shorter timescales than expected, providing a potential solution to the long-standing puzzle of early black hole growth.
April 16, 2025
Davy Kirkpatrick (Caltech/IPAC)
The (Sub)Stellar Initial Mass Function (8 MSun to 5 MJup) Based on the 20-pc Census
Abstract: In recent years, we have taken dramatic steps forward in constructing a more complete view of the stellar and substellar constituents within the immediate vicinity of the Sun. Gaia has measured exquisite parallaxes on objects as cold as mid-L within 20 pc and continues to provide updated multiplicity statistics. A combination of WISE discoveries plus Spitzer and ground-based parallaxes has enabled us to fill out the rest of the Sun's nearest census, at least down to early-Y dwarfs. Using this 20-pc census, we are now able to study the initial mass function in the field across a range of masses from 8 MSun down to at least 5 MJup. As this census represents the closest and most thoroughly studied sample we have, our aim is to avoid the multiplicity biases that have plagued other surveys by decomposing each system into its individual components and providing mass estimates for each. In this talk, I discuss the process of constructing the census, how we deduce the shape of the mass function from the statistics, and what the results could mean for the underlying star formation process itself.
April 23, 2025
Jim Dunlop (University of Edinburgh)
Charting early galaxy formation and growth with JWST and ALMA
Abstract: The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the
PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with
other public JWST imaging, is now enabling us to chart the emergence of the galaxy
population back to within ~300 million years of the Big Bang. Specifically, I will present and
discuss the first robust determination of the evolving UV galaxy luminosity function extending
out to redshifts z~13, as well as new measurements of the galaxy stellar mass function
reaching out to z~9. I will then interpret these results in the context of our current
understanding of the evolving dark matter halo mass function and the efficiency with which
galaxies are able to convert their baryons into stars. Finally, I will discuss new results on the
early growth of dust-enshrouded star formation, and the prospects for future progress
exploiting the combined power of JWST and the Atacama Large Millimeter Array (ALMA).
April 30, 2025
Tomer Yavetz (IAS)
Bars and Spirals: Deciphering the Dynamical Clues Hidden in the Gaia Data
Abstract:
The striking morphologies of spiral galaxies have long captured the imagination of both the general public and professional astrophysicists. Yet even basic questions about their diversity of dynamical structures — how they form, how they evolve, and why they appear in some disk galaxies but not others — remain largely unresolved. Our own Milky Way, with its central rotating bar and spiral arms, offers a unique window into the dynamical underpinnings of these features. 
As the only galaxy where we can trace the motions of individual stars in detail, it holds the key to understanding spiral structure and bar dynamics more broadly. But our vantage point within the disk complicates the picture: even with Gaia’s extraordinary six-dimensional stellar data, our view is limited to a local patch, and much of the global dynamical structure remains obscured. Still, that local patch is rich with dynamical clues and hints. Armed with new theoretical tools — adapted from fields ranging from plasma physics to cosmology — our ability to extract global information from local measurements has made significant progress. In this talk, I will show how new projections of the Gaia data, guided by a deeper theoretical understanding of galactic dynamics, are allowing us to reveal the transient nature of the Milky Way’s spiral arms, and to constrain the mass, pattern speed, and slowdown rate of its central bar.
May 7, 2025
Fei Dai (U. Hawaii)
Resonant Chains as the Initial Configuration of Kepler-like Planetary Systems
Abstract:
Ancient philosophers like Pythagoras and Kepler speculated that the planets of the Solar System might be arranged in resonant orbits—where their periods follow simple integer ratios (e.g., 3:2, 2:1)—as part of a Grand Design. However, except for Neptune and Pluto, the current-day Solar System planets are largely non-resonant. Similarly, most of the ~1000 confirmed exoplanetary systems (typical age of ~5 Gyr old) also exhibit non-resonant configurations. Our work recently revealed that the vast majority (~80%) of planetary systems younger than 100 Myr appear near (but not in) mean-motion resonance. We also found the fraction of resonant systems decreases steadily with time. 
In this talk, I will discuss how convergent disk migration naturally leads to the formation of resonant planetary chains and how subsequent dynamical evolution disrupts these resonances. I will also highlight how transit timing variations (TTVs) can serve as a powerful tool to probe the dynamic state of young planetary systems. Resonant-chain planetary systems serve as ideal testbeds for the evolution of planetary atmospheres. Finally, I will compare and contrast the non-resonant Solar System with near-resonant young planetary systems.
Brief bio: Dr. Dai is currently a faculty member at the Institute for Astronomy, University of Hawaii. Previously, he was a NASA Sagan Fellow at Caltech. He earned his PhD in Physics from MIT and his Bachelor's degree from the University of Cambridge.
May 14, 2025
May 21, 2025
Julian Munoz (University of Texas)
New cosmological insights from JWST
Abstract: The James Webb Space Telescope (JWST) has revolutionized our understanding of the early universe through its unprecedented infrared capabilities. This talk will explore how early JWST observations fit — or don’t — within our cosmological and galaxy-formation models. I will first discuss the higher-than-expected abundance of early (z>9) galaxies in JWST, and how clustering measurements are key for understanding its origin. I will then show how to use Hubble data to constrain the so-called "universe breaker" galaxies believed to be too massive to exist in LCDM. Finally, I will examine how recent JWST determinations of cosmic reionization may be at odds with complementary cosmological probes, including the CMB and the Lyman-alpha forest, and discuss potential solutions and implications for both cosmology and astrophysics.
May 28, 2025
Yoram Lithwick (Northwestern)
Enceladus and its Limit Cycle
Abstract: Enceladus is a moon of Saturn. It has some remarkable features, including geysers that spray water into space; a global ocean beneath its surface ice shell; and a large luminosity. I will describe how these phenomena are consequences of Enceladus being in a "limit cycle": it ice shell freezes and thaws on a timescale of around 10 million years, as its orbit gets closer and further to its neighbor moon Dione.
June 4, 2025
Tommaso Treu (UCLA)
To announce new H0 results