How Galaxies Like the Milky Way Became What They Are Link to heading

Other Research Themes: How do galaxies quench star formation? | How do galaxies acquire their mass?

Overview Link to heading

Galaxies are shaped by the interplay between internal dynamics and external accretion over billions of years. To understand how the Milky Way and similar galaxies achieved their current structure, we must study the dynamical imprints of past feedback events and the circumgalactic medium at low redshifts. By examining nearby and low-redshift galaxies, we reconstruct their assembly histories and reveal how gravitational processes, star formation, and supermassive black hole activity have left lasting signatures on galactic structure and kinematics.

Why It Matters Link to heading

The Milky Way is our laboratory for understanding galaxy evolution. However, many of the most dramatic processes shaping galaxies happen at vast distances from the galactic center and are invisible to conventional observations. Studying the extended gaseous halos and dynamical outflows around galaxies reveals the mechanisms by which galaxies regulate their own growth, preventing runaway star formation and maintaining the balance between gravitational assembly and feedback-driven outflows.

Understanding Milky Way evolution also provides critical context for interpreting distant, high-redshift galaxies. The structural features we observe in the Milky Way today—its disk, halo, streams, and giant outflows—are the fossil record of its evolutionary journey.

Our Approach Link to heading

We employ ultraviolet and optical spectroscopy to trace gas kinematics and physical conditions in galactic outflows and circumgalactic gas. The Fermi Bubbles, giant lobes of plasma extending above and below the Galactic Center, serve as powerful tracers of recent nuclear activity in the Milky Way. Through UV spectroscopy of background quasars and stars, we measure outflow velocities, constrain kinematic ages, and estimate the mass and energy budget of these dynamic structures.

Complementary studies of the low-redshift circumgalactic medium around nearby galaxies, traced through absorption line spectroscopy, reveal the extended gas reservoirs that regulate galaxy evolution. By mapping these halos in detail, we understand how past feedback events shaped the gaseous environments we observe today.

The Fermi Bubbles: A Window Into Recent Galactic History Link to heading

The Fermi Bubbles are among the most striking features of the Milky Way’s circumgalactic medium. These giant structures extend approximately 55° above and below the Galactic plane, visible across gamma-ray, microwave, and radio wavelengths. Using ultraviolet absorption-line spectroscopy from the Hubble Space Telescope, we constrain the velocity and properties of gas within these bubbles.

Our work revealed that all sightlines passing through the Fermi Bubbles exhibit high-velocity absorption components indicative of outflowing gas, with velocities ranging from -265 km/s near the Galactic Center to -91 km/s at larger distances. Through careful kinematic modeling, we determine that these bubbles were created by a powerful nuclear outflow approximately 6-9 million years ago, with outflow velocities reaching 1000-1300 km/s.

These observations illustrate how a single feedback event can reshape an entire galaxy. The Fermi Bubbles tell the story of how energy and momentum injected from the Galactic Center propagate outward, sweeping up ambient gas and creating the dramatic structures we observe today.

Low-Redshift Circumgalactic Medium: Reading the Fossil Record Link to heading

While the Fermi Bubbles show us active, ongoing processes, the circumgalactic medium of nearby galaxies preserves evidence of earlier evolutionary phases. By studying the spatial distribution, kinematics, and chemical composition of gas around nearby and low-redshift galaxies, we reconstruct their assembly histories.

Key findings from our low-redshift CGM studies include:

  • Extended gas reservoirs: Cool circumgalactic gas extends to distances of 100+ kpc around galaxies, containing metal masses comparable to or exceeding the metal mass in galactic stars and interstellar media.

  • Azimuthal asymmetry: The gas distribution around disk-dominated galaxies exhibits strong dependence on azimuthal angle, revealing the presence of bipolar outflows aligned along disk rotation axes—a signature of feedback-driven winds.

  • Dependence on galaxy properties: The strength and extent of circumgalactic gas depends on stellar mass, star formation rate, and galaxy color, demonstrating that galaxy evolution is imprinted on the gaseous halos that surround them.

These observations show that low-redshift galaxies are still settling from past dynamical events, and that their circumgalactic media contain crucial information about their evolutionary histories.

Key Results & Publications Link to heading

A New High-latitude H I Cloud Complex Entrained in the Northern Fermi Bubble Discovery of a new population of cold, neutral-hydrogen clouds surviving inside the hot Fermi Bubbles at large distances from the Galactic Center, with implications for the age and dynamics of the nuclear outflow. Bordoloi et al. 2025

Spatially Resolved Gas Flows Around the Milky Way UV spectroscopy reveals that the Milky Way is predominantly accreting gas while simultaneously losing material through powerful outflows. Gas flows vary dramatically across the sky, with inflows concentrated in well-defined structures and outflows distributed more uniformly. Clark, Bordoloi, & Fox 2022

Mapping the Nuclear Outflow of the Milky Way UV spectroscopy of 47 background QSOs constrains the outflow age to 6-9 Myr and minimum cool gas mass in the Fermi Bubbles to millions of solar masses. Bordoloi et al. 2017

Probing the Fermi Bubbles in Ultraviolet Absorption High-velocity metal absorption components detected toward a unique QSO sightline PDS456 reveal a biconical outflow with an opening angle of ~110°, directly matching hard X-ray observations and constraining the outflow velocity to ~1000 km/s. Fox et al. 2015

The COS-Dwarfs Survey: CGM census around low-z low mass galaxies We map the circumgalactic medium of a large sample of low-mass galaxies reveals carbon (C IV) extending to ~100 kpc, with circumgalactic carbon mass (>1.2 × 10⁶ M☉) exceeding that locked in stellar populations. These observations demonstrate that low-mass galaxies maintain substantial gas reservoirs essential for star formation. Bordoloi et al. 2014

The Team Link to heading

This research involves close collaborations with colleagues at NC State University and partner institutions, including contributions from graduate and undergraduate students. Team members bring expertise in ultraviolet spectroscopy, numerical modeling, and observational techniques.