Do Bursting Galaxies Leave Ghosts in Their Halos?
Galaxies don’t exist in isolation. They sit inside vast halos of gas that act like a living halo around a cosmic city—cool, hungry, and full of sudden weather changes. The circumgalactic medium CGM is where inflows supply fuel and outflows fling metals, heat, and dust far from the galactic disk. Astronomers have learned to listen for subtle shadows that cool gas casts in the light of distant quasars. A new study led by Zoe Harvey at the University of St Andrews, with partners from Cambridge and a cadre of SDSS researchers, treats those shadows as a diary of a galaxy’s recent life. By stacking thousands of quasar spectra, the team can map the presence of cool gas around some of the universe’s most massive and historically important galaxies, tracing how the gas content evolves with distance and with the galaxy’s recent star forming history.
What makes this project especially gripping is not just the gas itself but the link to a galaxy’s recent past. The researchers separate host galaxies into three flavors: star forming, quiescent, and post starburst—galaxies that had a spectacular burst of star formation that shut down quickly. The standout surprise is that post starburst galaxies harbor more cool gas in their halos, out to roughly one million parsecs, than their quieter or more actively star forming peers. The work uses data from the Sloan Digital Sky Survey SDSS, weaving together the DR7 quasar catalog and the CMASS galaxy catalog from DR12 to survey such massive galaxies with stellar masses above roughly 10^11.4 solar masses, spanning redshifts about 0.4 to 0.8. The study reminds us that the memory of a galaxy’s moment of glory can echo across entire halos, long after the fireworks fade. The authors are Zoe Harvey and colleagues from the University of St Andrews, with substantial collaboration from the University of Cambridge and others.
What Mg II tells us about the CGM around massive galaxies
The team’s main workhorse is a spectral fingerprint: the Mg II absorption doublet at rest frame wavelengths around 2796 and 2803 Angstroms. When a cool, dense cloud lies between us and a bright background quasar, magnesium ions absorb light at these telltale wavelengths. The absorption strength, measured as an equivalent width, is a proxy for the amount of cool gas along the line of sight. But the signal is faint when you look at the outskirts of a galaxy’s halo, and individual sightlines are noisy. So the authors use stacking: they align thousands of quasar spectra at the redshift of a foreground galaxy and average them. The result is a clearer glow of Mg II that reveals how much cool gas sits around the galaxy, out to distances of up to 9 million parsecs—nearly 30 million light years away from the galaxy itself.
The analysis shows a clean and expected trend: the Mg II absorption, a sign of cool gas, gets weaker as you move farther from the galaxy. Inward, the gas is denser, and the halo glows brighter in Mg II; outward, the gas thins and the signal fades. This radial pattern has been seen before in various surveys, but Harvey and colleagues push it further by separating the signal into different galaxy populations. They also prove the robustness of the signal by testing the measurement against random redshifts and by bootstrap resampling to estimate uncertainties. A crucial part of the puzzle is the scale: beyond about 1 Mpc the strength of Mg II absorption levels off, suggesting a transition from the circumgalactic medium to the general intergalactic medium IGM, at roughly the halo’s virial radius. In short, the halo’s grip weakens as you step into the cosmic field, and the matter you detect there becomes less tied to the galaxy’s current life cycle.
The methodology relies on a careful census of galaxies, too. The CMASS sample is dominated by very massive galaxies, with a broad but well-characterized distribution in redshift and mass. The authors classify galaxies using a data driven MFICA decomposition that teases apart light from young, intermediate, and old stellar populations. The K component tracks old, low mass stars while AF captures intermediate age stars. Star forming galaxies show a mix with a strong OB signature, quiescent galaxies are dominated by the old stars, and post starburst galaxies light up with the AF component. In this mass regime, the quiescent population makes up the majority, but there is a nontrivial fraction of star forming and post starburst galaxies too. The careful separation of these populations is essential because the team’s goal is to connect the CGM’s properties to recent star formation histories rather than to instantaneous current rates alone.
Post-starburst galaxies stand out in their halos
The standout result is stark and specific: within roughly 1 Mpc of the galaxy, post-starburst hosts exhibit much stronger Mg II absorption than either star forming or quiescent galaxies at the same stellar mass. In the inner 0 to 71 kpc, the equivalent width for all galaxies sits around 0.39 Angstroms on average, but when you break it down by type, post starbursts JUMP to about 1.37 Angstroms, while star formers hover near 0.49 and quiescent galaxies near 0.18. Stepping out to larger radii, the difference persists but gradually narrows; by 1.1 to 2.2 Mpc the three populations converge toward similar, modest levels of Mg II absorption (on the order of a few hundredths of an Angstrom).
What could explain this peaking of cool gas around post starbursts? The authors sketch several dynamical possibilities. One is that gas left over from the prior burst remains in the halo, a relic of the event that produced the starburst and the rapid quenching. A second possibility is that the post-starburst environment—likely shaped by mergers and the presence of neighboring galaxies—favors more cool gas in the halo. A third possibility is that strong, fast outflows driven by the burst sweep up, accelerate, or condense cool gas into the circumgalactic region, extending its reach. All of these are plausible in the context of what we know about post-starburst systems, which often arise from gas-rich mergers that funnel gas to the center and provoke intense, short-lived star formation before the feedback eventually arrests it.
There is observational support for a wind-driven picture: local post-starburst galaxies show fast cool gas outflows with speeds near a thousand kilometers per second. The new results take that idea and ask, could such winds push or seed cool gas far beyond the optical galaxy, into the CGM and even into the outskirts of the halo? The answer seems to be yes, at least for the timescales involved. The authors emphasize that the post-starburst phase is relatively brief on cosmic timescales, lasting less than about a gigayear, but within that window the winds and the halo’s reaction could align to produce a larger, more extended reservoir of cool gas than we see around other galaxies of the same mass.
It is important to note that the interpretation is nuanced. Post-starburst halos could reflect gas left over from the starburst, a denser environment with more satellites, or a halo in which outflows have interacted with the ambient gas to promote cooling and precipitation of Mg II bearing clouds. The team carefully weighs these possibilities, but the data alone cannot pin down a single mechanism. What the results do firmly establish is a real, measurable link between a galaxy’s recent burst history and the mass and distribution of cool gas in its halo—a memory that lasts long after the star formation has quenched.
The big flattening beyond 1 Mpc and why it matters
A second major thread in the paper is the behavior of Mg II absorption beyond the near halo. The radial profile shows a clear flattening: beyond a projected distance of about 1 Mpc, the equivalent widths stop falling steeply and become roughly independent of the galaxy’s star formation history. In practical terms, the strong differences that appear inside the halo’s inner regions between post starburst, star forming, and quiescent galaxies fade away as you move into the cosmic field. The authors translate this as the CGM giving way to the IGM—a transition that happens near the virial radius of these massive halos, which simulations place around 0.7 to 1 Mpc at these redshifts and masses. This boundary is not a hard wall but a gradual shift from a gas reservoir that feels the galaxy’s influence to a broader intergalactic medium largely governed by cosmic structure rather than a single galaxy.
The flattening is also a useful touchstone for theory. Cosmological simulations routinely predict a decline of cool gas with distance, but the exact extent and strength of the Mg II signal depend on a spectrum of sub-grid physics: the ionizing background radiation, the resolution, the treatment of multiphase gas, and the details of feedback. The observed global trend—from a galaxy type dependent inner halo to an almost universal, IGM-like baseline beyond 1 Mpc—provides a clean benchmark for simulations to match. It also highlights a limit of current models: while they can qualitatively reproduce the drop in Mg II with radius, the precise balance of cooling, heating, and cloud survival at large scales remains a delicate, model-dependent question.
Implications: quenching, winds, and the cosmic gas cycle
Why should a result about a shadowy line in a spectrum matter for the bigger story of galaxy evolution? Because it ties together the life cycle of stars, gas, and metals in a single narrative across cosmic distances. Post-starburst galaxies, as the study notes, are a potentially significant channel through which massive galaxies become quiescent. Their short lives in the postburst phase, followed by a passive, red, elliptical future, make them crucial for understanding how star formation ceases in the universe. If a burst of star formation is capable of driving cool gas far into the halo or even into the neighboring intergalactic medium, then feedback is not merely blowing gas away from the disk. It is reshaping the environment in which future gas accretes, cools, or condenses, thereby influencing how long a galaxy can stay quiet or—alternately—reignite activity under the right conditions.
Another layer is the distribution of metals. Mg II traces magnesium, a metal produced in stars and released by supernovae and winds. If post-starburst halos retain or push this cool gas toward large distances, they are spreading metals into the CGM and possibly beyond, seeding future structure formation with the building blocks of planets and atmospheres. This is a tangible reminder that a galaxy’s dramatic life events do not stay contained; they ripple through its surroundings on scales that rival the size of the halo itself. And while active galactic nuclei could play a role in driving winds and ionizing gas, the present sample’s post-starburst galaxies are not dominated by luminous AGN, suggesting that the connection between bursts, winds, and extended cool gas does not require a bright central engine. The physics could be a tapestry of winds, shocks, condensation, and halo dynamics, all interwoven over hundreds of millions of years.
The study also reinforces a broader theme in modern galaxy evolution: the precise form of a galaxy’s recent star formation history matters as much as its instantaneous rate. Continuous star formation and a single burst followed by quenching can leave very different imprints on the surrounding gas, and those imprints can persist on megaparsec scales. We are learning to read these imprints not as scattered clues but as structured, time-resolved stories about how galaxies regulate their own growth and how the cosmos redistributes baryons and metals across the web.
What comes next: simulations, surveys, and open questions
The work opens a host of exciting avenues. First up is expanding the sample of post-starburst galaxies, especially at the highest stellar masses and across a broader redshift range. Large surveys like DESI in the near future will dramatically increase the number of PSB galaxies with enough statistical power to tease apart how halo mass, environment, and starburst age influence the CGM. The authors note that green valley galaxies, which occupy the transitional lane between star forming and quiescent states, show more ambiguous CGM signatures in current data; more data may clarify whether their CGM mirrors quenched systems or still carries echoes of ongoing transformation.
On the theory side, the big challenge is to model post-starburst galaxies with enough fidelity to predict not just the inner halo gas but the fate of cool clouds as they travel through the halo and into the IGM. Do winds drag cool gas outward intact, or do hot winds break the clouds apart and encourage new cooling? Can pre-existing CGM gas condense under the influence of merger-driven disturbances to produce the observed excess Mg II out to 1 Mpc? Simulations will need to grapple with how ionizing backgrounds, radiation from any residual star formation, and possible low-luminosity AGN shape the cool gas census at these scales. The observational result—an extended, enhanced cool CGM around post-starburst galaxies—sets a clear target for theory to test and explain.
The study also hints at a larger, interdisciplinary promise: by mapping the CGM around different galaxy types and tying it to recent star formation histories, we gain a new diagnostic for feedback and metal transport. The CGM is not a passive reservoir; it is part of the baryon cycle that governs how galaxies grow, pause, resume, or shut down. If bursts can reverberate through halos out to a Mpc, then the cosmic ecosystem is more interconnected than we often imagine. That realization—galaxies and their halos speaking to each other across vast distances—feels like a natural, if humbling, culmination of decades of spectral sleuthing.
The work is a collaboration drawing on SDSS data and the power of modern data-driven galaxy classification. It anchors a narrative that the field has long suspected but is only now beginning to quantify with such clarity: the aftermath of a galaxy’s starburst can shape its halo for a time that spans more than a galaxy’s lifetime in a single phase. The University of St Andrews leads the effort with Zoe Harvey at the helm, complemented by colleagues at Cambridge and other institutions, demonstrating how a shared data commons and a patient, statistically rigorous approach can reveal the subtle dynamics that govern how galaxies live and breathe in a universe of gas and gravity.
