Astronomers have discovered a massive halo of hot, X-ray emitting gas extending tens of thousands of light-years from the spiral galaxy NGC 5746. The finding, based on a deep observation using the European Space Agency’s XMM-Newton space observatory, reveals a biconical or hourglass-shaped structure indicative of a powerful gaseous outflow powered by stellar activity within the galaxy. This detection provides compelling evidence that even relatively quiet galaxies, not just energetic starburst galaxies, can drive significant outflows that shape their evolution and enrich their surroundings.
The discovery reopens the case of NGC 5746’s halo, which has been a subject of debate for nearly two decades. An initial detection in 2006 with NASA’s Chandra X-ray Observatory was later retracted after researchers attributed the signal to instrumental artifacts. This new, more sensitive observation confirms the halo’s existence and provides the first clear characterization of its nature, suggesting it was created by stellar feedback rather than the accretion of primordial gas from the intergalactic medium, as was first theorized. The results challenge previous assumptions about the energy required to sustain such halos and suggest the star-forming activity in NGC 5746 is higher than previously estimated.
A Deeper View with XMM-Newton
The breakthrough was made possible by a long-duration observation totaling approximately 250,000 seconds with the XMM-Newton’s European Photon Imaging Camera (EPIC). This extended exposure provided unprecedented sensitivity, allowing astronomers to detect faint, diffuse X-ray emissions that had eluded previous, shorter observations. NGC 5746, a massive spiral galaxy located about 29 megaparsecs (approximately 95 million light-years) away, is viewed nearly edge-on, which provides an ideal orientation for distinguishing between gas in its disc and gas in the surrounding halo.
Researchers created detailed X-ray images and surface brightness profiles to map the extent and structure of the gas. The analysis showed the halo is not spherical but features two distinct “bubbles” of enhanced X-ray emission extending perpendicularly to the east and west of the galactic disc. This biconical shape is a classic signature of a galactic wind or outflow, where energy released in the disc pushes gas out into the halo. The hot plasma is detectable out to an average distance of about 30 kiloparsecs, and extends even further—beyond 40 kiloparsecs, or 130,000 light-years—in the direction of the bubbles.
Revisiting a Prior Cosmic Claim
The history of studying NGC 5746’s halo is a compelling example of the scientific process, involving claims, retractions, and new discoveries with superior technology. In 2006, a team using data from a 36.8-kilosecond Chandra observation first reported a hot halo around the galaxy. At the time, NGC 5746 was considered a quiescent galaxy with a low star formation rate (SFR) of about 0.8 to 1.0 solar masses per year, a level thought insufficient to power a massive outflow. This led the original discoverers to conclude the halo was most likely formed by the slow, ongoing accretion of primordial gas from the intergalactic medium, a key prediction of galaxy formation models.
A Retraction and a New Beginning
However, in a follow-up study in 2009, the same team retracted their finding. After applying newer calibration data, they concluded the detected signal was not astrophysical but rather an instrumental effect related to the quantum efficiency of the Chandra detectors. For more than a decade, the galaxy was no longer considered a prime example of a hot gaseous halo.
The new XMM-Newton data unambiguously reverses that conclusion. Its superior sensitivity and larger field of view were crucial in confirming the halo’s existence and, more importantly, in characterizing its shape as an outflow. The authors of the new study argue that the non-detection by the re-analyzed Chandra data was likely due to its sensitivity limit, not a lack of emission.
The Engine Behind the Outflow
A key puzzle presented by the new findings is the energy source driving such a massive outflow. Galactic winds are typically associated with intense bursts of star formation, where the combined energy from thousands of supernova explosions can eject gas from the disc. The previously measured low SFR of NGC 5746 seemed to contradict the existence of such a powerful outflow.
To investigate this, the research team used the X-ray data to derive a new estimate for the galaxy’s star formation rate. By analyzing the non-thermal X-ray emission from high-mass X-ray binaries—systems that trace recent star formation—they calculated a significantly higher SFR of approximately 2.9 solar masses per year. This revised rate is above the critical threshold thought necessary to create multi-phase halos and provides a viable energy source for the observed outflow. It suggests the stellar activity in NGC 5746 is more vigorous than infrared-based indicators had previously suggested.
Structure and Temperature of the Halo
The spectral analysis of the X-ray emission provides further clues about the halo’s nature. The gas within the halo has a temperature of about 0.56 keV, or over 6 million degrees Celsius, which is hotter than typical values for spiral galaxies and more characteristic of galaxies with active outflows. The gas within the galactic disc itself is even hotter, at around 0.70 keV.
The two bubble-like structures are particularly bright and appear to originate from the galactic core, spreading out as they move into the halo. This morphology, combined with the lack of corresponding bright emission in H-alpha and radio wavelengths from previous surveys, suggests the outflow may be an older event. The hot, X-ray emitting gas cools very slowly over hundreds of millions of years, while the warmer gas traced by H-alpha would have cooled and dispersed much faster. The bubbles seen today may be the relaxed remnants of a more energetic outflow that occurred in the past.
Broader Implications for Galaxy Evolution
The definitive detection of a luminous, outflow-driven X-ray halo in a non-starburst galaxy has significant implications. It demonstrates that massive spiral galaxies can host such halos, supporting theoretical models of galaxy formation that predict their existence. The previous scarcity of such detections may be a result of observational limits rather than their actual absence in the universe.
These outflows play a crucial role in the lifecycle of galaxies. They regulate star formation by expelling the gaseous fuel needed to form new stars and enrich the circumgalactic medium with heavy elements forged inside stars. The discovery in NGC 5746 suggests this feedback mechanism is at work even in galaxies without extreme starburst activity, highlighting a key process that shapes how galaxies evolve. The finding underscores the need for more deep observations of similar galaxies to determine just how common these halos are.
