Large radio bubble detected in galaxy NGC 4217

A grayscale background map shows the JVLA 3 GHz continuous radio emission in NGC 4217’s halo at 7′ angular resolution. Credit: Heesen et al., 2024.
An international team of astronomers has conducted radio observations of a star-forming galaxy known as NGC 4217. Through this observation activity, a large radio bubble was detected in the galaxy’s halo. The findings were reported in a paper published on September 23 on the preprint server arXiv.
Located about 61.6 million light-years away, NGC 4217 is a nearby star-forming spiral galaxy. Previous observations of this galaxy have shown that it contains dozens of absorbing dust structures of various morphologies. Additionally, a radio halo has been observed extending approximately 16,000 light-years from the galaxy’s star-forming disk.
Recently, a group of astronomers led by Volker Heesen from the University of Hamburg in Germany took a closer look at NGC 4217 in the radio band using the Jansky Very Large Array (JVLA) and the Low Frequency Array (LOFAR).
“We are using the new CHANG-ES (Nearby Galaxy Continuum HAloes – EVLA Survey) in the S-band (2-4 GHz) for galaxy NGC 4217, combined with archival LoTSS-DR2 (LOFAR 2 meter sky survey data). 2) release data at 144 MHz,” the researchers wrote in their paper.
The observations detected a significant spread of continuous radio emission in NGC 4217’s northwest halo. Further examination of this emission allowed Heesen’s team to identify a very widespread faint component that had previously evaded detection.
The images show that this component has the form of a bright-edged bubble that extends up to 65,000 light-years from the star-forming disk.
According to the paper, radiation increases along the walls of NGC 4217’s radio bubble, causing the center of the bubble to be slightly depressed. The northeastern edge of the bubble is particularly prominent, and the images collected suggest the possible presence of shells on this side.
The study found that the scale height of the radio bubble is 19,200 light-years and 9,400 light-years at 144 MHz and 3 GHz, respectively. Therefore, they are several times larger than the typical scale height of radio bubbles in edge-on galaxies. The total magnetic field strength of the bubble within NGC 4217 was measured to be approximately 11 μG.
Furthermore, astronomers estimate that at the edge of the bubble, wind speeds increase from about 300 km/s to 600 km/s, which is about the same level as NGC 4217’s escape velocity.
This result suggests that the bubble can be expanded by about 10% of the kinetic energy injected by the supernova over a dynamic time scale of 35,000 years. However, the researchers note that not all of the kinetic energy can be used to expand the bubbles, as much of it can be radiated.
Further information: V. Heesen et al, CHANG-ES XXXIV: a 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217, arXiv (2024). DOI: 10.48550/arxiv.2409.15449
Magazine information: arXiv
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