Large radio bubble detected in galaxy NGC 4217

An international team of astronomers has performed radio observations of a star-forming galaxy known as NGC 4217. The observational campaign detected a large radio bubble in the galaxy’s halo. The finding was reported in a paper published September 23 on the pre-print server arXiv.

Located some 61.6 million light years away, NGC 4217 is a nearby edge-on star-forming spiral galaxy. Previous observations of this galaxy have found that it contains dozens of absorbing dust structures with various morphologies. Moreover, a radio halo extending to about 16,000 light years from the galaxy’s star-forming disk has been identified.

Recently, a group of astronomers led by Volker Heesen of Hamburg University in Germany, has employed the Jansky Very Large Array (JVLA) and with LOw Frequency ARray (LOFAR), to take a closer look at NGC 4217 in radio band.

“We present new observations of the CHANG-ES [Continuum HAloes in Nearby Galaxies—an EVLA Survey] galaxy NGC 4217 in S-band (2–4 GHz) which we combine with archival LoTSS-DR2 [LOFAR Two-metre Sky Survey data release 2] data at 144 MHz,” the researchers wrote in the paper.

The observations detected a conspicuous extension of radio continuum emission in the north-western halo of NGC 4217. Further inspection of this emission allowed Heesen’s team to identify a very extended faint component that had previously evaded detection.

The images show that this component has the morphology of an edge-brightened bubble extending out to a distance of 65,000 light years from the star-forming disk.

According to the paper, the emission is boosted along the walls of NGC 4217’s radio bubble with a slight depression in the center of the bubble. The northeastern edge of the bubble is in particular prominent and the collected images suggest the potential presence of a shell on this side.

The study found that the scale heights of the radio bubble are 19,200 and 9,400 light years, at 144 MHz and 3 GHz, respectively. Therefore, they are a few times larger than the typical scale heights of radio bubbles in edge-on galaxies. The total magnetic field strength of the bubble in NGC 4217 was measured to be about 11 µG.

Furthermore, the astronomers estimated that at the edge of the bubble, the wind speed rises from approximately 300 to 600 km/s, which is approximately at a level of the escape velocity of NGC 4217.

This result suggests that the bubble can be inflated by about 10% of the kinetic energy injected by supernovae over its dynamical time-scale of 35,000 years. However, the researchers note that not all the kinetic energy can be used to inflate the bubble as large fractions may be radiated away.

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