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Galaxies grow by accumulating gas from outside. This gas then fuels the star formation. Observations have shown that galaxies do not seem to grow indefinitely despite the availability of gas. It is believed that many physical processes in action from within could potentially stop it from growing.

Supermassive black holes that inhabit the centres of galaxies may give rise to one such process. When matter falls onto these black holes, they become ‘active’. These active galactic nuclei
(AGNs) emit energy, sometimes as electromagnetic radiation and sometimes as plasma ejected in the form of collimated jets seen in radio wavelengths (radio jets), and sometimes both together. The amount of energy is so enormous that various models of galaxy evolution assume this to be one of the main factors inhibiting the growth of galaxies in two ways: first, strong electromagnetic radiation drives the cold gas out of the galaxy and second, the large radio jets which can grow to sizes larger than the galaxy can heat the gas in the outskirts to prevent it from cooling and falling inside, essentially cutting off the supply.

The distribution of molecular gas in the galaxy B2 0258+35. The figure is colour coded to represent the velocity of the gas. The large ring follows the rotation of the galaxy while the circumnuclear gas is entirely distinct. Most of it is, in fact, in the form of a fast outflow. Credits: S. Murthy et al.

However, recently various studies have shown that low-power radio jets may also help in clearing the cold gas within the host galaxy making just as an important contribution as the strong radiation. If this is confirmed, then it means that the models of galaxy evolution will be required to take this effect into account, potentially affecting their outcome. Nevertheless, the observations so far have not been able to cleanly separate out the contribution of radiation and radio jets in driving such outflows of potentially star-forming gas. The sources whose radio jets do appear to make a contribution in driving gas outflows also exhibit strong radiation, whose contribution cannot be ignored.

An international team of scientists used the Northern Extended Millimeter Array (NOEMA) to study the cold gas and how it is impacted by the jets in the galaxy named B2 0258+35. They found the presence of fast outflowing cold gas in the central region of this galaxy. NOEMA's spatial resolution allowed to pinpoint the location of the outflow and the scientists found that it overlaped neatly with one of the radio jets. Most importantly, this AGN does not emit strong electromagnetic radiation. Thus, they were able to conclude that the radio jet is the sole driver of this outflow.

The impact of the radio jet is considerable on the galaxy: it is driving out nearly 75% of the cold gas in the central region. At this rate, the central few kiloparsecs of the galaxy, a region that can fit about 40 million solar systems, will be voided of cold gas in only a few million years – a very short time span for a galaxy that is billions of years old. The velocities of the gas clouds being driven out is high – about 500 km/s, but not high enough for them to completely exit the gravitational potential of the galaxy. In the next many millions of years, after being pushed out of the central region, the clouds may rain back onto the galaxy at other locations, just as the stream of water from a fountain returns to the pond somewhat further away.

The discovery also confirms the predictions of numerical simulations that low-power radio jets still propagating within the host galaxy do have a considerable impact on the gas around them. Moreover, the radio jet in this case is of low power and thus representative for the majority of the radio jets found in massive galaxies. This means that this population of radio AGNs could, in fact, have an important role to play in galaxy evolution and theoretical models may be required to consider them more seriously.

Original Publication:

S. Murthy et al.: Cold gas removal from the centre of a galaxy by a low-luminosity jet, 2022, Nature Astronomy

Contact:

Suma Murthy
Email: murthy@jive.eu
Phone: +31(0)521-596509

Raffaella Morganti
Email: morganti@astron.nl
Phone: +31(0)521-595793