Astronomers have long asked: what makes galaxies stop forming new stars? What causes them to transition from blue, star-forming systems to red and dead galaxies? One mechanism often favoured by cosmological simulations that attempt to reproduce the observed Universe is the energy released when matter falls onto the supermassive black holes at the centres of galaxies. When these black holes enter active phases – known as active galactic nuclei (AGN) – the energy they release can heat and push gas away in the form of winds, preventing the gas from cooling and forming new stars.  

Astronomers study the gas entrained in these winds using spectroscopy – by decomposing the light when it passes through a prism and splits into colours, similar to what happens with rainbows. Several studies have detected winds in AGN, but most of them are restricted to visible light and to a particular type of gas known as warm ionised gas.

To understand how these AGN-driven winds affect star formation in galaxies – widely known as AGN feedback – we need to observe different gas phases simultaneously. This allows us to see how the winds interact with the gas and how much material they heat or drag away.

This is the goal of the QSOFEED project, led by Cristina Ramos Almeida, a researcher from the IAC and the Universidad de La Laguna. The team uses observations obtained with different telescopes, including the Gran Telescopio Canarias (GTC) at the Observatorio del Roque de los Muchachos, to study obscured quasars, which are highly energetic AGN whose bright central light is obscured by dust, allowing astronomers to better study the gas and stellar properties of their host galaxies. 

In this study, led by Pedro Henrique Cezar, the team presents observations of six obscured quasars in the near-infrared (light invisible to our eyes), obtained with the EMIR spectrograph installed on the GTC at the Observatorio del Roque de los Muchachos (ORM). These observations allow astronomers to simultaneously study the warm molecular gas (at temperatures of ~727ºC), warm ionised gas (~9,727ºC), and hot ionised gas (~15,576ºC). 

The researchers found winds reaching velocities of up to 9 million km/h (0.8% of the speed of light) in the warm ionised and hot ionised gas phases in the six quasars. Their observations suggest that both gas phases are tracing the same wind: 

“AGN have episodic winds, associated with periods of high accretion onto the central supermassive black hole. What we found is that the winds in the warm ionised and hot ionised phases have similar velocities and spatial extents across the galaxy, suggesting that they are tracing the same wind event. The difference is that the warm ionised wind carries, on average, six times more mass than the hot ionised wind,” says Pedro Henrique Cezar, lead author of the study. 

The surprise came when they did not find warm molecular winds in any of the quasars. This elusive gas phase appears to require a large reservoir of molecular gas in the quasars to show outflows, as otherwise they might remain undetected. Larger samples of quasars and other less luminous active galaxies need to be investigated to better understand the behaviour of the warm molecular gas in AGN. 

Article: P. H. Cezar, M. Coloma Puga, C. Ramos Almeida, J. A. Acosta-Pulido, G. Speranza, L. R. Holden, C. N. Tadhunter, M. V. Zanchettin, A. Audibert. QSOFEED: Investigating warm molecular, low- and high-ionization atomic gas in six type-2 quasars with GTC/EMIR. A&A, 2026. DOI: https://doi.org/10.1051/0004-6361/202557799

IAC contacts:

• Pedro Henrique Cezar (pcezar [at] iac.es (pcezar[at]iac[dot]es))
• Cristina Ramos Almeida (cra [at] iac.es (cra[at]iac[dot]es))
• Jose Antonio Acosta Pulido (jap [at] iac.es (jap[at]iac[dot]es))