Galactic outflows drive the evolution of dwarf galaxies

Stellar feedback is expected to play a key role in regulating the evolution of low-mass galaxies by producing galactic-scale winds (also known as outflows) that push the gas away from the interstellar medium, eventually preventing from the formation of new stars. In this respect, an international team of astronomers led by NCBJ scientist, have published a work on the Astronomy & Astrophysics journal addressing the impact of galactic outflows on the baryonic cycle of nearby dwarf galaxies.

Feedback from star formation and/or active galactic nuclei is expected to have a huge influence on the evolution of low-mass galaxies. Such mechanisms produce large-scale winds able to escape the relatively shallow gravitational potential of these systems, possibly entraining a large amount of gas and dust in the intergalactic medium (IGM). During the last decades, feedback has been considered as the main actor to alleviate a number of tensions between theoretical predictions and the observed properties of low-mass galaxies. Therefore, a precise understanding of it is crucial for a better description of the evolution of galaxies across cosmic time.

In a work just published on the journal Astronomy & Astrophysics, an international team of astronomers has investigated the role of galactic outflows in regulating the interplay between star formation and gas/dust depletion in tens of local dwarf galaxies. The research was led by Dr Michael Romano, a post-doc at the National Centre for Nuclear Research (NCBJ), in collaboration, among the others, with Dr Ambra Nanni, head of the research group at NCBJ and Principal Investigator of the SONATA BIS NCN project DINGLE.

The team made use of the Herschel Space Observatory to detect the light emitted at 158 μm by a specific carbon ion (an atom of carbon which lost an electron), called [CII]. Carbon atoms are mostly ionised by the ultraviolet radiation produced by young stars enshrouded by dust clouds in the interstellar medium of galaxies. The light arising from[CII] can thus provide information on the star-formation rate within the galaxies, and its line profile can be inspected to search for outflowing gas powered by star-formation activity.

„We analyzed the[CII] emission lines of about 30 local dwarf galaxies (that is, galaxies with low stellar mass) searching for an excess of carbon in the high-velocity wings of their spectra (modeled with a broad Gaussian component) with respect to the emission due to star formation (modeled with a narrow Gaussian component).” – says Dr Michael Romano. „This excess is detectable only if an outflow is pushing the gas away from the interstellar medium of the galaxy at relatively high velocity, and it is therefore a clear evidence of stellar feedback acting in these systems regulating their baryonic cycle.”

Scientists found that, on average, feedback in these galaxies is able to bring ~40% of the outflowing gas out in the IGM, where it cannot be used anymore for the production of new stars. At the same time, the[CII] emission observed in galaxies seems to be almost two times more extended than their ultraviolet emission, suggesting that a significant amount of atomic gas can still reside in the CGM of these sources, bound to their gravitational potential, and likely serving as fuel for new star formation.

„Outflows are essential ingredients to regulate galaxy evolution across cosmic time” – describes Dr Ambra Nanni. „Both cosmological simulations and chemical evolution models suggest that galactic outflows are crucial to reproduce the observed content of gas and dust in the interstellar medium of low-mass galaxies. Therefore, observational constraints on galactic outflows, as the ones we obtained, are fundamental for theoretician to simulate the baryon cycle in galaxies that allow to interpret observations and make predictions to be tested with different facilities such as ALMA, NOEMA and JWST.”

The results of this work highlight the ubiquity of galactic outflows in low-mass galaxies. Furthermore, as highlighted by the team, they offer a link to the study primordial galaxies formed after only one billion years after the Big Bang, which share comparable physical properties to those of local dwarf sources, and are ruled by similar feedback mechanisms.

Full results of this research are available in the article: Star-formation-driven outflows in local dwarf galaxies as revealed from[CII] observations by Herschel; M. Romano, A. Nanni, D. Donevski, M. Ginolfi, G. C. Jones, I. Shivaei, Junais, D. Salak, P. Sawant; A&A; DOI: https://doi.org/10.1051/0004-6361/202346143