Title: GA-NIFS: Multi-phase outflows in a star-forming galaxy at z ∼ 5.5
Authors: Eleonora Parlanti, Stefano Carniani, Giacomo Venturi, Rodrigo Herrera-Camus, Santiago Arribas, Andrew J. Bunker, Stephane Charlot, Francesco D’Eugenio, Roberto Maiolino, Michele Perna, Hannah Übler, Torsten Böker, Giovanni Cresci, Mirko Curti, Gareth C. Jones, Isabella Lamperti, Sandra Zamora
First Author’s Institution: Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
Status: Submitted to A&A [open access]
Galaxies can be windy places, and these winds are essential to the story of how galaxies evolve. Galactic winds, or outflows, sweep up gas, regulating the birth of new stars and spreading elements across space (e.g., see this bite). But observing these winds in the early Universe, when galaxies were still in their infancy, has been a formidable challenge. Thanks to the combined power of the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA), we now have a clearer view.
The authors of today’s paper used JWST to look deep into HZ4, a galaxy so distant that we see it as it was just 1 billion years after the Big Bang (at a redshift of 5.5). With JWST, they focused on the warm, ionised gas within the galaxy, glowing brightly in the rest-frame ultraviolet and optical wavelengths. With ALMA, they observed the cooler, neutral gas traced by far-infrared emission. By studying both types of gas, they uncovered the full story of the galaxy’s outflows.
Unveiling the winds
When we look at the ALMA and JWST spectra of this galaxy, we see that the emission lines all contain a broad component. This is due to Doppler broadening, indicating a high gas temperature, and also due to the high velocity dispersion and orbital velocity of the gas, as is typical for a galactic outflow. The authors carefully analysed the gas’s velocity, temperature, and emission patterns, ruling out alternative explanations like Active Galactic Nuclei (AGN) activity. Their analysis instead favours the interpretation that these massive outflows are a direct result of the galaxy’s intense star-forming activity, and that these outflows are carrying both warm ionised gas and cold neutral gas away from the galaxy. In addition, the authors found that neutral gas, crucial for future star formation, is being expelled at a higher rate than ionised gas. This has significant implications for the galaxy’s future ability to form new stars and sheds new light on how galaxies may have regulated their growth in the early Universe. However, we need to know a little more about the galaxy’s behaviour in order to interpret these broad components correctly.
A change in the narrative
Prior to this study, ALMA data had been used to classify HZ4 as a rotating galaxy. A rotating galaxy is often identified by a smooth velocity gradient (i.e. a smooth transition from positive to negative velocities, or from red to blue in the right-hand side maps in Figure 1). Lower resolution ALMA observations revealed a very convincing smooth velocity gradient (top right map in Figure 1). However, higher resolution JWST observations have now revealed that the galaxy is formed of three clumps with more complex kinematics (indicated by the cross-shaped markers in the lower panel of Figure 1). These clumps could indicate that the galaxy is instead undergoing a merger, which has implications for the interpretation of these observations.
Different outflow scenarios
Three scenarios are proposed to explain the broad lines and clumpy structure:
- Two different outflows, in different directions: The observed broad components may be due to outflows of ionised and neutral gas moving in different directions, likely driven by star formation and feedback processes within star-forming regions. The blue-shifted ionised gas outflow and the red-shifted neutral gas outflow could be originating from different clumps within the merging galaxies, with the possibility that the redshifted outflow is obscured in optical wavelengths but visible in the [CII] line.
- Galactic fountain: Another possibility is that the redshifted [CII] emission represents a galactic fountain, where gas expelled by star formation-driven winds cools down and eventually falls back onto the galaxy. However, proving the presence of such a fountain is challenging, especially at high redshift.
- Inflow or gravitational interaction (merger): The broad [CII] component might also represent inflowing gas from the cosmic web, enriched in [CII] and potentially connected to the galaxy’s growth. However, the high velocities observed (around 300 km/s) are more typical of outflows than inflows, which would be slower. Another explanation could be that these broad features are due to tidal interactions between merging galaxies.
The galaxy HZ4 likely involves a mix of outflows, inflows, and gravitational interactions, with outflows dominating the observed high velocities. Further study is needed to clarify these processes.
These findings are a crucial piece of the puzzle in understanding how galaxies grow and change over time. The early universe was a time of rapid transformation, and galactic winds like those in HZ4 played a key role in shaping the galaxies we see today. This discovery, made possible by the advanced capabilities of JWST and ALMA, offers a new perspective on the forces that governed the early universe.
As we continue to explore the cosmos with these powerful instruments, we can expect to uncover even more about the origins of galaxies and the cosmic winds that have sculpted them.
Astrobite edited by Katherine Lee
Featured image credit: ESO/M. Kornmesser
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