Caught in the act: Observations of a merging protocluster

Title: GA-NIFS: JWST/NIRSpec IFS view of the z~3.5 galaxy GS5001 and its close environment at the core of a large-scale overdensity

Authors: Lamperti, I., Arribas, S., Perna, M., et al.

First Author’s Institution: Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain.

Status: Submitted to A&A [open access]

Two panels showing different views of GS5001. The left hand panel shows a three-colour image of GS5001, which is bright at the centre and extends from the top left corner to the bottom right corner. The right panel shows a colourmap where the brightness of each pixel corresponds to the flux of [O III]. The shape of GS5001 is the same in both panels, and in the left panel, green circles show the north, main, and south components. Within the northern component there are three blue circles labelled n3, n2, n1 (left to right), and in the southern component there are three blue circles labelled s1, s2, and s3. There's seventh blue circle on the edge of the central green circle, labelled s4.
Figure 1: These two panels show two different views of GS5001 and its companions. The left image is taken by JWST NIRCam. The right panel shows the brightness of an oxygen emission line in each spaxel, with different regions circled in green, and individual companions circled in blue. Adapted from Figure 1 of today’s paper.

Spectroscopy is one of the most powerful tools in an observer’s toolbelt. By separating light from a galaxy into different wavelengths, we can accurately measure the galaxy’s redshift, study its chemical composition, and characterise the movement of gas in the galaxy. However, standard spectra don’t give us detailed spatial information. Typically, just one spectrum is obtained for the entire galaxy, and we assume that the total spectrum is representative of the galaxy’s general conditions.

Instead, we can use a technique known as integral field spectroscopy (IFS), which combines imaging and spectroscopy. For each pixel in an image, a spectrum is also taken to create a “spaxel,”  which means  that we can compare the spectra of different regions of a galaxy. The authors of today’s paper use the IFS mode of the near-infrared spectrograph (NIRSpec) onboard the James Webb Space Telescope (JWST) to observe GS5001, a galaxy at a redshift of ~3.5 (~11.9 billion years ago) that’s at the centre of an over-dense region in space, known as a protocluster. With the use of IFS, the authors are able to study the kinematics, metallicity, and star formation of the main galaxy, and can also compare this with the properties of nearby, smaller companion galaxies.

Are outflows preventing star formation?

Due to the Doppler effect, gas that’s moving towards us will be observed as having a shorter wavelength (blueshifted), while gas that’s moving away from us will be observed as having a longer wavelength (redshifted). Astronomers can use the wavelength offset of the light emitted by the gas in a galaxy to calculate the velocity of clouds of gas within that galaxy. When today’s authors applied this method to GS5001, they found evidence for an outflow of gas from the galaxy, likely driven by star formation.

A key open question when it comes to outflows is what effect that they may or may not have on star formation. If an outflow can carry enough gas out of the galaxy, it has the potential to quench star formation, which would significantly impact a galaxy’s evolution. In the case of GS5001, the authors found that only 15% of the outflowing gas has a high enough velocity to completely escape the galaxy’s gravitational potential well. Therefore, the majority of gas will fall back onto the galaxy and could eventually form new stars, suggesting that the outflow is probably not having a significant effect on the galaxy’s total star formation rate.

Companions of GS5001

GS5001 has four fully resolved companions – three in the south (s1, s2, and s3 in Figure 1) and one in the north (the green circle labelled north in Figure 1). There’s a possible additional component in the south (s4), and the northern component might consist of three individual companions, but they aren’t distinct enough to be confirmed companions. There is also a potential fourth southern component that can’t be fully resolved but is likely a merging companion. By comparing the spectrum of each companion to the spectrum of GS5001, the authors are able to show that the southern companions are blueshifted and the northern companion is redshifted relative to the GS5001’s velocity.

Since specific elements emit at specific wavelengths, a spectrum can be used to infer a galaxy’s chemical composition. The authors find that the companions are all more metal-poor than GS5001, which is consistent with a known relationship between stellar mass and metallicity – less massive galaxies have shallower potential wells, so outflows remove metals from the interstellar medium more efficiently, resulting in a lower metallicity.

A colourmap showing the metallicity of each spaxel. The northern component is circled in red, and is largely metal-poor, as indicated by its cyan colour. The main component is circled in orange, and the top edge is more metal-rich, as indicated by its magenta colour, and the bottom edge is more metal poor. The southern component is circled in blue, and it largely metal-poor.
Figure 2: This map shows the metallicity associated with each spaxel. The northern (red circle) and southern (blue circle) components are noticeably more metal poor than GS5001 (orange circle). Also note that the southern end of GS5001 is more metal poor than the rest of the galaxy. Adapted from Figure 7 of today’s paper.

Evidence for mergers and interactions

The kinematics and metallicity patterns in GS5001 show evidence of mergers and interactions between the central galaxy and its companions. GS5001 has tidal tails (see Figure 1), which are a telltale signature of galaxy interactions. Additionally, the interstellar medium of GS5001 is more turbulent close to the northern companion, as evidenced by broader emission lines in northern spaxels, suggesting that gas from the main galaxy and the companion is mixing and interacting. 

GS5001 shows a metallicity gradient (see Figure 2) – it’s more metal poor in the south and more metal-rich in the north. The low metallicity in the south may be due to an inflow of metal-poor gas in the unresolved southern clump (labelled s4 in Figure 1). Interaction between GS5001 and the northern companion may have triggered an episode of star formation in the past. The most massive stars from this episode would have already died in supernova explosions, enriching the interstellar medium, and creating a more metal-rich environment. 

Mergers are known to be crucial in setting the course for a galaxy’s evolution. They allow for rapid growth and can result in high star formation rates. JWST allows astronomers to observe mergers and interactions at higher and higher redshifts, allowing us to test our predictions for how mergers impact galaxy evolution over long periods of cosmic time. GS5001 is an example of such a system, and the use of IFS has allowed today’s authors to thoroughly characterise the kinematics, chemical composition, and interaction history of the protocluster. 

Astrobite edited by Cesiley King

Featured image credit: NASA/ESA

About Nathalie Korhonen Cuestas

Nathalie Korhonen Cuestas is a first year PhD student at Northwestern University, where her research focuses on the chemical evolution of galaxies.

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