MAGPI Magic III: Witnessing an early stage of galaxy evolution

By Tamal Mukherjee
[email protected]

One of the major fundamental questions in modern astronomy is how galaxies formed and evolved over cosmic time. To unveil these mysteries, astronomers often utilize Lyman-alpha (Lyα) emission from distant galaxies to investigate their formation mechanisms and evolution. This specific spectral line, associated with the transition of the electron in a hydrogen atom, provides a distinctive signature observable even in the distant, early universe due to the expansion of the Universe. Lyα emission is closely linked with processes occurring within galaxies, particularly in regions of active star formation. By detecting Lyα emission from galaxies using telescopes equipped with integral field spectrographs (IFS) that providing imaging and spectroscopy simultaneously, we have investigated the velocity structure, and dynamics of gas clouds associated with star forming regions. 

In a recently published paper, we report the detection of three distant Lyα emitting galaxies in the data obtained as part of the Middle Ages Galaxy Properties with Integral field spectroscopy (MAGPI) survey. This is a large program on the European Southern Observatory (ESO) Very Large Telescope (VLT), which uses the Multi Unit Spectroscopic Explorer (MUSE) instrument to obtain spatially-resolved spectroscopy. What sets these findings apart is the identification of a striking “double-peak” spectral feature in the Lyα spectra of these galaxies (see Figs. 1 and 2 below), shedding light on a crucial early evolutionary phase. These double peaks are not merely aesthetic; they signify a distinctive signature of gas movement within the galaxy. Dominant 2nd peaks (a.k.a. red peaks) are widely seen, but more strikingly we have discovered cases of strong blue (shorter wavelength) peaks suggesting a dominant flow of cold gas into the galaxy, offering unprecedented insights into the mechanisms driving star formation activity.

MAGPI 1534
Fig 1: Image showing Ly-alpha emission from a MAGPI galaxy (ID 1534).
Ly-alpha spec
Fig 2: Corresponding Ly-alpha spectrum of the galaxy.

We observe diverse spatial distributions of Lyα emission, where two sources exhibit extensive spatial extensions (Fig. 1 above shows Lyα emission from one such extended galaxy; ID 1534) while the third is quite compact. Through the power of IFS, we have unveiled substantial spatial variations in observed spectra, revealing how the strength and separation of the Lyα peaks evolve as we move away from the galaxy centres. Figs. 3 and 4 below show that the strength of the blue peak and the separation in velocity between the two peaks in a galaxy (ID 1208) decreases with increasing radius.

We have uncovered a fascinating trend in our observations: as we move outward from the galactic center, the separation between the spectral peaks decreases. This phenomenon appears to be linked to the amount of neutral hydrogen surrounding the galaxy. The strength of these peaks, on the other hand, is influenced by the gas’s radial motion—whether it’s flowing inward or outward. When gas flows toward the galactic core, it causes a shift in the Lyα photon’s wavelength, making it appear bluer. The dominance of the shorter wavelength peak at the core suggests ongoing cold gas accretion, fueling star formation processes. This offers us a unique glimpse into the early stages of these galaxies, providing valuable insights into their formation and evolution.

1208 ratio
Fig. 3: Pixel by pixel map of the ratio of strength of two Ly-alpha peaks in a MAGPI galaxy (ID 1208).
1208 vel sep
Fig. 4: Velocity separation map between the two Ly-alpha peaks in ID 1208.


Michael Murphy is the Australian representative on the ESO Science Technical Committee. Contact: [email protected]

Sarah Sweet is the Australian representative on the ESO Users Committee. Contact: [email protected]

Stuart Ryder is a Program Manager with AAL. Contact: [email protected]

Guest posts are also welcome – please submit these to [email protected]