KMOS reveals galactic-scale outflows in the early Universe

By Anshu Gupta
[email protected]

The first galaxies in the universe created bubbles of ionized gas that overlapped with each other, which led to the largest phase transition in the history of the universe known as the epoch of reionization (EoR). However, we don’t know much about the galaxies involved in the phase transition due to the high opacity of the neutral intergalactic medium.

The MOSEL survey looks at galaxies that have similar properties to those in the neutral universe but are present in the ionised universe, the “EoR analogues”. In Gupta et al. (2023) we analysed the strength of galactic-scale outflows in the EoR analogues by stacking the [O III] emission line spectra. The data were taken by the K-band Multi-Object Spectrograph (KMOS) on the  ESO VLT in Chile. Multiple kinematic components in the emission profile would suggest that gas is either flowing in or out of the galaxy.

Galactic-scale outflows like those seen in M82 below play a crucial role in the evolution of galaxies and the surrounding environment. These outflows occur when gas is pushed out of a galaxy by the energy released from star formation, or from a supermassive black hole at the center of the galaxy. They transport energy and heavy elements created by stars, and thus are essential for the chemical enrichment of the intergalactic medium. They also create channels in the interstellar medium of galaxies, enabling the escape of ionising radiation. Theoretical models assume strong outflows in the early universe, due to the high star formation densities in the first galaxies.

Hubble Space Telescope image of galaxy M82 with blue and green indicating visible light, and red indicating infrared. The biconical and extended emission in infrared emission is produced by outflowing gas driven by the starburst activity. Image credit: NASA/ESA.

However, we could only detect weak outflow in the EoR analogues with KMOS, despite their high star formation rates. In fact, the outflow strength in EoR analogues is similar to galaxies of similar mass, but with star formation rates 10 times lower. Our result suggests that either the gas is too hot to contain oxygen atoms in the lower ionisation state (>105 K), or that efficient cooling in a dense medium prevents transport of gas to large distances.

In addition to weak outflows in the EoR analogues, we made another interesting observation in one subset of the sample. We found a secondary kinematic component that appears redshifted to the systematic velocity of the sample (below, left), which means it was coming from the inflowing gas around the low-mass galaxies (below, right). This finding is puzzling because there are currently no detections of ionized gas inflows around galaxies, and it is difficult to heat up the inflowing gas to temperatures high enough to produce [O III] emission (~104 K). Gas inflows are more prominent in the early universe, and this observation could provide new insights into the inflow-outflow balance in the interstellar medium of galaxies.

Fig 2
Left: Stacked [O III] spectrum overlaid with best-fit 1-component (blue) and 2-component (red) spectrum. The 1-component fit has clear residuals redshifted to 5010 Angstroms that may be coming from the inflowing gas. Right: Illustration of radial velocity of gas around a galaxy. The velocity components from all the gas in the galaxy are shown as the black curve, the inflowing gas as a red curve, and the outflowing gas as a blue curve.


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]