A View Through the Intra-Group Medium

It has been apparent for some time from both observations and simulations that galaxies big and small are embedded in a network of gaseous structures known as the “cosmic web”. This has been highlighted to impressive effect in the last few generations of cosmological hydrodynamical simulations, where such structures are now modeled in exquisite detail.

A zoom in of hot gas modeled in the EAGLE simulation forming gaseous halos around a pair of galaxies. Credit: The EAGLE Project

Observing this cosmic web in it’s full 3-dimensional detail is however a tricky prospect. The cosmic web gas emits little light and has only recently been traced successfully in UV/optical emission - albeit at relatively high density and at specific wavelengths - through recent advances in instrumentation [1].

Tracing the broader distribution of the cosmic web in a wide range of cosmic structures (e.g. from isolated galaxies, through galaxy groups and galaxy clusters) requires the now relatively mature technique of identifying the gas through it’s absorption of the light from ultra-luminous background sources such as quasars. Such studies are thus reliant on chance alignments of these background sources with features in the cosmic web, such as filaments and galaxy halos. Again technological advances are playing their part in such studies, with the MUSE integrated-field spectrograph on the Very Large Telescope (VLT) on Cerro Paranal, in particular, playing a key role in studying the cosmic web.

A deep high resolution image of a patch of sky around a distant bright quasar (bright central source) taken using the Wide-Field-Camera 3 on the Hubble Space Telescope as part of our QSAGE survey.

MUSE has enhanced the possibility of ‘shot in the dark’ observations, where the entirety of the galaxy population is captured in observations of distant bright quasars. The instrument is indeed perfect for probing the gas structures within groups of galaxies at low-redshift, an exciting and dramatic environment for studying the galaxy population where a dramatic decline in star-formation has been seen in the past few billion years [2]. It has also inspired a new approach with the Hubble Space Telescope, where we are using thegrism facility on the Wide-Field Camera 3 to perform similar ‘shot in the dark’ style observations as part of our Quasar Sightline and Galaxy Evolution (QSAGE) survey.

Data from MUSE on the VLT reveal the dynamical properties of group galaxies around the line of sight to a background quasar. The light from the quasar is absorbed by gas within the group halo, revealing discrete structures containing magnesium, iron and silicon in abundance.

Two exciting studies have published results in the last few months using MUSE data to identify galaxy groups and analyzing the intra-group environment using the light from background quasars [3,4]. The MUSE data reveals the galaxy population in exquisite detail, showing the rotation of individual galaxies within the groups as they orbit around their mutual centre of gravity. From such data, we see how diffuse material moves with respect to the galaxies: falling into the gravitational well; being blown out through powerful supernovae winds; or orbiting alongside the galaxy population. Indeed, the data suggest a complex mix of these scenarios, with the gas seemingly remaining in cool clumps as it speeds through the group environment alongside the galaxy population. As one would expect from a mature environment such as a galaxy group, we see extensive material that has resulted from past star-formation, traced by such elements as iron, magnesium and silicon.

It is clear with recent advancements in observational capabilities such as MUSE, that the study of how the cosmic web traces, and interacts with, the galaxy population will be a fruitful field in the coming years, as in recent years. With the James Webb Space Telescope just around the corner, there’s plenty more to come.

Further Reading:

[1] Cantalupo et al. “A cosmic web filament revealed in Lyman-α emission around a luminous high-redshift quasar” (arXiv:1401.4469)

[2] Erfanianfar et al. 2014, “The evolution of star formation activity in galaxy groups” (arxiv:1409.5795)

[3] Bielby et al. 2017, “Probing the intra-group medium of a z = 0.28 galaxy group” (arxiv:1607.03386)

[4] Peroux et al. 2017, “Nature of the absorbing gas associated with a galaxy group at z~0.4” (arXiv:1609.07389)

Note: My research is funded by the Science & Technology Facilities Council.