the Oldest Known Galaxies with MUSE
astrobites | Daily Paper Summaries | 2020 Sep 14
Ali Crisp wrote:
The distance astronomers can “see” is limited by the age of the universe, in addition to being limited by current technology. Despite seeming instantaneous, light travels at a finite speed – specifically 3×108 m/s, or 186,282 miles per second for fans of the imperial system. In our daily lives, this isn’t a big deal because the distances the light needs to travel are negligible compared to the speed of light. However, since space is “vastly, hugely, mind-bogglingly big,” the distance is no longer negligible, and astronomers can only observe objects whose light has had enough time to reach us. For example, our Sun is about 150 million km away (or 93.5 million miles), and the light we receive from it takes 8 minutes to travel from the Sun to the Earth and 5.5 hours to reach Pluto. On a larger scale, since the universe is around 13 billion years old, light from the beginning of the universe has been traveling for around 13 billion years. That means the furthest things we can observe are the oldest, and we see them as they were around 13 billion years ago rather than how they are now. This makes them good indicators of what the early universe was like.
Using spectroscopic studies, some of these early objects have been classified as Lyman-α Emitters (LAEs). LAEs, perhaps unsurprisingly, are galaxies that emit “Lyman-α radiation” from hydrogen, but what does that even mean? Well, spectroscopy in general is the study of the electromagnetic spectrum that different objects give off, specifically the emission and absorption of photons when electrons transition between energy levels in an atom. The Lyman series – consisting of Lyman-α, Lyman-β, Lyman-γ, etc. – is a special set of ultraviolet emission lines that result from the transition of electrons from higher energy levels to the ground state in the hydrogen atom. Lyman-α specifically is the emission line from the electron transitioning from the first excited state to the ground state. Lyman-α Emitters are generally extremely distant and extremely young, and can be used to study the Epoch of Reionization and early galaxy formation. Usually, Lyman-α is so energetic that it is absorbed by neutral hydrogen, which was predominant in the early universe, after expansion and cooling. That means that LAEs must have formed later, just as the universe began to reionize, because Lyman-α radiation can make it through ionized hydrogen. This period of reionization corresponds to when star and galaxy formation really kicked in, so the increased formation rates are thought to be the source of the reionization. So, if we look at an LAE, we’re looking at potential progenitors for reionization and galaxy formation.
Amongst these LAEs is the galaxy Cosmos Redshift 7 (CR7), named as a nod to Portuguese footballer Cristiano Ronaldo (see, some astronomers do care about sports!). Discovered in 2015 using the Very Large Telescope (VLT), CR7 is one of brightest known LAEs at a redshift of z > 6.0 and was initially thought to show some indicators of both Population III stars – the first generation of stars, containing very few “metals” – and of a direct collapse black hole. CR7 has three distinct UV regions (Figure 2), two of which are made up of older The authors of today’s paper use the Multi-Unit Spectroscopic Explorer (MUSE) instrument on the VLT combined with near-infrared archival data from Hubble’s Wide Field Camera 3 (WFC3) and ESO’s UltraVISTA to do an in-depth study of CR7’s morphology. ...
The Nature of CR7 Revealed with MUSE: a Young Starburst
Powering Extended Lyman-α Emission at z=6.6 ~ Jorryt Matthee et al