Turn off your inner star formation

Card title: Can the luminous alpha Lyman emitters az ~ 5.7 and z ~ 6.6 suppress star formation?

Authors: Daryl Joe D. Santos, Tomotsugu Goto, Tetsuya Hashimoto, Seong Jin Kim, Ting-Yi Lu, Yi-Hang Valerie Wong, Simon C.-C. Ho, Tiger Y.-Y. Hsiao

Establishment of the first author: Institute of Astronomy, National Tsing Hua University, Taiwan and Max Planck Institute for Extraterrestrial Physics, Germany

State: Accepted in MNRAS [closed access]

How does the lonely and rural galaxy evolve from that of a crowded city full of lights? The role of the environment on the formation and evolution of galaxies is a complex problem that remains unclear. How densely surrounded galaxies – by other galaxies and radiation – have implications for how large structures and clusters of galaxies are formed, as well as how star formation is triggered and subsequently deactivated or extinguished.

Star-forming galaxies, which are commonly detected across an emission line from the Lyman-alpha transition (n = 2 to n = 1) of hydrogen, are a source of high-energy radiation and are potentially a clustering site of galaxies . The impact of the environment on the evolution of these Lyman-alpha emitters (LAEs) is still confusing enough that many studies have somewhat conflicting conclusions about what is happening. Do they tend to cluster together? Does their presence improve or suppress star formation? In today’s article, the authors venture to answer these questions by taking a statistical sample of LAEs in the early days, about a billion years after the Big Bang.

More specifically, their investigation focuses on the impact of ionizing radiation in the ultraviolet wavelength range on star evolution and formation within galaxies in the early Universe. Although the physical processes to suppress or extinguish ongoing star formation are also not fully understood, the presence of strong ultraviolet radiation is a potential quenching method, as it prevents gas from collapsing in stars.

If you emit it, you should put a ring on it

A distinctive feature of LAEs is their ionizing emission in the ultraviolet wavelength range. This radiation can come from ongoing star formation and / or from an active central supermassive black hole. From their prominent Lyman-alpha emission lines, LAEs can be selected via precise spectroscopic observations or via narrowband imaging, which uses a filter to acquire an image while collecting light only around a specific wavelength range ( in other words, a narrow band of the electromagnetic spectrum). If a galaxy has excessive brightness in the narrowband image and we already have some hypothesis that the source of the detection is a Lyman-alpha emission line, we can calculate an approximate redshift.

This paper builds on this technique for a wide-area narrowband survey with Hyper Suprime-Cam on the Subaru telescope, which produced a large sample of candidate LAEs at z ~ 5.7 and z ~ 6.6. These specific redshifts are derived from the redshifted Lyman-alpha line corresponding to the wavelength of the narrowband filters used. Once the sample was built, with about 1000 LAEs in each redshift container, the authors analyzed the number of faint LAEs around the bright LAEs (hence, those with more ionizing radiation) within rings of increasing radius around to the central LAEs. They then placed these rings at certain distances from the bright central LAE to probe their environments on different scales, from the level of the single galaxy to the large voids in the cosmic web.

Why should we care about the environment

For the sample z ~ 5.7, the authors found that the density of faint LAEs around bright central LAEs decreased as the intrinsic brightness of the central galaxy increased (see Figure 1). Their interpretation: the ionizing photons of the central LAEs suppress the star formation of galaxies around them. With their star formation off, the galaxies around them tend not to emit Lyman-alpha, so fewer galaxies are observed in the environment such as LAEs. While stronger radiation within smaller rings explains greater suppression efficiency of the faint closest LAEs, the authors caution that, based on their quick analytical calculations, this physical explanation is not sufficient to explain the amount of suppression here. view.

In the beginning (the highest redshift z ~ 6.6 sample), the image is slightly different and the density of the faint LAEs tends to be more constant across the brightness of the central LAEs. The authors note that the faint LAEs here may be outside the ionized gas bubble that surrounds the central LAEs. Since the Lyman-alpha photons are opaque to the still dominant neutral hydrogen gas at first, galaxies outside the bubble may not be detectable as LAEs. However, this scenario is still not sufficient to explain their findings.

Left with questions about the underlying physics, the authors remind us that LAE environments are subject to complex processes, including feedback from star formation and black hole activity, time scales of activity, movements of galaxies, metals and dust content, and neutral gas opacity for Lyman-alpha photons. Overall, their results support a scenario where fewer LAEs exist around brighter central LAEs, with the suppression of star formation not only caused by Lyman-alpha photons. Future work is critical to unraveling all the complex effects involved in the environmental suppression of star formation, but the big picture is that the environment is at work for the formation of galaxies in the early Universe.

Astrobite by Jessie Thwaites

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