A Galactic Prison Break: Tracing Lyα Escape at High Redshift

Title: UV and Lyα halos of Lyα emitters across environments at z = 2.84

Authors: Satoshi Kikuta, Yuichi Matsuda, Shigeki Inoue, Charles C. Steidel, Renyue Cen, Zheng Zheng, Hidenobu Yajima,  Rieko Momose,  Masatoshi Imanishi,  and Yutaka Komiyama

First Author’s Institution: National Astronomical Observatory of Japan, Tokyo, Japan

Status: Accepted for publication in ApJ, open access on arXiv

Astrophysics is an observational science. At its most basic level, the science we do requires light from the object we’re studying to reach our telescopes! If the light is produced only at a specific wavelength, as is the case for a spectral line, that’s even better, because it can be used to probe the properties of the object we’re trying to study in detail.

A Useful Escapee

Hydrogen, which is the most common atom in galaxies (and in the universe as a whole, by a lot) produces several spectral lines. The one we’re looking at today is produced by its electron transitioning from its first excited state down to its lowest-energy (‘ground‘) state. This is known as the Hydrogen Lyman-α (Lyα) transition, and it produces light in the ultraviolet (UV) range (at a wavelength of 1216 Å) in its rest frame. If the transition happens far enough away, however, the expansion of the universe redshifts the light to longer wavelengths in the visible range. Because there’s so much hydrogen in galaxies, this spectral line is very bright, and because many modern-day telescopes are designed to capture visible light, this line is very easy to see.


Based on all this, Lyα should be the perfect type of light to use when studying galaxies! There’s one issue, however – that light still has to reach the telescope. This is harder than it sounds, for the same reason that so much Lyα emission is produced by galaxies in the first place. Hydrogen is very, very common in galaxies, and hydrogen atoms can suck the Lyα light back up (absorb it) just as easily as they emit it in the first place. Because hydrogen is so common, Lyα light has to rattle around for a long time, being absorbed and re-emitted, before it can finally escape the galaxy in which it was produced. This means, by the time it reaches our telescopes, Lyα light isn’t a great tracer of the regions where it was emitted, but it is a very good tracer of the size and shape of the hydrogen gas across the entire galaxy.

Figure 1: The positions on the sky of the 3490 LAEs being studied in today’s paper. The points are coloured by the strength of the Lyα emission from each galaxy, and their size corresponds to the strength of each galaxy’s UV emission. The gray contours show the regions where the galaxies are most densely grouped together. Figure 1 of the paper.

Some Very Efficient Observations

In today’s paper, the authors investigate a resolved image of the Lyα emission from a still-forming cluster of galaxies at redshift 2.84 (light that was emitted 11.4 billion years ago). Because so many galaxies are clumped together in this galaxy cluster, the 3490 Lyman-Alpha Emitting galaxies (LAEs) in the image could all be observed in just 8.5 hours with the Hyper-Supreme Cam (HSC) on the Subaru telescope. Observations used the g-band optical filter (which shows redshifted UV emission) and a narrowband filter intended to pick up emission exclusively from the Lyα spectral line. All of the LAEs detected in the image are shown in Figure 1, coloured by their equivalent width (a measure of the strength of the emission). The dense region at the center of the image contains an extremely bright quasi-stellar object (QSO; a galaxy with a black hole in its center sucking up matter so fast that the matter becomes brighter than the galaxy itself), HS1549+1919, which is how the cluster was discovered in the first place.

Stacking Up the Evidence

The authors of this paper wanted to investigate how various galaxy properties change the way Lyα photons escape from a galaxy. To do this, they split the full sample of LAEs into subsamples based on other galaxy properties, such as the luminosity of the Lyα and UV emission, the equivalent width of the Lyα line, or the distance from the central QSO. They then stacked together the images of all the galaxies in each subsample to produce a single, average image with a much better signal-to-noise ratio for each subsample. From these averaged images, shown in Figure 2, the authors can calculate the amount of Lyα emission as a function of distance from the center (a ‘radial profile’, like the ones shown in Figure 3) to calculate the average size and shape of the Lyα emission from the included galaxies. They can thus deduce how Lyα escape is affected by the galaxy properties that change between subsamples.

Figure 2: Stacked images of the Lyα emission from the subsamples of LAEs defined by the authors to explore the effects of various galaxy properties on Lyα emission. From top to bottom, the subsamples are split based on: the magnitude of the UV emission, the Lyα luminosity, the Lyα equivalent width, and the distance from the central QSO. Contours are provided, to show the Lyα shape more clearly. Adapted from Figure 10 of the paper.

Factors Affecting Escape

The authors found that Lyα escape matched their expectations well in general: galaxies with more UV or Lyα luminosity, or with lower equivalent widths (all indicators that a galaxy is making new stars quickly) had much more extended Lyα emission than their lower-luminosity counterparts. This makes sense – bigger galaxies form more stars, so the ones forming stars the fastest should have the most extended hydrogen gas, and thus the most extended Lyα emission. 

Trends in the subsamples split up by the galaxy’s environment, however, weren’t quite so well behaved. The size of the Lyα emission didn’t change much as the galaxies got more distant from the central QSO, but the shape (the ‘spatial distribution’) of the emission definitely did. Galaxies far from the QSO had a lot of Lyα escape in the center of the galaxy, but the amount dropped off very fast towards the outskirts (the blue and green lines in Figure 3). Galaxies close to the QSO, however, didn’t have as steep a drop-off (the red line in Figure 3).

Figure 3: The average radial profiles (amount of emission as a function of distance from the center) of the different subsamples, split based on distance from the QSO. Note that, in the circled region, there’s more emission from the close-in galaxies (the red line) than the distant ones (the blue and green lines). Adapted from Figure 20 of the paper.

The authors believe that these galaxies have the same amount of hydrogen gas surrounding them as the ones farther away from the QSO, but that gas has been excited by the energetic photons emitted by the QSO. This would change how the hydrogen atoms absorb, and let the Lyα escape more easily even far away from the center of the galaxy. This trend is subtle, and the authors do note that it could be due to an observational bias where galaxies moving quickly in relation to the QSO are missed by the narrowband filter they use.

What’s Next?

High-redshift protoclusters like the one studied here are very important to understanding galaxy evolution, because they will evolve into galaxy clusters like the one we live in now. They are, however, very difficult to study effectively, because they’re so far away. It’s therefore critical to develop techniques like the one being used by the authors here, that study many galaxies at once using a minimum amount of telescope time. In the future, the authors plan to do similar analyses on different clusters using different telescopes (such as JWST) to really nail down the factors affecting Lyα escape.

Astrobite edited by: Karthik Yadavalli

Featured image credit: NASA/CXC/M.Weiss

About Delaney Dunne

I'm a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency Line Intensity Mapping experiment based in California's Owens Valley.

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