What is a Lyman-alpha blob?

Giant clouds of gas billions of light-years away are emitting radiation. What’s powering them?

One of the most powerful tools in an astronomer’s toolkit is spectroscopy. By studying the features of an object’s spectrum, a number of its properties can be determined, which might include its composition, temperature, and velocity along the observer’s line of sight. The use of spectroscopy at a variety of wavelengths has proved enormously useful in observational astronomy, and has been a part of the discovery of many classes of object.

LAB-1, the first Lyman-alpha blob discovered. Image credit: ESO/M. Hayes, via the Creative Commons Attribution 3.0 Unported license.

Depending on the part of the spectrum being utilized, certain specific spectral lines might be the target of an observation. For instance, the 21-centimeter line is useful for some radio astronomers studying gas clouds, while the visible and ultraviolet Balmer series is handy for classifying stars. Today, I’m going to use the Lyman-alpha line, found at ultraviolet wavelengths. It arises from an electron in a hydrogen atom transitioning from the second-lowest to the lowest energy level, releasing a photon in the process. Lyman-alpha emission is often used to study galaxies, and related objects, including my topic for today, a class of extremely large objects called Lyman-alpha blobs.

The catch? Nobody knows for sure why these blobs — often several times as large as the Milky Way — emit this radiation. They clearly contain hydrogen, but to show such strong Lyman-alpha emission requires a significant power source. So what could energize such a massive blob of gas?

A fortunate discovery

Lyman-alpha blobs were only discovered quite recently, at the start of the new millenium. They were discovered by accident, in a search for Lyman-break galaxies. Lyman break galaxies show no emission at wavelengths less than 912 Angstroms, a wavelength marking a spot in the set of spectral lines related to the Lyman-alpha line. The cause is an abundance of hydrogen gas absorbing this light; even though Lyman-break galaxies are undergoing rapid star formation, most of this light is simply blocked.

Lyman-break galaxies disappear at short wavelengths — see the first frames! Figure 2, Burgarella et al. (2006).

These galaxies are often found to have their light redshifted, meaning that the spectral lines are shifted to longer wavelengths, thanks to the Doppler effect. This means that they’re farther away from Earth — in some cases, billions of light-years. High-redshift objects are always interesting, because they allow us to peer back in time. While searching for more Lyman-break galaxies at the Palomar Observatory, a team of astronomers (Steidel et al. (2000)) found two objects that they simply called “blobs”. The blobs were found at the same redshift as an overdensity of the galaxies being studied, but their spectra were strange.

Figure 6, Steidel et al. (2000). The first images of the blobs.

The Lyman-alpha emission from these blobs was strong, but all other emission, including at radio wavelengths, was quite faint, which was unusual. If there was a very luminous source powering the emission, like a supermassive black hole, it might be expected to show emission in other portions of the spectrum. The team explored several other possibilities, including the idea that large-scale star formation could be happening in the blobs. The stars would then ionize the surrounding hydrogen, and the electrons would recombine with protons, causing Lyman-alpha emission along the way. Observations at other wavelengths were not able to confirm this hypothesis. Another idea was that the emission could be from cooling gas, something that had been observed at different wavelengths at smaller redshifts.

Not one of a kind

The blobs discovered by the team at Palomar were only the first of their kind to be found. After the observations of LAB-1 — the name given to the larger blob — were announced, the region at the same redshift (z=3.1) was studied in more detail (Matsuda et al. (2005)). The group looked at the location where the blobs were found, SSA 22, which was now thought to be a galaxy protocluster — a group of galaxies forming together. They were curious about the structure of the region, and whether any more Lyman-alpha blobs or Lyman-break galaxies might be there.

A NASA visualization of the SSA22 Protocluster, showing the filaments found by Matsuda et al. (2005).

The results were startling. In addition to the two known blobs, with diameters of 100,000 parsecs (a few times that of the Milky Way), there were dozens of smaller Lyman-alpha blobs in the protocluster, along with plenty of Lyman-break galaxies. Moreover, the cluster appeared to have a structure of three converging filaments, with the two large blobs at the center. Simulations predicted that the filaments should collapse into extremely massive galaxies, formed thanks to the blobs.

Towards the end of the decade, more Lyman-alpha blobs had been discovered, and the idea that they were protogalaxies was quite popular. Interestingly enough, though, the blobs hadn’t been seen at higher redshifts — not that there had been many searches at those distances. Galaxies at higher redshifts should display higher star-formation rates, as they would naturally be younger. The first high-redshift Lyman- alpha blob found was discovered in 2009 by a team observing at a variety of wavelengths (Ouchi et al. (2009)). Named Himiko, this blob was measured to have a redshift of z=6.6, making it almost 13 billion years old.

Figure 2, Ouchi et al. (2009). A composite image of Himiko.

The group found no evidence that Himiko is powered by an active galactic nucleus (AGN), like a quasar, but they were also unable to rule it out. In addition, they considered that perhaps gravitational lensing — which can magnify and brighten high-redshift objects — could have artificially inflated its luminosity, but successfully ruled out number of lensing scenarios; Himiko’s luminosity is intrinsic, not magnified.

Of course, this didn’t resolve the conundrum of what Lyman-alpha blobs actually are. In addition to the cooling and stellar ionization scenarios, Ouchi et al. considered three other possibilities:

• A cluster of smaller ionized HII regions, like those found in normal galaxies, in a very massive galaxy.
• Mergers of a number of individually luminous galaxies with strong Lyman-alpha emission, also known as Lyman-alpha emitters.
• Photoionization of gas by an active galactic nucleus.

The AGN and massive galaxy scenarios were discarded as unlikely, but the cooling gas, merger, and stellar ionization explanations remained.

A composite image of LAB-1, showing x-ray emission in blue. Image credit: NASA.

The culprit

A possible breakthrough came in 2011 with another study of LAB-1. Hayes et al. (2011) used the Very Large Telescope to detect polarized Lyman-alpha emission from the blob. In some places, up to 20% of the light was polarized. The authors argued that the radiation must therefore have been, as they put it, “preferentially emitted” in a particular direction, ruling out the idea that the gas alone could be the source. They concluded, therefore, that there must be galaxies inside LAB-1, and that they must hold sources responsible for the observed emission — ruling out variants of the gas hypotheses.

In light of the Hayes et al. detection, the possibilities of star formation, merging galaxies and AGN activity all seemed palatable. The AGN hypothesis still had the deficiencies from the analysis of Himiko — namely, that signs of it had not been observed by that team — but those same observations hadn’t ruled it out. However, evidence that might clinch the stellar scenario arrived in 2016, in the form of observations from the Atacama Large Millimeter Array (ALMA), a radio interferometer. Geach et al. (2016) found not one but three separate components inside LAB-1 at sub-millimeter wavelengths, surrounded by clusters of ultraviolet sources (found by observations from the Hubble Space Telescope). The conclusions were clear: Young galaxies with high rates of star formation were the source of the Lyman-alpha emission, and the gaseous blobs around them were scattering it.

At last, the culprit! The three components found by ALMA. Figure 1, Geach et al. (2016)

After a decade and a half of study, it seems that the culprit behind Lyman-alpha blobs has finally been found — and in the first blob found, no less! Ironically, though the discovery team detected no radio emission from LAB-1, it was radio waves that proved the key identifiers in the recent results. The dozens of studies in the interim have yielded a substantial amount of information about these still-peculiar objects, denizens of the early universe.