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LABs are luminous extended nebulae of hydrogen gas in the early universe (z ≥ 3) found in overdensities of LAEs (lyman-alpha emitters) and LBGs (lyman-break galaxies). I was wondering if LABs could be detected using JWST data, but the only narrowband observations with exposure time long enough to find LABs and wavelength short enough to realistically find these objects have been concentrated in observations towards one supernova–nowhere near the survey volume to find these objects, especially at a narrowband at 12.5 redshift for Lya emission.

I was wondering if it was instead possible to use the commonly-used Lyman-break method (in which you can use narrowband images to identify LBGs) to also identify LABs. What would be the challenges with this method and is it even feasible?

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  • $\begingroup$ Perhaps I misunderstand, but a narrowband excess of flux in Lyα is usually how LABs are found. The differences are that 1) there isn't really any continuum, just a lower limit, and 2) the Lyα flux is very low, typically falling below ~1e-19 erg/s/cm2/arsec2 farther away than ~10 kpc from the center. Probing this spectroscopically can be done with VLT's MUSE, but not much lower than 1e-18 (see e.g. Herentz+ 2020). To probe the surface brightness profile out to large distances, one can instead stack multiple LABs. Charles Charles Steidel had a paper on this, in 2010 I think. $\endgroup$
    – pela
    Commented Sep 11, 2023 at 11:12
  • $\begingroup$ Just wanted to clarify–which properties of lyman-alpha blobs make them not possess a lyman-limit break? Thank you so much for answering! $\endgroup$
    – Ankit Biswas
    Commented Sep 11, 2023 at 20:21
  • $\begingroup$ You're welcome, though I'm still having a little trouble understanding the sentence starting with "I was wondering…", but I tried answering below :) $\endgroup$
    – pela
    Commented Sep 12, 2023 at 10:38

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Lyman α blobs

Lyman alpha blobs (LABs) are huge (~0.1–1 million lightyears' scale) regions surrounding a galaxy or galaxy group, where we see a higher-than-average flux of Lyman alpha (or "Lyα", i.e. the $\lambda=1216\,\mathrm{Å}$ photon originating from the 2→1 transition of neutral hydrogen). The physical origin has been debated, but is thought to be Lyα emitted from the central galaxies (or a quasar) and scattered by the surrounding hydrogen, and/or produced locally either by numerous smaller galaxies or even accreting gas that cools. The Lyα flux is very low, typically falling below $\sim10^{-19}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}\,\mathrm{arcsec}^{-2}$ farther away than $\sim 10\,\mathrm{kpc}$ from the center.

Probing this spectroscopically can be done with VLT's MUSE, but not much lower than $10^{-18}$ cgs units (see e.g. Herenz et al. 2020). To probe the surface brightness profile out to large distances, one can instead stack multiple LABs (e.g. Steidel et al. 2011).

Lyman break galaxies

A Lyman break galaxy (LBG) is, formally, a galaxy detected due to its lack of flux blueward of the Lyman break at $\lambda=921\,\mathrm{Å}$. The physical reason is that photons of shorter wavelengths are able to ionize the surrounding hydrogen. Steidel (1996) revolutionized high-redshift astronomy with this technique, because you can take an image of a large field through different filters, and then look for a galaxy that is invisible in filters blueward of the (restframe) Lyman break. This is called the "drop-out technique".

At high redshifts where both the density and the neutral fraction of hydrogen is larger, the break is seen not at $\lambda=921\,\mathrm{Å}$, but at the Lyman alpha wavelength at $\lambda=1216\,\mathrm{Å}$. Galaxies with a Lyα break can also be found with the drop-out technique, and somewhat confusingly such galaxies are also occasionally referred to a "LBGs", but it's important to realize that the physics are different, and that the break occurs at a different wavelength.

The answer

So, LBGs are found because there is an underlying (stellar) continuum that is "broken" at a specific wavelength. In contrast, LABs are found by comparing broadband and narrowband images, looking for regions where there is an excess flux in the narrowband. There (usually) isn't any continuum to be broken in a LAB region.

So, if I understand your question correctly, the answer is no.

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    $\begingroup$ Thank you so much! This clears it up a lot. $\endgroup$
    – Ankit Biswas
    Commented Sep 12, 2023 at 20:44

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