<h3>Lyman α blobs</h3>
<p>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 &quot;Lyα&quot;, i.e. the <span class="math-container">$\lambda=1216\,\mathrm{Å}$</span> 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 <span class="math-container">$\sim10^{-19}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}\,\mathrm{arcsec}^{-2}$</span> farther away than <span class="math-container">$\sim 10\,\mathrm{kpc}$</span> from the center.</p>
<p>Probing this spectroscopically can be done with VLT's MUSE, but not much lower than <span class="math-container">$10^{-18}$</span> cgs units (see e.g. <a href="https://ui.adsabs.harvard.edu/abs/2020A%26A...642A..55H/abstract" rel="nofollow noreferrer">Herenz et al. 2020</a>). To probe the surface brightness profile out to large distances, one can instead stack multiple LABs (e.g. <a href="https://ui.adsabs.harvard.edu/abs/2011ApJ...736..160S/abstract" rel="nofollow noreferrer">Steidel et al. 2011</a>).</p>
<h3>Lyman break galaxies</h3>
<p>A Lyman break galaxy (LBG) is, formally, a galaxy detected due to its lack of flux blueward of the Lyman break at <span class="math-container">$\lambda=921\,\mathrm{Å}$</span>. The physical reason is that photons of shorter wavelengths are able to ionize the surrounding hydrogen. <a href="https://ui.adsabs.harvard.edu/abs/1996ApJ...462L..17S/abstract" rel="nofollow noreferrer">Steidel (1996)</a> 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 &quot;drop-out technique&quot;.</p>
<p>At high redshifts where both the density and the neutral fraction of hydrogen is larger, the break is seen not at <span class="math-container">$\lambda=921\,\mathrm{Å}$</span>, but at the Lyman <em>alpha</em> wavelength at <span class="math-container">$\lambda=1216\,\mathrm{Å}$</span>. 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 &quot;LBGs&quot;, but it's important to realize that the physics are different, and that the break occurs at a different wavelength.</p>
<h3>The answer</h3>
<p>So, LBGs are found because there is an underlying (stellar) continuum that is &quot;broken&quot; 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.</p>
<p>So, if I understand your question correctly, the answer is no.</p>

CC BY-SA 4.0
2023-09-12T20:44:09.237Z
Thank you so much! This clears it up a lot.
Ankit Biswas
CC BY-SA 4.0
2023-09-12T08:31:33.947Z
2023-09-12T08:31:33.947Z
pela
<p>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.</p>
<p>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?</p>

CC BY-SA 4.0
2023-09-11T11:12:14.243Z
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.
pela
CC BY-SA 4.0
2023-09-11T20:21:07.967Z
Just wanted to clarify–which properties of lyman-alpha blobs make them not possess a lyman-limit break? Thank you so much for answering!
Ankit Biswas
CC BY-SA 4.0
2023-09-12T10:38:33.820Z
You're welcome, though I'm still having a little trouble understanding the sentence starting with "I was wondering…", but I tried answering below :)
pela
<galaxy><photography><photometry><james-webb-space-telescope><nebula>
Is it possible to detect lyman alpha blobs (LABs) with the lyman-break method?
Ankit Biswas
# Is it possible to detect lyman alpha blobs (LABs) with the lyman-break method?

(Asked by: username_0 on 2023-09-11)

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?

**Tags:** galaxy, photography, photometry, james-webb-space-telescope, nebula

> **Comments:**
> **username_1**: 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.  
> **username_0**: Just wanted to clarify–which properties of lyman-alpha blobs make them not possess a lyman-limit break? Thank you so much for answering!  
> **username_1**: You're welcome, though I'm still having a little trouble understanding the sentence starting with "I was wondering…", but I tried answering below :)  

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## 1 answers

### Answer by username_1 on 2023-09-12. Score: 3 | Accepted answer

### 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](https://ui.adsabs.harvard.edu/abs/2020A%26A...642A..55H/abstract)). To probe the surface brightness profile out to large distances, one can instead stack multiple LABs (e.g. [Steidel et al. 2011](https://ui.adsabs.harvard.edu/abs/2011ApJ...736..160S/abstract)).

### 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)](https://ui.adsabs.harvard.edu/abs/1996ApJ...462L..17S/abstract) 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.

> **Comments:**
> **username_0**: Thank you so much! This clears it up a lot.  

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