MAMMOTH-LyC: Investigating the Role of Galaxy Mergers in a Strong Lyman Continuum Leaker at z=2.39

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Hi, I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "MAMMOTH-LyC: Investigating the Role of Galaxy Mergers in a Strong Lyman Continuum Leaker at z=2.39".

Jocelyn: Here's the gist of the paper. The gist: The MAMMOTH-LyC survey discovered J1244-LyC1, a strong Lyman continuum (LyC) leaker at z = 2.39 exhibiting clear merger signatures, which is the first confirmed high-redshift LyC-leaking merger and the most massive LyC emitter known to date Discovery and Identification of J1244-LyC1 The MAMMOTH-LyC survey obtained 18-orbit-deep WFC3/UVIS F225W imaging in two massive galaxy protocluster fields at z ∼ 2.2 The survey reported the discovery of J1244-LyC1, a strong Lyman continuum (LyC) leaker at z = 2.39, exhibiting clear merger signatures This source has a highly significant (10σ) LyC detection, corresponding to an absolute escape fraction of fesc = 36% ± 4% (1σ) With a stellar mass of 10 10.2M⊙, J1244-LyC1 is both the first confirmed high-redshift LyC-leaking merger and the most massive LyC emitter known to date The LyC emission is spatially resolved into multiple peaks that coincide with the system’s disturbed morphology, confirming genuine multi-site LyC leakage Physical Properties and Spectroscopic Confirmation The spectroscopic analysis confirmed the redshift of J1244-LyC1 to be z = 2.39, with no evidence found for a low-z interloper The emission lines detected include

O ii: , Hγ, Hβ, and Hα The two Keck/MOSFIRE K band observations provided a clear separation of the double velocity components in the Hα emission This separation yields a relative velocity difference of 116 ± 3.6 km s−1 between the two components The best-fit SED model indicates that J1244-LyC1 is dominated by a young stellar population with an age of 330 ± 120 Myr and a recent SFR of 65 ± 33 M⊙ yr−1 The derived physical parameters are listed in Table 2 LyC Escape Fraction Calculation The escape fraction fesc was constrained observationally by comparing the observed LyC flux to the intrinsic LyC flux expected from stellar population models The relative escape fraction is calculated using the formula fesc,rel = (LLyC/LUV)int (FLyC/FUV)obs × tIGM −1 IGM, where tIGM = e−τIGM represents the transmission fraction of ionizing photons through the IGM After applying dust correction, the absolute escape fraction is derived as fesc,abs = fesc,rel × 10−0.4 AUV The calculated absolute escape fraction is fesc,abs = 0.36 ± 0.04, which is consistent with the SED result Spatial Resolution and Merger Signatures Two-dimensional surface brightness modeling using GALFIT revealed two bright components separated by a projected distance of 2.5 kpc (0.3′′) and tidal-tail features The analysis identified three independent LyC-emitting regions—clumps A, B, and C—each with SNR > 3 Clump A lies between the two UV-bright centers, while clumps B and C exhibit spatial offsets from the main merger system The presence of clump A distinguishes J1244-LyC1 from other merger-featured LyC candidates because its LyC emission shows no significant spatial offset from the UV morphology Role of Galaxy Mergers in Reionization Galaxy mergers are proposed as a physical mechanism connected to LyC escape, where tidal interactions can disrupt the interstellar medium, creating multiple low-column-density pathways that facilitate LyC escape The LaCOS survey found that more than 41% of confirmed LyC leakers reside in merging systems In contrast to low-mass galaxies, massive galaxies rely more heavily on the strong ISM disturbances induced by mergers to open low-opacity channels for LyC leakage This suggests that merger-driven processes become far more important for enabling LyC escape in massive galaxies The diffuse LyC-band emission observed in J1244-LyC1 provides a unique opportunity to investigate this issue in the high-redshift Universe Environmental Effects and Future Directions The detection of J1244-LyC1 is particularly intriguing because, although it does not reside within the BOSS1244 protocluster, it is still affected by the IGM environment associated with the foreground protocluster The presence of background LyC leakers may provide additional information on the IGM along these lines of sight Further follow-up observations with HST and ground-based adaptive-optics IFUs, such as Keck/OSIRIS, will be crucial for further understanding the dynamical structure, Lyα line profile, and spatial distribution of Lyα emission in J1244-LyC1 This work presents the first results from the MAMMOTH-LyC ultra-deep HST WFC3/UVIS imaging survey targeting the core regions of two massive galaxy protoclusters at z ∼ 2.2 The total LyCband signal is detected at a 10σ level, with an estimated escape fraction of fesc ∼ 36%, and shows no spatial offset relative to the UV-band imaging (F475W) This work is supported by the National Key R&D Program of China No.2025YFF0510603, the National Natural Science Foundation of China (grant 12373009), the CAS Project for Young Scientists in Basic Research Grant No.

Vera: YSBR-062, the China Manned Space Program with grant no.

Paper summary: Vera: So, looking at J1244-LyC1, this object found by the MAMMOTH survey is really significant because it’s a strong Lyman continuum leaker with clear merger features at z equals two point three nine <ref:2512.23202#pg1>.

Jocelyn: And that means the authors of this paper are focusing on how these mergers actually drive those ionizing photons out into space <ref:2512.23202#pg1>.

Subrahmanyan: Yeah, they're connecting the stellar mass of this galaxy, which is around one zero to ten to the power ten point two solar masses, with how much gas disruption happens during a merger <ref:2512.23202#pg1>.

Vera: Exactly because they found that tidal interactions create these multiple pathways for LyC escape—it’s not just one big hole in the cloud you're looking at <ref:2512.23202#pg1>.

Jocelyn: And they show us this absolute escape fraction of thirty six percent plus or minus six percent, which is a solid number to work with <ref:2512.23202#pg1>.

Subrahmanyan: That percentage tells us that for massive galaxies, mergers are the main way we can open up those low-opacity channels for ionizing radiation <ref:2512.23202#pg3>.

Vera: It's also important because this is the first confirmed high-redshift LyC leaking merger they’ve found, and it’s currently the most massive one known to science <ref:2512.23202#pg1>.

Jocelyn: So, what does this tell us about reionization? Does this object represent a typical way that galaxies got ready for cosmic reionization <ref:2512.23202#pg1>?

Subrahmanyan: It suggests that merger-driven processes might be far more important for enabling LyC escape in massive galaxies than we thought when you look at lower mass systems <ref:2512.23202#pg3>.

Vera: They are still using HST and ground-based adaptive optics instruments to get a better look at the dynamical structure of this system <ref:2512.23202#pg1>.

Jocelyn: And that means they're still working on getting a really detailed picture of how all that gas is moving around inside J1244-LyC1 <ref:2512.23202#pg1>.

Subrahmanyan: Next up, we’ll take a look at some of the theoretical models trying to predict how much ionizing radiation gets lost in these kinds of disturbed environments <ref:2512.23202#pg3>.

Conclusion: Vera: So, to wrap up this part of the discussion, we're looking at the paper titled "MAMMOTH-LyC: Investigating the Role of Galaxy Mergers in a Strong Lyman Continuum Leaker at z=two point three nine".

Jocelyn: The authors are really focused on J1244-LyC1 and how it fits into the bigger picture of galaxy mergers and ionizing radiation escape.

Subrahmanyan: They're using this specific object to test a hypothesis about how much tidal disruption in a merger actually changes the physics of photon escape, which is key for reionization models.

Vera: The main point is that this galaxy shows clear evidence—like those multiple LyC peaks and the spatial offset between clumps A, B, and C—that its strong ionizing signal comes from several different places due to a disturbed structure.

Jocelyn: And they pin down an absolute escape fraction of f esc = zero point three six plus or minus zero point zero four, which is solid observational data showing how much light actually gets out of the galaxy versus what's inside.

Subrahmanyan: That result supports the idea that for massive galaxies, mergers are a critical mechanism for creating those low-opacity pathways we need to get ionizing photons through the interstellar medium.

Vera: It really paints a picture where tidal interactions don't just build structure, they actively rewire how easily radiation can escape in these high-redshift environments.

Jocelyn: This object is a prime example of what we might expect to see when looking at the galaxy population driving reionization around z = two point four.

Subrahmanyan: It sets a benchmark for us: if we want to understand how galaxies contribute to the ionizing background, we need to prioritize these merger-driven processes in massive systems.

Vera: But they still flag that more detailed follow-up observations with instruments like Keck/OSIRIS are needed to fully map out all those gas dynamics and confirm the exact structure.

Jocelyn: That means while we have a very strong signal here, we don't yet have the complete dynamical story of J1244-LyC1.

Subrahmanyan: So this paper gives us a concrete anchor point for merger physics in reionization, but the next step is getting those deeper structural measurements to see exactly how that disruption works on smaller scales.