Weekly archive
LRDigestWeekly digests of little red dots and JWST-selected AGN papers, with curated reading notes and archive pages.
654 indexed papers · 430 research notes
The authors propose supermassive dark stars as a unified, still-unconfirmed explanation for early compact luminous sources and massive black holes. Their argument combines tentative helium absorption candidates with parameterized growth tracks, rather than a population-level observational test.
This paper proposes that feedback-free cluster formation, dry cluster migration, and later wet compaction can jointly make compact, black-hole-hosting LRDs at z=4–8. The central prediction remains a partly untested synthesis because no simulation yet follows all processes together.
JWST/MIRI-MRS spectroscopy of four luminous type-1 quasars at z=7.08–7.64 yields mutually consistent broad-line black-hole masses of (4–15)×10^8 M⊙ and directly detects hot-torus emission, without resolving the early-SMBH growth tension.
A nuclear-star-cluster model suggests that dense, gas-fed clusters can grow an intermediate-mass seed into an LRD-scale massive black hole by z~5, while predicting tidal disruptions more frequently than stellar-mass-BH captures at z=4–6.
This two-component SIDM model identifies parameter space that authors argue meets relic-density, velocity-dependent scattering, linear-power, and representative direct-detection constraints; a subpercent ultra-interacting component is proposed to seed early black holes relevant to little red dots.
MESA quasi-star models predict a long-lived, cool late stage whose continuum broadly resembles two JWST Little Red Dots, including one measured Balmer break, but mismatched extreme breaks and simplified atmospheres prevent identification of LRDs as quasi-stars.
For 70 JWST broad-line AGN at 3<z<7, the authors infer black holes overmassive relative to local broad-line AGN, with the offset and BH-to-stellar-mass ratio increasing toward high redshift; LRD-rich sample composition contributes substantially.
BRAHMA constrained simulations find that heavy-seed abundance and merger dynamics govern black-hole assembly above z≈9, whereas permissive late gas accretion is needed to reach z≈6 quasar masses; extreme z≈9–10 BH-to-stellar ratios remain difficult under subgrid dynamical friction.
BlackTHUNDER presents JWST/NIRSpec IFU spectroscopy of UNCOVER_20466 (z=8.50), confirming its LRD nature and an overmassive central black hole. Balmer decrements indicate little dust, yielding an Hβ-based bolometric output and Eddington ratio only ~10% of earlier estimates—consistent with a currently subdued/dormant phase. Strong Lyα (f_esc ≈ 30%) and an extreme [O III]4363/Hγ imply AGN photoionization with ultra-dense gas (n_e ~ 10^7 cm^-3). The source sits on local M_BH–σ_* and M_BH–M_dyn relations and is compatible with growth from a heavy seed (~10^3 M⊙), marking it as a likely progenitor core of a later galaxy.
Analyzes Little Red Dots in a stellar-only framework by mapping collision, relaxation, and age timescales and folding in recent N-body results for extreme stellar systems. Finds typical LRD cores sit in the tage ~ tcoll < trelax regime, triggering core runaway collisions that form a massive black hole; in more extreme cases with tcoll < trelax the whole system enters a “Forbidden Stellar Zone” and collapses. This pathway yields high BH formation efficiencies (~10–50% of the stellar mass), naturally explaining LRD transience, frequent X-ray non-detections prior to seed formation, and over-massive remnants once formed. Concludes LRDs are prime sites to catch nascent, potentially already supermassive, BH seeds at z≈4–8.
Ultra-deep JWST/NIRSpec G395M spectroscopy behind Abell S1063 enables a systematic search for faint broad-line nuclei, fitting narrow components with an [O III] λ5007 template and validating the method with mocks. The authors identify ten broad-line AGN at 4.5<z<7.0 (eight secure, two tentative) with broad Hα–based virial masses down to ~10^5.5 Msun; composite galaxy+AGN SEDs yield host masses implying M_BH/M_* ≲ 0.1% and placements consistent with the local M_BH–M_* relation. From these they build a BH mass function that agrees with prior results above ~10^6.5 Msun but shows an excess at ~10^5.5 Msun relative to a Schechter extrapolation. They interpret the low-mass excess as evidence for descendants of direct-collapse seeds in the reionization era.
Proposes a two-component origin for LRD SEDs during assembly of Mh ≳ 1e10–11 Msun halos at z ≃ 4–10: a super-Eddington–fed seed BH (growing to ~1e6–1e7 Msun) inflates a dense envelope at Teff ≃ 5000 K that makes the red optical, while a coeval ~1e7 Msun nuclear starburst supplies the blue UV. This naturally yields the V-shaped UV–optical continuum and the weakness of X-ray/IR/radio signatures, with broad lines arising despite envelope scattering. A key prediction is a nearly constant Lopt/LUV ≃ 2–10 for ≃15 Myr (~one-third Salpeter) until SNe expel nuclear gas and quench the envelope. Population-wide, the ratio stays uniform at z ≃ 4–8 but declines toward lower z as red envelopes fail, linking LRD phases to later normal AGN states.
Builds a seed-to-SMBH pathway inside a two-phase (fast/slow halo growth) framework: Pop III remnants are bred in mini-halos at the H2- (or delayed atomic-) cooling thresholds, then grow via nuclear-burst super-Eddington episodes, SGC sub-cloud capture, and finally mergers. Implemented on subhalo merger trees, the model produces a broad seed spectrum (10–10^5 Msun at z≈20–30) and a multi-piece, redshift-dependent M_BH–M_* relation set by the dominant channel. A key result is that early nuclear bursts enable rapid mass gain, naturally yielding compact accretors reminiscent of JWST little red dots before merger-dominated quiescence. The framework links LW background and halo assembly rate to BH demographics and matches current constraints.
Assuming the star-only interpretation of compact, red z∼5 Little Red Dots, the authors model ultradense cores where dynamical friction for 10 M⊙ stars in the inner 0.1 pc is <0.1 Myr, triggering rapid mass segregation. A Fokker–Planck core-evolution calculation, an analytic collisional-growth model, and direct N-body simulations all converge on runaway mergers that build a very massive star of 9×10^3–5×10^4 M⊙ within <1 Myr. The VMS then contracts on a ∼8000 yr Kelvin–Helmholtz timescale and collapses to an ∼10^4 M⊙ black hole. This route yields seed number densities above direct-collapse expectations and, with a dense residual core, can power elevated tidal disruption event rates.
Strong lensing plus JWST/NIRSpec IFS resolves narrow Hα kinematics and spectroastrometric shifts in the lensed Little Red Dot Abell2744‑QSO1 at z=7.04, enabling a direct rotation curve from ~10–200 pc. The curve is Keplerian and rules out compact extended mass profiles (e.g., MW‑like nuclear star cluster, Plummer, or a DM cusp), implying a central point mass of order 5×10^7 M⊙ consistent with Balmer‑line virial estimates. The inferred stellar content is minimal, with MBH/M* > 2, pointing to a near‑“naked” black hole in a pristine environment. This result validates virial BH masses for LRDs at the EoR and captures a massive BH seed in its earliest growth phase.
Preliminary MELIORA cosmological simulations implement a direct-collapse BH formation model that tracks Lyman–Werner irradiation and halo inflow rates to test whether newborn heavy seeds can resemble Little Red Dots. The simulated population of newly formed DCBHs drops sharply at z<6 as inflows wane, mirroring the observed emergence-and-decline trend of LRDs. DCBH birth coincides with a strong gas-compaction event and a short, bright AGN phase lasting ~200 Myr, after which the first Pop III stars appear and the AGN fades. If LRDs trace this nursery phase, their weak X-ray and hot-dust signatures follow naturally from the scenario.
Using the A-SLOTH semi-analytic framework, the authors generate a DCBH-seeded SMBH population and benchmark it against Little Red Dot (LRD) demographics and spectra. Heavy (DCBH) seeds reproduce the observed LRD BH mass functions, number densities, and host-halo trends better than stellar-remnant seeds. For the most extreme early LRDs (e.g., MoM-BH*-1), DCBH scenarios with either strong dust attenuation or dense, dust-free gas can match the distinctive V-shaped, Balmer-break–dominated SEDs, whereas even super-Eddington growth on light seeds cannot. The work points to gas metallicity as a crucial observable, beyond BH-to-stellar mass ratios, to pin down LRD progenitors.
Cosmological radiation‑hydrodynamical simulations test quasar SEDs (near‑UV to hard X‑ray) impinging from 10–1000 kpc onto a neighboring halo and quantify how AGN radiation regulates primordial collapse. X‑rays keep the free‑electron fraction high, catalyzing H2 despite LW dissociation, so collapse is delayed until ≲10^7 Msun of pristine gas accumulates—yielding a massive Pop III cluster for the weaker two fields and a DCBH for the strongest. Runaway collapse occurs at z ≃ 24.99, 15.46, and 12.63 (A→C), directly tying radiation intensity to later collapse and larger available gas reservoirs. Predicted He II 1640 luminosities match the GN‑z11 Pop III candidate and should be detectable with NIRSpec to z ≈ 15, outlining an AGN‑proximity pathway to luminous Pop III clusters or early DCBH seeds.
High-resolution hydrodynamics around ∼10^6 M⊙ primordial black holes show that accretion-driven LW fields plus strong baryon–DM streaming (v_bχ ≳ 0.8 σ_bχ) push gas onto an atomic-cooling track and trigger collapse in the PBH wake. The collapsing clouds sustain inflow rates ≳0.01–0.1 M⊙ yr⁻¹, enabling ∼10^5 M⊙ supermassive stars that rapidly form direct-collapse BHs at z ∼ 20–10. These secondary DCBHs pair with the primary PBH to yield in-situ SMBH binaries with initial q ≈ O(0.1) at ∼10 pc separations. The channel predicts compact, LRD-like sources and low-frequency GW emitters accessible to JWST/ALMA and LISA/TianQin.
Models little red dots as supermassive stars fed by massive self‑gravitating discs assembled in gas‑rich major mergers, reproducing the V‑shaped SED as two blackbodies and attributing Balmer broadening to disc rotation—without invoking extra AGN, stellar, or dust components. MCMC fits to two representative LRDs (J0647‑1045 and COS‑756434) yield sub‑Eddington disc accretion and SMS masses of a few ×10^6 Msun with physically motivated parameters. The framework predicts a luminosity cut‑off of order few ×10^44 erg/s and ties the observed redshift distribution of LRDs to merger rates, matching current trends. Acknowledged caveat: reproducing the absolute abundance of LRDs remains challenging, though the fits are robust to observed diversity.
This work builds an improved, unified framework for the halo mass function seeded by cosmic string loops, treating arbitrary loop-velocity distributions and capturing both spherical and filamentary growth. From this mass function, it isolates halos that meet direct-collapse conditions at high redshift, producing black-hole seeds of ~10^4–10^5 Msun. For reasonable string parameters, the predicted seed abundance reproduces the observed number density of JWST’s Little Red Dots, offering a non-Gaussian small-scale route to early SMBH growth. Because loop formation lacks large acoustic waves, the scenario avoids CMB spectral-distortion bounds, and a velocity–tension phase diagram (including fragmentation into “beads”) clarifies where this channel dominates.
Semi-analytic models, anchored to stellar-evolution and GR collapse simulations, follow ejecta–CSM shocks from exploding supermassive Pop III stars to predict light curves, spectra, and photometry. The events peak at ~10^45–10^47 erg/s and last 10–200 yr in the source frame (observed 250–3000 yr), yielding quasi-persistent variability indistinguishable from little red dots/AGN on 0.5–9(1+z) yr baselines. Bright explosions should be detectable in long-wavelength JWST filters to z≤20 at 24–26 mag, with pulsating SMSs to z≤15; Euclid and Roman reach z<11–12. Deep fields could constrain the event rate to ~10^-11 Mpc^-3 yr^-1 and yield several hundred (Euclid) to dozens (Roman) detections, directly testing heavy-seed SMBH pathways.
First-principles synthetic spectra for non-rotating, metal-free supermassive stars (up to 10^6 M⊙) are used to test whether LRDs are direct photospheres of SMSs. The model’s luminosity at 4050 Å (Lλ ≈ 1.7×10^44 erg s^-1 μm^-1) and NLTE physics reproduce hallmark LRD traits—a V-shaped Balmer break and strong Hβ emission with other Balmer lines in absorption—with wind/macroturbulent broadening matching observed profiles. Quantitative fits to sources at z=7.76 and z=3.55 include an Hβ width match for MoM‑BH*-1 to within 4%, offering a self-consistent alternative to multi-component obscured AGN. A luminosity-dependent visibility window of ~10^4 yr for the brightest systems and 10^5–10^6 yr for objects 1–2 dex fainter implies LRDs may capture brief, pre-collapse stages of SMBH progenitors.
Proposes LRD heavy seeds as runaway-merger products of primordial black holes initially clustered by small-scale long–short mode coupling. Monte Carlo solutions of the Smoluchowski equation yield merger timescales sufficient to assemble 10^5–10^8 Msun SMBHs within the first Gyr while evading CMB distortion limits. The scenario predicts a distinctive stochastic gravitational-wave background and high birth spins inherited from cluster tidal torques. Interpretation assumes LRDs host obscured AGN.
Proposes ultra-strongly self-interacting dark matter (uSIDM) as a route to rapid gravothermal core collapse in early halos, yielding massive black hole seeds (≳10^5 Msun) by z≳5 that can power Little Red Dots. Builds a semi-analytic framework tracking halo evolution, collapse redshifts z_coll, and the SMBH mass function with stochastic growth via a log-normal Eddington ratio and finite duty cycle. Finds that uSIDM reproduces LRD abundance, compactness, and characteristic BH masses, with a z≈5 SMBH mass function matching LRD data and SIDM merger-tree results, especially at m_BH≳10^7 Msun. Implies LRDs could be incisive tracers of non-gravitational dark sector physics and that early SMBH assembly may depart from CDM expectations.
High-resolution zoom-in simulations with Pop III.1 progenitor preheating follow 10^5 Msun seeds through a suite of AGN feedback modes (Eddington-limited thermal; non-Eddington thermal, kinetic, and radiative). Across all models the seeds grow efficiently to ~10^7 Msun by z≈8, with growth largely unimpeded by feedback; the radiative case shows brief super-Eddington bursts that launch fast, mass-loaded outflows (>2500 km s^-1) to ~50 kpc and slightly suppress stellar mass. The simulated systems resemble JWST moderate-luminosity quasars, yielding overmassive BHs, dynamical masses ~10^9.5 Msun, and short-lived high Eddington fractions. This argues that early AGN feedback coexists with rapid SMBH build-up while exporting energy and metals deep into the CGM/IGM.
Normalizing a thin-disk+corona spectrum to a J1120-like quasar grown in the Smidt et al. (2018) simulation, the authors predict continuum NIR AB magnitudes from z = 6–15 in Euclid, Roman, and JWST bands. They find Euclid can photometrically select such SMBHs to z ≈ 11 (Wide), z ≈ 13 (Deep), and z ≈ 14 (Ultra Deep), while Roman reaches z ≈ 14.5 in Y/J/H and its F213 band extends selection to z ≈ 18. JWST’s sensitivity enables spectroscopic redshift confirmation to z ≈ 15 and even detects ≳10^6 M⊙ BHs accreting near Eddington. This Euclid/Roman discovery plus JWST follow-up pipeline could inventory early SMBHs (including analogs of UHZ1 and GHZ9) and discriminate their growth channels beyond z > 10.
The authors post-process a direct-collapse pre-SMBH inflow–outflow geometry with a Monte Carlo Lyα transfer that includes photon destruction and two-photon emission, finding that a radiatively driven shell plus a biconical polar funnel enables substantial escape. The emergent Lyα is highly anisotropic and line shapes depend on funnel opening angle and outer column density, yielding strong asymmetry with an extended red tail and measurable peak shifts/cuspiness. The escaping fraction can exceed 95% for a z=10 pre-SMBH object, with only the red wing detectable. Such sources are in principle reachable with NIRSpec/MOS in ~10^4 s at 10σ, and their line profiles provide a diagnostic to separate them from high-z galaxies and quasars.
Proposes an SIDM-driven seeding pathway for JWST little red dots in which gravothermal core collapse, sped up by central baryons, makes a tiny seed that grows first via baryonic Eddington accretion and then rapidly through dark Bondi inflow of SIDM. Demonstrates that black holes can reach ~10^7 Msun within ~500 Myr after the formation of ~10^9 Msun halos at z≈4–11, with most of the final mass supplied by dark matter rather than gas. Fluid simulations with and without a Plummer-fit baryonic potential show that baryons substantially shorten the collapse timescale and establish conditions for efficient dark-sector Bondi growth. The framework naturally explains the overmassive BH-to-stellar ratios in LRDs and links them to SIDM cross sections consistent with galaxy-scale structure data.
JWST reveals BiRD, a luminous little red dot at z=2.33 in the J1030 field, flagged as a bright outlier in the F200W−F356W vs F356W diagram. NIRCam/WFSS detects He I λ10830 and Paγ with both narrow and broad components (FWHM ≳ 2000 km s−1), plus a blueshifted He I absorption indicating dense outflowing gas with N(He I*, 2^3S) ≈ 0.5–1.2×10^14 cm−2 at Δv ≈ −830 km s−1; the source lacks X-ray and radio emission. The Paγ broad component implies MBH ≈ 10^8 M⊙ and Lbol ≈ 2.9×10^45 erg s−1, echoing GN-28074 and RUBIES-BLAGN-1 where similar He I absorption is seen. A first JWST-based census places the LRD space density at z<3 within a factor ∼2–3 of UV-selected quasars at comparable Lbol and MBH, arguing that LRDs contribute meaningfully to AGN demographics at cosmic noon and may trace rapid seed growth.
JWST/NIRSpec-IFS dissects the triply imaged Little Red Dot Abell2744-QSO1 at z=7.04, isolating the narrow Hβ to map line ratios. [OIII]5007 is exceptionally weak—[OIII]/Hβn ≈ 0.6 within r<150 pc and <0.32 at 150–300 pc—implying Z ≈ 8×10^-3 Z⊙ centrally and Z<6×10^-3 Z⊙ in the extended gas, while broad Balmer lines/variability yield MBH ≈1.5×10^7 M⊙ in a host with Mdyn ≈10^7.1–10^8 M⊙. This places a massive black hole in an almost pristine, very low-mass galaxy only ~700 Myr post-Big Bang. The chemistry and mass budget strain heavy-seed or super‑Eddington growth paths, motivating alternatives such as primordial black-hole seeds that remain to be fully tested.
Builds a multi-wavelength photoionization grid that co-models IMBH accretion and ULX populations in metal-poor, highly star-forming dwarfs, with BH mass tied to seeding channel and metallicity and ULXs evolving with post-starburst age and Z. Finds broadband X-rays and common UV lines carry little leverage on MBH and cannot reliably separate ULXs from IMBHs, while many optical diagrams also fail. Redefines optical cuts using He II λ4686 and [O I] λ6300, and highlights mid-IR ratios (e.g., [Ar II] 6.98μm, [Ne V] 14.3μm, [O IV] 25.9μm) as the cleanest discriminants with sensitivity to MBH and fAGN. Implication: relying on strong-line diagrams alone biases dwarf AGN demographics and may inflate claims of over-massive BHs in some JWST-selected high-z samples.
CAPERS-LRD-z9 is a PRIMER-selected little red dot spectroscopically confirmed with NIRSpec/PRISM as a z=9.288 broad-line AGN via a ~3500 km s−1 Hβ plus narrow [O III] λλ4959,5007. The broad Hβ implies a canonical black-hole mass log(MBH/M⊙)=7.58±0.15 (allowing 6.65–8.50 with systematics). A pronounced Balmer break requires a dense (~10^10 cm−3) neutral-gas cocoon around the nucleus; SED modeling (CLOUDY+bagpipes) limits the host to M*<10^9 M⊙, hinting at MBH/M* possibly >5% and a UV continuum that may be AGN-dominated. Together, these properties push early BH growth scenarios toward massive seeds or super‑Eddington episodes within ≲500 Myr of cosmic time.
Semi-analytic merger-tree modeling with observationally calibrated X-ray irradiation (guided by HERA’s 21 cm non-detections) shows that strong X-ray backgrounds boost H2 formation, steering most overdense halos off the classic direct-collapse path. Including baryonic streaming motion preserves an atomic-cooling route in a subset of cases, still yielding massive seeds. The model predicts seeds with M_BH ≳ 10^4 Msun by z ≃ 15 and an abundance ≈10^-4 Mpc^-3, sufficient to account for SMBHs verified by JWST spectroscopy at 3<z<6. X-ray ionization prevents formation of extremely overmassive BHs comparable to their hosts, while establishing BH-to-stellar mass ratios ≃0.01–0.1 at seeding.
Using Monte Carlo merger trees, the authors model black hole seeding via gravothermal core-collapse in self-interacting dark matter halos and their subsequent growth at early times. Collapse occurs in rare, high-concentration halos at a characteristic mass scale set by the velocity-dependent cross section, naturally producing seeds early enough to explain compact, galaxy-poor little red dots. With simple growth assumptions and a 100% duty cycle, the model reproduces the observed LRD black hole mass function for σ0m ~ 30 cm^2 g^-1 and ω ~ 80 km s^-1, consistent with independent local constraints. If σ0m or ω are higher, the resulting overproduction at one end of the mass function can be mitigated by a reduced duty cycle or lower Eddington ratios, suggesting high‑z BH demographics as a probe of SIDM physics.
JWST/NIRSpec spectroscopy of the point-like source MoM-BH*-1, observed ~660 Myr after the Big Bang, reveals an enormous Balmer break, broad multi-peaked Hβ, and deep Balmer-line absorption. The authors model it as a “black hole star”: a supermassive black hole enshrouded by extremely dense, turbulent, dust-free gas whose atmosphere imprints the break and absorption while dominating the observed light; narrow [OIII] and residual UV are attributed to a faint host. The data are well matched by a simple Cloudy setup that reproduces the break strength, Balmer EWs, UV faintness, and MIRI continuum, with line ratios consistent with high densities that can collisionally suppress [OIII] and a hint of variability reinforcing the SMBH interpretation. They further argue that Little Red Dots can be explained as BH*s embedded in comparably bright hosts, implying current LRD black hole masses—derived with steep dust corrections and standard scaling relations—may be overestimated by orders of magnitude.
Cosmological RHD zoom-ins of a dwarf progenitor with a variable, cloud-scale SFE (calibrated on AU-scale RMHD) produce burstier star formation, a richer massive cluster population, and higher total stellar mass than constant-SFE runs. Dense clouds reach up to ~80% SFE, yielding bound star clusters (Σ~10^2–10^4 M_sun pc^-2, r<=3 pc) with a flat mass function (dN/dlogM ∝ M^Γ, Γ≈−0.4); the most massive (10^4–10^5 M_sun) assemble via mergers and show 0.05–0.1 dex metallicity spreads. A nuclear star cluster is seeded by z≈8.7 and grows to ~2.4×10^5 M_sun, with 83% of its mass built through cluster mergers, reaching ~20% of the galaxy’s stellar mass. This early, merger-driven NSC pathway provides a natural route to SMBH seeding and links to the compact “little red dots” population at z≳5.
Using 341 Little Red Dots (39 with spectroscopic redshifts) compiled from JWST surveys, this paper models their occurrence across cosmic time. The counts peak at z ~ 6–8 and fall steeply by z < 4, well described by a log‑normal distribution interpreted as the first one or two accretion episodes of newborn seed black holes. Dense circumnuclear gas naturally explains broad Balmer-line absorption, X‑ray weakness, and brief super‑Eddington bursts that yield overmassive BHs relative to local relations. Summing recurrent episodes reproduces the global AGN abundance evolution ∝ (1+z)^(−5/2) and provides an analytic form for the LRD redshift dependence for future tests.
A semi-analytic growth model tied to dark-matter halo assembly follows M_BH/M_star for both heavy and light seeds under feedback-limited, episodic super-Eddington accretion. Despite mass loss in winds, the ratio first increases and then converges to a universal ~0.1–0.3 set by the balance between nuclear gas feeding and star-formation efficiency, forming an attractor in the M_BH–M_star plane. On this attractor, overmassive BHs grow more slowly than their hosts while undermassive seeds catch up rapidly, erasing seed-memory by z~4–6. The model predicts that 10^5–10^6 Msun BHs at higher redshift that deviate from this trend would reveal their birth conditions.
Proposes that Little Red Dots are recurrent tidal disruption events triggered during runaway collapse of the first dense star clusters, forming intermediate-mass black hole seeds. Matching JWST LRD number densities to seed-formation predictions implies a required TDE rate of ~10^-4 yr^-1; analytic (loss-cone) and simulation-based scalings show this rate is feasible in dense clusters. The scenario naturally explains compact, UV-bright emission with broad H-alpha and links LRDs to early black-hole growth. A top-heavy IMF may be needed to satisfy ΛCDM mass-function limits, and the hypothesis is testable via TDE-like variability in JWST follow-up.