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1. Quantifying the Impact of Incompleteness on Identifying and Interpreting Galaxy Protocluster Populations with the TNG-Cluster Simulation

   https://doi.org/10.3847/1538-4357/adf644  

   Baxter, Devontae C; Coil, Alison L; Nadler, Ethan O; Nelson, Dylan; Pillepich, Annalisa; Forrest, Ben; Giddings, Finn; Golden-Marx, Emmet; Lemaux, Brian C; Sikorski, Derek (September 2025, The Astrophysical Journal)

   Abstract We use the TNG-Cluster simulation to investigate how stellar mass and star formation rate (SFR) incompleteness affect the identification of density peaks within galaxy protoclusters at different redshifts. Our analysis focuses on a sample of 352 protoclusters, defined as the progenitor populations of galaxies that reside within the virialized region ofz= 0 clusters with $M_{\mathrm{200}}^{z=0}\sim 10^{14.2-15.5}{M}_{\odot }$ . For comparison, we define our “baseline” protocluster population as galaxies withM_⋆ > 10^8.5M_⊙at any redshift. We find thatM_⋆-limited (M_⋆ > 10^9.5M_⊙) and SFR-limited (SFR > 10M_⊙yr⁻¹) subpopulations recover the baseline highest galaxy density peak in roughly ∼60% of cases within an accuracy of 1.0 pMpc (corresponding to an angular scale of $\sim 2^{\mathrm{\prime }}-2\stackrel{\mathrm{\prime }}{.}5$ ) atz > 2. This recovery fraction drops to ∼40%−50% when restricting to galaxies withM_⋆ > 10^10.0M_⊙. We find that the baseline highest galaxy density peaks typically coincide with the highest dark matter and stellar mass density peaks, with separations less than 0.5 pMpc in ∼60%−75% of cases atz > 2. This agreement drops to ∼45%−50% when restricting to galaxies withM_⋆ > 10^10.0M_⊙. These results indicate that identifying the densest regions of protoclusters—i.e., the core—is highly sensitive to stellar mass and SFR completeness limits. Nevertheless, atz > 2, we find that the baseline highest galaxy density peaks are generally sites of enhanced star formation and accelerated mass growth relative to the remainder of the protocluster, consistent with some observational studies. 

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2. The Accelerating Decline of the Mass Transfer Rate in the Recurrent Nova T Pyxidis*

   https://doi.org/10.3847/1538-4357/ad7463  

   Godon, Patrick; Sion, Edward M; Williams, Robert E; Darnley, Matthew J; Sokoloski, Jennifer L; Lawrence, Stephen S (October 2024, The Astrophysical Journal)

   Abstract The recurrent nova T Pyxidis (T Pyx) has erupted six times since 1890, with its last outburst in 2011, and the relatively short recurrence time between classical nova explosions indicates that T Pyx must have a massive white dwarf (WD) accreting at a high rate. It is believed that, since its outburst in 1890, the mass transfer rate in T Pyx was very large due to a feedback loop where the secondary is heated by the hot WD. The feedback loop has been slowly shutting off, reducing the mass transfer rate, and thereby explaining the magnitude decline of T Pyx from ∼13.8 (before 1890) to 15.7 just before the 2011 eruption. We present an analysis of the latest Hubble Space Telescope far-ultraviolet and optical spectra, obtained 12 yr after the 2011 outburst, showing that the mass transfer rate has been steadily declining and is now below its preoutburst level by about 40%: $\dot{M}\sim 1-3\times {10}^{-7}{M}_{\odot }$ yr⁻¹for a WD mass of ∼1.0–1.4M_⊙, an inclination of 50°–60°, reddening ofE(B−V) = 0.30 ± 0.05, and a Gaia Data Release 3 distance of ${2860}_{-471}^{+816}$ pc. This steady decrease in the mass transfer rate in the ∼decade after the 2011 outburst is in sharp contrast with the more constant preoutburst ultraviolet continuum flux level from archival International Ultraviolet Explorer spectra. The flux (i.e., $\dot{M}$ ) decline rate is 29 times faster now in the last ∼decade than observed since 1890 to ∼2010. The feedback loop shut off seems to be accelerating, at least in the decade following its 2011 outburst. In all eventualities, our analysis confirms that T Pyx is going through an unusually peculiar short-lived phase. 

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3. Shapiro Delay Measurements from Fifteen Years of PSR J1231−1411 Radio Observations

   https://doi.org/10.3847/1538-4357/ae3d95  

   Cromartie, H Thankful; Kerr, Matthew; Ransom, Scott M; Ray, Paul S; Guillemot, Lucas; Cognard, Ismaël; Fonseca, Emmanuel; Theureau, Gilles (February 2026, The Astrophysical Journal)

   Abstract We present 15 yr of Nançay and Green Bank radio telescope timing observations for PSR J1231−1411. This millisecond pulsar is a primary science target for the Neutron Star Interior Composition Explorer telescope (NICER, which discovered its X-ray pulsations), has accumulated near-continuousγ-ray data since the Fermi-Large Area Telescope’s launch, and has been studied extensively with the Green Bank and Nançay radio telescopes. We have undertaken a campaign with the Green Bank Telescope targeting specific orbital phases designed to improve our constraint on the pulsar’s mass through the detection of a relativistic Shapiro delay. Both frequentist and Bayesian techniques—the latter incorporating priors from white dwarf binary evolution models—are applied to 15 yr of radio observations, yielding relatively weak constraints on the companion and pulsar masses of $0.23_{-0.06}^{+0.09}$ M_⊙and $1.87_{-0.67}^{+1.11}$ M_⊙, respectively (68.3% CI from Bayesian fits); however, the orbital inclination is measured to better relative precision ( $79.80_{-4.70}^{+3.47}$ °). Restricting the maximum allowed pulsar mass to 3M_⊙improves the constraint and lowers the measured mass to $1.71_{-0.56}^{+0.70}$ M_⊙. A fully generalized Bayesian fit that simultaneously samples the noise and timing models yields a pulsar mass in close agreement with this value. While our radio-derived inclination result has informed recent NICER X-ray studies of J1231−1411, the lessons learned from this troublesome pulsar will also bolster future high-precision mass measurement campaigns and resulting constraints on the neutron star interior equation of state. 

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4. VLBI Observations of SN 2012AU Reveal a Compact Radio Source a Decade Post Explosion

   https://doi.org/10.3847/1538-4357/ae517f  

   Lazda, Mattias; Nimmo, Kenzie; Drout, Maria R; Marcote, Benito; Hessels, Jason_W T; Wiston, Eli; Margutti, Raffaella; Ould-Boukattine, Omar; Laskar, Tanmoy; Vink, Jacco; et al (April 2026, The Astrophysical Journal)

   Abstract Three leading models have been put forth to justify the observed radio re-brightening associated with stripped-envelope supernovae (SESNe) years post-explosion: radiation from an emerging pulsar wind nebula (PWN), shock interaction with a dense circumstellar medium (CSM), or emission from off-axis, relativistic jets. SN 2012au is a particularly intriguing SESN in this regard, as observations obtained ≳6 yr post-explosion have shownboth(i) optical emission features consistent with a young PWN and (ii) a radio re-brightening. We present the results of our very long baseline interferometric (VLBI) observations of SN 2012au performed between 8 and 13 yr post core-collapse. Our VLBI observations reveal a luminous, steadily fading radio source that remains compact (≤1.4 × 10¹⁷cm) and stationary (≤0.36c) over the course of our campaign. Overall, we find that our VLBI measurements can be readily explained by a ∼decade-old PWN, potentially explained by shock interaction with specific CSM geometries, and are unlikely to be explained by emission from an off-axis, relativistic jet. Assuming a PWN origin, our observations require that the initial spindown luminosity of the central pulsar be between $10^{36}\mathrm{erg}{\mathrm{s}}^{-1}\le {\dot{E}}_0\le 4\times {10}^{42}\mathrm{erg}{\mathrm{s}}^{-1}$ and radio efficiency factor beη_R≥ 3 × 10⁻⁷(both quoted at the 99.7% confidence interval). These results are consistent with independent inferences obtained using optical spectroscopy of SN 2012au, alongside inferences of known Galactic systems. If a PWN origin is confirmed, SN 2012au would represent the first extragalactic PWN emerging from a modern-day SN, providing a novel opportunity to study the formation properties of a decade-old pulsar. 

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5. Metallicity Dependence of Pressure-regulated Feedback-modulated Star Formation in the TIGRESS-NCR Simulation Suite

   https://doi.org/10.3847/1538-4357/ad59ab  

   Kim, Chang-Goo; Ostriker, Eve C; Kim, Jeong-Gyu; Gong, Munan; Bryan, Greg L; Fielding, Drummond B; Hassan, Sultan; Ho, Matthew; Jeffreson, Sarah_M R; Somerville, Rachel S; et al (August 2024, The Astrophysical Journal)

   Abstract We present a new suite of numerical simulations of the star-forming interstellar medium (ISM) in galactic disks using the TIGRESS-NCR framework. Distinctive aspects of our simulation suite are (1) sophisticated and comprehensive numerical treatments of essential physical processes including magnetohydrodynamics, self-gravity, and galactic differential rotation, as well as photochemistry, cooling, and heating coupled with direct ray-tracing UV radiation transfer and resolved supernova feedback and (2) wide parameter coverage including the variation in metallicity over $Z\prime \equiv Z/Z_{\odot }\sim 0.1-3$ , gas surface density Σ_gas∼ 5–150M_⊙pc⁻², and stellar surface density Σ_star∼ 1–50M_⊙pc⁻². The range of emergent star formation rate surface density is Σ_SFR∼ 10⁻⁴–0.5M_⊙kpc⁻²yr⁻¹, and ISM total midplane pressure isP_tot/k_B= 10³–10⁶cm⁻³K, withP_totequal to the ISM weight $\mathcal{W}$ . For given Σ_gasand Σ_star, we find ${\mathrm{\Sigma }}_{\mathrm{SFR}}\propto Z{\prime }^{0.3}$ . We provide an interpretation based on the pressure-regulated feedback-modulated (PRFM) star formation theory. The total midplane pressure consists of thermal, turbulent, and magnetic stresses. We characterize feedback modulation in terms of the yield ϒ, defined as the ratio of each stress to Σ_SFR. The thermal feedback yield varies sensitively with both weight and metallicity as ${\mathrm{\Upsilon }}_{\mathrm{th}}\propto {\mathcal{W}}^{-0.46}Z{\prime }^{-0.53}$ , while the combined turbulent and magnetic feedback yield shows weaker dependence ${\mathrm{\Upsilon }}_{\mathrm{turb}+\mathrm{mag}}\propto {\mathcal{W}}^{-0.22}Z{\prime }^{-0.18}$ . The reduction in Σ_SFRat low metallicity is due mainly to enhanced thermal feedback yield, resulting from reduced attenuation of UV radiation. With the metallicity-dependent calibrations we provide, PRFM theory can be used for a new subgrid star formation prescription in cosmological simulations where the ISM is unresolved. 

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