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1. Balmer Absorption in Iron Low-ionization Broad Absorption Line Quasars

   https://doi.org/10.3847/1538-4357/ae04df  

   Leighly, Karen M; Gallagher, Sarah C; Choi_최, Hyunseop 현섭; Terndrup, Donald M; Voelker, Julianna R; Richards, Gordon T; Morabito, Leah K (October 2025, The Astrophysical Journal)

   Abstract While Civis the most common absorption line in broad absorption line quasar spectra, Balmer absorption lines (BALs) are among the rarest. We present analysis of Balmer absorption in a sample of 14 iron low-ionization BAL quasars (FeLoBALQs); eight are new identifications. We measured velocity offset, width, and apparent optical depth. The partial covering that is ubiquitous in BAL quasar spectra alters the measured Balmer optical depth ratios; accounting for this, we estimated the true H(n= 2) column density. We found the anticipated correlation between Eddington ratio and outflow speed, but it is weak in this sample because nearly all of the objects have the low outflow speeds characterizing loitering outflow FeLoBAL quasars, objects that are also found to have low accretion rates. Measurements ofdN/dv, the differential column density with respect to the outflow speed, are anticorrelated with the luminosity and Eddington ratio: the strongest absorption is observed at the lowest speeds in the lowest-luminosity objects. The absorption line width is correlated withα_oi, theF_λpoint-to-point slope between 5100 Å and 3μm. This parameter is strongly correlated with the Eddington ratio among low-redshift quasars. BALs have been recently found in the spectra of little red dots (LRDs), a class of high-redshift objects discovered by JWST. We note suggestive similarities between LRDs and FeLoBAL quasars in the emission-line shape, the presence of steep reddening and a scattered blue continuum, the lack of hot dust emission, and X-ray weakness. 

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2. Little Red Dots as Late-stage Quasi-stars

   https://doi.org/10.3847/1538-4357/ae274a  

   Begelman, Mitchell C; Dexter, Jason (December 2025, The Astrophysical Journal)

   Abstract We argue that the “Little Red Dots” (LRDs) discovered with the James Webb Space Telescope are quasi-stars in their late stages of evolution. Quasi-stars are hypothetical objects predicted to form following the core collapse of supermassive stars and consist of black holes (BHs) accreting from massive envelopes at a super-Eddington rate. We show that models of late-stage quasi-stars, with BH masses exceeding ∼10% of the total, predict thermal and radiative properties that are insensitive to both BH and envelope mass and spectrally resemble LRDs. Specifically, we show that they are likely to exhibit reddish colors, a strong Balmer break, and possess conditions favorable to the production of Balmer lines that are broadened by electron scattering. Their huge electron column densities suppress any X-rays. Late-stage quasi-stars, with BH masses ≳10⁶M_⊙, should dominate the overall quasi-star population. Their short predicted lifetimes (tens of Myr), coupled with the high observed comoving density of LRDs, suggest that most or all supermassive BHs go through a quasi-star/LRD phase during their formation and growth. 

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3. Evidence for Intrinsic X-Ray Weakness among Red Quasars at Cosmic Noon

   https://doi.org/10.3847/1538-4357/ad710c  

   Ma_马, Yilun 逸伦; Goulding, Andy; Greene, Jenny E; Zakamska, Nadia L; Wylezalek, Dominika; Jiang_姜, Yan-Fei 燕飞 (October 2024, The Astrophysical Journal)

   Abstract Quasar feedback is a key ingredient in shaping galaxy evolution. A rare population of extremely red quasars (ERQs) atz= 2−3 are often associated with high-velocity [Oiii]λ5008 outflows and may represent sites of strong feedback. In this paper, we present an X-ray study of 50 ERQs to investigate the link between the X-ray and outflow properties of these intriguing objects. Using hardness ratio analysis, we confirm that the ERQs are heavily obscured systems with gas column density reachingN_H= 10²³⁻²⁴cm⁻². We identify 20 X-ray-nondetected ERQs at high mid-infrared (MIR) luminosities ofνL_ν,6μm≳ 3 × 10⁴⁶erg s⁻¹. By stacking the X-ray observations, we find that the nondetected ERQs are on average underluminous in X-rays by a factor of ∼10 for their MIR luminosities. We consider such X-ray weakness to be due to both heavy gas absorption and intrinsic factors. Moreover, we find that the X-ray-weak sources also display higher-velocity outflows. One option to explain this trend is that weaker X-rays facilitate more vigorous line-driven winds, which then accelerate the [Oiii]-emitting gas to kiloparsec scales. Alternatively, super-Eddington accretion could also lead to intrinsic X-ray weakness and more powerful continuum-driven outflow. 

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4. The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the X-Rays and Radio

   https://doi.org/10.3847/2041-8213/ae0b4d  

   A_J, Nayana; Margutti, Raffaella; Wiston, Eli; Laskar, Tanmoy; Migliori, Giulia; Chornock, Ryan; Galvin, Timothy J; LeBaron, Natalie; Hajela, Aprajita; Christy, Collin T; et al (October 2025, The Astrophysical Journal Letters)

   Abstract We present X-ray (0.3–79 keV) and radio (0.25–203 GHz) observations of the most luminous fast blue optical transient (LFBOT) AT 2024wpp atz= 0.0868, spanning 2–280 days after first light. AT 2024wpp shows luminous (L_X≈ 1.5 × 10⁴³erg s⁻¹), variable X-ray emission with a Compton hump peaking atδt≈ 50 days. The X-ray spectrum evolves from a soft (F_ν∝ν^−0.6) to an extremely hard state (F_ν∝ν^1.26) accompanied by a rebrightening atδt≈ 50 days. The X-ray emission properties favor an embedded high-energy source shining through asymmetric expanding ejecta. We detect radio emission peaking atL_{9 GHz}≈ 1.7 × 10²⁹erg s⁻¹Hz⁻¹atδt≈ 73 days. The spectral evolution is unprecedented: the early millimeter fluxes rise nearly an order of magnitude duringδt≈ 17–32 days, followed by a decline in spectral peak fluxes. We model the radio emission as synchrotron radiation from an expanding blast wave interacting with a dense environment ( $\dot{M}\sim 10^{-3}{M}_{\odot }{\mathrm{yr}}^{-1}$ forv_w= 1000 km s⁻¹). The inferred outflow velocities increase from Γβc≈ 0.07cto 0.42cduringδt≈ 32–73 days, indicating an accelerating blast wave. We interpret these observations as a shock propagating through a dense shell of radius ≈10¹⁶cm and then accelerating into a steep density profileρ_CSM(r) ∝r^−3.1. All radio-bright LFBOTs exhibit similar circumstellar medium (CSM) density profiles (ρ_CSM∝r⁻³), suggesting similar progenitor processes. The X-ray and radio properties favor a progenitor involving super-Eddington accretion onto a compact object launching mildly relativistic disk wind outflows. 

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5. Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic disk

   https://doi.org/10.1051/0004-6361/202553998  

   Rugel, M R; Beuther, H; Soler, J D; Goldsmith, P; Anderson, L; Hafner, A; Dawson, J R; Wang, Y; Bihr, S; Wiesemeyer, H; et al (August 2025, Astronomy & Astrophysics)

   The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new high-sensitivity absorption line observations of the four ground state hyperfine-splitting transitions of OH at 18 cm toward four Galactic and extragalactic continuum sources as follow-up to the THOR survey. We compared these to deep observations of the [C II] 158 µm line at 1.9 THz obtained with the upGREAT instrument on SOFIA, observations of the neutral atomic hydrogen (H I) 21 cm line with the VLA, and CO (J= 2–1) lines obtained with the APEX PI230 receiver at the APEX 12 m sub-mm telescope. We trace OH over a large range of molecular hydrogen column densities of 7.9 × 10¹⁹cm^-2to 4.7 × 10²²cm^-2, and derive OH abundances with respect to molecular and total hydrogen column densities ofX_OH,H2=N_OH/N_H2= 1.2_−0.2^+0.3× 10⁻⁷andX_OH,H=N_OH/N_H= 4.8_−0.8^+0.9× 10⁻⁸, respectively. Increased sensitivity and spectral resolution allowed us to detect weak and narrow features with the lowest column density detected atN_OH= 3.7 × 10¹³cm^-2. The increase in sensitivity is a factor of five in direct comparison at the resolution the OH observations in the THOR survey (1.5 km s^-1). We identify only one OH absorption component out of 23 without CO counterpart, yet several with intermediate molecular gas fractions (f_mol≤ 0.8). A potential association of [C II] 158 μm emission with an OH absorption component is seen toward one sightline. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark molecular gas feature appears to be tracing only the upper end of the distribution of CO-dark OH features found by previous studies. We conclude that if OH absorption was to be used as a CO-dark molecular gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark molecular gas tracer, and yet it is not an exclusive tracer of CO-dark molecular gas. For OH hyperfine-splitting transitions in the vicinity of photodissociation regions in W43-South, we detect a spectral and spatial offset between the peak of the inversion of the OH 1612 MHz line and the absorption of the OH 1720 MHz line on the one hand, and the absorption of the OH main lines on the other hand, which provides additional constraints on the interpretation of the OH 18 cm line signatures typical of HII regions. 

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