Taotao Fang

Fast radio bursts (FRBs) provide sensitive probes of ionized baryons through their dispersion measures (DMs). In this work, we test whether localized FRB sight lines intersecting foreground galaxy groups exhibit a measurable excess in cosmic DM relative to clean nonintersecting sight lines. We crossmatch 117 localized FRBs with measured DMs and host redshifts against a foreground group catalog from the DESI Legacy Imaging Surveys. Within the catalog footprint and at z _host < 0.5, we identify 23 sight lines intersecting foreground groups and 18 clean nonintersecting sight lines for the main comparison. A joint fit to this sample gives a positive offset of 143 . 3 − 53.0 + 53.2 pc cm − 3 in the inferred cosmic DM, DM _cosmic , for the group-intersecting sight lines relative to the nonintersecting control sample. A template fit for the foreground halos gives A halo = 0.3 4 − 0.50 + 0.51 , with a positive posterior median but a weakly constrained detailed halo contribution. A mock forecast within the DESI footprint indicates that larger samples of localized FRBs would improve the foreground halo-template constraint.

X-ray absorption-line spectroscopy provides a powerful method for probing the extended hot circumgalactic medium (CGM) of individual galaxies. In this study, we present the first survey of O vii and O viii absorption lines toward the hot halo of M31, using 15 and 18 background active galactic nuclei, with impact parameters ranging from R _imp ∼ 300–730 kpc and ∼190–730 kpc, respectively. We find a marginal excess absorption above the expected Milky Way (MW) foreground toward the innermost sightlines, consistent with an additional hot-CGM contribution associated with M31. Comparing sightlines inside and outside R _imp = 300–350 kpc, the excess corresponds to a mean equivalent width of ∼4–9 mÅ for the inner O vii sightlines at R _imp ∼ 310 kpc and ∼17 mÅ for the inner O viii sightlines at R _imp ∼ 200 kpc, with a nominal combined significance level of 2.2 σ –2.3 σ . The hot-CGM mass inferred from this excess is highly model dependent, especially on the assumed CGM boundary, and therefore does not yet provide a robust baryon census. Attributing the excess entirely to hot gas confined within approximately the virial radius of M31 would require a CGM mass exceeding the nominal “missing” baryon budget. The inferred mass decreases to a few ×10 ¹¹ M _⊙ when the assumed CGM boundary is extended to 400–500 kpc. Deeper observations, both through longer exposures of existing sightlines and the inclusion of additional inner-halo targets, will be essential to robustly constrain the properties of the hot CGM of M31.

Detecting active galactic nuclei (AGNs) in dwarf galaxies is crucial for probing low-mass black hole demographics, yet remains intrinsically and observationally challenging because most diagnostics are calibrated for massive systems. We analyze a sample of low-luminosity galaxies ( L _{24 μ m} < 10 ⁹ L _⊙ ) and identify five radio-excess sources using the empirical infrared–radio criterion ratio q 24 = log ( f 24 μ m / f 1.4 GHz ) < 0.902 . Detailed spectral energy distribution (SED) modeling reveals that two sources show evidence for modest AGN components (∼10% and ∼30% luminosity contributions, respectively). Both sources evade classical optical and mid-infrared AGN selection criteria. We therefore further examine their time-domain behavior using NEOWISE and Zwicky Transient Facility (ZTF) data. The NEOWISE light curves show mild long-term mid-infrared variability, while our ZTF difference-image aperture photometry reveals significant nonconstant nuclear optical variability. Notably, these candidates have much lower star formation rates than previously reported radio-excess dwarf AGNs, reducing stellar contamination, and their radio fluxes substantially exceed predictions from the infrared–radio correlation, supporting an AGN interpretation. These systems represent promising low-mass AGN candidates and underscore the potential of radio-excess selection combined with SED modeling and time-domain diagnostics for uncovering weak or obscured AGNs in low-mass galaxies.

Galaxy clusters represent extreme cosmic laboratories where environmental processes dramatically reshape their constituent galaxies, yet their effect on the gaseous halos of central galaxies remains poorly constrained. Here we present the first statistical mapping of cool gas around massive brightest cluster galaxies (BCGs) at z ≈ 0.55. Using Mg II absorption in stacked sight-line spectra from over a million background quasars observed by the Dark Energy Spectroscopic Instrument, we compare BCGs to a matched sample of field galaxies and trace the radial profile from 40 kpc to 15 Mpc. Our analysis reveals a striking dual environmental signature: within 200 kpc, the circumgalactic medium around BCGs is significantly suppressed compared to that of field galaxies, while at larger radii (200 kpc–10 Mpc) a pronounced excess of cool gas emerges. This clear transition from suppression in the core to enhancement on such large scales delineates a novel observed pattern for gas regulation by the dense environment. It suggests that clusters may not only strip gas in the core but also facilitate its accumulation in the outskirts. Our results provide key observational constraints on theoretical models of environmental processing in and around the most massive dark matter halos.

The Hundred-deg ² H I Deep (HD ² ) survey carried out with the Five-hundred-meter Aperture Spherical Telescope is planned to map a contiguous region within the DESI DR1 footprint, achieving an effective integration time of 20 minutes for each pointing and a uniform detection sensitivity of 0.28 mJy beam ⁻¹ at 4.8 km s ⁻¹ resolution. We present early results from the pilot HD ² survey: a 10 deg ² field overlapping with the Hyper Suprime-Cam Subaru Strategic Program and the DESI early data release SV3, observed with an integration time of 7.3 minutes beam ⁻¹ and an rms of 0.45 mJy beam ⁻¹ at 4.8 km s ⁻¹ resolution. We identify 339 H I sources at z < 0.09, corresponding to ∼34 detections per deg ² , nearly 6 times higher than the detection rate of the wide-field surveys. Optical counterparts are primarily identified using DESI redshifts, yielding a matching rate and correctness exceeding 90% for galaxies with r < 19.5 mag, a substantial improvement over the Sloan Digital Sky Survey. Under the constraint of r < 17.8 mag and 0.01 < z < 0.05, nearly 50% of galaxies in the DESI Bright Galaxy Survey samples have H I detections in this pilot survey. The optical properties of these H I -detected galaxies span nearly the entire parameter range of the DESI sample. The gas fraction scaling relations versus stellar mass, stellar mass surface density, near-UV (NUV) − r , and specific star formation rate are consistent with previous surveys, e.g., ALFALFA, DINGO, and xGASS. These results justify the feasibility of the full HD ² survey, which will build a high-completeness H I census over a contiguous area to probe the cold gas scaling relations of galaxies over different scales.

We present the evolution of galaxy size and surface brightness in the rest-frame J band (1.22 μ m), tracing the stellar mass distribution, over 0.5 ≤ z ≤ 3, using a sample of 15,420 galaxies with stellar masses M _⋆ = 10 ¹⁰ –10 ^11.5 M _⊙ from the JWST COSMOS-Web survey. The rest-frame J -band effective radius ( R _{e , J} ) is obtained from previous measurements and mapped from the available JWST/NIRCam filters, while the surface brightness ( μ _J ) is corrected for dust extinction and cosmological dimming. At a characteristic mass of M _⋆ = 5 × 10 ¹⁰ M _⊙ , star-forming galaxies (SFGs) exhibit a size evolution of R _{e , J} ∝ (1 + z ) ^β with β = −0.92 ± 0.04, falling between previously reported shallower and steeper measurements. Quiescent galaxies evolve more rapidly, with β = −1.34 ± 0.05, consistent with earlier studies. Among SFGs, lower-mass systems (10 ¹⁰ –10 ^10.5 M _⊙ ) show slower ( β = −0.66 ± 0.02) size evolution compared to their higher-mass counterparts. Furthermore, the surface brightness brightens toward higher redshifts, scaling as μ J ∝ − 2.5 log ( 1 + z ) γ . We find γ = 3.07 ± 0.08 for SFGs and γ = 3.70 ± 0.08 for quiescent galaxies. We also find that massive SFGs ( M _⋆ > 10 ^10.5 M _⊙ ) exhibit similar μ _J values at fixed redshift, independent of mass. Finally, we demonstrate that the observed surface brightness evolution is driven by the combined evolution of galaxy luminosity and size.

Recently, we performed a systematic study of active galactic nucleus (AGN) feedback in a disk galaxy within the MACER framework. Various model predictions, including the AGN duty cycle, the correlation between black hole accretion rates and star formation rates, and the (cold) gas fraction, have been compared with observations and will be presented in a series of papers. As the second paper in this series, without adjusting any model parameters, we directly use the simulation data introduced in Paper I to compute the predicted X-ray surface brightness profile and compare it with eROSITA observations of circumgalactic medium emission around galaxies, which provide important constraints on AGN feedback models. For this comparison, we adopt two stacked eROSITA radial profiles of X-ray surface brightness: (1) distant galaxies with log ( M * / M ⊙ ) = 10.5 − 11.0 at z ≈ 0.02–0.10 from Y. Zhang et al. and (2) nearby L ^* galaxies within 50 Mpc from L. He & Z. Li. We find that the average simulated profile over time is in good agreement with the stacked measurements of Y. Zhang et al. over a broad radial range (out to ∼100 kpc). Our model predictions also match the results of L. He & Z. Li at projected radii from ∼20 to 120 kpc. While our simulations, which predict only thermal emission, are consistent with these recent X-ray observations, the limitations in our current model mean that this agreement does not preclude a potential contribution from nonthermal emission, e.g., from an extended halo of cosmic rays.

We present a multiwavelength follow-up study of 621 winged radio galaxies (WRGs) recently identified from LoTSS DR2, constituting the largest statistically significant samples of X-shaped (XRGs) and Z-shaped (ZRGs) radio galaxies to date. Our results show that WRGs are predominantly strongly radio dominated, with XRGs on average more radio luminous than ZRGs. Their optical hosts are massive elliptical galaxies residing in moderate-density environments. For 270 of XRGs, we measure angular offsets between the radio wings and the optical major axis. While most XRGs show large misalignments consistent with hydrodynamic backflow along the host minor axis, a substantial fraction (25%) exhibits small offsets (<30°), indicating that additional processes, such as jet reorientation, may also play a role. ZRGs, in contrast, are characterized by strongly antisymmetric deformations of their radio lobes pointing toward a coherent mechanism affecting both jets, modulated by local environmental interactions at the lobe termini. Mid-infrared diagnostics indicate merger-related cold gas in many WRGs, particularly XRGs, which also more frequently host powerful active galactic nuclei, while ZRGs are more often classified as low-excitation radio galaxies. This is consistent with our previous results showing that, although most WRGs exhibit FR II morphologies, FR I sources are almost exclusively ZRGs, suggesting that Z-shaped structures are statistically associated with lower jet power and are therefore more susceptible to perturbations. Nevertheless, the physical processes responsible for shaping XRGs and ZRGs need not be fundamentally different. Instead, the final morphology likely reflects the interplay between jet power, jet stability, and the surrounding environment.

Atomic hydrogen (H i ) plays a fundamental role in fueling star formation in galaxies. However, the behavior of H i gas in interacting systems, particularly galaxy pairs, remains elusive. In this work, we investigate the H i content of major mergers by crossmatching the extragalactic H i catalog from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) All-Sky H i Survey with a previously established sample of isolated galaxy pairs. With the superior sensitivity of FAST, we have constructed the largest sample of major mergers with H i detections, consisting of 440 galaxy pairs: 364 spiral-spiral (S+S) and 76 spiral-elliptical (S+E) systems. We examine the H i gas fraction ( f _{H I} ), star formation rate (SFR), and H i star formation efficiency (SFE _{H I} = SFR/ M _{H I} ) for individual galaxies in pairs. The control sample is matched in both stellar mass and redshift. We find that paired galaxies, particularly those in pairs with small projected separations ( d _p < 50 h ⁻¹ kpc), exhibit systematically lower (by 8.8%) H i gas fractions compared to the control galaxies. The SFR is enhanced for galaxies in the S+S pairs. The SFE _{H I} is ∼15% higher for galaxies in the S+S pairs than in the control galaxies, while spiral galaxies in the S+E pairs show no significant difference in SFE _{H I} compared to the control sample. These findings suggest that the merger process triggers efficient H i gas depletion and enhances star formation, especially in close S+S pairs. Notably, our sample includes 26 red spirals in paired systems. These galaxies exhibit H i deficiency and suppressed star formation activity compared to the isolated galaxies, indicating that interactions may affect quiescent spirals differently, potentially due to mechanisms similar to those of ellipticals.