2025 Science Highlights

The broad frequency coverage, high sensitivity and resolution, and temporal flexibility keep the NRAO facilities at the forefront of studies of the most extreme energetic events in the Cosmos. Radio observations have long been critical to multi-messenger astronomy, and to understanding the origin of photons, particles, and neutrinos at the highest energies. Recent VLBA observations have illuminated the physical processes involved in the prominent neutrino emitting blazar Active Galactic Nuclei (AGN) at z = 0.6, PKS1424+240, in unprecedented detail. A stack of 42 epochs of VLBA observations at 15 GHz reveals the persistent, parsec-scale emission, with a morphology that implies that the object is viewed inside the jet cone, i.e. straight down the axis of its relativistic jet, with a viewing angle of <0.6^o. This projection effectively maximizes Doppler boosting to values ∼30 and greatly boosts the electromagnetic and neutrino emission in the direction of the observer. The polarized emission implies a toroidal component in the magnetic field of the jet, suggesting a current-carrying jet that flows directly toward our line of sight. Blazars with very small jet viewing angles offer a solution to the Doppler factor crisis, i.e., to the longstanding mismatch between Doppler factors inferred from the low apparent jet speeds determined by Very Long Baseline Interferometry (VLBI) with higher speeds derived from observations of ultra-high energy photons (eg gamma-rays and beyond). Relativistic beaming clearly plays the critical role in the gamma-ray and neutrino emission of blazars.

Figure: Left: stacked 42 epoch total intensity (contours) and linear polarization (color) of the VLBA imacge at 0.8mas resolution of the z=0.6 neutrino blazar PKS1424+340. The position angle of the electric vectors are shown as line segments. Right: the magnetic field direction in a linear integral convolution projection (Kovalev et al. 2025, A&A 700, L12).

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) survey, anchored by the GBT, continues to break new ground in the study of gravitational waves from merging supermassive black holes. Most recently, NANOGrav has set limits on free-floating objects (FFO) in interstellar space— rogue planets, brown dwarfs, and large asteroids that are not gravitationally bound to any star. Such objects have been seen in microlensing surveys, as well as through the discovery of interstellar asteroids passing through our solar system on hyperbolic trajectories. Pulsar timing can potentially show signatures of such objects passing near the pulsar. The NANOGrav 15-year data set has set an upper limit to the number density of such encounters in our Galactic neighborhood. For example, the upper limit on the number density for Jupiter-mass FFOs is < 6x10⁵ pc^-3.

Figure: Upper limits (95% confidence) on the number density of FFOs for different mass ranges in the vicinity of 10 of the longest timed pulsars in the NANOgrav dataset. The combined upper limits on the FFO number density from all 68 pulsars are also shown with black squares (Dey et al. 2025 arXiv:2507.19475).

The VLA Sky Survey (VLASS) has become the gold-standard for high resolution, high sensitivity, synoptic surveys of the radio sky. A recent cross correlation of VLASS AGN candidates with galaxies in the Sloan Digital Sky Survey (SDSS) has made the most reliable measurement of the demographics of off-nuclear (i.e. outside the galaxy center), massive black holes (MBH, 10⁵ to 10⁹ M_⊙) to date. These systems likely represent the remnants of galaxy mergers (major and minor), where the MBH may wander indefinitely within the merging galaxy halos. Tracing this population is key to understanding the efficiency of binary MBH formation (a key parameter in nanoHz gravitational wave statistics), and the rates at which MBHs are seeded in low-mass satellite galaxies. The cross correlation identified 328 offset AGN candidates, of which about 30% are likely chance projections. The offset AGN occupation fraction is positively correlated with host galaxy stellar mass, consistent with predictions that most offset MBH will reside in massive halos. This trend vanishes, and may reverse, at the lowest stellar masses, potentially reflecting the weaker host galaxy gravitational potentials. The results suggest a binary MBH formation rate of < 0.5 per merger.

Figure: Two examples of offset AGN identified through cross correlation of the VLASS and the SDSS. In these cases, the radio AGN has an optical counterpart. Color frames are SDSS g+r+I and the grayscale image is the VLASS. The scale bars are 10’’ (Barrows &amp; Comerford 2025, arXiv:2509.09768).

Tidal disruption events (TDE) occur when a star falls into the gravitational potential of a massive black hole, being subsequently torn apart by the extreme tidal forces of a massive black hole. The debris forms an accretion disk, driving broadband electromagnetic emission, possibly a strong wind from the disk, and even a relativistic jet. TDEs provide information of MBH demographics, and the stellar populations in their vicinity, as well as physical processes in transient accretion disks and relativistic jet formation. The VLA and ALMA, with an assortment of other telescopes, have recently discovered the first off-nuclear TDE, indicating association with an MBH originating from the merger of a smaller satellite galaxy. The off-nuclear TDE exhibits double-peaked radio light curves and the fastest-evolving radio emission observed from a TDE to date. The observations suggest a self- and free-free absorbed synchrotron spectrum. The source may be explained by either a single outflow launched 80 days after the event, or including a second outflow launched 100 days later. The fast evolution and double peak may relate to the off-nuclear position and the local stellar and gaseous environment, as well as a potentially lower mass for the MBH (10⁵ M_⊙).

Figure: Left: VLA images of the off-nuclear TDE at z = 0.045 before and after the advent of the radio source. Right: Radio light curves showing the double peak and the unprecedented fast temporal evolution (Margutti et al. ApJ 992, L18).

Radio observations provide unique probes of the thermal and non-thermal processes of Solar system objects. Recent ALMA observations of Jupiter’s moon Callisto demonstrate the power of such observations to penetrate surface properties in unprecedented detail. Callisto is particularly interesting for its very quiescent, ancient surface (> 4 billion years), and a liquid water sub-surface ocean, with no evidence for crustal recycling. Surface structure is therefore predominantly exogenic in origin (i.e. impacts and dust infall). Thermal observations of Callisto’s leading and trailing hemispheres were obtained at 0.87 mm to 3 mm, with resolutions of 0.09” to 0.24” (420–1100 km). Callisto’s millimeter emissivities are high, 0.85–0.97, compared to 0.75–0.85 for Europa and Ganymede. The surface thermal modeling requires either two thermal inertia components or a variable electrical skin depth. Residuals from the global best-fit models show thermal anomalies, including impact craters with brightness temperatures that are locally 3–5 K colder than surrounding regions, such as the giant Valhalla impact basin and a suite of large craters, such as Lofn, appearing as cold anomalies.

Figure: ALMA images of Callisto after subtracting mean model disk emission (in K), at 0.09’’ to 0.24’’ resolution. The images are scaled such that the horizontal and vertical axes span -3500 km to +3500 km (Cordiner et al. arXiv:2508.05925).

Comets originating in the Oort cloud provide our most direct view of conditions in the nascent solar system. Recent ALMA observations of the Halley-type comet Pons-Brooks indicate outgassing of both H₂0 and HDO directly from the nucleus. The D/H ratio of 1.7x10^-4 is consistent with the D/H ratio in the Earth’s oceans, suggesting a common heritage between the Oort cloud’s water ice reservoir, and the water that was delivered to the young Earth during the early history of the solar system.

Figure: ALMA emission maps of H2O and HDO in comet 12P/Pons-Brooks at 1” resolution. Insets show the spectra (Camarca et al. arXiv:2507.12671).

Solar prominences (PR) are common features on the solar surface, corresponding to magnetic loops anchored in the photosphere, and extending an order of 100,000 km into the corona, forming on timescales of days and persisting for weeks. They play an important role in space weather, both participating in heating the corona and powering coronal mass ejections. Their physical origin remains a puzzle. ALMA single dish maps have recently been combined with broader radio frequency measurements to probe the hot thermal gas emission and absorption from a solar prominence. The density in the prominence was found to be about 100 times higher than the quiescent disk, and the temperature 150 times lower, and evidence was found for departures from hydrostatic equilibrium. The results suggest stability is most likely maintained by a dynamically dominant magnetic field of order 10 G.

Figure: Left: Hα image of solar prominence taken with Cerro Tololo optical telescope on December17, 2015, with PR boundaries (red contours). Right: Concurrent ALMA Band 3 total power image. The blue circle is the ALMA Band 3 single dish beam size (Matkovic et al. arXiv:2509.08605).

The NRAO facilities continued to enable major discoveries in the areas of star and planet formation, and the physics of the interstellar medium, through the unique capabilities to study the gas, dust, and star formation on scales of AU to kiloparsecs. One major advance this year was the final data release of the Green Bank Ammonia Survey, mapping all significant star forming regions in the Gould Belt. The survey reveals close correlation between the NH₃ emission with the dust distribution derived by Herschel observations, with NH₃ extending beyond the typical 0.1 pc length scales of dense cores, into the transition zone between dense cores and turbulent clouds. The higher column density regions typically display subsonic non-thermal velocity dispersions. Maps of the gas kinematics, temperature, and NH₃ column density show that the velocity dispersion and kinetic temperature increase with increasing star formation activity. 

Figure: Green Bank Ammonia Survey in the Perseus molecular cloud at 32’’ resolution (Black contours, Pineda et al. arXiv:2510.0607). The color scale shows N(H2) derived from Herschel.

Magnetic fields play a key role in stellar evolution and stellar atmospheres, but to date, no direct measurement has been made of field strengths near the photospheres of massive protostars. The VLA has, for the first time, detected circularly polarized emission from a massive protostar, IRAS 18162–2048. The fractional CP varies between 3–5% across the observed frequency range of 4–6 GHz. Possible physical origins for this emission include gyrosynchrotron emission and/or Faraday conversion due to turbulence in the magnetic medium, both arising from mildly relativistic electrons. The results provide the first estimate of the magnetic field strength of B ~ 20 to 35 G in the immediate vicinity of the surface of the massive protostar. The Lorentz factor of the low-energy electrons is estimated to be in the range –7 for gyrosynchrotron emission, and 80–100 for Faraday conversion.

Figure: Left: VLA image of the region around the massive protostar IRAS 18162–2048 at 5 GHz, 6” resolution (Cheriyan et al. 2025 ApJ 988, L9). Right: color scale now shows the stokes V circular polarization

On scales of molecular protostellar cores and clumps, a recent survey of 17 massive protostellar cores and clumps at 230 GHz with ALMA down to 0.4” resolution (1000 AU) shows linearly polarized thermal emission from aligned dust grains. A bimodal distribution is seen for the implied magnetic field orientation relative to the dust structural elongation: either parallel or perpendicular. The parallel systems are typically seen at the highest column density (> 10²³ cm⁻²). The underlying cause of this bimodal distribution is likely gravitational collapse at higher gas densities, which drags and reorients the magnetic field, consistent with an initially sub-Alvenic cloud that becomes magnetically supercritical and super-Alvenic as the cloud collapses to form stars.

Figure: ALMA observations of linearly polarized emission from aligned dust grains at 230 GHz in protostellar cores and clumps in NGC6334, down to 0.4” resolution (Zhang et al. 2025, ApJ 992, 103).

Radio studies lead the field of large molecule astrochemistry and the search for organic and pre-biotic molecules. Key to these studies are parallel laboratory experiments to determine the spectra of such complex molecules. The GBT Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM) program at the GBT represents the state-of-the-art for such studies, combining laboratory experiments and GBT high resolution broadband spectroscopy. This program has unambiguously identified the seven-ring polycyclic aromatic hydrocarbon (PAH) cyanocoronene (C₂₄H₁₁CN), in the Torus Molecular Cloud. PAHs are critical to ISM chemistry because they sequester up to 25% of the interstellar carbon. The derived column density is 2.7x10¹² cm^-2, and the temperature is 6.0 K. Cyanocoronene is the largest PAH discovered in space to date, and yet its column density is comparable to smaller PAHs, defying the trend of decreasing abundance with increasing molecular size and complexity found for carbon chains. Comparisons to organics in the Murchison meteorite and in return samples from the asteroid Ryugu suggest a substantial inheritance of PAH from the ISM, possibly produced in the cold (T∼10 K) conditions that occur ∼1 Myr before the birth of the Sun. PAHs represent a promising source of carbon for forming terrestrial worlds, to which carbon is supplied in the form of solid-state organics in the natal clouds.

Figure: The seven-ring PAH Cyanocoronene structure and GBT GOTHAM (black) + laboratory (orange) spectra. This is the largest PAH discovered outside the solar system to date (Wenzel et al. 2025 ApJ 984, L36).

The era of high resolution (sub-kiloparsec) studies of the most distant galaxies is upon us, with the advent of the near-IR capabilities of the JWST to study the stars and ionized gas, working with ALMA and the VLA probing the cooler gas, dust, and non-thermal continuum. These studies have been pushed to within 500 Myr of the Big Bang, or z > 10, where observations show a much higher abundance of galaxies than expected so early in the Universe. Recent JWST and ALMA observations at z = 11.1 have shed new light on the earliest galaxy formation process through study of a lower luminosity, relatively ‘normal’ galaxy pair. [OIII] emission has been detected with ALMA in this system, but the non-detection of dust continuum emission sets an upper limit to the star formation rate of 6 M_⊙ yr^-1. The system resembles a low-z metal poor dwarf galaxy, although the presence of oxygen indicates an even earlier epoch of star formation in this forming galaxy.

Figure: ALMA [OIII] image of a z = 11.1 galaxy pair at 0.8” resolution at 280 GHz (red contours), plus the JWST near-IR images (Witstok et al. 2025, arXiv:2507.22888).

A further serious challenge to models of very early galaxy formation has come to the fore based on the JWST discovery of galaxies with large stellar masses, ~ 10¹⁰ M_⊙, within a few hundred Myr of the Big Bang, thereby requiring an unnaturally efficient conversion of gas into stars. Again, ALMA has been a key tool in addressing this puzzle. ALMA and JWST have recently observed a protocluster of five galaxies at z=7.9, all within a 10 kpc region. The observations reveal a dynamic cycle of merger induced gas stripping, which drives gas cycling among the group galaxies, and efficient star formation. The new observations represent the first comprehensive evidence of a massive galaxy forming through gas-rich, multiple-galaxy mergers enhanced by the dense protocluster environment, just 650 Myrs after the Big Bang. The results suggest that the protocluster core is indeed one of the main drivers of efficient massive galaxy formation and rapid evolution in the early Universe.

Figure: ALMA + JWST images of the stars and [CII] 158um line emission from a z = 7.9 merging galaxy cluster (Fudamoto et al. arXiv:2510.11770), elucidating the details of massive galaxy formation in the early Universe.

ALMA is also critical for obtaining sub-kpc information on gas dynamics and galaxy rotation curves and dynamical masses in the early Universe. Recent observations of the dust and [CII] 158um line emission at 0.4 kpc resolution of an interacting pair of galaxies at z ~ 6 show clear signatures of rotation in the gas dynamics, as well as strong tidal interactions. The dynamical masses of the galaxies are 10.6 and 2.0 x10¹⁰ M_⊙, or five times larger than the stellar masses derived from JWST observations.

Figure: Left: ALMA observations of the dust continuum and [CII] 158 um emission from a z = 5.85 massive galaxy pair at 70mas resolution (contours), plus the JWST near-IR images (Akins et al. arXiv:2508.06607). Right: the velocity fields of the galaxies measured with the [CII] line, plus best-fit rotation curves.

The VLA and VLBA remain crucial for the study of AGN in massive galaxies, and the role of jets in gas heating on tens of kpc scales. Recent VLA + VLBA observations of a massive, ‘cooling flow’ cluster at z = 0.5 reveal both a young powerful radio jet AGN on 30pc scales, plus radio emission associated with a 10kpc scale starburst in the host galaxy with a star formation rate of 150 M_⊙ yr^-1. This system, CHIPS 1911+4455, represents a transitional phase in cluster evolution, where the AGN in the central galaxy has just begun to respond to the inflow of cooling cluster gas, and star formation is still very active in the evolving giant elliptical galaxy.

Figure: Left: Chandra image of the X-ray emitting hot gas plus the HST image of the stars in a z = 0.5 brightest cluster galaxy and cooling flow cluster. Center: the JVLA L band image at 1” resolution, and the VLBA 5 GHz image at 1.4mas x 4.3mas resolution. Right: the HST image of the stars and the JVLA image at 1.4 GHz of the parent galaxy (Ubertosi et al. 2025, ApJ 989, 128).

ALMA has also been used to map the velocity fields of molecular gas at parsec-scale resolution in nearby galaxies, to isolate the dynamical sphere of influence of the supermassive black hole (SMBH). Imaging of the CO 2-1 emission at 75mas resolution (7pc) from the nearby barred lenticular galaxy NGC 1574, are the first to spatially resolve the SMBH’s sphere of influence, resulting in an unambiguous detection of the Keplerian velocity increase due to the SMBH at the center of the galaxy, with an implied black hole mass of 6x10⁷ M_⊙. The black hole mass is slightly below that expected from the bulge mass–black hole mass relationship in the nearby Universe.

Figure: Left: ALMA image CO 2-1 velocity field at 75mas (7pc) resolution of the barred lenticular galaxy NGC1574, which hosts a supermassive black hole. Center: the velocity field along the major axis, plus the best fit model including a SMBH. Right: separating the mass contributions from the galaxy and the SMBH, indicating a black hole mass of 6.2x107 M⊙ (Zhang et al. arXiv:2507.10662).