Feed aggregator

Nature, Published online: 10 September 2025; doi:10.1038/d41586-025-02872-5

Discovery joins a list of the greatest hits of the LIGO detector, which ten years ago became the first to detect gravitational waves.

An exceptionally loud collision between two black holes has been detected by the LIGO gravitational wave observatory, enabling physicists to test a theorem postulated by Stephen Hawking in 1971

Title to be confirmed

Abstract not available

• Speaker: Madalina Tudorache (IoA)
• Friday 12 December 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

arXiv:2506.12141v2 Announce Type: replace Abstract: We report the discovery of a little red dot (LRD), dubbed BiRD ('big red dot'), at $z=2.33$ in the field around the $z=6.3$ quasar SDSSJ1030+0524. Using NIRCam images, we identified it as a bright outlier in the $F200W-F356W$ color vs $F356W$ magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from HeI$\lambda 10830$ and PaG line, both showing a narrow and a broad ($FWHM\gtrsim 2000\ \rm kms^{-1}$) component. The HeI line is affected by an absorption feature, tracing dense gas with HeI column density in the $2^3S$ level $N\sim 0.5-1.2\times 10^{14}\rm cm^{-2}$, depending on the location of the absorber, which is outflowing at the speed of $\Delta v \sim -830\ \rm kms^{-1}$. As observed in the majority of LRDs, BiRD does not show X-ray or radio emission. The BH mass and the bolometric luminosity, both inferred from the PaG broad component, amount to $M_{\rm BH}\sim 10^8\rm M_{\odot}$ and $L_{\rm bol}\sim 2.9\times 10^{45}\rm ergs^{-1}$, respectively. Intriguingly, BiRD presents strict analogies with other two LRDs spectroscopically confirmed at cosmic noon, GN-28074 ("Rosetta Stone") at $z=2.26$ and RUBIES-BLAGN-1 at $z=3.1$. The blueshifted HeI absorption detected in all three sources suggests that gas outflows may be common in LRDs. We derive a first estimate of the space density of LRDs at $z<3$ based on JWST data, as a function of $L_{\rm bol}$ and BH mass. The space density is only a factor of $\sim 2-3$ lower than that of UV-selected quasars with comparable $L_{\rm bol}$ and $z$, meaning that the contribution of LRDs to the broader AGN population is also relevant at cosmic noon. A similar trend is also observed in terms of BH masses. If, as suggested by recent theories, LRDs probe the very first and rapid growth of black hole seeds, our finding may suggest that the formation of black hole seeds remains efficient at least up to cosmic noon.

arXiv:2509.07064v1 Announce Type: new Abstract: The James Webb Space Telescope (\textit{JWST}) opened a new observational window on the primordial Universe. Here we present new JWST NIRSpec integral field spectroscopy (IFS) observations of the $z=7.54$ quasar ULAS J1342+0928 obtained as part of the Galaxy Assembly with NIRSpec IFS (GA-NIFS) GTO programme. The new data-set obtained with both the prism ($R\sim100$) and the high-resolution grating ($R\sim2700$) allow for a complete description of the quasar emission from the rest-frame UV to optical bands. The low-resolution data reveal the presence of [\ion{O}{iii}] emission on $\sim$7 kpc scales, well above the typical galaxy size at this redshift, likely associated with a past outflow event. Additionally, the high-resolution observations show a more energetic ionised outflow on nuclear scales ($\lesssim 0.6$ kpc). The total ionised mass outflow rate ranges between 50 and 300 $\rm M_{\odot} \, yr^{-1}$ where the significant spread is mostly due to the lack of tight constraints on the electron density. This range overlaps in part with the star formation rate range (85--545 $\rm M_{\odot} \, yr^{-1}$), implying that the nuclear outflow could ultimately lead to an early star formation quenching. By employing an accretion disc modelling, for the first time on \textit{JWST} data, we manage to robustly estimate the black hole mass and the bolometric luminosity, $\rm \log(M_{BH}/(M_{\odot}))=9.2\pm 0.2$ and $\rm \log(L_{bol}/(erg \, s^{-1}))=46.8\pm 0.1$, respectively. We derive an Eddington ratio of $\rm \lambda_{Edd}\sim 0.4$, challenging the paradigm of widespread super-Eddington accretion in quasars at the epoch of reionisation.

Mapping the Cosmic Web in Lyα emission around quasars and non-AGN galaxies

Abstract not available

• Speaker: Davide Tornotti (Milan)
• Friday 31 October 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

From Primordial Magnetic Fields to AGN-Driven Magnetized Outflows

Magnetic fields are ubiquitous across cosmic scales, yet their role in galaxy formation remains elusive. In this talk, I show that primordial magnetic fields, originating before the end of recombination, can amplify small perturbations, accelerating the collapse of the first dark matter halos. This results in an early onset and efficient star formation, consistent with JWST observations. However, primordial fields are not the only source of magnetization at high redshifts. Using RAMSES -RTMHD cosmological simulations, I compare the role of primordial fields and AGN -driven outflows in shaping the magnetic landscape of the high-redshift universe.

• Speaker: Mahsa Sanati (Oxford)
• Friday 14 November 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Davide Tornotti (Milan)
• Friday 31 October 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

Novel approaches to simulating the observable Universe

Abstract not available

• Speaker: Chris Lovell (KICC)
• Friday 10 October 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

Probing early star formation and ionized bubble growth with JWST

Abstract not available

• Speaker: Lily Whitler (KICC)
• Friday 03 October 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

Add to your calendar or Include in your list

arXiv:2503.11457v3 Announce Type: replace Abstract: We derive Fe-abundance ratios of 7 galaxies at $z=9-12$ with $-22

arXiv:2509.05724v1 Announce Type: cross Abstract: Recent advances in neural density estimation have enabled powerful simulation-based inference (SBI) methods that can flexibly approximate Bayesian inference for intractable stochastic models. Although these methods have demonstrated reliable posterior estimation when the simulator accurately represents the underlying data generative process (GDP), recent work has shown that they perform poorly in the presence of model misspecification. This poses a significant problem for their use on real-world problems, due to simulators always misrepresenting the true DGP to a certain degree. In this paper, we introduce robust variational neural posterior estimation (RVNP), a method which addresses the problem of misspecification in amortised SBI by bridging the simulation-to-reality gap using variational inference and error modelling. We test RVNP on multiple benchmark tasks, including using real data from astronomy, and show that it can recover robust posterior inference in a data-driven manner without adopting tunable hyperparameters or priors governing the misspecification.

arXiv:2509.05459v1 Announce Type: new Abstract: The JWST has enabled the discovery of Active Galactic Nuclei at high redshifts. The intrinsic UV spectrum of GN-z11 at redshift z = 10.6 has a spectral slope compatible with a standard accretion disc. By fitting a disc model to its spectrum, we find that the mass of the black hole must be above 1.6e7 Msun in order that it lies below the Eddington limit. We define this mass as the Eddington mass of the black hole. We note that the spectral shape is consistent with that of accreting stellar mass black holes sources in their soft state, for which no variability is expected. Mom-z14 is a more distant object at z = 14.44 and has a similar UV slope. Disc model-fitting gives a similar result but lower mass accretion rate. We also examine 3 further high redshift objects: GS z14-1, GHZ2 and GS-z11-1 at z = 13.86, 12.34 and 11.28, again obtaining similar results. If sub-Eddington accretion discs are indeed the origin of much of the UV emission from these objects, then the existence of massive black holes less than 440 and 290 Myr after the Big Bang point either to exceptional black hole seeds or to primordial black holes. The observed spread of UV spectral slopes in high redshift objects suggests that our approach may be relevant to about half of that population.

arXiv:2509.05423v1 Announce Type: new Abstract: JWST has revealed a previously unknown population of low-luminosity active galactic nuclei (AGN) in the early Universe. These JWST-AGN at high redshifts are characterised by a set of peculiar properties, including unusually weak X-ray emission. Here we investigate the apparent lack of X-ray emission in the framework of the ``AGN radiative dusty feedback'' scenario based on the effective Eddington limit for dust. We analyse how the boundary in the $N_\mathrm{H} - \lambda$ plane, defined by the column density versus the Eddington ratio, is modified as a function of the dusty gas parameters (metallicity, dust grain size and composition). Low metallicity gas with little dust content tends to survive against radiation pressure, and likely accumulates in the nuclear region. We suggest that such dust-poor gas can provide long-lived absorption and may lead to heavy X-ray obscuration, as observed in early JWST-AGN. The blowout vs. stalling condition of the obscuring clouds indicates that higher metallicities are required to eject heavier column densities, while large columns of gas can stall in low metallicity environments. Therefore the metallicity may play a key role in the AGN radiative dusty feedback scenario. We discuss how other peculiar properties of JWST-AGN --such as Balmer absorption features and weak radio emission-- may be naturally interpreted within the same physical framework.

Title to be confirmed

Abstract not available

• Speaker: Andreas Schachner (Munich U. and Cornell U.)
• Monday 24 November 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Gonzalo Villa.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Joseph Conlon (Oxford U.)
• Monday 27 October 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Gonzalo Villa.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Lucien Heurtier (King's College London)
• Monday 06 October 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Gonzalo Villa.

Add to your calendar or Include in your list

Title to be confirmed

Abstract not available

• Speaker: Ivonne Zavala (Swansea U.)
• Monday 13 October 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Gonzalo Villa.

Add to your calendar or Include in your list

arXiv:2509.04336v1 Announce Type: cross Abstract: We present a GPU-accelerated implementation of the gravitational-wave Bayesian inference pipeline for parameter estimation and model comparison. Specifically, we implement the `acceptance-walk' sampling method, a cornerstone algorithm for gravitational-wave inference within the bilby and dynesty framework. By integrating this trusted kernel with the vectorized blackjax-ns framework, we achieve typical speedups of 20-40x for aligned spin binary black hole analyses, while recovering posteriors and evidences that are statistically identical to the original CPU implementation. This faithful re-implementation of a community-standard algorithm establishes a foundational benchmark for gravitational-wave inference. It quantifies the performance gains attributable solely to the architectural shift to GPUs, creating a vital reference against which future parallel sampling algorithms can be rigorously assessed. This allows for a clear distinction between algorithmic innovation and the inherent speedup from hardware. Our work provides a validated community tool for performing GPU-accelerated nested sampling in gravitational-wave data analyses.

arXiv:2509.03593v1 Announce Type: new Abstract: Transient events associated with supermassive black holes provide rare opportunities to study accretion and the environments of supermassive black holes. We present a multiwavelength study of AT2020adpi (ZTF20acvfraq), a luminous optical/UV transient in the nucleus of the galaxy WISEA J231853.77$-$103505.6 ($z=0.26$) that exhibits the properties of an ambiguous nuclear transient. Near peak, its spectral energy distribution is well described by a power law ($\lambda L_\lambda \propto \lambda^{-\alpha}$, $\alpha = 0.44 \pm 0.04$), with a maximum $g$-band luminosity of $(3.6 \pm 0.6)\times10^{44}$ erg s$^{-1}$, which is consistent with luminous AGN flares. We detect a strong mid-infrared flare ($L_\mathrm{peak}^{\mathrm{MIR}} = (2.3 \pm 0.05)\times10^{44}$ erg s$^{-1}$) delayed by $\sim$240 rest-frame days, indicating a hot dust echo from material at $\sim$0.2 pc. The optical and near-infrared spectra show broad H, He I, [OIII] lines, as well as narrow Fe II, and prominent Mg II, which is a combination not typical of TDEs. Taken together, these features suggest AT2020adpi is an ambiguous nuclear transient, where an accretion episode was triggered by stellar disruption of an accretion disk or instabilities within an active nucleus. This source demonstrates the need for careful multiwavelength analysis to distinguish between extreme AGN variability and TDEs.