Rajeshwari Dutta

We conducted eMERLIN and Gemini integral field unit (IFU) observations towards TXS 1033+026, a post-merger ultraluminous infrared galaxy, to study the radio continuum and H α and H I 21 cm lines at sub-kiloparsec scales. We find a symmetric, two-sided core-jet structure in both L - (≈1.4 GHz) and C - (≈4.8 GHz) band continuum images. The active galactic nucleus likely has a double-double radio morphology, indicating possible multiple episodes of activity. The Gemini IFU observations provided sub-arcsecond resolution. Results from these observations suggest the presence of a kiloparsec-scale circumnuclear, warped H α disc that rotates around the core. The H I 21 cm maps reveal a complex kinematic structure, with indications of two counter-rotating gas components orbiting the core-jet axis; one of these components counter-rotates relative to the kiloparsec-scale H α disc. However, the true morphology of this structure may be more complex than our interpretation suggests. We estimate a H I 21 cm velocity-integrated optical depth of 90 ± 3 km s ⁻¹ . We note that these are the first results to probe H α and cold neutral hydrogen gas in TXS 1033+026 at sub-kiloparsec scales.

We investigate the multiphase circumgalactic medium during the epoch of reionization ( z > 6) by cross-correlating cool- C II and warm-ionized C IV absorption systems with star-forming [O III ] emitters. JWST/NIRCam wide-field slitless spectroscopy from the EIGER survey was combined with medium- and high-resolution optical and NIR spectra of six background quasars, including new VLT/X-Shooter observations of PSO J159-02. We analyzed the relation between 16 C II and 14 C IV absorbers (log( N ) > 13.0) and 136 galaxies within an impact parameter of R _⊥ ≤ 1000 pkpc and a line-of-sight separation of Δ v ≤ 500 km s ⁻¹ . We detected a statistically significant excess of both ions around galaxies compared to a randomized background. The C IV covering fraction remains enhanced up to ∼1 pMpc, whereas C II drops to the background level beyond ∼0.5 pMpc, demonstrating that the warm-ionized phase is more spatially extended than cooler gas. Jointly, the 3D galaxy-absorber spatial clustering is significantly weaker than the galaxy-galaxy autocorrelation. This provides direct physical evidence that early carbon enrichment is not confined to the virial radius of massive star-forming systems; rather, a substantial fraction of these metals permeates the diffuse intergalactic medium or is injected by a widespread population of faint undetected dwarf galaxies. Finally, we note a rapid radial decline of the C IV covering fraction compared with lower-redshift samples at z < 2 and z ∼ 3 − 4, which indicates an evolving circumgalactic medium ionization structure where early metals predominantly reside in lower ionization states. In conclusion, we determine a conservative lower limit for the observed carbon mass of M _{C  II +C  IV} ≥ 2.8 × 10 ⁶ M _⊙ within 300 pkpc.

Context. Current and future large spectroscopic surveys are significantly enhancing the volume and resolution of quasar spectra that are observed, motivating the creation of efficient and precise automated techniques to detect absorption features. Aims. This study focuses on the detection of metal absorption features using a novel U-Net model on WEAVE-like mock spectra in the quasar rest-frame wavelength interval 1230 Å ≤ λ _RF ≤ 3095 Å. We test the network performance for absorption detection both on ideal data and after simulating the continuum fitting step as applied on real data. The ability to recover the correct line centers is also studied. Methods. The performance of these architectures is evaluated by the completeness, purity, and F1 score reached in bins of signal-to-noise (S/N _line ) for the absorption lines and with the absolute fractional flux error for the continuum. Results. The U-Net model reaches scores of ≈90% for all metrics (completeness, purity, and F1 score) at S/N _line ≈ 4. All false positive detections with S/N _line ≥ 5 fall in the tails of the broad Ly α absorbers distribution of damped Ly α systems. The combination of continuum fitting and line detections has negligible effects on the detection performance at S/N _line ≥ 4. Conclusions. Our proposed U-Net architecture offers a competitive tool for the analysis of absorption lines in current and upcoming large spectroscopic surveys and is well suited to the identification of any absorption line feature.

We present a complete and homogeneous analysis of the Ly α emission properties of the cosmic web at 3 ≲ z ≲ 5 as a function of the overdensity of Ly α emitters (LAEs) in the MUSE Ultra Deep Field (MUDF) and the MUSE Extremely Deep Field (MXDF). We identified 41 overdensities, probing environments that are δ ≈ 2 − 10 times denser than the field. We searched for extended emission down to surface brightness (SB) levels of 3 − 5 × 10 ⁻²⁰ erg s ⁻¹ cm ⁻² arcsec ⁻² , revealing filamentary structures beyond the scale of the circumgalactic medium, confirming that LAEs act as signposts of the cosmic web. A trend with overdensity emerges: non-detections are mainly at δ < 2, a ≈1 dex scatter in SB appears for 2 ≲ δ ≲ 3.5, and detections reach a maximum intrinsic SB of ≈2 × 10 ⁻¹⁹ erg s ⁻¹ cm ⁻² arcsec ⁻² for δ ≳ 3.5. Emitting regions occupy a fraction of projected area f _s ≈ 0.20 − 0.25 inside filaments, yielding a cosmological incidence for Ly α emission in the cosmic web ℓ (Ly α ) of ≈1.22, which is similar to the one of partially neutral Lyman limit systems. This analysis indicates that the emitting gas is partially ionized at moderate densities ( n _H ≈ 10 ⁻³ − 10 ⁻¹ cm ⁻³ ), likely tracing the denser spines of intergalactic filaments and embedded substructures. Finally, we forecast how oriented stacking in larger samples of overdensities from shallower observations could yield an expanded view of the cosmic web before the next-generation wide-field spectroscopic instruments become operational.

We present the largest catalogue to date of Galaxies On Top Of Quasars (GOTOQs)—systems where the sight line to a background quasar passes directly through or very close to a foreground galaxy. Using ∼1.1 million quasar spectra from the SDSS-DR16 and DESI Early Data Release, we identify 1345 unique GOTOQs over 0.0045 ≤ z ≤ 1.09, more than quadrupling the number previously known. The catalogue combines both absorption-agnostic and absorption-selected searches, enabling a robust characterisation of gas in galaxies at projected separations of D ≲ 20 kpc. The GOTOQ emission-line ratios indicate that their host galaxies are predominantly normal, star-forming disks with typical dust extinctions of A _V ≃ 0.5 mag. We measure the Mg ii covering fraction at D ≲ 20 kpc and find it to be remarkably high, f c = 98 . 2 − 4.5 + 1.3 %, indicating that these systems trace gas-rich, metal-enriched regions at the disk-halo interface. The median colour excess towards the quasar line of sight of E ( B − V ) = 0.087 is significantly higher than that of typical Mg ii absorbers, underscoring the dust-rich nature of GOTOQ sight lines. From a high signal-to-noise composite spectrum, we report the first statistical detection of the diffuse interstellar band at λ 4428 ( W _r = 0.055 ± 0.011 Å) and at λ 5780 ( W _r = 0.051 ± 0.012 Å), revealing the presence of complex organic molecules at the disk-halo interface. This catalogue provides a powerful reference sample for future multiwavelength studies of gas flows, cold gas, and dust evolution in the inner circumgalactic medium.

Recent JWST observations have unveiled a large number of quiescent galaxies at z ≳ 3, bringing potential challenges to current galaxy formation models. Since star formation is expected to be fed by external gas accretion, knowledge about the circumgalactic media (CGM) of these galaxies is essential to understanding how they quench. In this work, we present the discovery of a massive, passive galaxy ( M _★ ≃ 10 ¹¹ M _⊙ ) within the MUSE Quasar Nebula 01 (MQN01) large-scale structure at z ≃ 3.25, containing one of the largest overdensities of galaxies and active galactic nuclei (AGNs) found so far at z ≳ 3. This passive galaxy has a star formation rate of 4 ₋₂ ⁺⁶ M _⊙ yr, placing it more than 1 dex below the star-forming main sequence, and it has no detectable molecular gas ( M _H2 < 2 × 10 ¹⁰ M _⊙ within a 700 km s ⁻¹ window, if assuming r ₄₁ = 0.34 and α _CO = 4 M _⊙ K ⁻¹ km ⁻¹ s pc ² ). Surprisingly, it is located at the center of a large cool gas reservoir, as traced by bright Ly α and H α emission. Deep multiwavelength information unique to this field, including spectroscopic and Chandra X-ray observations, suggests that an external mechanism could keep this cool gas reservoir in a dynamically hot, turbulent state, thus reducing its ability to efficiently accumulate within the galaxy. In particular, we discuss the possibility that the agent of this external energy injection is the AGN jet from a nearby star-forming galaxy located at a projected distance of 48 kpc and detected in both X-ray and radio. Additional or alternative energy injection mechanisms could involve the gravitational interaction between these two galaxies. In all scenarios, the elevated ionizing field provided by the AGN overdensity, including the nearby AGN, can illuminate the passive galaxy’s cool CGM and make it visible through fluorescent emission. Our study demonstrates that the star formation rates of high-redshift galaxies could be substantially reduced and maintained at a low level even within gas-rich and overdense environments in particular situations.

The formation and evolution of galaxies is regulated by the exchange of gas with the surrounding large-scale structures on circum- and intergalactic scales, yet little is known about the complex processes shaping the cycle of baryons in and out of galaxies. In this work, we present a multiline study of the gas surrounding a z ≈ 3.66 quasar known to host one of the brightest Ly α nebulae at high redshift, MUSE Quasar Nebula 04 (MQN04). By combining a high-resolution MUSE detection of nonresonant He II emission with a precise measurement of the redshift of the quasar host via the ALMA CO(4–3) line, we study the kinematics of the cool ionized gas down to #x2248; 1 kpc from the quasar. The MUSE observations reveal complex clumpy structures as well as diffuse emission extended over ≈100 kpc and blueshifted by ≈0 − 800 km s ⁻¹ relative to the quasar systemic redshift, suggesting that the circumgalactic medium is highly asymmetric. The analysis of the He II /Ly α line ratio, and the presence of a low-column density (≈ 10 ^14.6 cm ⁻² ) H I absorber along the quasar sightline suggests that MQN04 resides in a highly ionized medium. This is also supported by the gas kinematics, which, except in the most central region, shows consistent velocity shifts across the different tracers, indicative of relatively weak radiative transfer effects. Based on its morphology and kinematics, we conclude that the extended He II emission may arise from merger-driven tidal stripping or inflows of gas illuminated by the quasar radiation. On comoving megaparsec scales, we discover a large concentration ( δ ≈ 41) of star-forming galaxies lying within |Δ v _QSO | ≲ 1000 km s ⁻¹ from the quasar. MQN04 is therefore one of the most overdense environments discovered at this epoch.

We use deep MUSE data from the MUDF survey to investigate the cool gas around galaxies at redshifts 0.5 ≲ z ≲ 2. We constructed two samples: one sample for a down-the-barrel analysis, probing outflows via Mg II absorption against galaxy continua, and the other sample for projected galaxy pairs to examine the gas around the foreground galaxies in the transverse direction. From down-the-barrel stacked spectra, we detected blueshifted Mg II absorption, indicative of outflows, in which the absorption strength increases with stellar mass and star formation rate. Lower-mass galaxies exhibit weaker absorption, but higher outflow velocities, whereas higher-mass systems retain more cool gas with slower outflows. In the transverse direction, the absorption of Mg II decreases with the impact parameter, following a shallow profile. Comparing observations with radiative transfer models, we found that extrapolating an expanding halo model constrained with down-the-barrel measurements to halo scales overestimates the observed equivalent widths, likely due to the outflow geometry and the absence of the interstellar medium in the model. Our results highlight that mass, outflow geometry, and gas retention shape the cool circumgalactic medium, and that the combination of absorption and emission diagnostics provides powerful constraints on the properties of the cold halo gas.

A systematic search for H i 21 cm absorption in quasar–galaxy pairs (QGPs) provides a powerful means to map the distribution of cold gas around high-redshift star-forming galaxies. Fiber spectroscopy of high-redshift quasars enables the serendipitous detection of foreground star-forming galaxies at extremely small impact parameters, forming a unique subset of QGPs known as galaxies on top of quasars (GOTOQs). In this study, we present results from a pilot upgraded Giant Metrewave Radio Telescope survey of three GOTOQs, where we achieved a remarkable 100% detection rate of H i 21 cm absorption. By combining our findings with the existing literature, we establish that GOTOQs constitute a distinct population in terms of H i 21 cm absorption, with significantly higher detection rates than those observed in damped Ly α -based or metal absorption-based searches. For the GOTOQs, we find a strong correlation between the line-of-sight reddening and the H i 21 cm optical depth, characterized by ∫ τ d v ( km s − 1 ) = 13.5 8 − 2.35 + 2.75 E ( B − V ) + 0.6 8 − 1.27 + 1.06 , consistent with the Milky Way sightlines. We also show that the H I 21 cm optical depth declines with the impact parameter, and find a tentative trend for the H I 21 cm detection rates to also decline with the impact parameter. With upcoming wide-field spectroscopic surveys expected to substantially expand the catalog of known GOTOQs, the success of this pilot survey lays the foundation for constructing a statistically significant sample of intervening H i 21 cm absorbers.

Context. Ongoing and upcoming large spectroscopic surveys are drastically increasing the number of observed quasar spectra, making the development of fast and accurate automated methods to estimate spectral continua necessary. Aims. This study evaluates the performance of three neural networks (NNs) – an autoencoder, a convolutional NN (CNN), and a U-Net – in predicting quasar continua within the rest frame wavelength range of 1020 Å to 2000 Å. The ability to generalize and predict galaxy continua within the range of 3500 Å to 5500 Å is also tested. Methods. We evaluated the performance of these architectures using the absolute fractional flux error (AFFE) on a library of mock quasar spectra for the WEAVE survey and on real data from the early data release observations of the Dark Energy Spectroscopic Instrument (DESI) and the VIMOS Public Extragalactic Redshift Survey (VIPERS). Results. The autoencoder outperforms U-Net, achieving a median AFFE of 0.009 for quasars. The best model also effectively recovers the Lyα optical depth evolution in the DESI quasar spectra. With minimal optimization, the same architectures can be generalized to the galaxy case, with the autoencoder reaching a median AFFE of 0.014 and reproducing the D4000n break in DESI and VIPERS galaxies.