David M. Alexander

We present the largest statistical spectroscopic sample of radio quasars at 4.7 ≤ z ≤ 6.7 to date. These have been selected from a dedicated Dark Energy Spectroscopic Instrument target program and provide a robust sample to study the influence of radio jets on early quasar evolution. Using low-frequency radio data from the LOFAR Two Metre Sky Survey Data Release 3 at 144 MHz, we identify 83 high- z radio quasars, 26 of which are classified as radio-loud (5%–15% of 646 confirmed high- z quasars, defined as R = F _5GHz / F _4400Å > 10). With radio luminosities of ∼10 ²⁵⁻²⁸ W Hz ⁻¹ , the radio emission in all 83 quasars is expected to be jet dominated rather than powered by star formation. Analysis of their rest-frame ultraviolet spectra reveals broadly similar composite spectral properties and C iv velocity shifts compared to radio-nondetected quasars. The radio-detected and radio-loud quasars exhibit slightly smaller Ly α and C iv equivalent widths on average; however, we find no significant evidence (∼1.3 σ ) for a higher fraction of weak-line quasars (∼10% ± 4% versus ∼4% ± 1% for radio-nondetected quasars). We further find that the broad absorption line (BAL) quasars are predominantly radio-quiet, and no correlation is observed between absorption index and radio-loudness, suggesting that radio emission is not produced by the BAL outflows themselves and is a physically distinct phenomenon. Our results demonstrate that powerful radio jets were already well established by z ∼ 5.

Circumnuclear structures in active galactic nuclei (AGN) respond to accretion variability over different spatial and temporal scales, but whether compact narrow-line gas responds on decade timescales has not been systematically established. We use repeat Sloan Digital Sky Survey and Dark Energy Spectroscopic Instrument spectroscopy of 473 variability-selected galactic nuclei with evidence for recent nuclear brightening. We identify 19 sources whose narrow [O III ] λ 5007 flux increased by more than a factor of 2 over rest-frame baselines of 12.0–20.4 yr, which we term narrow-line ionization echoes (NIEs). In equal-weight stacked spectra, [O III ] increased by 229% ± 22%, while lower-ionization lines remained nearly unchanged or declined, indicating an ionization-dependent response rather than uniform spectral rescaling. The NIE hosts also moved coherently toward higher excitation in optical diagnostic diagrams, with mean increases of 0.46 ± 0.18 dex in log ( [ O III ] / H β ) and 0.58 ± 0.13 dex in log ( [ O III ] / [ O II ] ) . Photoionization models reproduce this evolution with a higher ionization parameter and a harder AGN-like ionizing spectrum at fixed gas abundance. For 13 sources with well-defined mid-infrared (MIR) outbursts, the integrated W1+W2 excess energy correlates strongly with the absolute [O III ] luminosity change ( r = 0.87), linking the dust and narrow-line responses to the same nuclear event. The MIR timescale and spectroscopic baseline constrain the variable [O III ]-emitting gas to characteristic scales of 1.5–5.2 pc. Compact narrow-line gas therefore appears to form an intermediate, decade-scale response layer between the infrared dust echo and the long-lived extended narrow-line region.

Context . Feedback from dusty quasars is thought to be a key driver in galaxy evolution. Multiple studies have reported a link between dust extinction and radio emission in quasars, possibly attributed to winds or jets interacting with the interstellar medium. Aims . We use optical spectra from the Dark Energy Spectroscopic Instrument (DESI) to test whether ionised outflows could be driving the observed radio–dust connection and to determine whether dust and/or colour play a role in producing high-velocity ionised outflows in quasars. Methods . Based on a sample of 3418 DESI quasars at 0.5 < z < 0.9, we constrained the [O III ]5007 kinematics by fitting the spectra with PyQSOFit and comparing the observed trends among the [O III ] outflow velocity and the line-of-sight dust extinction E ( B − V ), optical-to-mid-infrared (MIR) colour g − W 2, and L _6µm / L _{5100 Å} (a proxy for MIR excess). Utilising radio data from the LOFAR Two-metre Sky Survey (LoTSS) DR2, we also explored the link between radio, dust, and outflows. Results . We find that the link between radio emission and dust obscuration is driven by sources hosting high-velocity ionised outflows. However, we find no significant trend between the [O III ] outflow velocity and E ( B − V ), which suggests dust extinction alone is not a significant factor in determining whether a quasar hosts a powerful ionised outflow. On the other hand, we find a clear positive trend between g − W 2 and outflow velocity, even after controlling for luminosity. We suggest this is due to a connection between a MIR excess (boosting the W 2 band) and outflows, which is supported by a positive trend between L _6µm / L _{5100 Å} and outflow velocity. Conclusions . Our results point to a scenario in which outflows shock the surrounding dust and gas, heating the dust and boosting both the MIR and radio emission.

Compact and red sources in the high-redshift ( z ∼ 5) Universe, known as “Little Red Dots” (LRDs), are among JWST’s most intriguing discoveries. These sources have broad Balmer emission lines, weak X-ray emission, and unique spectral energy distributions (SEDs) poorly fit by either stellar or active galactic nucleus (AGN) templates. Local LRD-like objects allow for detailed studies of the underlying physical processes with archival multiwavelength datasets unavailable in the high- z Universe. We show that the Sloan Digital Sky Survey (SDSS) ugriz filters at z ≈ 0.4, 0.8 overlap well with the JWST filters used to select LRDs at z ∼ 5. We use SDSS quasars to define a sample of ∼1300 Local Red Dots (LoRDs) that share the same photometric colors of LRDs. A subset of the LoRD sample selected to have V-shaped continua ( N = 244) show prominent higher-order Balmer absorption features and [Ne v ] λ 3426 emission, both of which would likely be missed in JWST/PRISM observations given the low spectral resolution. A composite SED of the LoRDs differs from a typical quasar SED in the rest-frame UV/optical, but the two agree with each other in the near-infrared. The LoRD SED matches well with a stack of LRDs and can be modeled either as a reddened AGN combined with a host galaxy, or as a reddened AGN combined with a host galaxy and a cool blackbody. Interestingly, the LoRDs are X-ray detected at a rate comparable to typical quasars. However, the probability that LoRDs and typical quasars would go undetected, if subject to the LRD X-ray upper limits, is >50%.

Using the first spectroscopic data release from the Dark Energy Spectroscopic Instrument (DESI DR1), we conduct the largest systematic search for active galactic nuclei (AGN) signatures in ≈1.7 million low-redshift (0.001 ≤ z ≤ 0.45) line-emitting galaxies. Based on the [N ii ]/H α versus [O iii ]/H β emission-line ratio diagnostic, we identify AGN in 314,245/1,211,573 (25.9%) high-mass ( log ( M ⋆ / M ⊙ ) > 9.5 ) and 9648/467,214 (2.1%) dwarf ( log ( M ⋆ / M ⊙ ) ≤ 9.5 ) galaxies. The AGN fraction in line-emitting galaxies increases monotonically with stellar mass, from ∼1.4% at the low-mass end to ∼93.3% at the high-mass end. Among these AGN, 17,949 exhibit broad H α emission (BL-AGN), which we use to estimate their black hole (BH) masses via single-epoch virial methods. Leveraging this large BL-AGN sample, we extend the M _BH − M _⋆ scaling relation down to log ( M ⋆ / M ⊙ ) ≈ 7.8 and log ( M BH / M ⊙ ) ≈ 4.4 . We find a bimodal distribution of galaxies in this space, characterized by two distinct empirical relations. The relation for galaxies with high stellar and BH masses extrapolates to overlap with the high-redshift overmassive BH candidates. Our results suggest at least two broad evolutionary pathways for these overmassive BHs and their host galaxies across cosmic time. With this paper, we release the EmFit value-added catalog, containing emission-line flux and width measurements for ∼7.4 million galaxies, the largest catalog with emission-line decomposition into narrow, broad, and outflow components to date. This work significantly expands upon the early DESI results and provides a statistical sample for probing the galaxy−BH connection in the low-mass galaxy regime.

In 2021 May the Dark Energy Spectroscopic Instrument (DESI) collaboration began a 5 yr spectroscopic redshift survey to produce a detailed map of the evolving three-dimensional structure of the Universe between z = 0 and z ≈ 4. DESI’s principal scientific objectives are to place precise constraints on the equation of state of dark energy, the gravitationally driven growth of large-scale structure, and the sum of the neutrino masses, and to explore the observational signatures of primordial inflation. We present DESI DR1, which consists of all data acquired during the first 13 months of the DESI main survey, as well as a uniform reprocessing of the DESI Survey Validation data, which were previously made public in the DESI Early Data Release. The DR1 main survey includes high-confidence redshifts for 18.7M objects, of which 13.1M are spectroscopically classified as galaxies, 1.6M as quasars, and 4M as stars, making DR1 the largest sample of extragalactic redshifts ever assembled. We summarize the DR1 observations, the spectroscopic data-reduction pipeline and data products, large-scale structure catalogs, value-added catalogs, and describe how to access and interact with the data. In addition to fulfilling its core cosmological objectives with unprecedented precision, we expect DR1 to enable a wide range of transformational astrophysical studies and discoveries.

The slitless spectroscopy mode of the on board the Euclid telescope has enabled efficient spectroscopy of objects within a large field of view. Nevertheless, the relatively low spectral resolution, overlapping spectra, and contamination pose challenges to source classification and redshift determination using the spectra alone. In this work, we present a large and homogeneous sample of bright quasars identified from the Euclid Quick Data Release (Q1), constructed by combining high-purity candidate selections from to wavelengths without telluric lines, which will be pivotal to quasar spectral analysis. We obtained an empirical spectroscopic depth of JE łesssim 21.5 and ̋E łesssim 21.3 at the sensitivity of the Wide Field Survey, beyond which the number of securely identified quasars declines sharply. Accordingly, the sample presented in this paper comprises spectroscopically confirmed quasars brighter than these limits. We analysed morphological parameters from the Visible Camera (VIS) using Sérsic and model-independent (CAS) metrics, and a deep-learning point spread function fraction to track nuclear dominance. The VIS morphologies show a clear redshift dependence: at low redshift (z<0.5), obvious host structures are common and a single Sérsic model fits about half of the sources; at intermediate redshift (0.5<z<2), the nuclear component dominates, with 90% of the Sérsic fits saturating at the upper index limit. In this intermediate redshift regime, f_ nisp nisp and WISE with the new spectroscopic capabilities of Euclid. Through visual inspection of the Euclid spectra of these quasar candidates, we identify approximately 3500 quasars and determine reliable redshifts in the range of 0< z łesssim4.8. Of these, 2686 are new spectroscopic identifications relative to existing public compilations. We generated the first Euclid composite spectrum of quasars covering rest-frame nuv nir nir PSF is available, and we use it as a more reliable compactness measure than the single-Sérsic and CAS parameters to quantify nuclear versus host emission. We also explore the novel Euclid colour space and discuss the role of these quasars in refining active galactic nucleus selection techniques for future Euclid data releases. nir

From a parent catalog of 561 changing-look active galactic nuclei (CL-AGNs) identified by W.-J. Guo et al., we investigate the evolutionary sequence of broad emission lines using a redshift-selected subset (0.35 < z < 0.45) of 54 CL-AGNs whose Dark Energy Spectroscopic Instrument (DESI) spectra simultaneously cover the H α , H β , and Mg ii emission lines. To provide a baseline for comparison, we construct a control sample of 19,897 normal Type 1 AGNs within the same redshift range from the DESI Year 1 data. Through stacked spectral analysis and line-continuum luminosity correlations, we identify a clear evolutionary sequence in all AGNs where broad H β fades first, followed by Mg ii, and then H α , as the AGN luminosity declines—consistent with expectations from reverberation mapping. This trend reflects a radially stratified broad-line region (BLR), where each line’s responsivity depends on its ionization potential and radial distance from the central engine. In addition, we find that more massive supermassive black holes require lower Eddington ratios to fully suppress broad emission lines, suggesting that the critical accretion threshold for the changing-look phenomenon is mass-dependent. Our results present the first statistical confirmation of a stratified broad-line fading sequence in AGNs, reinforcing the central role of accretion state in shaping BLR structure and visibility.

From a parent catalog of 561 changing-look active galactic nuclei (CL-AGNs) identified by Guo et al. (2025), we investigate the evolutionary sequence of broad emission lines using a redshift-selected subset (0.35 < z < 0.45) of 54 CL-AGNs whose Dark Energy Spectroscopic Instrument (DESI) spectra simultaneously cover the Hα, H\b{eta}, and Mg II emission lines. To provide a baseline for comparison, we construct a control sample of 19,897 normal Type 1 AGNs within the same redshift range from the DESI Year 1 data. Through stacked spectral analysis and line-continuum luminosity correlations, we identify a clear evolutionary sequence in all AGN where broad H\b{eta} fades first, followed by Mg II, and then Hα, as the AGN luminosity declines - consistent with expectations from reverberation mapping. This trend reflects a radially stratified broad line region (BLR), where each line's responsivity depends on its ionization potential and radial distance from the central engine. In addition, we find that more massive supermassive black holes (SMBHs) require lower Eddington ratios to fully suppress broad emission lines, suggesting that the critical accretion threshold for the CL phenomenon is mass-dependent. Our results present the first statistical confirmation of a stratified broad line fading sequence in AGNs, reinforcing the central role of accretion state in shaping BLR structure and visibility.