Background Quasars Research Articles

Canonical Hubble-Tension-Resolving Early Dark Energy Cosmologies Are Inconsistent with the Lyman-α Forest.

Current cosmological data exhibit discordance between indirect and some direct inferences of the present-day expansion rate H_{0}. Early dark energy (EDE), which briefly increases the cosmic expansion rate prior to recombination, is a leading scenario for resolving this "Hubble tension" while preserving a good fit to cosmic microwave background (CMB) data. However, this comes at the cost of changes in parameters that affect structure formation in the late-time universe, including the spectral index of scalar perturbations n_{s}. Here, we present the first constraints on axionlike EDE using data from the Lyman-α forest, i.e., absorption lines imprinted in background quasar spectra by neutral hydrogen gas along the line of sight. We consider two independent measurements of the one-dimensional Lyα forest flux power spectrum from the Sloan Digital Sky Survey (SDSS eBOSS) and from the MIKE/HIRES and X-Shooter spectrographs. We combine these with a baseline dataset comprised of Planck CMB data and baryon acoustic oscillation (BAO) measurements. Combining the eBOSS Lyα data with the CMB and BAO dataset reduces the 95% confidence level (C.L.) upper bound on the maximum fractional contribution of EDE to the cosmic energy budget f_{EDE} from 0.07 to 0.03 and constrains H_{0}=67.9_{-0.4}^{+0.4} km/s/Mpc (68% C.L.), with maximum aposteriori value H_{0}=67.9 km/s/Mpc. Similar results are obtained for the MIKE/HIRES and X-Shooter Lyα data. Our Lyα-based EDE constraints yield H_{0} values that are in >4σ tension with the SH0ES distance-ladder measurement and are driven by the preference of the Lyα forest data for n_{s} values lower than those required by EDE cosmologies that fit Planck CMB data. Taken at face value, the Lyα forest severely constrains canonical EDE models that could resolve the Hubble tension.

Characterizing cool, neutral gas, and ionized metals in the outskirts of low-z galaxy clusters

ABSTRACT We present the first detection of cool, neutral gas in the outskirts of low-z galaxy clusters using a statistically significant sample of 3191 z ≈ 0.2 background quasar-foreground cluster pairs by cross-matching the Hubble Spectroscopic Legacy Archive quasar catalogue with optically- and SZ-selected cluster catalogues. The median cluster mass of our sample is ≈1014.2 $\rm {\rm M}_{\odot }$ with a median impact parameter (ρcl) of ≈5 Mpc. We detect significant Lyα, marginal C iv, but no O vi absorption in the signal-to-noise ratio weighted mean stacked spectra with rest-frame equivalent widths of 0.096 ± 0.011, 0.032 ± 0.015, and <0.009 Å (3σ) for our sample. The Lyα REW shows a declining trend with increasing ρcl (ρcl/R500), which is well explained by a power-law with a slope of −0.79 (−0.70). The covering fractions (CFs) measured for Lyα (21 per cent), C iv (10 per cent), and O vi (10 per cent) in cluster outskirts are significantly lower than in the circumgalatic medium (CGM). We also find that the CGM of galaxies that are closer to cluster centres or that are in massive clusters is considerably deficient in neutral gas. The low CF of the Lyα along with the non-detection of Lyα signal when the strong absorbers ($N(\rm{H\,{\small I}}) > 10^{13}$ cm−2) are excluded, indicate the patchy distribution of cool gas in the outskirts. We argue that the cool gas in cluster outskirts in combination arises from the circumgalactic gas stripped from cluster galaxies and to large-scale filaments feeding the clusters with cool gas.

DPQP: A Detection Pipeline for Quasar Pair Candidates Based on QSO Photometric Images and Spectra

Quasars have an important role in the studies of galaxy evolution and star formation. The rare close projection of two quasars in the sky allows us to study the environment and matter exchange around the foreground quasar (QSOfg) and the background quasar (QSObg). This paper proposes a pipeline DPQP for quasar pair (QP) candidates’ detection based on photometric images and the corresponding spectra. The pipeline consists of three main parts: a target source detector, a regressor, and a discriminator. In the first part, the target source detection network–YOLOv4 (TSD-YOLOv4) and the target source classification network (TSCNet) are used in sequence to detect quasars in SDSS photometric images. In the second part, a depth feature extraction network of quasar images (DE-QNet) is constructed to estimate the redshifts of quasars from photometric images. In the third part, a quasar pair score (Q-Score) metric is proposed based on the spectral analysis. The larger the Q-Score, the greater the possibility of two pairs being a quasar pair. The experimental results show that between redshift 1.0 and 4.0, the MAE of DE-QNet is 0.316, which is 16.1% lower than the existing method. Samples with |Δz| < 0.15 account for 77.1% of the test dataset. A new table with 1025 QP candidates is provided by traversing 50,000 SDSS photometric images.

The BarYon Cycle project (ByCycle): identifying and localizing Mg ii metal absorbers with machine learning

ABSTRACT The upcoming ByCycle project on the VISTA/4MOST multi-object spectrograph will offer new prospects of using a massive sample of ∼1 million high spectral resolution (R = 20 000) background quasars to map the circumgalactic metal content of foreground galaxies (observed at R = 4000–7000), as traced by metal absorption. Such large surveys require specialized analysis methodologies. In the absence of early data, we instead produce synthetic 4MOST high-resolution fibre quasar spectra. To do so, we use the TNG50 cosmological magnetohydrodynamical simulation, combining photo-ionization post-processing and ray tracing, to capture Mg ii (λ2796, λ2803) absorbers. We then use this sample to train a convolutional neural network (CNN) which searches for, and estimates the redshift of, Mg ii absorbers within these spectra. For a test sample of quasar spectra with uniformly distributed properties ($\lambda _{\rm {Mg\, {\small II},2796}}$, $\rm {EW}_{\rm {Mg\, {\small II},2796}}^{\rm {rest}} = 0.05\!-\!5.15$ Å, $\rm {SNR} = 3\!-\!50$), the algorithm has a robust classification accuracy of 98.6 per cent and a mean wavelength accuracy of 6.9 Å. For high signal-to-noise (SNR) spectra ($\rm {SNR \gt 20}$), the algorithm robustly detects and localizes Mg ii absorbers down to equivalent widths of $\rm {EW}_{\rm {Mg\, {\small II},2796}}^{\rm {rest}} = 0.05$ Å. For the lowest SNR spectra ($\rm {SNR=3}$), the CNN reliably recovers and localizes EW$_{\rm {Mg\, {\small II},2796}}^{\rm {rest}}$ ≥0.75 Å absorbers. This is more than sufficient for subsequent Voigt profile fitting to characterize the detected Mg ii absorbers. We make the code publicly available through GitHub. Our work provides a proof-of-concept for future analyses of quasar spectra data sets numbering in the millions, soon to be delivered by the next generation of surveys.

Model selection with baryonic acoustic oscillations in the Lyman-α forest

The recent release of the final, complete survey of Lyman-α baryonic acoustic oscillation measurements provides the most significant and accurate data base for studying cosmic geometry at an effective redshift , which is inaccessible to other sources. In this paper, we use these data to select among four distinct cosmologies: Planck ΛCDM, the universe, the Milne universe and the Einstein-de Sitter universe. Given the breadth and depth of the Lyman-α study, this BAO measurement alone provides a strong model comparison, complementary to previous studies that combined Lyman-α data with measurements at lower redshifts. Though both approaches are useful, the latter tends to dilute the disparity between model predictions and the observations. We therefore examine how the models compare to each other strictly based on the BAO scale measured in the Lyman-α forest and background quasars. We find that the Milne universe and the Einstein-de Sitter universe are strongly ruled out by these data. There is also strong evidence disfavoring the standard model. The Lyman-α measurements are completely consistent with the cosmic geometry predicted by . As such, evidence continues to grow that the zero active mass condition from general relativity ought to be an essential ingredient in ΛCDM.

Discovery of Super-enriched Gas ∼1 Gyr after the Big Bang

Abundances of chemical elements in the interstellar and circumgalactic media of high-redshift galaxies offer important constraints on the nucleosynthesis by early generations of stars. Damped Lyα absorbers (DLAs) in spectra of high-redshift background quasars are excellent sites for obtaining robust measurements of element abundances in distant galaxies. Past studies of DLAs at redshifts z > 4 have measured abundances of ≲0.01 solar. Here we report the discovery of a DLA at z = 4.7372 with an exceptionally high degree of chemical enrichment. We estimate the H i column density in this absorber to be log (N H I/cm−2) = 20.48 ± 0.15. Our analysis shows unusually high abundances of carbon and oxygen ([C/H] = 0.88 ± 0.17, [O/H] = 0.71 ± 0.16). Such a high level of enrichment a mere 1.2 Gyr after the Big Bang is surprising because of insufficient time for the required amount of star formation. To our knowledge, this is the first supersolar absorber found at z > 4.5. We find the abundances of Si and Mg to be [Si/H] = and [Mg/H] = , confirming the metal-rich nature of this absorber. By contrast, Fe shows a much lower abundance ([Fe/H] = ). We discuss implications of our results for galactic chemical evolution models. The metallicity of this absorber is higher than that of any other known DLA and is >2 orders of magnitude above predictions of chemical evolution models and the N H I-weighted mean metallicity from previous studies at z > 4.5. The relative abundances (e.g., [O/Fe] = 2.29 ± 0.05, [C/Fe] = 2.46 ± 0.08) are also highly unusual compared to predictions for enrichment by early stars.

Circumgalactic Medium on the Largest Scales: Detecting X-Ray Absorption Lines with Large-area Microcalorimeters

The circumgalactic medium (CGM) plays a crucial role in galaxy evolution as it fuels star formation, retains metals ejected from the galaxies, and hosts gas flows in and out of galaxies. For Milky Way–type and more-massive galaxies, the bulk of the CGM is in hot phases best accessible at X-ray wavelengths. However, our understanding of the CGM remains largely unconstrained due to its tenuous nature. A promising way to probe the CGM is via X-ray absorption studies. Traditional absorption studies utilize bright background quasars, but this method probes the CGM in a pencil beam, and, due to the rarity of bright quasars, the galaxy population available for study is limited. Large-area, high spectral resolution X-ray microcalorimeters offer a new approach to exploring the CGM in emission and absorption. Here, we demonstrate that the cumulative X-ray emission from cosmic X-ray background sources can probe the CGM in absorption. We construct column density maps of major X-ray ions from the Magneticum simulation and build realistic mock images of nine galaxies to explore the detectability of X-ray absorption lines arising from the large-scale CGM. We conclude that the O VII absorption line is detectable around individual massive galaxies at the 3σ–6σ confidence level. For Milky Way–type galaxies, the O VII and O VIII absorption lines are detectable at the ∼ 6σ and ∼ 3σ levels even beyond the virial radius when coadding data from multiple galaxies. This approach complements emission studies, does not require additional exposures, and will allow for probing the baryon budget and the CGM at the largest scales.

Cosmo-tomography toward PKS 1830−211: Variability of the quasar and of its foreground molecular absorption monitored with ALMA

Context. Time variability of astronomical sources provides crude information on their typical size and on the implied physical mechanisms. PKS 1830−211 is a remarkable radio-bright lensed quasar with a foreground molecular absorber in the lens galaxy at z = 0.89. Small-scale morphological changes in the core–jet structure of the quasar – which is magnified by the lensing – result in a varying illumination of the absorber screen, which in turn causes variations in the absorption profile. Aims. We aim to study the time variations of the system (the two main lensed images of the quasar and the two corresponding sightlines in the absorber) in order to obtain constraints on both the quasar activity and small-scale structures in the interstellar medium of the absorber. Methods. We used ALMA to monitor the submillimeter continuum emission of PKS 1830−211, together with the absorption spectra of the H2O and CH molecules, with 17 visits spread over six months in 2016. Complementing this, we used available ALMA data to investigate changes in the system in the period 2012−2022. Results. From the continuum data, we followed the evolution of the flux density, flux-density ratio, spectral index, and differential polarization between the two lensed images of the quasar; all quantities show significant variations related to the intrinsic activity of the quasar. We propose a simple parametric model of a core plus a ballistic plasmon to account for the continuum evolution, from which we constrain a time delay of 25 ± 3 days between main lensed images. The spectral lines reveal significant variations in the foreground absorption profile. A principal component analysis highlights apparent wavy time variations, possibly linked to the helical jet precession period of the quasar. From the deep averaged spectra towards the southwest image, we detect the absorption of the rare isotopolog 13CH and estimate an abundance ratio of 12CH/13CH ∼ 150. We also measure the oxygen isotopic ratios, 16O/18O = 65.3 ± 0.7 and 18O/17O = 11.5 ± 0.5 in the z = 0.89 absorber. Finally, we find a remarkable continuous shallow trough in the water absorption spanning a velocity interval of nearly 500 km s−1. This broad absorption could be the signature of an extra-planar molecular component. Conclusions. All together, the system formed by the quasar PKS 1830−211 and its foreground lens–absorber acts as a powerful gravitational microscope, providing us with the possibility to dissect small-scale structures in both the ISM of the foreground absorber and the jet of the background quasar.

MUSE-ALMA Haloes XI: gas flows in the circumgalactic medium

ABSTRACT The flow of gas into and out of galaxies leaves traces in the circumgalactic medium which can then be studied using absorption lines towards background quasars. We analyse 27 ${{\log [N({\textrm {H}}\, {\small {I}})/\rm {cm}^{-2}]}} > 18.0$ H i absorbers at z = 0.2 to 1.4 from the MUSE-ALMA Haloes survey with at least one galaxy counterpart within a line of sight velocity of ±500 km s−1. We perform 3D kinematic forward modelling of these associated galaxies to examine the flow of dense, neutral gas in the circumgalactic medium. From the VLT/MUSE, HST broad-band imaging, and VLT/UVES and Keck/HIRES high-resolution UV quasar spectroscopy observations, we compare the impact parameters, star-formation rates, and stellar masses of the associated galaxies with the absorber properties. We find marginal evidence for a bimodal distribution in azimuthal angles for strong H i absorbers, similar to previous studies of the Mg ii and O vi absorption lines. There is no clear metallicity dependence on azimuthal angle, and we suggest a larger sample of absorbers is required to fully test the relationship predicted by cosmological hydrodynamical simulations. A case-by-case study of the absorbers reveals that ten per cent of absorbers are consistent with gas accretion, up to 30 per cent trace outflows, and the remainder trace gas in the galaxy disc, the intragroup medium, and low-mass galaxies below the MUSE detection limit. Our results highlight that the baryon cycle directly affects the dense neutral gas required for star-formation and plays a critical role in galaxy evolution.

Proximate molecular quasar absorbers

Proximate molecular quasar absorbers (PH2) are an intriguing population of absorption systems that was recently uncovered through strong H2absorption at a small velocity separation from the background quasars. We performed a multi-wavelength spectroscopic follow-up of 13 such systems with VLT/X-shooter. Here, we present the observations and study the overall chemical enrichment measured from the H I, H2, and metal lines. We combined this with an investigation of the neutral gas kinematics with respect to the quasar host. We find gas-phase metallicities in the range 2% to 40% of the solar value, that is, in the upper-half range of H I-selected proximate damped Lyman-αsystems, but similar to what is seen in intervening H2-bearing systems. This is likely driven by similar selection effects that play against the detection of most metal- and molecule-rich systems in absorption. Differences are seen in the abundance of dust (from [Zn/Fe]) and its depletion pattern when compared to intervening systems, however, possibly indicating different dust production or destruction close to the active galactic nucleus. We also note the almost ubiquitous presence of a high-ionisation phase traced by N Vin proximate systems. In spite of the hard UV field from the quasars, we found no strong overall deficit of neutral argon, at least when compared to intervening damped Lyman-αsystems. The reason likely is that argon is mostly neutral in the H2phase, which accounts for a large fraction of the total amount of metals. We measured the quasar systemic redshifts through emission lines from both ionised gas and CO(3–2) emission, the latter being detected in all six cases for which we obtained 3 mm data from complementary NOEMA observations. For the first time, we observe a trend between the line-of-sight velocity with respect to systemic redshift and metallicity of the absorbing gas. This suggests that high-metallicity neutral and molecular gas is more likely to be located in outflows, while low-metallicity gas could be more clustered in velocity space around the quasar host, possibly with an infalling component.

ALMACAL – X. Constraints on molecular gas in the low-redshift circumgalactic medium

ABSTRACT Despite its crucial role in galaxy evolution, the complex circumgalactic medium (CGM) remains underexplored. Although, it is known to be multiphase, the importance of the molecular gas phase to the total CGM mass budget is, to date, unconstrained. We present the first constraints on the molecular gas covering fraction in the CGM of low-redshift galaxies, using measurements of CO column densities along sightlines towards mm-bright background quasars with intervening galaxies. We do not detect molecular absorption against the background quasars. For the individual, low-redshift, ‘normal’ galaxy haloes probed here, we can therefore rule out the presence of an extremely molecular gas-rich CGM, as recently reported in high-redshift protoclusters and around luminous active galactic nuclei. We also set statistical limits on the volume filling factor of molecular material in the CGM as a whole, and as a function of radius. Interstellar medium-like molecular clouds of ∼30 pc in radius with column densities of N(CO) ≳ 1016 cm−2 have volume filling factors of <0.2 per cent. Large-scale smooth gas reservoirs are ruled out much more stringently. The development of this technique in the future will allow deeper constraining limits to be set on the importance (or unimportance) of molecular gas in the CGM.

The Study of the Circumgalactic Medium with Quasar Pairs

We have collected 10,025 foreground–background quasar pairs with projected distances d p < 500 kpc from the large quasar catalog of the Sloan Digital Sky Survey DR16Q. We investigate the properties of the Mg ii absorption lines with W r > 0.15 Å around foreground quasars, including both the line of sight (LOS; of foreground quasars) and transverse (TRA; perpendicular to the LOS) absorptions. Both the equivalent width (the correlation coefficient ρ = −0.915 and the probability P < 10−4 of no correlation) and incident rate (ρ = −0.964 and P < 10−6) of TRA Mg ii absorption lines are obviously anticorrelated with projected distance. The incident rate of TRA Mg ii absorption lines is obviously (>4σ) greater than that of LOS Mg ii absorption lines at projected distances d p < 200 kpc, while the TRA and LOS Mg ii both have similar (<3σ) incident rates at scales d p > 200 kpc. The anisotropic radiation from quasars would be the most possible interpretation for the anisotropic absorption around quasars. This could also indicate that the quasar radiation is not obviously impacting the gas halos of quasars at scales d p > 200 kpc.

MusE GAs FLOw and Wind (MEGAFLOW) IX. The impact of gas flows on the relations between the mass, star formation rate, and metallicity of galaxies

ABSTRACTWe study the link between gas flow events and key galaxy scaling relations: the relations between star formation rate (SFR) and stellar mass (the main sequence, MS); gas metallicity and stellar mass (the mass–metallicity relation, MZR); and gas metallicity, stellar mass, and SFR (the fundamental metallicity relation, FMR). Using all star-forming galaxies (SFGs) in the 22 MUSE fields of the MusE GAs FLOw and Wind (MEGAFLOW) survey, we derive the MS, MZR, and FMR scaling relations for 385 SFGs with $M_\star = 10^{8} - 10^{11.5}\, {\rm M}_\odot$ at redshifts 0.35 &amp;lt; z &amp;lt; 0.85. Using the MUSE data and complementary X-Shooter spectra at 0.85 &amp;lt; z &amp;lt; 1.4, we determine the locations of 21 SFGs associated with inflowing or outflowing circumgalactic gas (i.e. with strong Mg ii absorption in background quasar spectra) relative to these scaling relations. Compared to a control sample of galaxies without gas flows (i.e. without Mg ii absorption within 70 kpc of the quasar), SFGs with inflow events (i.e. Mg ii absorption along the major axis) are preferentially located above the MS, while SFGs with outflow events (i.e. Mg ii absorption along the minor axis) are preferentially more metal rich. Our observations support the scenario in which gas accretion increases the SFR while diluting the metal content and where circumgalactic outflows are found in more metal-rich galaxies.

Stellar mass fraction and quasar accretion disk size in SDSS J1004+4112 from photometric follow-up

AbstractThe gravitational lens SDSS J1004+4112 was the first discovered system where a background quasar is lensed by a galaxy cluster instead of a single galaxy. We use the 14.5-year r-band light curves together with the recently measured time delay of the fourth brightest quasar image (Munõz et al. (2022)) and the mass model from Forés-Toribio et al. (2022) to study the microlensing effect in this system. We constrain the quasar accretion disk size to light-days at 2407Å in the restframe which is compatible with most previous estimates. We also infer the fraction of mass in stars at the positions of the quasar images: $${\alpha _A} = 0.058_{ - 0.032}^{ + 0.024},{\alpha _B} = 0.048_{ - 0.014}^{ + 0.032},{\alpha _C} = 0.018_{ - 0.018}^{ + 0.015}$$ and $${\alpha _D} = 0.008_{ - 0.008}^{ + 0.033}$$ . The stellar fraction estimates are reasonable for intracluster medium although the stellar fractions at images A and B are slightly larger, suggesting the presence of a near undetected galaxy.

New strong lensing modelling of SDSS J2222+2745 enhanced with VLT/MUSE spectroscopy

Context. SDSS J2222+2745, at z = 0.489, is one of the few currently known lens clusters with multiple images (six) of a background (z = 2.801) quasar with measured time delays between two image pairs (with a sub-percent relative error for the longer time delay). Systems of this kind can be exploited as alternative cosmological probes through high-precision and accurate strong lensing models. Aims. We present recent observations from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) and new total mass models of the core of the galaxy cluster SDSS J2222+2745. Methods. We combine archival multi-band, high-resolution imaging from the Hubble Space Telescope (HST) with our VLT/MUSE spectroscopic data to securely identify 34 cluster members and 12 multiple images from 3 background sources. We also measure the stellar velocity dispersions of 13 cluster galaxies, down to HST F160W = 21 mag, enabling an independent estimate of the contribution of the sub-halo mass component to the lens total mass. By leveraging the new spectroscopic dataset, we build improved strong lensing models. Results. The projected total mass distribution of the lens cluster is best modelled with a single large-scale mass component, a galaxy-scale component, anchored by the VLT/MUSE kinematic information, and an external shear component. The best-fit strong lensing model yields a root mean square separation between the model-predicted and observed positions of the multiple images of 0″​​.29. When analysing the impact of systematic uncertainties, stemming from modelling assumptions and used observables, we find that the resulting projected total mass profile, the relative weight of the sub-halo mass component, and the critical lines are consistent, within the statistical uncertainties. The predicted magnification and time-delay values are, instead, more sensitive to the local details of the lens total mass distribution, and vary significantly among lens models that are similarly good at reproducing the observed multiple image positions. In particular, the model-predicted time delays can differ by a factor of up to ∼1.5. Conclusions. SDSS J2222+2745 is a promising lens cluster for cosmological applications. However, due to its complex morphology, the relatively low number of secure ‘point-like’ multiple images, and current model degeneracies, it becomes clear that additional information (from the observed surface brightness distribution of lensed sources and the measured time delays) needs to be included in the modelling for accurate and precise cosmological measurements. The full VLT/MUSE secure spectroscopic catalogue presented in this work is made publicly available.

Cospatial 21 cm and metal-line absorbers in the epoch of reionization – I. Incidence and observability

ABSTRACTf Overdense, metal-rich regions, shielded from stellar radiation might remain neutral throughout reionization and produce metal as well as 21 cm absorption lines. Simultaneous absorption from metals and 21 cm can complement each other as probes of underlying gas properties. We use Aurora, a suite of high-resolution radiation-hydrodynamical simulations of galaxy formation, to investigate the occurrence of such ‘aligned’ absorbers. We calculate absorption spectra for 21 cm, O I, C II, Si II, and Fe II. We find velocity windows with absorption from at least one metal and 21 cm, and classify the aligned absorbers into two categories: ‘aligned and cospatial absorbers’ and ‘aligned but not cospatial absorbers’. While ‘aligned and cospatial absorbers’ originate from overdense structures and can be used to trace gas properties, ‘aligned but not cospatial absorbers’ are due to peculiar velocity effects. The incidence of absorbers is redshift dependent, as it is dictated by the interplay between reionization and metal enrichment, and shows a peak at z ≈ 8 for the aligned and cospatial absorbers. While aligned but not cospatial absorbers disappear towards the end of reionization because of the lack of an ambient 21 cm forest, aligned and cospatial absorbers are associated with overdense pockets of neutral gas that can be found at lower redshift. We produce mock observations for a set of sightlines for the next generation of telescopes like the ELT and SKA1-LOW, finding that given a sufficiently bright background quasar, these telescopes will be able to detect both types of absorbers throughout reionization.

Observations of a Magellanic Corona

The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are the closest massive satellite galaxies of the Milky Way. They are probably on their first passage on an infalling orbit towards our Galaxy1 and trace the continuing dynamics of the Local Group2. Recent measurements of a high mass for the LMC (Mhalo ≈ 1011.1–11.4 M⊙)3–6 imply that the LMC should host a Magellanic Corona: a collisionally ionized, warm-hot gaseous halo at the virial temperature (105.3–5.5 K) initially extending out to the virial radius (100–130 kiloparsecs (kpc)). Such a corona would have shaped the formation of the Magellanic Stream7, a tidal gas structure extending over 200° across the sky2,8,9 that is bringing in metal-poor gas to the Milky Way10. Here we show evidence for this Magellanic Corona with a potential direct detection in highly ionized oxygen (O+5) and indirectly by means of triply ionized carbon and silicon, seen in ultraviolet (UV) absorption towards background quasars. We find that the Magellanic Corona is part of a pervasive multiphase Magellanic circumgalactic medium (CGM) seen in many ionization states with a declining projected radial profile out to at least 35 kpc from the LMC and a total ionized CGM mass of log10(MH II,CGM/M⊙) ≈ 9.1 ± 0.2. The evidence for the Magellanic Corona is a crucial step forward in characterizing the Magellanic group and its nested evolution with the Local Group.

A Giant Arc on the Sky

ABSTRACT We present the serendipitous discovery of a ‘Giant Arc on the Sky’ at $z$ ∼ 0.8. The Giant Arc (GA) spans ∼1 Gpc (proper size, present epoch) and appears to be almost symmetrical on the sky. It was discovered via intervening Mg ii absorbers in the spectra of background quasars, using the catalogues of Zhu &amp; Ménard. The use of Mg ii absorbers represents a new approach to the investigation of large-scale structures (LSSs) at redshifts $0.45 \la z \la 2.25$. We present the observational properties of the GA, and we assess it statistically using methods based on (i) single-linkage hierarchical clustering (∼4.5σ); (ii) the Cuzick-Edwards test (∼3.0σ); and (iii) power-spectrum analysis (∼4.8σ). Each of these methods has distinctive attributes and powers, and we advise considering the evidence from the ensemble. We discuss our approaches to mitigating any post hoc aspects of analysing significance after discovery. The overdensity of the GA is δρ/ρ ∼ 1.3 ± 0.3. The GA is the newest and one of the largest of a steadily accumulating set of very large LSSs that may (cautiously) challenge the Cosmological Principle, upon which the ‘standard model’ of cosmology is founded. Conceivably, the GA is the precursor of a structure like the Sloan Great Wall (but the GA is about twice the size), seen when the Universe was about half its present age.

Discovery of a Cool, Metal-rich Gas Reservoir in the Outskirts of z ≈ 0.5 Clusters

We built the first-ever statistically significant sample of ≈80,000 background quasar–foreground cluster pairs to study the cool, metal-rich gas in the outskirts (>R 500) of z ≈ 0.5 clusters with a median mass of ≈1014.2 M ⊙. The sample was obtained by crossmatching the Sloan Digital Sky Survey (SDSS) cluster catalog of Wen & Han and the SDSS quasar catalog of Lyke et al. The median impact parameter (ρ cl) of the clusters from the quasar sightlines is 2.4 Mpc (median ρ cl/R 500 = 3.6). A strong Mg II, along with marginal Fe II, absorption is detected in the mean and median stacked spectra of the quasars with a total Mg II rest-frame equivalent width () of 0.034 ± 0.005 Å (7σ) and 0.010 ± 0.003 Å (3σ), respectively. The shows a declining trend with increasing ρ cl and ρ cl/R 500, but does not show any significant trend with mass (M 500) or redshift (z cl) within the small M 500 and z cl ranges probed here. The Mg II absorption signal and the trends persist even if we exclude the quasar–cluster pairs where the background quasars may be probing the circumgalactic medium of bright galaxies with impact parameters <300 kpc. The Mg II (and Fe II) absorption reported here is the first detection of its kind. It indicates the presence of a cool, metal-rich gas reservoir surrounding galaxy clusters out to several R 500. We suggest that the metal-rich gas in the cluster outskirts arise from stripped materials and that gas stripping may be important out to large clustocentric distances (>3R 500).

Exploratory Study of the Transverse Proximity Effect around BAL Quasars

We aim to determine why there exists anisotropic H i absorption around quasars; i.e., the environments around quasars are highly biased toward producing strong H i absorption in the transverse direction while there exists a significant deficit of H i absorption within a few megaparsecs of quasars along the line of sight. The most plausible explanation for this opposite trend is that the transverse direction is shielded from quasar UV radiation by dust torus. However, a critical weakness of this explanation is that we do not have any information on the inclination angle of our sightline relative to the torus. In this study, we examine environments of quasars with broad-absorption troughs in their spectra (i.e., BAL quasars) because it is widely believed that BAL troughs are observed if the central continuum is viewed from the side through their powerful outflows near the dust torus. With closely separated 12 projected quasar pairs at different redshifts with a separation angle of θ < 120″, we examine H i absorption at foreground BAL quasars in the spectra of background quasars. We confirm that there exists optically thick gas around two of 12 BAL quasars, and that the mean H i absorption strength is EWrest ∼ 1 Å. This is consistent with past results of studies of non-BAL quasars, although not statistically significant. The origins of optically thick H i absorbers around BAL and non-BAL quasars could be different since their column densities are different by ∼3 orders of magnitude. A larger sample is required to narrow down possible scenarios explaining the anisotropic H i absorption around quasars.