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Andra Stroe

Publications and source records attributed to Andra Stroe.

At least 19 recordsLinked to original sources
arXiv2025

SDSS-C4 3028: The Nearest Blue Galaxy Cluster Devoid of an Intracluster Medium

SDSS-C4 3028 is a galaxy cluster at $z=0.061$, notable for its unusually high fraction of star-forming galaxies with 19 star-forming and 11 quiescent spectroscopically-confirmed member galaxies. From Subaru/HSC imaging, we derived a weak lensing mass of $M_{200} = (1.3 \pm 0.9) \times 10^{14} \rm M_\odot$, indicating a low-mass cluster. This is in excellent agreement with its dynamical mass of $M_{200} = (1.0\pm0.4)\times10^{14} \rm M_\odot$, derived from SDSS spectroscopic data. XMM-Newton observations reveal that its X-ray emission is uniform and fully consistent with the astrophysical X-ray background, with no evidence for an intracluster medium (ICM). The 3$\sigma$ upper limit of $L_{\rm X}(0.1-2.4\rm keV)=7.7\times10^{42}$ erg s$^{-1}$ on the cluster's X-ray luminosity falls below the value expected from the $L_{\rm X}-M_{\rm halo}$ scaling relation of nearby galaxy clusters. We derived star-formation histories for its member galaxies using the photometric spectral energy distribution from SDSS, 2MASS, and WISE data. Most of its quiescent galaxies reside within the central 300 kpc, while star-forming ones dominate the outer region (300 kpc - 1 Mpc). The core region has formed the bulk of its stellar mass approximately 1.5 Gyr earlier than the outskirts. We infer a long quenching time of $>3$ Gyr for its quiescent galaxies, consistent with slow quenching mechanisms such as galaxy-galaxy interaction or strangulation. These findings suggest that SDSS-C4 3028 may have undergone an "inside-out" formation and quenching process. Its ICM may have been expelled by intense AGN feedback after core formation but before full cluster assembly. The high fraction ($\sim$0.63) of star-forming members likely results from the absence of ram pressure stripping in this blue cluster, supporting the important role of ram pressure stripping in quenching galaxies in clusters.

astro-ph.GA↗
arXiv2025

PSZ2 G181.06+48.47 III: weak-lensing analysis and merging scenario reconstruction of a low-mass cluster with exceptionally-distant radio relics

The galaxy cluster PSZ2 G181.06+48.47 ($z=0.234$) is a post-merging system that exhibits symmetric double radio relics separated by ~2.7 Mpc. We present the first weak-lensing analysis of PSZ2 G181.06+48.47 and propose possible merging scenarios using numerical simulations. Our analysis with Subaru Hyper Suprime-Cam imaging identifies a binary dark matter structure consisting of northern and southern components, separated by ~500 kpc. Assuming Navarro-Frenk-White (NFW) halos, the masses for the northern and southern subclusters are $M_{200c}^{N} = 0.88_{-0.30}^{+0.35} \times 10^{14} M_{\odot}$ and $M_{200c}^{S} = 2.71_{-0.48}^{+0.51} \times 10^{14} M_{\odot}$, respectively. By superposing the two NFW halos, we determine the total mass of the cluster to be $M_{200c} = 4.22_{-1.00}^{+1.10} \times 10^{14} M_{\odot}$ ($M_{500c} = 2.90_{-0.69}^{+0.75} \times 10^{14} M_{\odot}$). Our mass estimate suggests that the two relics are located around the cluster $R_{200c}$, where the density of the intracluster medium is very low. Our idealized simulations find that an off-axis collision of a 3:1 major merger can simultaneously reproduce the observed relic and dark matter halo separations. From these findings, we suggest that the system is observed ~0.9 Gyr after the first pericenter passage and is returning from the first apocenter.

astro-ph.CO↗
arXiv2025

PSZ2 G181.06+48.47 II: radio analysis of a low-mass cluster with exceptionally-distant radio relics

We report upgraded Giant Metrewave Radio Telescope and Karl J. Jansky Very Large Array radio observations of a low-mass merging galaxy cluster PSZ2 G181.06+48.47. This exceptional galaxy cluster hosts two megaparsec-scale diffuse sources, symmetrically located with respect to the cluster center and separated by about 2.6 Mpc in projection. We detect these low surface brightness sources in our new high-frequency observations (0.3-2 GHz) and classify them as radio relics associated with merger-driven shock fronts. The southwest relic exhibits an inverted morphology and shows evidence of spectral steepening in the post-shock region, potentially tracing a high Mach number shock ($\sim 4$) under the framework of diffusive shock acceleration. The northeast relic is found to be highly polarized with a 22% average polarization fraction at 1.5 GHz and aligned magnetic field vectors. Its spectral and polarization properties, along with the presence of a nearby tailed galaxy, support re-acceleration scenarios. The merger axis defined by the two relics is tilted by $\sim 45$ degree with respect to the plane of the sky, which implies an unprecedented physical separation of $\sim 3.5$ Mpc. We also detect a possible faint radio halo, suggesting weak turbulence in the central cluster region. We conclude that the faint double relics can be best explained by two outward moving shock waves in which particles are (re-)accelerated and that the cluster is in an evolved merger state. PSZ2 G181.06+48.47 presents a unique opportunity to investigate particle acceleration in low mass systems characterized by large relic separations.

astro-ph.CO↗
arXiv2025

PSZ2 G181.06+48.47 I: X-ray exploration of a low-mass cluster with exceptionally-distant radio relics

Relics are diffuse, highly-polarized radio sources that trace merger-driven shocks at the periphery of merging galaxy clusters. The LOFAR survey recently discovered a rare example of double relics in the low-mass cluster PSZ2 G181.06+48.47. Through a detailed exploration of new Chandra and XMM-Newton observations, we reveal that PSZ2 G181.06+48.47 has a lower mass ($M_{500,X}=2.32^{+0.29}_{-0.25}\times10^{14}$ M$_{\odot}$) than previously thought. Despite its cool global temperature of $kT_{500}=3.62^{+0.15}_{-0.07}$ keV, PSZ2 G181.06+48.47 is one of the most disturbed clusters in the Planck sample, with a complex morphological and thermodynamic structure. We discover a set of three discontinuities within <500 kpc of the cluster center, and, from a surface brightness analysis, place $5\sigma$ upper limits of $M_{NE}<1.43$ and $M_{SW}<1.57$ for any shock associated with the relic locations. We also revise established scaling relations for double radio-relics by adding 12 new systems not included in previous work. The PSZ2 G181.06+48.47 relics have the widest separation (scaled for $r_{500}$) of all known double-relic systems. The exceptional distance from the cluster center ($>r_{200}$), indicates the relics may be associated with shocks in the ``run-away" phase. We propose that this late-stage, post-apocenter merger is captured as the two subclusters with a mass ratio of 1.2-1.4 fall back into each other. The outer relic shocks were likely produced at the first core passage, while the inner discontinuities are associated with the second infall.

astro-ph.HE↗
arXiv2023

Weak-Lensing Analysis of the Complex Cluster Merger Abell 746 with Subaru/Hyper Suprime-Cam

The galaxy cluster Abell 746 (A746; $z$=0.214), featuring a double radio relic system, two isolated radio relics, a possible radio halo, disturbed V-shaped X-ray emission, and intricate galaxy distributions, is a unique and complex merging system. We present a weak-lensing analysis of A746 based on wide-field imaging data from Subaru/Hyper Suprime-Cam observations. The mass distribution is characterized by a main peak which coincides with the center of the X-ray emission. At this main peak, we detect two extensions toward the north and west, tracing the cluster galaxy and X-ray distributions. Despite the ongoing merger, our estimate of the A746 global mass $M_{500}=4.4\pm1.0\times10^{14}~M_{\odot}$ is consistent with the previous results from SZ and X-ray observations. We conclude that reconciling the distributions of mass, galaxies, and intracluster medium with the double radio relic system and other radio features remains challenging.

astro-ph.GA↗
arXiv2023

AMUSE-antlia I: Nuclear X-ray properties of early-type galaxies in a dynamically young galaxy cluster

To understand the formation and growth of supermassive black holes (SMBHs) and their co-evolution with host galaxies, it is essential to know the impact of environment on the activity of active galactic nuclei (AGN). We present new Chandra X-ray observations of nuclear emission from member galaxies in the Antlia cluster, the nearest non-cool core and the nearest merging galaxy cluster, residing at D = 35.2 Mpc. Its inner region, centered on two dominant galaxies NGC 3268 and NGC 3258, has been mapped with three deep Chandra ACIS-I pointings. Nuclear X-ray sources are detected in 7/84 (8.3%) early-type galaxies (ETG) and 2/8 (25%) late-type galaxies with a median detection limit of 8x10^38 erg/s. All nuclear X-ray sources but one have a corresponding radio continuum source detected by MeerKAT at the L-band. Nuclear X-ray sources detected in early-type galaxies are considered as the genuine X-ray counterpart of low-luminosity AGN. When restricted to a detection limit of logLx(erg/s) > 38.9 and a stellar mass of 10 < log Ms(Msun) <11.6, 6/11 (54.5%) ETG are found to contain an X-ray AGN in Antlia, exceeding the AGN occupation fraction of 7/39 (18.0%) and 2/12 (16.7%) in the more relaxed, cool core clusters, Virgo and Fornax, respectively, and rivaling that of the AMUSE-Field ETG of 27/49 (55.1%). Furthermore, more than half of the X-ray AGN in Antlia are hosted by its younger subcluster, centered on NGC 3258. We believe that this is because SMBH activity is enhanced in a dynamically young cluster compared to relatively relaxed clusters.

astro-ph.GA↗
arXiv2022

The Host Galaxies of Hybrid Morphology Radio Sources

Based on their differing radio morphologies, powerful radio galaxies can be separated into the Fanaroff-Riley I (FR-I) and II (FR-II) classes. Hybrid morphology radio sources (HyMoRS) contain morphologies consistent with each type of jet on either side: a powerful, highly relativistic FR-II-like jet terminating in a hotspot on one side and an FRI-like plume on the other. HyMoRS present a unique opportunity to study the conditions which give rise to the dichotomy. Using host galaxy properties, we conduct the first multiwavelength investigation into whether orientation can explain HyMoRS morphology. Through optical spectroscopy and mid-infrared photometry, we analyze the emission characteristics, and evaluate the broad characteristics of five HyMoRS host galaxies at intermediate redshifts (0.4 < z < 1.5). The HyMoRS host galaxies in our sample have properties consistent with typical host galaxies of FR-II sources, suggesting that the observed hybrid morphologies may be caused by a dense, cluster-like environment bending FR-II jets combined with a favorable orientation which can make one side appear similar to an FR-I jet. Our results thus support the hypothesis that HyMoRS are mainly caused by environment and orientation.

astro-ph.GA↗
arXiv2021

ENISALA: II. Distinct Star Formation and Active Galactic Nucleus Activity in Merging and Relaxed Galaxy Clusters

The growth of galaxy clusters is energetic and may trigger and/or quench star formation and black hole activity. The ENISALA project is a collection of multiwavelength observations aimed at understanding how large-scale structure drives galaxy and black hole evolution. Here, we introduce optical spectroscopy of over 800 H$α$ emission-line galaxies, selected in 14 z~0.15-0.31 galaxy clusters, spanning a range of masses and dynamical states. We investigate the nature of the emission lines in relation to the host galaxy properties, its location within the cluster, and the properties of the parent cluster. We uncover remarkable differences between mergers and relaxed clusters. The majority of H$α$ emission-line galaxies in merging cluster fields are located within 3 Mpc of their center. A large fraction of these line-emitters in merging clusters are powered by star formation irrespective of cluster-centric radius, while the rest are powered by active galactic nuclei. Star-forming galaxies are rare within 3 Mpc of relaxed clusters and active galactic nuclei are most abundant at their outskirts (~1.5-3 Mpc). We discover a population of star-forming galaxies with large equivalent widths and blue UV-optical colors, found exclusively in the merging clusters in our sample. The widespread emission-line activity in merging clusters is likely supported by triggered activity in recently-accreted, gas-rich galaxies. By contrast, our observations for relaxed clusters match established models, in which black hole activity is enhanced at the virial radius and star-formation is quenched within the infall region. We conclude that emission-line galaxies experience distinct evolutionary paths in merging and relaxed clusters.

astro-ph.GA↗
arXiv2021

GLEAM: Galaxy Line Emission & Absorption Modeling

We present GLEAM (Galaxy Line Emission & Absorption Modeling), a Python tool for fitting Gaussian models to emission and absorption lines in large samples of 1D extragalactic spectra. GLEAM is tailored to work well in batch mode without much human interaction. With GLEAM, users can uniformly process a variety of spectra, including galaxies and active galactic nuclei, in a wide range of instrument setups and signal-to-noise regimes. GLEAM also takes advantage of multiprocessing capabilities to process spectra in parallel. With the goal of enabling reproducible workflows for its users, GLEAM employs a small number of input files, including a central, user-friendly configuration in which fitting constraints can be defined for groups of spectra and overrides can be specified for edge cases. For each spectrum, GLEAM produces a table containing measurements and error bars for the detected spectral lines and continuum, and upper limits for non-detections. For visual inspection and publishing, GLEAM can also produce plots of the data with fitted lines overlaid. In the present paper, we describe GLEAM's main features, the necessary inputs, expected outputs, and some example applications, including thorough tests on a large sample of optical/infra-red multi-object spectroscopic observations and integral field spectroscopic data. gleam is developed as an open-source project hosted at https://github.com/multiwavelength/gleam and welcomes community contributions.

astro-ph.IM↗
arXiv2020

The First Integral Field Unit Spectroscopic View of Shocked Cluster Galaxies

Galaxy clusters grow by merging with other clusters, giving rise to Mpc-wide shock waves that travel at 1000-2500 km/s through the intra-cluster medium. To study the effects of merger shocks on the properties of cluster galaxies, we present the first spatially resolved spectroscopic view of 5 H$α$ emitting galaxies located in the wake of shock fronts in the low redshift (z~0.2), massive (~2$\times10^{15}$ M$_\odot$), post-core passage merging cluster, CIZA J2242.8+5301 (nicknamed the `Sausage'). Our Gemini/GMOS-N integral field unit (IFU) observations, designed to capture H$α$ and [NII] emission, reveal the nebular gas distribution, kinematics and metallicities in the galaxies over >16 kpc scales. While the galaxies show evidence for rotational support, the flux and velocity maps have complex features like tails and gas outflows aligned with the merger axis of the cluster. With gradients incompatible with inside-out disk growth, the metallicity maps are consistent with sustained star formation (SF) throughout and outside of the galactic disks. In combination with previous results, these pilot observations provide further evidence of a likely connection between cluster mergers and SF triggering in cluster galaxies, a potentially fundamental discovery revealing the interaction of galaxies with their environment.

astro-ph.GA↗
arXiv2020

The X-ray and radio activity of typical and luminous Lya emitters from z~2 to z~6: evidence for a diverse, evolving population

Despite recent progress in understanding Lyman-alpha (Lya) emitters (LAEs), relatively little is known regarding their typical black hole activity across cosmic time. Here, we study the X-ray and radio properties of ~4000 LAEs at 2.2 3) and find an average luminosity of 10^{44.31+-0.01} erg/s and an average black hole accretion rate (BHAR) of 0.72+-0.01 Msun/yr, consistent with moderate to high accreting AGN. We detect 120 sources in deep radio data (radio AGN fraction of 3.2%+-0.3%). The global AGN fraction (8.6%+-0.4%) rises with Lya luminosity and declines with increasing redshift. For X-ray detected LAEs, Lya luminosities correlate with the BHARs, suggesting that Lya luminosity becomes an accretion rate indicator. Most LAEs (93.1+-0.6%) at 2<z<6 have no detectable X-ray emission (BHARs<0.017 Msun/yr). The median star formation rate (SFR) of star-forming LAEs from Lya and radio luminosities is 7.6^{+6.6}_{-2.8} Msun/yr. The black hole to galaxy growth ratio (BHAR/SFR) for LAEs is <0.0022, consistent with typical star forming galaxies and the local BHAR/SFR relation. We conclude that LAEs at 2<z<6 include two different populations: an AGN population, where Lya luminosity traces BHAR, making them bright in Lya, and another with low SFRs which remain undetected in even the deepest X-ray stacks but is detected in the radio stacks.

astro-ph.GA↗
arXiv2019

VIS3COS: III. environmental effects on the star formation histories of galaxies at z~0.8 seen in [OII], H$δ$, and Dn4000

[ABRIDGED] We present spectroscopic observations of 466 galaxies in and around a superstructure at $z\sim0.84$ targeted by the VIMOS Spectroscopic Survey of a Supercluster in the COSMOS field (VIS$^{3}$COS). We use [OII]$λ$3727, H$δ$, and $D_n4000$ to trace the recent, mid-, and long-term star formation histories and investigate how stellar mass and the local environment impacts those. By studying trends both in individual and composite galaxy spectra, we find that both stellar mass and environment play a role in the observed galactic properties. We find that the median [OII] equivalent width (|EW$_\mathrm{[OII]}|$) decreases from $27\pm2$ Å to $2.0_{-0.4}^{+0.5}$ Å and $D_n4000$ increases from $1.09\pm0.01$ to $1.56\pm0.03$ with increasing stellar mass (from $\sim10^{9.25}$ to $\sim10^{11.35}\ \mathrm{M_\odot}$). Concerning the dependence on the environment, we find that at fixed stellar mass |EW$_\mathrm{[OII]}|$ is tentatively lower in higher density environments. Regarding $D_n4000$, we find that the increase with stellar mass is sharper in denser environments, hinting that such environments may accelerate galaxy evolution. Moreover, we find larger $D_n4000$ values in denser environments at fixed stellar mass, suggesting that galaxies are on average older and/or more metal-rich in such dense environments. This set of tracers depicts a scenario where the most massive galaxies have, on average, the lowest sSFRs and the oldest stellar populations (age $\gtrsim1$ Gyr, showing a mass-downsizing effect). We also hypothesize that the observed increase in star formation (higher EW$_\mathrm{[OII]|}$, higher sSFR) at intermediate densities may lead to quenching since we find the quenched fraction to increase sharply from the filament to cluster-like regions at similar stellar masses.

astro-ph.GA↗
arXiv2019

Unveiling the cause of hybrid morphology radio sources (HyMoRS)

Hybrid morphology radio sources (HyMoRS) are a rare group of radio galaxies in which differing Fanaroff & Riley morphologies (FR I/II) are observed for each of the two lobes. While they potentially provide insights into the formation of lobe structure, particle acceleration, and the FR dichotomy, previous work on HyMoRS has mainly been limited to low-resolution studies, searches for new candidates, and milliarcsecond-scale VLBI observations of the core region. In this paper, we use new multi-array configuration Very Large Array (VLA) observations between 1 and 8 GHz to determine the morphology of HyMoRS on arcsecond scales and perform the first well-resolved spectral study of these unusual sources. We find that while the apparent FR I lobe is centre-brightened, this is the result of a compact acceleration region resembling a hotspot with a spectrum more consistent with an FR II ("strong-flavour") jet. We find that the spectra of the apparent FR I lobes are not similar to their classical counterparts and are likely the result of line-of-sight mixing of plasma across a range of spectral ages. We consider possible mechanisms that could lead to the formation of HyMoRS under such conditions, including environment asymmetry and restarted sources, concluding through the use of simple modelling that HyMoRS are the result of orientation effects on intrinsically FR II sources with lobes non-parallel to the inner jet.

astro-ph.GA↗
arXiv2019

Chandra Observations of the Spectacular A3411-12 Merger Event

We present deep Chandra observations of A3411-12, a remarkable merging cluster that hosts the most compelling evidence for electron re-acceleration at cluster shocks to date. Using the $Y_X-M$ scaling relation, we find $r_{500} \sim 1.3$ Mpc, $M_{500} = (7.1 \pm 0.7) \times 10^{14} \ M_{\rm{\odot}}$, $kT=6.5\pm 0.1$ keV, and a gas mass of $M_{\rm g,500} = (9.7 \pm 0.1) \times 10^{13} M_\odot$. The gas mass fraction within $r_{500}$ is $f_{\rm g} = 0.14 \pm 0.01$. We compute the shock strength using density jumps to conclude that the Mach number of the merging subcluster is small ($M \leq 1.15_{-0.09}^{+0.14}$). We also present pseudo-density, projected temperature, pseudo-pressure, and pseudo-entropy maps. Based on the pseudo-entropy map we conclude that the cluster is undergoing a mild merger, consistent with the small Mach number. On the other hand, radio relics extend over Mpc scale in the A3411-12 system, which strongly suggests that a population of energetic electrons already existed over extended regions of the cluster.

astro-ph.HE↗
arXiv2019

Large Molecular Gas Reservoirs in Star Forming Cluster Galaxies

We present CO(2-1) observations of 72 galaxies in the nearby, disturbed Antlia galaxy cluster with the Atacama Pathfinder Experiment (APEX) telescope. The galaxies in our sample are selected to span a wide range of stellar masses ($10^{8}M_{\odot}\lesssim M_{\star} \lesssim 10^{10}M_{\odot}$) and star formation rates ($0.0005M_{\odot}\text{yr}^{-1}<\text{SFR}<0.3M_{\odot}\text{yr}^{-1}$). Reaching a depth of $23\text{mJy}$ in $50\text{km}\text{s}^{-1}$ channels, we report a total CO detection rate of $37.5\%$ and a CO detection rate of $86\%$ for sources within 1 dex of the main sequence. We compare our sample with a similar sample of galaxies in the field, finding that, for a fixed stellar mass and SFR, galaxies in the Antlia cluster have comparable molecular gas reservoirs to field galaxies. We find that $\sim41\%$ (11/27) of our CO detections display non-Gaussian CO(2-1) emission line profiles, and a number of these sources display evidence of quenching in their optical images. We also find that the majority of our sample lie either just below, or far below the main sequence of field galaxies, further hinting at potential ongoing quenching. We conclude that the Antlia cluster represents an intermediate environment between fields and dense clusters, where the gentler intracluster medium (ICM) allows the cluster members to retain their reservoirs of molecular gas, but in which the disturbed ICM is just beginning to influence the member galaxies, resulting in high SFRs and possible ongoing quenching.

astro-ph.GA↗
arXiv2019

VIS3COS: II. Nature and nurture in galaxy structure and morphology

We study the impact of local density and stellar mass on the morphology of ~500 quiescent (Q) and SFGs from the VIS3COS survey. We perform B/D decomposition of the SBPs and find ~41% of 10^10 Msun galaxies to be best fitted with 2 components. We complement our analysis with non-parametric measurements and qualitative visual classifications. We find that galaxy morphology depends on stellar mass and environment for our sample as a whole. We only find an impact of the environment on galaxy sizes for galaxies more massive than 10^11 Msun. We find higher n and B/T in high-density regions when compared to low-density counterparts at similar stellar masses. We also find that higher stellar mass galaxies have steeper light profiles compared to the lower ones. Using visual classifications, we find a morphology-density relation at z~0.84 for galaxies more massive than 10^10 Msun, with elliptical galaxies being dominant at high-density regions and disks more common in low-density regions. However, when splitting the sample into colour-colour selected SF and Q sub-populations, there are no statistically significant differences between low- and high-density regions. We find that Q galaxies are smaller, have higher n, and higher B/T when compared to SF counterparts. We confirm these trends with non-parametric quantities, finding Q galaxies to be smoother and having most of their light over smaller areas than SFGs. Overall, we find that the dependence of morphology on stellar mass is stronger than with local density and these relations are strongly correlated with the quenching fraction. The change in average morphology corresponds to a change in the relative fractions of blue disk-like galaxies and red elliptical galaxies with stellar mass and environment. We hypothesize that the processes responsible for the quenching of SF must also affect the galaxy morphology on similar timescales.

astro-ph.GA↗
arXiv2018

VIS3COS: I. survey overview and the role of environment and stellar mass on star formation

We present the VIMOS Spectroscopic Survey of a Supercluster in the COSMOS field (VIS3COS) at z~0.84. We use VIMOS high-resolution spectra (GG475 filter) to spectroscopically select 490 galaxies in and around the super-structure and an additional 481 galaxies in the line of sight. We present the redshift distribution, the catalog to be made public and the first results on the properties of individual galaxies and stacked spectra (3500Å<λ<4200Å rest-frame). We probe a wide range of densities and environments (from low-density field to clusters and rich groups). We find a decrease in the median star formation rate from low to high density environments in all bins of stellar mass and a sharp rise of the quenched fraction (from ~10% to ~40-60%) of intermediate stellar mass galaxies $(10<\log_{10}\left(M_\star/\mathrm{M_\odot}\right)<10.75)$ from filaments to clusters. The quenched fraction for massive galaxies shows little dependence on the environment being constant at ~30-40%. We find a break in the strength of the [OII] emission, with nearly constant line equivalent widths at lower densities (~-11Å) and then a drop to ~-2.5Å towards higher densities. The break in the [OII] line strength happens at similar densities ($\log_{10}(1+δ)\sim0.0-0.5$) as the observed rise in the quenched fraction. Our results may provide further clues regarding the different environmental processes affecting galaxies with different stellar masses and highlight the advantages of a single dataset in the COSMOS field probing a wide range of stellar masses and environments. We hypothesize that quenching mechanisms are enhanced in high-density regions.

astro-ph.GA↗
arXiv2018

The nature of luminous Lyman-alpha emitters at z~2-3: maximal dust-poor starbursts and highly ionising AGN

Deep narrow-band surveys have revealed a large population of faint Lyman-alpha (Lya) emitters (LAEs) in the distant Universe, but relatively little is known about the most luminous sources ($L_{Lyα}>10^{42.7}$ erg/s; $L_{Lyα}>L^*_{Lyα}$). Here we present the spectroscopic follow-up of 21 luminous LAEs at z~2-3 found with panoramic narrow-band surveys over five independent extragalactic fields (~4x10$^6$ Mpc$^{3}$ surveyed at z~2.2 and z~3.1). We use WHT/ISIS, Keck/DEIMOS and VLT/X-SHOOTER to study these sources using high ionisation UV lines. Luminous LAEs at z~2-3 have blue UV slopes ($β=-2.0^{+0.3}_{-0.1}$), high Lya escape fractions ($50^{+20}_{-15}$%) and span five orders of magnitude in UV luminosity ($M_{UV}\approx-19$ to -24). Many (70%) show at least one high ionisation rest-frame UV line such as CIV, NV, CIII], HeII or OIII], typically blue-shifted by ~100-200 km/s relative to Lya. Their Lya profiles reveal a wide variety of shapes, including significant blue-shifted components and widths from 200 to 4000 km/s. Overall, 60+-11% appear to be AGN dominated, and at $L_{Lyα}>10^{43.3}$ erg/s and/or $M_{UV}<-21.5$ virtually all LAEs are AGN with high ionisation parameters (log U=0.6+-0.5) and with metallicities of ~0.5-1 Zsun. Those lacking signatures of AGN (40+-11%) have lower ionisation parameters ($\log U=-3.0^{+1.6}_{-0.9}$ and $\logξ_{\rm ion}=25.4\pm0.2$) and are apparently metal-poor sources likely powered by young, dust-poor "maximal" starbursts. Our results show that luminous LAEs at z~2-3 are a diverse population and that 2xL$^*_{Lyα}$ and 2xM$_{UV}^*$ mark a sharp transition in the nature of LAEs, from star formation dominated to AGN dominated.

astro-ph.GA↗