Distant galaxy clusters provide important tests of the growth of large-scale structure in addition to highlighting the process of galaxy evolution in a consistently defined environment at large ...look-back time. We present a sample of 22 distant (z > 0.8) galaxy clusters and cluster candidates selected from the 9 deg2 footprint of the overlapping X-ray Multi Mirror (XMM) Large Scale Structure (LSS), CFHTLS-Wide and Spitzer-SWIRE surveys. Clusters are selected as extended X-ray sources with an accompanying overdensity of galaxies displaying optical to mid-infrared photometry consistent with z > 0.8. Nine clusters have confirmed spectroscopic redshifts in the interval 0.8 < z < 1.2, four of which are presented here for the first time. A further 11 candidate clusters have between 8 and 10 band photometric redshifts in the interval 0.8 < z < 2.2, while the remaining two candidates do not have information in sufficient wavebands to generate a reliable photometric redshift. All of the candidate clusters reported in this paper are presented for the first time. Those confirmed and candidate clusters with available near-infrared photometry display evidence for a red sequence galaxy population, determined either individually or via a stacking analysis, whose colour is consistent with the expectation of an old, coeval stellar population observed at the cluster redshift. We further note that the sample displays a large range of red fraction values indicating that the clusters may be at different stages of red sequence assembly. We compare the observed X-ray emission to the flux expected from a suite of model clusters and find that the sample displays an effective mass limit M
200 ∼ 1 × 1014 M with all clusters displaying masses consistent with M
200 < 5 × 1014 M. This XMM distant cluster study represents a complete sample of X-ray-selected z > 0.8 clusters. We discuss the importance of this sample to investigate the abundance of high-redshift clusters and to provide a relatively unbiased view of distant cluster galaxy populations.
ABSTRACT
The binary fraction of a stellar population can have pronounced effects on its properties, and, in particular, the number counts of different massive star types, and the relative subtype ...rates of the supernovae (SNe) that end their lives. Here we use binary population synthesis models with a binary fraction that varies with initial mass to test the effects on resolved stellar pops and SNe, and ask whether these can constrain the poorly-known binary fraction in different mass and metallicity regimes. We show that Wolf–Rayet (WR) star subtype ratios are valuable binary diagnostics, but require large samples to distinguish by models. Uncertainties in which stellar models would be spectroscopically classified as WR stars are explored. The ratio of thermonuclear, stripped-envelope, and other core-collapse SNe may prove a more accessible test and upcoming surveys will be sufficient to constrain both the high- and low-mass binary fraction in the z < 1 galaxy population.
Gamma-ray bursts (GRBs) are powerful probes of early stars and galaxies, during and potentially even before the era of reionization. Although the number of GRBs identified at z 6 remains small, they ...provide a unique window on typical star-forming galaxies at that time, and thus are complementary to deep field observations. We report the identification of the optical drop-out afterglow of Swift GRB 120923A in near-infrared Gemini-North imaging, and derive a redshift of from Very Large Telescope/X-shooter spectroscopy. At this redshift the peak 15-150 keV luminosity of the burst was 3.2 × 1052 erg s−1, and in this sense it was a rather typical long-duration GRB in terms of rest frame luminosity. This burst was close to the Swift/Burst Alert Telescope detection threshold, and the X-ray and near-infrared afterglow were also faint. We present ground- and space-based follow-up observations spanning from X-ray to radio, and find that a standard external shock model with a constant-density circumburst environment of density n 4 × 10−2 cm−3 gives a good fit to the data. The near-infrared light curve exhibits a sharp break at t 3.4 days in the observer frame which, if interpreted as being due to a jet, corresponds to an opening angle of . The beaming-corrected γ-ray energy is then erg, while the beaming-corrected kinetic energy is lower, erg, suggesting that GRB 120923A was a comparatively low kinetic energy event. We discuss the implications of this event for our understanding of the high-redshift population of GRBs and their identification.
ABSTRACT We present late time multi-wavelength observations of Swift J1644+57, suggested to be a relativistic tidal disruption flare (TDF). Our observations extend to >4 years from discovery and show ...that 1.4 years after outburst the relativistic jet switched off on a timescale less than tens of days, corresponding to a power-law decay faster than t−70. Beyond this point weak X-rays continue to be detected at an approximately constant luminosity of LX ∼ 5 × 1042 erg s−1 and are marginally inconsistent with a continuing decay of t−5/3, similar to that seen prior to the switch-off. Host photometry enables us to infer a black hole mass of MBH = 3 × 106 M , consistent with the late time X-ray luminosity arising from sub-Eddington accretion onto the black hole in the form of either an unusually optically faint active galactic nucleus or a slowly varying phase of the transient. Optical/IR observations show a clear bump in the light curve at timescales of 30-50 days, with a peak magnitude (corrected for host galaxy extinction) of MR ∼ −22 to −23. The luminosity of the bump is significantly higher than seen in other, non-relativistic TDFs and does not match any re-brightening seen at X-ray or radio wavelengths. Its luminosity, light curve shape, and spectrum are broadly similar to those seen in superluminous supervnovae, although subject to large uncertainties in the correction of the significant host extinction. We discuss these observations in the context of both TDF and massive star origins for Swift J1644+5734 and other candidate relativistic tidal flares.
ABSTRACT
The Gravitational-wave Optical Transient Observer (GOTO) is an array of wide-field optical telescopes, designed to exploit new discoveries from the next generation of gravitational wave ...detectors (LIGO, Virgo, and KAGRA), study rapidly evolving transients, and exploit multimessenger opportunities arising from neutrino and very high energy gamma-ray triggers. In addition to a rapid response mode, the array will also perform a sensitive, all-sky transient survey with few day cadence. The facility features a novel, modular design with multiple 40-cm wide-field reflectors on a single mount. In 2017 June, the GOTO collaboration deployed the initial project prototype, with 4 telescope units, at the Roque de los Muchachos Observatory (ORM), La Palma, Canary Islands. Here, we describe the deployment, commissioning, and performance of the prototype hardware, and discuss the impact of these findings on the final GOTO design. We also offer an initial assessment of the science prospects for the full GOTO facility that employs 32 telescope units across two sites.
We present a comprehensive analysis of the presence of very massive stars (VMS > 100 M ⊙ ) in the integrated spectra of 13 UV-bright star-forming galaxies at 2.2 ≲ z ≲ 3.6 taken with the Gran ...Telescopio Canarias (GTC). These galaxies have very high UV absolute magnitudes ( M UV ≃ −24), intense star formation (star formation rate ≃100 − 1000 M ⊙ yr −1 ), and metallicities in the range of 12 + log(O/H) ≃ 8.10 − 8.50 inferred from strong rest-optical lines. The GTC rest-UV spectra reveal spectral features indicative of very young stellar populations with VMS, such as strong P-Cygni line profiles in the wind lines N V λ 1240 and C IV λ 1550 along with intense and broad He II λ 1640 emission with equivalent width (EW 0 ) ≃ 1.40 − 4.60 Å, and full width half maximum (FWHM) ≃1150 − 3170 km s −1 . A Comparison with known VMS-dominated sources and typical galaxies without VMS reveals that some UV-bright galaxies closely resemble VMS-dominated clusters (e.g., R136 cluster). The presence of VMS is further supported by a quantitative comparison of the observed strength of the He II emission with population synthesis models with and without VMS, where models with VMS are clearly preferred. Employing an empirical threshold for EW 0 (He II ) ≥ 3.0 Å, along with the detection of other VMS-related spectral profiles (N IV λ 1486, 1719), we classify nine out of 13 UV-bright galaxies as VMS-dominated sources. This high incidence of VMS-dominated sources in the UV-bright galaxy population (≈70%) contrasts significantly with the negligible presence of VMS in typical L UV ∗ LBGs at similar redshifts (< 1%). Our results thus indicate that VMS are common in UV-bright galaxies, suggesting a different initial mass function (IMF) with upper mass limits between 175 M ⊙ and 475 M ⊙ .
ABSTRACT
Accurately determining the age of H ii regions and the stars they host is as important as it is challenging. Historically, the most popular method has been isochrone fitting to ...Hertzsprung–Russell diagrams or colour–magnitude diagrams. Here, we introduce a different method for age determination using BPASS (Binary Population and Spectral Synthesis) and hoki . We infer the most likely ages of the regions D118 and D119 NGC 300 to be log(age/yr) = $6.86^{+0.05}_{-0.06}$ and we also deduce stellar mass and number counts by comparison with the BPASS models. We compare how our binary and single-star models perform and find that the latter are unable to predict 20 per cent (±10 per cent) of our sample. We also discuss how results obtained from isochrone fitting would differ. We conclude that ages could be underestimated by ∼0.2 dex and that the limitations of the isochrone method is not solely due to the lack of binary stars. We propose that the method presented here is more reliable and more widely applicable since it can be used on smaller samples. Alongside this study, we release new hoki features to allow easy implementation of this method.
High-redshift quasars can be used to trace the early growth of massive galaxies and may be triggered by galaxy–galaxy interactions. We present Multi-Object Spectroscopic Explorer (MUSE) science ...verification data on one such interacting system consisting of the well-studied z = 3.2 PKS1614+051 quasar, its AGN companion galaxy and bridge of material radiating in Ly α between the quasar and its companion. We find a total of four companion galaxies (at least two galaxies are new discoveries), three of which reside within the likely virial radius of the quasar host, suggesting that the system will evolve into a massive elliptical galaxy by the present day. The MUSE data are of sufficient quality to split the extended Ly α emission line into narrow velocity channels. In these the gas can be seen extending towards each of the three neighbouring galaxies suggesting that the emission-line gas originates in a gravitational interaction between the galaxies and the quasar host. The photoionization source of this gas is less clear but is probably dominated by the two AGN. The quasar's Ly α emission spectrum is double peaked, likely due to absorbing neutral material at the quasar's systemic redshift with a low column density as no damping wings are present. The spectral profiles of the AGN and bridge's Ly α emission are also consistent with absorption at the same redshift indicating that this neutral material may extend over >50 kpc. The fact that the neutral material is seen in the line of sight to the quasar and transverse to it, and the fact that we see the quasar and it also illuminates the emission-line bridge, suggests that the quasar radiates isotropically and any obscuring torus is small. These results demonstrate the power of MUSE for investigating the dynamics of interacting systems at high redshift.
ABSTRACT
The temporal and spectral evolution of gamma-ray burst (GRB) afterglows can be used to infer the density and density profile of the medium through which the shock is propagating. In ...long-duration (core-collapse) GRBs, the circumstellar medium (CSM) is expected to resemble a wind-blown bubble, with a termination shock, separating the stellar wind and the interstellar medium (ISM). A long standing problem is that flat density profiles, indicative of the ISM, are often found at lower radii than expected for a massive star progenitor. Furthermore, the presence of both wind-like environments at high radii and ISM-like environments at low radii remains a mystery. In this paper, we perform a ‘CSM population synthesis’ with long GRB progenitor stellar evolution models. Analytic results for the evolution of wind blown bubbles are adjusted through comparison with a grid of 2D hydrodynamical simulations. Predictions for the emission radii, ratio of ISM to wind-like environments, wind, and ISM densities are compared with the largest sample of afterglow derived parameters yet compiled, which we make available for the community. We find that high ISM densities of n ∼ 1000 cm−3 best reproduce observations. If long GRBs instead occur in typical ISM densities of n ∼ 1 cm−3, then the discrepancy between theory and observations is shown to persist at a population level. We discuss possible explanations for the origin of variety in long GRB afterglows, and for the overall trend of CSM modelling to over-predict the termination shock radius.
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