Stephen J. Warren

The giant Chinguetti meteorite that Gaston Ripert reported seeing in 1916 has never been found. A radionuclide analysis by Welten et al (2001) of the 4.5kg mesosiderite that Ripert recovered, supposedly sitting on the larger object, has convinced many that Ripert was mistaken, and interest in the giant meteorite has subsequently faded. Aspects of Ripert's account of the giant meteorite are nevertheless compelling, particularly his description of ductile metal needles in one area of the surface explored. Several visual searches for the giant meteorite, beginning in 1924, might have failed because the object was already by then covered in sand. Using DEM data we have measured dune heights and established their drift speed. This has allowed us to create a map of locations where the meteorite could lie. The 2004 PRISM-I aeromagnetics surveys, acquired by Fugro for the Mauritanian Government for Mining Sector capacity building purposes, have the necessary area coverage, spatial resolution, and sensitivity to establish if the meteorite exists. In Jan 2023 we requested the PRISM data from the Ministry of Petroleum Energy and Mines, explaining, under Confidentiality, the scientific purpose of the request. To date the data have not been made available to us.

We present the results of a new, deeper, and complete search for high-redshift 6.5 < z < 9.3 quasars over 977 deg² of the VISTA Kilo-Degree Infrared Galaxy (VIKING) survey. This exploits a new list-driven dataset providing photometry in all bands ZYJHK_s, for all sources detected by VIKING in J. We use the Bayesian model comparison (BMC) selection method of Mortlock et al., producing a ranked list of just 21 candidates. The sources ranked 1, 2, 3 and 5 are the four known z > 6.5 quasars in this field. Additional observations of the other 17 candidates, primarily DESI Legacy Survey photometry and ESO FORS2 spectroscopy, confirm that none is a quasar. This is the first complete sample from the VIKING survey, and we provide the computed selection function. We include a detailed comparison of the BMC method against two other selection methods: colour cuts and minimum-χ² SED fitting. We find that: i) BMC produces eight times fewer false positives than colour cuts, while also reaching 0.3 mag. deeper, ii) the minimum-χ² SED fitting method is extremely efficient but reaches 0.7 mag. less deep than the BMC method, and selects only one of the four known quasars. We show that BMC candidates, rejected because their photometric SEDs have high χ² values, include bright examples of galaxies with very strong [O iii] λλ4959,5007 emission in the Y band, identified in fainter surveys by Matsuoka et al. This is a potential contaminant population in Euclid searches for faint z > 7 quasars, not previously accounted for, and that requires better characterisation.

We use the large homogeneous sample of late M dwarfs, M7–M9.5, of Ahmed & Warren ( 2019) matched to Gaia DR2, to measure the relation between absolute magnitude and spectral type, and to infer the multiplicity fraction of the population, and the distribution of mass ratios in the binary systems. Binaries are identified photometrically as overluminous sources. In order to define a sample that is unbiased with respect to multiplicity we use distance limits that are a function of G − J colour to define a volume-complete sample of 2706 systems. The G − J colours are very precise, with random errors all less than 0.02. We measure absolute magnitudes M_J that are on average 0.5 mag brighter than previous determinations. We find evidence that the discrepancies arise from differences in spectral types in different samples. The measured binary fraction is |$16.5\pm 0.8{{\ \rm per\ cent}}$|⁠, of which |$98{{\ \rm per\ cent}}$| are unresolved: both values are consistent with results of previous studies. The distribution of excess flux in the binaries, compared to the singles, is used to infer the mass ratio distribution f(q)∝q^γ, where q = M_s/M_p. We infer a very steep distribution over this spectral range, with γ > 10 (⁠|$99{{\ \rm per\ cent}}$| probability). This says that unresolved ultracool M dwarf binaries reside almost exclusively in equal mass systems, and implies that the spectral types of the unresolved binaries match to with 0.5 spectral subtypes. The intrinsic scatter in absolute magnitude M_J for ultracool M dwarfs at fixed G − J colour is measured to be 0.21 mag.

This work investigates the vertical profile of the stars in the disk of the Milky Way. The models investigated are of the form sech^2/n(nz/(2H)) where, setting α = 2/n, the three functions of the sequence α= 0, 1, 2 correspond to exponential, sech, and sech² functions respectively. We consider symmetric models and asymmetric models, above and below the plane. The study uses the large sample of K and M stars of Ferguson et al. (2017) and applies the methods of Bayesian model comparison to discriminate between the 6 models. Two inconsistencies in Ferguson et al. (2017), concerning the vertical height cut and the model continuity across the plane, are noted and addressed. We find that (1) in the Milky Way the symmetric disc models are decisively ruled out, with northern thin disc scale heights about 25% larger than southern, (2) there is moderate evidence for the exponential and sech models over the sech² model, though a sample extending further into the Galactic mid-plane is needed to strengthen this result, (3) the photometric distances used by Ferguson et al. underestimate the GAIA distances by a factor of roughly 1.16, and (4) the increase of scale height with Galactic latitude observed by Ferguson et al. is due to incorrect cuts to the data.

We present a systematic analysis of the constraints σ_γ on the mass profile slope γ obtainable when fitting a singular power-law ellipsoid model to a typical strong lensing observation of an extended source. These results extend our previous analysis of circular systems, Paper I. We draw our results from 676 mock observations covering a range of image configurations, each created with a fixed signal to noise ratio S = 100 in the images. We analyse the results using a combination of theory and a simplified model which identifies the contribution to the constraints of the individual fluxes and positions in each of the lensed images. The main results are: 1. For any lens ellipticity, the constraints σ_γ for two image systems are well described by the results of Paper I, transformed to elliptical coordinates; 2. We derive an analytical expression for σ_γ for systems with the source aligned with the axis of the lens; 3. For both two-image systems and aligned systems, σ_γ is limited by the flux uncertainties; 4. The constraints for off-axis four-image systems are a factor of two to eight better, depending on source size, than for two-image systems, and improve with increasing lens ellipticity. We show that the constraints on γ in these systems derive from the complementary positional information of the images alone, without using flux. The complementarity improves as the offset of the source from the axis increases, such that the best constraints σ_γ < 0.01, for S = 100, occur when the source approaches the caustic.

We provide predictions of the yield of 7 < z < 9 quasars from theEuclidwide survey, updating the calculation presented in theEuclidRed Book in several ways. We account for revisions to theEuclidnear-infrared filter wavelengths; we adopt steeper rates of decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10^{k(z − 6)},k = −0.72, and a further steeper rate of decline,k = −0.92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make use of an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identification of fainter quasars, down toJ_AB ∼ 23. Quasars atz > 8 may be selected fromEuclidOYJHphotometry alone, but selection over the redshift interval 7 < z < 8 is greatly improved by the addition ofz-band data from, e.g., Pan-STARRS and LSST. We calculate predicted quasar yields for the assumed values of the rate of decline of the QLF beyondz = 6. If the decline of the QLF accelerates beyondz = 6, withk = −0.92,Euclidshould nevertheless find over 100 quasars with 7.0 < z < 7.5, and ∼25 quasars beyond the current record ofz = 7.5, including ∼8 beyondz = 8.0. The firstEuclidquasars atz > 7.5 should be found in the DR1 data release, expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8,M₁₄₅₀ < −25, using 8 m class telescopes to confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of the candidate lists is predicted to be modest even atJ_AB ∼ 23. The precision with whichkcan be determined over 7 < z < 8 depends on the value ofk, but assumingk = −0.72 it can be measured to a 1σuncertainty of 0.07.

In this series of papers we develop a formalism for constraining mass profiles in strong gravitational lenses with extended images, using fluxes in addition to positional information. We start in this paper with a circular power-law profile and show that the slope γ is uniquely determined by only two observables: the flux ratio f₁/f₂ and the image position ratio θ₁/θ₂ of the two images. We derive an analytic expression relating these two observables to the slope, a result which does not depend on the Einstein angle or the structure or brightness of the source. We then find an expression for the uncertainty on the slope σ_γ that depends only on the position ratio θ₁/θ₂ and the total S/N in the images. For example, in a system with position ratio θ₁/θ₂ = 0.5, S/N =100 and γ = 2 we find that γ is constrained to a precision of ±0.03. We then test these results against a series of mock observations. We invert the images and fit an 11 parameter model, including ellipticity and position angle for both lens and source and measure the uncertainty on γ. We find agreement with the theoretical estimate for all mock observations. In future papers we will examine the radial range of the galaxy over which the constraint on the slope applies, and extend the analysis to elliptical lenses.

The space density of late M dwarfs, subtypes M7–M9.5, is not well determined. We applied the photo-type method to iz photometry from the Sloan Digital Sky Survey and YJHK photometry from the UKIRT Infrared Deep Sky Survey, over an effective area of 3070 deg², to produce a new, bright J(Vega) < 17.5, homogeneous sample of 33 665 M7–M9.5 dwarfs. The typical S/N of each source summed over the six bands is > 100. Classifications are provided to the nearest half spectral subtype. Through a comparison with the classifications in the BOSS Ultracool Dwarfs (BUD) spectroscopic sample, the typing is shown to be accurately calibrated to the BUD classifications and the precision is better than 0.5 subtypes rms; i.e. the photo-type classifications are as precise as good spectroscopic classifications. Sources with large χ² > 20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example, L subdwarfs are misclassified towards earlier types by approximately two spectral subtypes. We estimate that this bias affects only ∼1% of the sources. Therefore the sample is well suited to measure the luminosity function and investigate the softening towards the Galactic plane of the exponential variation of density with height.