arXiv subjects

Lise Christensen

Publications and source records attributed to Lise Christensen.

At least 37 records · Page 2Linked to original sources

arXiv2021

Sub-damped Lyman alpha systems in the XQ-100 survey II -- Chemical evolution at 2.4<z<4.3

We present the measured gas-phase metal column densities in 155 sub-damped Lyman alpha systems (subDLAs) with the aim to investigate the contribution of subDLAs to the chemical evolution of the Universe. The sample was identified within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift range 2.4<=z<=4.3. Using all available column densities of the ionic species investigated (mainly CIV, SiII, MgII, SiIV, AlII, FeII, CII, and OI; in order of decreasing detection frequency), we estimate the ionization-corrected gas-phase metallicity of each system using Markov Chain Monte Carlo techniques to explore a large grid of Cloudy ionization models. Without accounting for ionization and dust depletion effects, we find that the HI-weighted gas-phase metallicity evolution of subDLAs are consistent with damped Lyman alpha systems (DLAs). When ionization corrections are included, subDLAs are systematically more metal-poor than DLAs (between ~0.5 sigma and ~3 sigma significance) by up to ~1.0 dex over the redshift range 3<=z<=4.3. The correlation of gas-phase [Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs, suggesting that the two classes of absorbers have a similar relative dust depletion pattern. As previously seen for Lyman limit systems, the gas-phase [C/O] in subDLAs remains constantly solar for all metallicities indicating that both subDLAs and Lyman limit systems could trace carbon-rich ejecta, potentially in circumgalactic environments.

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arXiv2020

High Molecular Gas Masses in Absorption-selected Galaxies at $z \approx 2$

We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3$-$2) or (4$-$3) emission from the fields of 12 high-metallicity ([M/H]~$\geq -0.72$\,dex) damped Lyman-$α$ absorbers (DLAs) at $z \approx 1.7-2.6$. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, $\rm M_{mol} \approx (1.3 - 20.7) \times (α_{\rm CO}/4.36) \times 10^{10} \; M_\odot$. The impact parameters of the CO emitters to the QSO sightline lie in the range $b \approx 5.6-100$~kpc, with the three new CO detections having $b \lesssim 15$~kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H]~$\gtrsim -0.3$\,dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[{\sc ii}]~158$μ$m detection rate in high-$z$, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-$z$ DLAs. The H{\sc i}-absorption criterion identifying DLAs selects the entire high-$z$ galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.

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arXiv2020

Into the Lyα jungle: exploring the circumgalactic medium of galaxies at z ~ 4-5 with MUSE

We present a study of the galaxy environment of 9 strong HI+CIV absorption line systems ($16.2<{\rm log}(N({\rm HI}))<21.2$) spanning a wide range in metallicity at $z\sim4-5$, using MUSE integral field and X-Shooter spectroscopic data collected in a $z\approx 5.26$ quasar field. We identify galaxies within a 250 kpc and $\pm1000$ km s$^{-1}$ window for 6 out of the 9 absorption systems, with 2 of the absorption line systems showing multiple associated galaxies within the MUSE field of view. The space density of Ly$α$ emitting galaxies (LAEs) around the HI and CIV systems is $\approx10-20$ times the average sky density of LAEs given the flux limit of our survey, showing a clear correlation between the absorption and galaxy populations. Further, we find that the strongest CIV systems in our sample are those that are most closely aligned with galaxies in velocity space, i.e. within velocities of $\pm500$ km s$^{-1}$. The two most metal poor systems lie in the most dense galaxy environments, implying we are potentially tracing gas that is infalling for the first time into star-forming groups at high redshift. Finally, we detect an extended Ly$α$ nebula around the $z\approx 5.26$ quasar, which extends up to $\approx50$ kpc at the surface brightness limit of $3.8 \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. After scaling for surface brightness dimming, we find that this nebula is centrally brighter, having a steeper radial profile than the average for nebulae studied at $z\sim3$ and is consistent with the mild redshift evolution seen from $z\approx 2$.

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arXiv2019

A Catalog of Emission-Line Galaxies from the Faint Infrared Grism Survey: Studying Environmental Influence on Star Formation

We present a catalog of 208 $0.3 < z < 2.1$ Emission Line Galaxies (ELG) selected from 1D slitless spectroscopy obtained using Hubble's WFC3 G102 grism, as part of the Faint Infrared Grism Survey (FIGS). We identify ELG candidates by searching for significant peaks in all continuum-subtracted G102 spectra, and, where possible, confirm candidates by identifying consistent emission lines in other available spectra or with published spectroscopic redshifts. We provide derived emission line fluxes and errors, redshifts, and equivalent widths (EW) for H$α$ $\lambda6563$, [OIII]$λ\lambda4959,5007$, and [OII]$λ\lambda3727$ emission lines, for emission line galaxies down to AB(F105W) $ > 28$ and $> 10^{-17}$ erg cm$^{-2}$ s$^{-1}$ line flux. We use the resulting line catalog to investigate a possible relationship between line emission and a galaxy's environment. We use 7th-nearest-neighbor distances to investigate the typical surroundings of ELGs compared to non-ELGs, and we find that [OIII] emitters are preferentially found at intermediate galaxy densities near galaxy groups. We characterize these ELGs in terms of the galaxy specific star formation rate (SSFR) versus stellar mass, and find no significant influence of environment on that relation. We calculate star formation rates (SFR), and find no dependence of SFR on local galaxy surface density for $0.3 < z < 0.8$ H$α$ emitters and for $0.8<z<1.3$ [OIII] emitters. We find similar rates of close-pair interaction between ELGs and non-ELGs. For galaxy surface densities $Σ\leq 30$ Mpc$^{-2}$, we find no consistent effect of environment on star formation.

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arXiv2019

MUSE Analysis of Gas around Galaxies (MAGG) -- I: Survey design and the environment of a near pristine gas cloud at z~3.5

We present the design, methods, and first results of the MUSE Analysis of Gas around Galaxies (MAGG) survey, a large programme on the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) which targets 28 z > 3.2 quasars to investigate the connection between optically-thick gas and galaxies at z~3-4. MAGG maps the environment of 52 strong absorption line systems at z > 3, providing the first statistical sample of galaxies associated with gas-rich structures in the early Universe. In this paper, we study the galaxy population around a very metal poor gas cloud at z~3.5 towards the quasar J124957.23-015928.8. We detect three Lyman alpha emitters within <200km/s of the cloud redshift, at projected separations <185 kpc (physical). The presence of star-forming galaxies near a very metal-poor cloud indicates that metal enrichment is still spatially inhomogeneous at this redshift. Based on its very low metallicity and the presence of nearby galaxies, we propose that the most likely scenario for this LLS is that it lies within a filament which may be accreting onto a nearby galaxy. Taken together with the small number of other LLSs studied with MUSE, the observations to date show a range of different environments near strong absorption systems. The full MAGG survey will significantly expand this sample and enable a statistical analysis of the link between gas and galaxies to pin down the origin of these diverse environments at z~3-4.

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arXiv2019

Metallicity has followed local gravitational potential of galaxies since z=3

The MZ relation between stellar mass (M*) and metallicity (Z) of nearby galaxies has been described as both a global and local property, i.e. valid also on sub-galaxy scales. Here we show that Z has remained a local property, following the gravitational potential, since z=3. In absorption the MZ relation has been well studied, and was in place already at z=5.1. A recent absorption study of GRB galaxies revealed a close match to Damped Lyα (DLA) galaxies, surprising due to their vastly different impact parameters and leading the authors to suggest that local metallicity follows the local gravitational potential. In this paper we formulate an observational test of this hypothesis. The test, in essence, forms a prediction that the velocity dispersion of the absorbing gas in galaxy halos, normalized by the central velocity dispersion, must follow a steep log scale slope of -0.015 dex/kpc as a function of impact parameter out to at least 20-30 kpc. We then compile an archival data and literature based sample of galaxies seen in both emission and absorption suitable for the test, and find that current data confirm the hypothesis out to 40-60 kpc. In addition we show that the distribution of the velocity offsets between z em and z abs favours a model where DLA systems are composed of individual sub-clouds distributed along the entire path through the halo, and disfavours a model where they are one single cloud with a bulk motion and internal sub-structure.

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arXiv2019

Sub-damped Lyman alpha systems in the XQ-100 survey I -- Identification and contribution to the cosmological HI budget

Sub-damped Lyman alpha systems (subDLAs; HI column densities of 19.0<=logN(HI)<20.3) are rarely included in the cosmic HI census performed at redshifts z>=1.5, yet are expected to contribute significantly to the overall HI mass budget of the Universe. In this paper, we present a blindly selected sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path length X=475) in the XQ-100 survey to investigate the contribution of subDLAs to the HI mass density of the Universe. The impact of X-Shooter's spectral resolution on sub-DLA identification is evaluated, and found to be sufficient for reliably finding absorbers with logN(HI)>=18.9. We compared the implications of searching for subDLAs solely using HI absorption versus the use of additional metal lines to confirm the identification, and found that metal-selection techniques would have missed 75 subDLAs. Using a bootstrap-Monte Carlo simulation, we computed the column density distribution function (f(N,X)) and the cosmological HI mass density of subDLAs and compared with our previous work based on the XQ-100 damped Lyman alpha systems. We do not find any significant redshift evolution in f(N,X) or cosmological HI mass density for subDLAs. However, subDLAs contribute 10-20 per cent of the total cosmological HI mass density measured at redshifts 2<z<5 (agreeing with previous measurements), and thus have a small but significant contribution to the HI budget of the Universe.

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arXiv2019

The Evolution of O I over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium

We present a survey for metal absorption systems traced by neutral oxygen over $3.2 < z < 6.5$. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199 QSOs with redshifts up to 6.6. In total we detect 74 O I absorbers, of which 57 are separated from the background QSO by more than 5000 km/s. We use a maximum likelihood approach to fit the distribution of O I $λ$1302 equivalent widths in bins of redshift, and from this determine the evolution in number density of absorbers with $W_{1302} > 0.05$ angstroms. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over $4.9 < z < 5.7$ is a factor of 1.7--4.1 lower (68% confidence) than over $5.7 < z < 6.5$, with a lower value at $z < 5.7$ favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at $z \sim 6$ than at lower redshifts. Absorption from highly ionized metals traced by C IV is also weaker in higher-redshift O I systems, supporting this picture. The evolution of O I absorbers implies that metal-enriched circumgalactic gas at $z \sim 6$ is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch.

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arXiv2019

Linking gas and galaxies at high redshift: MUSE surveys the environments of six damped Lyman alpha galaxies at z~3

We present results from a survey of galaxies in the fields of six z>3 Damped Lyman alpha systems (DLAs) using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT). We report a high detection rate of up to ~80% of galaxies within 1000 km/s from DLAs and with impact parameters between 25 and 280 kpc. In particular, we discovered 5 high-confidence Lyman alpha emitters associated with three DLAs, plus up to 9 additional detections across five of the six fields. The majority of the detections are at relatively large impact parameters (>50 kpc) with two detections being plausible host galaxies. Among our detections, we report four galaxies associated with the most metal-poor DLA in our sample (Z/Z_sun = -2.33), which trace an overdense structure resembling a filament. By comparing our detections with predictions from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological simulations and a semi-analytic model designed to reproduce the observed bias of DLAs at z>2, we conclude that our observations are consistent with a scenario in which a significant fraction of DLAs trace the neutral regions within halos with a characteristic mass of 10^11-10^12 M_sun, in agreement with the inference made from the large-scale clustering of DLAs. We finally show how larger surveys targeting ~25 absorbers have the potential of constraining the characteristic masses of halos hosting high-redshift DLAs with sufficient accuracy to discriminate between different models.

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arXiv2019

Gaia-assisted selection of a quasar reddened by dust in an extremely-strong Damped Lyman-α Absorber at z=2.226

Damped Lyman-α Absorbers (DLAs) as a class of QSO absorption-line systems are currently our most important source of detailed information on the cosmic chemical evolution of galaxies. However, the degree to which this information is biased by dust remains to be understood. One strategy is to specifically search for QSOs reddened by metal-rich and dusty foreground absorbers. In this Letter we present the discovery of a z=2.60 QSO strongly reddened by dust in an intervening extremely-strong DLA at z=2.226. This QSO was identified through a novel selection combining the astrometric measurements from ESA's Gaia satellite with extent optical and near/mid-infrared photometry. We infer a total neutral atomic-hydrogen column density of log N(HI)=21.95{\pm}0.15 and a lower limit on the gas-phase metallicity of [Zn/H]>-0.96. This DLA is also remarkable in exhibiting shielded neutral gas witnessed in CI and tentative detections of CO molecular bands. The Spectral Energy Distribution (SED) of the QSO is well-accounted for by a normal QSO-SED reddened by dust from a DLA with a 10%-of-Solar metallicity, dust extinction of A_V=0.82{\pm}0.02mag, and LMC-like extinction curve including the characteristic 2175Å extinction feature. Such QSO absorption-line systems have shown to be very rare in previous surveys, which have mostly revealed sight-lines with low extinction. The present case therefore suggests that previous samples have under-represented the fraction of dusty absorbers. Building a complete sample of such systems is needed to assess the significance of this effect.

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arXiv2019

FIGS: Spectral fitting constraints on the star formation history of massive galaxies since Cosmic Noon

We constrain the stellar population properties of a sample of 52 massive galaxies, with stellar mass log Ms>10.5, over the redshift range 0.5<z<2 by use of observer-frame optical and near-infrared slitless spectra from HST's ACS and WFC3 grisms. The deep exposures (~100 ks) allow us to target individual spectra of massive galaxies to F160W=22.5AB. Our spectral fitting approach uses a set of six base models adapted to the redshift and spectral resolution of each observation, and fits the weights of the base models, including potential dust attenuation, via an MCMC method. Our sample comprises a mixed distribution of quiescent (19) and star-forming galaxies (33). We quantify the width of the age distribution (Dt) that is found to dominate the variance of the retrieved parameters according to Principal Component Analysis. The population parameters follow the expected trend towards older ages with increasing mass, and Dt appears to weakly anti-correlate with stellar mass, suggesting a more efficient star formation at the massive end. As expected, the redshift dependence of the relative stellar age (measured in units of the age of the Universe at the source) in the quiescent sample rejects the hypothesis of a single burst (aka monolithic collapse). Radial colour gradients within each galaxy are also explored, finding a wider scatter in the star-forming subsample, but no conclusive trend with respect to the population parameters.

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arXiv2019

RELICS: Reionization Lensing Cluster Survey

Large surveys of galaxy clusters with the Hubble and Spitzer Space Telescopes, including CLASH and the Frontier Fields, have demonstrated the power of strong gravitational lensing to efficiently deliver large samples of high-redshift galaxies. We extend this strategy through a wider, shallower survey named RELICS, the Reionization Lensing Cluster Survey. This survey, described here, was designed primarily to deliver the best and brightest high-redshift candidates from the first billion years after the Big Bang. RELICS observed 41 massive galaxy clusters with Hubble and Spitzer at 0.4-1.7um and 3.0-5.0um, respectively. We selected 21 clusters based on Planck PSZ2 mass estimates and the other 20 based on observed or inferred lensing strength. Our 188-orbit Hubble Treasury Program obtained the first high-resolution near-infrared images of these clusters to efficiently search for lensed high-redshift galaxies. We observed 46 WFC3/IR pointings (~200 arcmin^2) with two orbits divided among four filters (F105W, F125W, F140W, and F160W) and ACS imaging as needed to achieve single-orbit depth in each of three filters (F435W, F606W, and F814W). As previously reported by Salmon et al., we discovered 322 z ~ 6 - 10 candidates, including the brightest known at z ~ 6, and the most distant spatially-resolved lensed arc known at z ~ 10. Spitzer IRAC imaging (945 hours awarded, plus 100 archival) has crucially enabled us to distinguish z ~ 10 candidates from z ~ 2 interlopers. For each cluster, two HST observing epochs were staggered by about a month, enabling us to discover 11 supernovae, including 3 lensed supernovae, which we followed up with 20 orbits from our program. We delivered reduced HST images and catalogs of all clusters to the public via MAST and reduced Spitzer images via IRSA. We have also begun delivering lens models of all clusters, to be completed before the JWST GO call for proposals.

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arXiv2019

Emission Line Metallicities From The Faint Infrared Grism Survey and VLT/MUSE

We derive direct measurement gas-phase metallicities of $7.4 < 12 + \log(O/H) < 8.4$ for 14 low-mass Emission Line Galaxies (ELGs) at $0.3 < z < 0.8$ identified in the Faint Infrared Grism Survey (FIGS). We use deep slitless G102 grism spectroscopy of the Hubble Ultra Deep Field (HUDF), dispersing light from all objects in the field at wavelengths between 0.85 and 1.15 microns. We run an automatic search routine on these spectra to robustly identify 71 emission line sources, using archival data from VLT/MUSE to measure additional lines and confirm redshifts. We identify 14 objects with $0.3 < z < 0.8$ with measurable O[III]$λ$4363 Å emission lines in matching VLT/MUSE spectra. For these galaxies, we derive direct electron-temperature gas-phase metallicities with a range of $7.4 < 12 + \log(O/H) < 8.4$. With matching stellar masses in the range of $10^{7.9} M_{\odot} < M_{\star} < 10^{10.4} M_{\odot}$, we construct a mass-metallicity (MZ) relation and find that the relation is offset to lower metallicities compared to metallicities derived from alternative methods (e.g.,$R_{23}$, O3N2, N2O2) and continuum selected samples. Using star formation rates (SFR) derived from the $Hα$ emission line, we calculate our galaxies' position on the Fundamental Metallicity Relation (FMR), where we also find an offset toward lower metallicities. This demonstrates that this emission-line-selected sample probes objects of low stellar masses but even lower metallicities than many comparable surveys. We detect a trend suggesting galaxies with higher Specific Star Formation (SSFR) are more likely to have lower metallicity. This could be due to cold accretion of metal-poor gas that drives star formation, or could be because outflows of metal-rich stellar winds and SNe ejecta are more common in galaxies with higher SSFR.

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arXiv2019

Detection of the Far-infrared [O III] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in the Heart of the Reionization Era

We present the Atacama Large Millimeter/submillimeter Array (ALMA) detection of the [O III] 88 $μ$m line and rest-frame 90 $μ$m dust continuum emission in a Y-dropout Lyman break galaxy (LBG), MACS0416_Y1, lying behind the Frontier Field cluster MACS J0416.1-2403. This [O III] detection confirms the LBG with a spectroscopic redshift of $z = 8.3118 \pm 0.0003$, making this object one of the furthest galaxies ever identified spectroscopically. The observed 850 $μ$m flux density of $137 \pm 26$ $μ$Jy corresponds to a de-lensed total infrared (IR) luminosity of $L_{\rm IR} = (1.7 \pm 0.3) \times 10^{11} L_{\odot}$ if assuming a dust temperature of $T_{\rm dust} = 50$ K and an emissivity index of $β= 1.5$, yielding a large dust mass of $4 \times 10^6 M_{\odot}$. The ultraviolet-to-far IR spectral energy distribution modeling where the [O III] emissivity model is incorporated suggests the presence of a young ($τ_{\rm age} \approx 4$ Myr), star-forming (SFR $\approx 60 M_{\odot}$ yr$^{-1}$), moderately metal-polluted ($Z \approx 0.2 Z_{\odot}$) stellar component with a mass of $M_{\rm star} = 3 \times 10^8 M_{\odot}$. An analytic dust mass evolution model with a single episode of star-formation does not reproduce the metallicity and dust mass in $τ_{\rm age} \approx 4$ Myr, suggesting a pre-existing evolved stellar component with $M_{\rm star} \sim 3 \times 10^9 M_{\odot}$ and $τ_{\rm age} \sim 0.3$ Gyr as the origin of the dust mass.

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arXiv2019

The nature of damped HI absorbers probed by cosmological simulations: satellite accretion and outflows

We use state-of-the-art cosmological zoom simulations to explore the distribution of neutral gas in and around galaxies that gives rise to high column density \ion{H}{i} \mbox{Ly-$α$} absorption (formally, sub-DLAs and DLAs) in the spectra of background quasars. Previous cosmological hydrodynamic simulations under-predict the mean projected separations $(b)$ of these absorbers relative to the host, and invoke selection effects to bridge the gap with observations. On the other hand, single lines of sight (LOS) in absorption cannot uniquely constrain the galactic origin. Our simulations match all observational data, with DLA and sub-DLA LOS existing over the entire probed parameter space ($-4\lesssim $[M/H]$\lesssim 0.5$, $b<50$ kpc) at all redshifts ($z\sim 0.4 - 3.0$). We demonstrate how the existence of DLA LOS at $b\gtrsim 20-30$ kpc from a massive host galaxy require high numerical resolution, and that these LOS are associated with dwarf satellites in the main halo, stripped metal-rich gas and outflows. Separating the galaxy into interstellar ("\ion{H}{i} disc") and circumgalactic ("halo") components, we find that both components significantly contribute to damped \ion{H}{i} absorption LOS. Above the sub-DLA (DLA) limits, the disc and halo contribute with $\sim 60 (80)$ and $\sim 40 (20)$ per cent, respectively. Our simulations confirm analytical model-predictions of the DLA-distribution at $z\lesssim 1$. At high redshift ($z\sim 2-3$) sub-DLA and DLAs occupy similar spatial scales, but on average separate by a factor of two by $z\sim 0.5$. On whether sub-DLA and DLA LOS sample different stellar-mass galaxies, such a correlation can be driven by a differential covering-fraction of sub-DLA to DLA LOS with stellar mass. This preferentially selects sub-DLA LOS in more massive galaxies in the low-$z$ universe.

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arXiv2018

A Two-Dimensional Spectroscopic Study of Emission Line Galaxies in the Faint Infrared Grism Survey (FIGS) I: Detection Method and Catalog

We present the results from the application of a two-dimensional emission line detection method, EMission-line two-Dimensional (EM2D), to the near-infrared G102 grism observations obtained with the Wide-Field Camera 3 (WFC3) as part of the Cycle 22 {\em Hubble Space Telescope} Treasury Program: the Faint Infrared Grism Survey (FIGS). Using the EM2D method, we have assembled a catalog of emission line galaxies (ELGs) with resolved star formation from each of the four FIGS fields. Not only can one better assess the global properties of ELGs, but the EM2D method allows for the analysis and an improved study of the individual emission-line region {\it within} each galaxy. This paper includes a description of the methodology, advantages, and the first results of the EM2D method applied to ELGs in FIGS. The advantage of 2D emission line measurements includes significant improvement of galaxy redshift measurements, approaching the level of accuracy seen in high-spectral-resolution data, but with greater efficiency; and the ability to identify and measure the properties of multiple sites of star-formation and over scales of $\sim$ 1 kpc within individual galaxies out to z $\sim$ 4. The EM2D method also significantly improves the reliability of high-redshift ($z\sim7$) Lyman-$α$ detections. Coupled with the wide field of view and high efficiency of space-based grism observations, EM2D provides a noteworthy improvement on the physical parameters that can be extracted from grism observations.

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arXiv2018

New Constraints on the free-streaming of warm dark matter from intermediate and small scale Lyman-$α$ forest data

We present new measurements of the free-streaming of warm dark matter (WDM) from Lyman-$α$ flux-power spectra. We use data from the medium resolution, intermediate redshift XQ-100 sample observed with the X-shooter spectrograph ($z=3 - 4.2$) and the high-resolution, high-redshift sample used in Viel et al. (2013) obtained with the HIRES/MIKE spectrographs ($z=4.2 - 5.4$). Based on further improved modelling of the dependence of the Lyman-$α$ flux-power spectrum on the free-streaming of dark matter, cosmological parameters, as well as the thermal history of the intergalactic medium (IGM) with hydrodynamical simulations, we obtain the following limits, expressed as the equivalent mass of thermal relic WDM particles. The XQ-100 flux power spectrum alone gives a lower limit of 1.4 keV, the re-analysis of the HIRES/MIKE sample gives 4.1 keV while the combined analysis gives our best and significantly strengthened lower limit of 5.3 keV (all 2$σ$ C.L.). The further improvement in the joint analysis is partly due to the fact that the two data sets have different degeneracies between astrophysical and cosmological parameters that are broken when the data sets are combined, and more importantly on chosen priors on the thermal evolution. These results all assume that the temperature evolution of the IGM can be modelled as a power law in redshift. Allowing for a non-smooth evolution of the temperature of the IGM with sudden temperature changes of up to 5000K reduces the lower limit for the combined analysis to 3.5 keV. A WDM with smaller thermal relic masses would require, however, a sudden temperature jump of $5000\,K$ or more in the narrow redshift interval $z=4.6-4.8$, in disagreement with observations of the thermal history based on high-resolution resolution Lyman-$α$ forest data and expectations for photo-heating and cooling in the low density IGM at these redshifts.

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arXiv2018

Extreme magnification of a star at redshift 1.5 by a galaxy-cluster lens

Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to ~50. Here we report an image of an individual star at redshift z=1.49 (dubbed "MACS J1149 Lensed Star 1 (LS1)") magnified by >2000. A separate image, detected briefly 0.26 arcseconds from LS1, is likely a counterimage of the first star demagnified for multiple years by a >~3 solar-mass object in the cluster. For reasonable assumptions about the lensing system, microlensing fluctuations in the stars' light curves can yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhalos or massive compact objects may help to account for the two images' long-term brightness ratio.

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