arXiv subjects

L. Christensen

Publications and source records attributed to L. Christensen.

At least 55 records · Page 3Linked to original sources

arXiv2018

Testing strong line metallicity diagnostics at z~2

High-z galaxy gas-phase metallicities are usually determined through observations of strong optical emission lines with calibrations tied to the local universe. Recent debate has questioned if these calibrations are valid in the high-z universe. We investigate this by analysing a sample of 16 galaxies at z~2 available in the literature, and for which the metallicity can be robustly determined using oxygen auroral lines. The sample spans a redshift range of 1.4 < z < 3.6, has metallicities of 7.4-8.4 in 12+log(O/H) and stellar masses 10^7.5-10^11 Msun. We test commonly used strong line diagnostics (R23, O3, O2, O32, N2, O3N2 and Ne3O2 ) as prescribed by four different sets of empirical calibrations, as well as one fully theoretical calibration. We find that none of the strong line diagnostics (or calibration set) tested perform consistently better than the others. Amongst the line ratios tested, R23 and O3 deliver the best results, with accuracies as good as 0.01-0.04 dex and dispersions of ~0.2 dex in two of the calibrations tested. Generally, line ratios involving nitrogen predict higher values of metallicity, while results with O32 and Ne3O2 show large dispersions. The theoretical calibration yields an accuracy of 0.06 dex, comparable to the best strong line methods. We conclude that, within the metallicity range tested in this work, the locally calibrated diagnostics can still be reliably applied at z~2.

astro-ph.GA↗

arXiv2018

Hunting for Metals Using XQ-100 Legacy Survey Composite Spectra

We investigate the NV absorption signal along the line of sight of background quasars, in order to test the robustness of the use of this ion as criterion to select intrinsic (i.e. physically related to the quasar host galaxy) narrow absorption lines (NALs). We build composite spectra from a sample of $\sim$ 1000 CIV absorbers, covering the redshift range 2.55 < z < 4.73, identified in 100 individual sight lines from the XQ-100 Legacy Survey. We detect a statistical significant NV absorption signal only within 5000 km s$^{-1}$ of the systemic redshift, z$\rm_{em}$. This absorption trough is $\sim$ 15$σ$ when only CIV systems with N(CIV) > 10$^{14}$ cm$^{-2}$ are included in the composite spectrum. This result confirms that NV offers an excellent statistical tool to identify intrinsic systems. We exploit the stacks of 11 different ions to show that the gas in proximity to a quasar exhibits a considerably different ionization state with respect to gas in the transverse direction and intervening gas at large velocity separations from the continuum source. Indeed, we find a dearth of cool gas, as traced by low-ionization species and in particular by MgII, in the proximity of the quasar. We compare our findings with the predictions given by a range of Cloudy ionization models and find that they can be naturally explained by ionization effects of the quasar.

astro-ph.GA↗

arXiv2018

Infrared molecular hydrogen lines in GRB host galaxies

Molecular species, most frequently H_2, are present in a small, but growing, number of gamma-ray burst (GRB) afterglow spectra at redshifts z~2-3, detected through their rest-frame UV absorption lines. In rare cases, lines of vibrationally excited states of H_2 can be detected in the same spectra. The connection between afterglow line-of-sight absorption properties of molecular (and atomic) gas, and the observed behaviour in emission of similar sources at low redshift, is an important test of the suitability of GRB afterglows as general probes of conditions in star formation regions at high redshift. Recently, emission lines of carbon monoxide have been detected in a small sample of GRB host galaxies, at sub-mm wavelengths, but no searches for H_2 in emission have been reported yet. In this paper we perform an exploratory search for rest-frame K band rotation-vibrational transitions of H_2 in emission, observable only in the lowest redshift GRB hosts (z<0.22). Searching the data of four host galaxies, we detect a single significant rotation-vibrational H_2 line candidate, in the host of GRB 031203. Re-analysis of Spitzer mid-infrared spectra of the same GRB host gives a single low significance rotational line candidate. The (limits on) line flux ratios are consistent with those of blue compact dwarf galaxies in the literature. New instrumentation, in particular on the JWST and the ELT, can facilitate a major increase in our understanding of the H_2 properties of nearby GRB hosts, and the relation to H_2 absorption in GRBs at higher redshift.

astro-ph.GA↗

arXiv2018

Stellar masses, metallicity gradients and suppressed star formation revealed in a new sample of absorption selected galaxies

Context. Absorbing galaxies are selected via the detection of characteristic absorption lines which their gas-rich media imprint in the spectra of distant light-beacons. The proximity of the typically faint foreground absorbing galaxies to bright background sources makes it challenging to robustly identify these in emission, and hence to characterise their relation to the general galaxy population. Aims. We search for emission to confirm and characterise ten galaxies hosting damped, metal-rich quasar absorbers at redshift z < 1. Methods. We identify the absorbing galaxies by matching spectroscopic absorption -and emission redshifts and from projected separations. Combining emission-line diagnostics with existing absorption spectroscopy and photometry of quasar-fields hosting metal-rich, damped absorbers, we compare our new detections with reference samples and place them on scaling relations. Results. We spectroscopically confirm seven galaxies harbouring damped absorbers (a 70% success-rate). Our results conform to the emerging picture that neutral gas on scales of tens of kpc in galaxies is what causes the characteristic Hi absorption. Our key results are: (I) Absorbing galaxies with $\log _{10} [M_\star ~(M_\odot)] \gtrsim 10$ have star formation rates that are lower than predicted for the main sequence of star formation. (II) The distribution of impact parameter with Hi column density and with absorption-metallicity for absorbing galaxies at $z\sim 2-3$ extends to $z\sim 0.7$ and to lower Hi column densities. (III) A robust mean metallicity gradient of $\langle Γ\rangle = 0.022 \pm 0.001~[dex~kpc^{-1}]$. (IV) By correcting absorption metallicities for $\langle Γ\rangle$ and imposing a truncation-radius at $12~\mathrm{kpc}$, absorbing galaxies fall on top of predicted mass-metallicity relations, with a statistically significant decrease in scatter.

astro-ph.GA↗

arXiv2018

The 2175 Å extinction feature in the optical afterglow spectrum of GRB 180325A at z=2.25

The UV extinction feature at 2175 Å is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 Å bump on the sightline to the γ-ray burst (GRB) afterglow GRB 180325A at z=2.2486, the only unambiguous detection over the past ten years of GRB follow-up, at four different epochs with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT)/X-shooter. Additional photometric observations of the afterglow are obtained with the Gamma-Ray burst Optical and Near-Infrared Detector (GROND). We construct the near-infrared to X-ray spectral energy distributions (SEDs) at four spectroscopic epochs. The SEDs are well-described by a single power-law and an extinction law with R_V~4.4, A_V~1.5, and the 2175 Å extinction feature. The bump strength and extinction curve are shallower than the average Galactic extinction curve. We determine a metallicity of [Zn/H]>-0.98 from the VLT/X-shooter spectrum. We detect strong neutral carbon associated with the GRB with an equivalent width of Wr(λ1656) = 0.85+/-0.05. We also detect optical emission lines from the host galaxy. Based on the Hαemission line flux, the derived dust-corrected star-formation rate is ~46+/-4 M_sun/yr and the predicted stellar mass is log M*/M_sun~9.3+/-0.4, suggesting the host galaxy is amongst the main-sequence star-forming galaxies.

astro-ph.GA↗

arXiv2018

ALMA observations of a metal-rich damped Lyα absorber at z = 2.5832: evidence for strong galactic winds in a galaxy group

We report on the results of a search for CO(3-2) emission from the galaxy counterpart of a high-metallicity Damped Ly-alpha Absorber (DLA) at z=2.5832 towards the quasar Q0918+1636. We do not detect CO emission from the previously identified DLA galaxy counterpart. The limit we infer on M_gas / M_star is in the low end of the range found for DLA galaxies, but is still consistent with what is found for other star-forming galaxies at similar redshifts. Instead we detect CO(3-2) emission from another intensely star-forming galaxy at an impact parameter of 117 kpc from the line-of-sight to the quasar and 131 km s^-1 redshifted relative to the velocity centroid of the DLA in the quasar spectrum. In the velocity profile of the low- and high-ionisation absorption lines of the DLA there is an absorption component consistent with the redshift of this CO-emitting galaxy. It is plausible that this component is physically associated with a strong outflow in the plane of the sky from the CO-emitting galaxy. If true, this would be further evidence, in addition to what is already known from studies of Lyman-break galaxies, that galactic outflows can be traced beyond 100 kpc from star-forming galaxies. The case of this z=2.583 structure is an illustration of this in a group environment.

astro-ph.GA↗

arXiv2018

Unidentified quasars among stationary objects from Gaia DR2

We here apply a novel technique selecting quasar candidates purely as sources with zero proper motions in the Gaia data release 2 (DR2). We demonstrate that this approach is highly efficient toward high Galactic latitudes with < 25% contamination from stellar sources. Such a selection technique offers a very pure sample completeness, since all cosmological point sources are selected regardless of their intrinsic spectral properties within the limiting magnitude of Gaia. We carry out a pilot-study by defining a sample compiled by including all Gaia-DR2 sources within one degree of the North Galactic Pole (NGP) selected to have proper motions consistent with zero within 2-sigma uncertainty. By cross-matching the sample to the optical Sloan Digital Sky Survey (SDSS) and the mid-infrared AllWISE photometric catalogues we investigate the colours of each of our sources. Together with already spectroscopically confirmed quasars we are therefore able to determine the efficiency of our selection. The majority of the zero proper motion sources have optical to mid-infrared colours consistent with known quasars. The remaining population may be contaminating stellar sources, but some may also be quasars with colours similar to stars. Spectroscopic follow-up of the zero proper motion sources is needed to unveil such a hitherto hidden quasar population. This approach has the potential to allow substantial progress on many important questions concerning quasars such as determining the fraction of dust-obscured quasars, the fraction of broad absorption line (BAL) quasars, and the metallicity distribution of damped Lyman-$α$ absorbers. The technique could also potentially reveal new types of quasars or even new classes of cosmological point sources.

astro-ph.GA↗

arXiv2018

A quasar hiding behind two dusty absorbers. Quantifying the selection bias of metal-rich, damped Lyman-alpha absorption systems

The cosmic chemical enrichment as measured from damped Ly$α$ absorbers (DLAs) will be underestimated if dusty and metal-rich absorbers have evaded identification. Here we report the discovery and present the spectroscopic observations of a quasar, KV-RQ\,1500-0031, at $z=2.520$ reddened by a likely dusty DLA at $z=2.428$ and a strong MgII absorber at $z=1.603$. This quasar was identified as part of the KiDS-VIKING Red Quasar (KV-RQ) survey, specifically aimed at targeting dusty absorbers which may cause the background quasars to escape the optical selection of e.g. the SDSS quasar survey. For the DLA we find an HI column density of $\log N$(HI) = $21.2\pm 0.1$ and a metallicity of [X/H] = $-0.90\pm 0.20$ derived from an empirical relation based on the equivalent width of SiII$λ$1526. We observe a total visual extinction of $A_V=0.16$ mag induced by both absorbers. We compile a sample of 17 additional dusty ($A_V > 0.1$ mag) DLAs toward quasars (QSO-DLAs) from the literature for which we characterize the properties of HI column density, metallicity and dust. From this sample we also estimate a correction factor to the overall DLA metallicity budget. We demonstrate that the dusty QSO-DLAs have high metal column densities ($\log N$(HI) + [X/H]) and are more similar to gamma-ray burst (GRB)-selected DLAs (GRB-DLAs) than regular QSO-DLAs. We evaluate the effect of dust reddening in DLAs as well as illustrate how the induced color excess of the underlying quasars can be significant (up to $\sim 1$ mag in various optical bands), even for low to moderate extinction values ($A_V \lesssim 0.6$ mag). Finally we discuss the direct and indirect implications of a significant dust bias in both QSO- and GRB-DLA samples. [Abridged]

astro-ph.GA↗

arXiv2018

Massive, Absorption-selected Galaxies at Intermediate Redshifts

The nature of absorption-selected galaxies and their connection to the general galaxy population have been open issues for more than three decades, with little information available on their gas properties. Here we show, using detections of carbon monoxide (CO) emission with the Atacama Large Millimeter/submillimeter Array (ALMA), that five of seven high-metallicity, absorption-selected galaxies at intermediate redshifts, $z \approx 0.5-0.8$, have large molecular gas masses, $M_{\rm Mol} \approx (0.6 - 8.2) \times 10^{10} \: {\rm M}_\odot$ and high molecular gas fractions ($f_{\rm Mol} \equiv \: M_{\rm Mol}/(M_\ast + M_{\rm Mol}) \approx 0.29-0.87)$. Their modest star formation rates (SFRs), $\approx (0.3-9.5) \: {\rm M}_\odot$ yr$^{-1}$, then imply long gas depletion timescales, $\approx (3 - 120)$ Gyr. The high-metallicity absorption-selected galaxies at $z \approx 0.5-0.8$ appear distinct from populations of star-forming galaxies at both $z \approx 1.3-2.5$, during the peak of star formation activity in the Universe, and lower redshifts, $z \lesssim 0.05$. Their relatively low SFRs, despite the large molecular gas reservoirs, may indicate a transition in the nature of star formation at intermediate redshifts, $z \approx 0.7$.

astro-ph.GA↗

arXiv2018

The X-shooter GRB afterglow legacy sample (XS-GRB)

In this work we present spectra of all $γ$-ray burst (GRB) afterglows that have been promptly observed with the X-shooter spectrograph until 31-03-2017. In total, we obtained spectroscopic observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimize biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneous sample of 93 afterglow spectra for GRBs discovered by the Swift satellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluric absorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swift sample. We constrain the fraction of dark bursts to be < 28 per cent and we confirm previous results that higher optical darkness is correlated with increased X-ray absorption. For the 42 bursts for which it is possible, we provide a measurement of the neutral hydrogen column density, increasing the total number of published HI column density measurements by $\sim$ 33 per cent. This dataset provides a unique resource to study the ISM across cosmic time, from the local progenitor surroundings to the intervening universe.

astro-ph.HE↗

arXiv2017

Cosmic evolution and metal aversion in super-luminous supernova host galaxies

The SUperluminous Supernova Host galaxIES (SUSHIES) survey aims to provide strong new constraints on the progenitors of superluminous supernovae (SLSNe) by understanding the relationship to their host galaxies. We present the photometric properties of 53 H-poor and 16 H-rich SLSN host galaxies out to $z\sim4$. We model their spectral energy distributions to derive physical properties, which we compare with other galaxy populations. At low redshift, H-poor SLSNe are preferentially found in very blue, low-mass galaxies with high average specific star-formation rates. As redshift increases, the host population follows the general evolution of star-forming galaxies towards more luminous galaxies. After accounting for secular evolution, we find evidence for differential evolution in galaxy mass, but not in the $B$-band and the far UV luminosity ($3σ$ confidence). Most remarkable is the scarcity of hosts with stellar masses above $10^{10}~M_\odot$ for both classes of SLSNe. In the case of H-poor SLSNe, we attribute this to a stifled production efficiency above $\sim0.4$ solar metallicity. However, we argue that, in addition to low metallicity, a short-lived stellar population is also required to regulate the SLSN production. H-rich SLSNe are found in a very diverse population of star-forming galaxies. Still, the scarcity of massive hosts suggests a stifled production efficiency above $\sim0.8$ solar metallicity. The large dispersion of the H-rich SLSNe host properties is in stark contrast to those of gamma-ray burst, regular core-collapse SN, and H-poor SLSNe host galaxies. We propose that multiple progenitor channels give rise to this sub-class.

astro-ph.GA↗

arXiv2017

ALMA + VLT observations of a Damped Lyman-α absorbing galaxy: Massive, wide CO emission, gas-rich but with very low SFR

We are undertaking an ALMA survey of molecular gas in galaxies selected for their strong HI absorption, so-called DLA/sub-DLA galaxies. Here we report CO(2-1) detection from a DLA galaxy at z = 0.716. We also present optical and near-infrared spectra of the galaxy revealing [OII], Hα and [NII] emission lines shifted by ~170 km/s relative to the DLA, and providing an oxygen abundance 3.2 times solar, similar to the absorption metallicity. We report low unobscured SFR ~1 Msun/yr given the large reservoir of molecular gas, and also modest obscured SFR=4.5(+4.4,-2.6) Msun/yr based on far-IR and sub-mm data. We determine mass components of the galaxy: log[M*/Msun] = 10.80(+0.07,-0.14), log[M mol-gas/Msun] = 10.37 +/-0.04, and log[M dust/Msun] = 8.45(+0.10,-0.30). Surprisingly, this HI absorption-selected galaxy has no equivalent objects in CO surveys of flux-selected samples. The galaxy falls off current scaling relations for the SFR to molecular gas mass and CO Tully-Fisher relation. Detailed comparison of kinematical components of the absorbing, ionized and molecular gas, combined with their spatial distribution, suggests that part of the CO gas is both kinematically and spatially de-coupled from the main galaxy. It is thus possible that a major star burst in the past could explain the wide CO profile as well as the low SFR. Support for this also comes from the SED favouring an instantaneous burst of age ~0.5 Gyr. Our survey will establish whether flux-selected surveys of molecular gas are missing a key stage in the evolution of galaxies and their conversion of gas to stars.

astro-ph.GA↗

arXiv2017

Mass and metallicity scaling relations of high redshift star-forming galaxies selected by GRBs

We present a comprehensive study of the relations between gas kinematics, metallicity, and stellar mass in a sample of 82 GRB-selected galaxies using absorption and emission methods. We find the velocity widths of both emission and absorption profiles to be a proxy of stellar mass. We also investigate the velocity-metallicity correlation and its evolution with redshift and find the correlation derived from emission lines to have a significantly smaller scatter compared to that found using absorption lines. Using 33 GRB hosts with measured stellar mass and metallicitiy, we study the mass-metallicity relation for GRB host galaxies in a stellar mass range of $10^{8.2} M_{\odot}$ to $10^{11.1} M_{\odot}$ and a redshift range of $ z\sim 0.3-3.4$. The GRB-selected galaxies appear to track the mass-metallicity relation of star forming galaxies but with an offset of 0.15 towards lower metallicities. This offset is comparable with the average error-bar on the metallicity measurements of the GRB sample and also the scatter on the MZ relation of the general population. It is hard to decide whether this relatively small offset is due to systematic effects or the intrinsic nature of GRB hosts. We also investigate the possibility of using absorption-line metallicity measurements of GRB hosts to study the mass-metallicity relation at high redshifts. Our analysis shows that the metallicity measurements from absorption methods can significantly differ from emission metallicities and assuming identical measurements from the two methods may result in erroneous conclusions.

astro-ph.GA↗

arXiv2017

Solving the conundrum of intervening strong MgII absorbers towards GRBs and quasars

Previous studies have shown that the incidence rate of intervening strong MgII absorbers towards GRBs were a factor of 2 - 4 higher than towards quasars. Exploring the similar sized and uniformly selected legacy data sets XQ-100 and XSGRB, each consisting of 100 quasar and 81 GRB afterglow spectra obtained with a single instrument (VLT/X-shooter), we demonstrate that there is no disagreement in the number density of strong MgII absorbers with rest-frame equivalent widths $W_r^{2796} >$ 1 Å towards GRBs and quasars in the redshift range 0.1 < z < 5. With large and similar sample sizes, and path length coverages of $Δ$z = 57.8 and 254.4 for GRBs and quasars, respectively, the incidences of intervening absorbers are consistent within 1 sigma uncertainty levels at all redshifts. For absorbers at z < 2.3 the incidence towards GRBs is a factor of 1.5$\pm$0.4 higher than the expected number of strong MgII absorbers in SDSS quasar spectra, while for quasar absorbers observed with X-shooter we find an excess factor of 1.4$\pm$0.2 relative to SDSS quasars. Conversely, the incidence rates agree at all redshifts with reported high spectral resolution quasar data, and no excess is found. The only remaining discrepancy in incidences is between SDSS MgII catalogues and high spectral resolution studies. The rest-frame equivalent width distribution also agrees to within 1 sigma uncertainty levels between the GRB and quasar samples. Intervening strong MgII absorbers towards GRBs are therefore neither unusually frequent, nor unusually strong.

astro-ph.GA↗

arXiv2017

The MUSE view of the host galaxy of GRB 100316D

The low distance, $z=0.0591$, of GRB 100316D and its association with SN 2010bh represent two important motivations for studying this host galaxy and the GRB's immediate environment with the Integral-Field Spectrographs like VLT/MUSE. Its large field-of-view allows us to create 2D maps of gas metallicity, ionization level, and the star-formation rate distribution maps, as well as to investigate the presence of possible host companions. The host is a late-type dwarf irregular galaxy with multiple star-forming regions and an extended central region with signatures of on-going shock interactions. The GRB site is characterized by the lowest metallicity, the highest star-formation rate and the youngest ($\sim$ 20-30 Myr) stellar population in the galaxy, which suggest a GRB progenitor stellar population with masses up to 20 -- 40 $M_{\odot}$. We note that the GRB site has an offset of $\sim$660pc from the most luminous SF region in the host. The observed SF activity in this galaxy may have been triggered by a relatively recent gravitational encounter between the host and a small undetected ($L_{Hα} \leq 10^{36}$ erg/s) companion.

astro-ph.GA↗

arXiv2017

The most distant, luminous, dusty star-forming galaxies: redshifts from NOEMA and ALMA spectral scans

We present 1.3- and/or 3-mm continuum images and 3-mm spectral scans, obtained using NOEMA and ALMA, of 21 distant, dusty, star-forming galaxies (DSFGs). Our sample is a subset of the galaxies selected by Ivison et al. (2016) on the basis of their extremely red far-infrared (far-IR) colours and low {\it Herschel} flux densities; most are thus expected to be unlensed, extraordinarily luminous starbursts at $z \gtrsim 4$, modulo the considerable cross-section to gravitational lensing implied by their redshift. We observed 17 of these galaxies with NOEMA and four with ALMA, scanning through the 3-mm atmospheric window. We have obtained secure redshifts for seven galaxies via detection of multiple CO lines, one of them a lensed system at $z=6.027$ (two others are also found to be lensed); a single emission line was detected in another four galaxies, one of which has been shown elsewhere to lie at $z=4.002$. Where we find no spectroscopic redshifts, the galaxies are generally less luminous by 0.3-0.4 dex, which goes some way to explaining our failure to detect line emission. We show that this sample contains amongst the most luminous known star-forming galaxies. Due to their extreme star-formation activity, these galaxies will consume their molecular gas in $\lesssim 100$ Myr, despite their high molecular gas masses, and are therefore plausible progenitors of the massive, `red-and-dead' elliptical galaxies at $z \approx 3$.

astro-ph.GA↗

arXiv2017

Magnifying the early episodes of star formation: super star clusters at cosmological distances

We study the spectrophotometric properties of a highly magnified (μ~40-70) pair of stellar systems identified at z=3.2222 behind the Hubble Frontier Field galaxy cluster MACS~J0416. Five multiple images (out of six) have been spectroscopically confirmed by means of VLT/MUSE and VLT/X-Shooter observations. Each image includes two faint (m_uv~30.6), young (<100 Myr), low-mass (<10^7 Msun), low-metallicity (12+Log(O/H)~7.7, or 1/10 solar) and compact (30 pc effective radius) stellar systems separated by ~300pc, after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow (σ_v <~ 25 km/s) high-ionization lines. These features may be the signature of very hot (T>50000 K) stars within dense stellar clusters, whose dynamical mass is likely dominated by the stellar component. Remarkably, the ultraviolet metal lines are not accompanied by Lya emission (e.g., CIV / Lya > 15), despite the fact that the Lya line flux is expected to be 150 times brighter (inferred from the Hbeta flux). A spatially-offset, strongly-magnified (μ>50) Lya emission with a spatial extent <~7.6 kpc^2 is instead identified 2 kpc away from the system. The origin of such a faint emission can be the result of fluorescent Lya induced by a transverse leakage of ionizing radiation emerging from the stellar systems and/or can be associated to an underlying and barely detected object (with m_uv > 34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Lya emission, suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Lya detection.

astro-ph.GA↗

arXiv2017

Steep extinction towards GRB 140506A reconciled from host galaxy observations: Evidence that steep reddening laws are local

We present the spectroscopic and photometric late-time follow-up of the host galaxy of the long-duration Swift gamma-ray burst GRB 140506A at z = 0.889. The optical and near-infrared afterglow of this GRB had a peculiar spectral energy distribution (SED) with a strong flux-drop at 8000 Å (4000 Å rest-frame) suggesting an unusually steep extinction curve. By analyzing the contribution and physical properties of the host galaxy, we here aim at providing additional information on the properties and origin of this steep, non-standard extinction. We find that the strong flux-drop in the GRB afterglow spectrum at < 8000 Å and rise at < 4000 Å is well explained by the combination of a steep extinction curve along the GRB line of sight and contamination by the host galaxy light so that the scenario with an extreme 2175 Å extinction bump can be excluded. We localise the GRB to be at a projected distance of approximately 4 kpc from the centre of the host galaxy. Based on emission-line diagnostics of the four detected nebular lines, Halpha, Hbeta, [O II] and [O III], we find the host to be a modestly star forming (SFR = 1.34 +/- 0.04 Msun yr^-1) and relatively metal poor (Z = 0.35^{+0.15}_{-0.11} Zsun) galaxy with a large dust content, characterized by a measured visual attenuation of A_V = 1.74 +/- 0.41 mag, thus unexceptional in all its physical properties. We model the extinction curve of the host-corrected afterglow and show that the standard dust properties causing the reddening seen in the Local Group are inadequate in describing the steep drop. We conclude that the steep extinction curve seen in the afterglow towards the GRB is of exotic origin, is sightline-dependent only and thus solely a consequence of the circumburst environment.

astro-ph.GA↗