arXiv subjects

A. De Cia

Publications and source records attributed to A. De Cia.

At least 19 recordsLinked to original sources

arXiv2026

Connecting the dusty dots: dust depletion and extinction of local interstellar clouds

Investigating the chemical complexity of the interstellar medium (ISM) is key for understanding its physical nature and evolution. In this work, we study parsec-scale interstellar dust clouds in the neutral ISM of the Milky Way using two different probes: dust depletion and dust extinction. We examine their relationship to investigate the distribution of metals and dust in the Solar neighbourhood, and how they are related to the Local Bubble. We use measurements of dust depletion for individual gas clouds along sight lines of sight towards bright O/B stars within 1.1 kpc of the Sun, derived from UV absorption-line spectra. We combine these with parsec-scale 3D dust extinction density maps out to 1.25 kpc. We assume a correlation between dust depletion and dust extinction density, which we use to imply that the absorption components are spatially associated with the peaks in dust extinction density, and to pinpoint the likely locations of the gas clouds in physical space. We identify peaks in the dust extinction curves, and then associate the stronger peaks with the strongest dust depletion components. Independent distance measurements along the line of sight towards one of our targets, theta1 Ori C, validates our results. In our sample, the minimum distance between clouds that have significantly different chemical properties is ~ 100pc, giving an indication on the physical scale on which chemical mixing remains incomplete in the ISM of the Milky Way. For five of the eight targets, we report dust depletion values for gas clouds associated with the Local Bubble. Additionally, we find a velocity gradient that is consistent with the expansion of the Local Bubble, further supporting our methodology. Overall, we show that it is possible to use complementary information from dust depletion and dust extinction to build more detailed maps of ISM metal and dust distributions.

astro-ph.GA↗

arXiv2025

A large, chemically enriched, neutral gas reservoir in a galaxy at z = 6.782

The chemical characterization of galaxies in the first billion years after the Big Bang is one of the central goals of current astrophysics. Optical/near-infrared spectroscopy of long gamma-ray bursts (GRBs) have been heralded as an effective diagnostic to probe the interstellar medium of their host galaxies and their metal and dust content, up to the highest redshift. An opportunity to fulfill this expectation was provided by the recent blast triggered by the Neil Gehrels Swift Observatory of GRB 240218A at redshift z=6.782. We study a high-redshift galaxy selected in a complementary way with respect to flux-limited surveys, not depending on galaxy luminosity and stellar mass. We present the VLT/X-shooter spectrum of its afterglow enabling the detection of neutral-hydrogen, low-ionization, high-ionization and fine-structure absorption lines. We determine the metallicity, kinematics and chemical abundance pattern, providing the first detailed characterization of the neutral gas of a galaxy at z>6.5. From the analysis of fine-structure lines we estimate the distance of the closest gas clouds as $d_{II}=620^{+230}_{-140}$ pc. We determine a high neutral hydrogen column density, $\log(N(HI)/cm^{-2})=22.5\pm0.3$, which is the highest one at z>6 determined so far for a GRB host galaxy, as well as a surprisingly high metal column density, $\log(N(ZnII)/cm^{-2})>14.3$. The observed metallicity of the host galaxy system is [Zn/H]>-0.8. We find evidence of a high amount of dust depletion and of aluminum overabundance, although a number of transitions are saturated. The high hydrogen column density, metal abundances and dust depletion in the neutral gas align with those of the ionized gas of very high-redshift galaxies unveiled by ALMA and JWST, testifying that a rapid build up of metals and dust, and massive neutral hydrogen reservoirs seem to be common features of galaxies in the early Universe.

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arXiv2025

Investigating chemical variations between interstellar gas clouds in the Solar neighbourhood

The interstellar medium (ISM) is a fundamental component of the Milky Way. Studying its chemical composition and the level of its chemical diversity gives us insight into the evolution of the Milky Way and the role of gas in the Galactic environment. In this paper, we use a novel simulation technique to model the distribution of total hydrogen between gas components, and therefore derive new constraints on the dust depletion and metallicity. We study individual gas components along the lines of sight towards eight bright O/B stars within 1.1 kpc of the Sun using high-resolution HST/STIS absorption spectra (R sim 114 000). We measure the level of dust depletion for these individual components and find components with higher levels of dust depletion compared to Milky Way sightlines in the literature. We find large ranges in the level of dust depletion among components along lines of sight, up to 1.19 dex. Although it is not possible to directly measure the metallicity of individual components due to the saturated and damped Ly-alpha line, we investigate possible metallicity ranges for individual gas components by exploring many different distributions of the total hydrogen gas between components. We select possible combinations of these gas fractions which produce the minimum metallicity difference between components, and for these cases we determine individual metallicities to accuracies that range between sim 0.1 to 0.4 dex. This work shows that full line-of-sight analyses wash out the level of diversity along lines of sight, and that component-by-component studies give a more in-depth understanding of the chemical intricacies of the interstellar medium.

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arXiv2024

Modelling of long gamma-ray burst host galaxies at cosmic noon from damped Lyman-α absorption statistics

We study the properties of long gamma-ray burst (GRB) host galaxies using a statistical modelling framework derived to model damped Lyman-$α$ absorbers (DLAs) in quasar spectra at high redshift. The distribution of NHI for GRB-DLAs is $\sim$10 times higher than what is found for quasar-DLAs at similar impact parameters. We interpret this as a temporal selection effect due to the short-lived GRB progenitor probing its host at the onset of a starburst where the interstellar medium may exhibit multiple overdense regions. Owing to the larger NHI, the dust extinction is larger with 29 per cent of GRB-DLAs exhibiting A(V)>1 mag in agreement with the fraction of 'dark bursts'. Despite the differences in NHI distributions, we find that high-redshift 2 < z < 3 quasar- and GRB-DLAs trace the luminosity function of star-forming host galaxies in the same way. We propose that their differences may arise from the fact that the galaxies are sampled at different times in their star formation histories, and that the absorption sightlines probe the galaxy haloes differently. Quasar-DLAs sample the full H I cross-section, whereas GRB-DLAs sample only regions hosting cold neutral medium. Previous studies have found that GRBs avoid high-metallicity galaxies ($\sim$0.5 $Z_{\odot}$). Since at these redshifts galaxies on average have lower metallicities, our sample is only weakly sensitive to such a threshold. Lastly, we find that the modest detection rate of cold gas (H$_2$ or C I) in GRB spectra can be explained mainly by a low volume filling factor of cold gas clouds and to a lesser degree by destruction from the GRB explosion itself.

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arXiv2024

ANDES, the high resolution spectrograph for the ELT: science goals, project overview and future developments

The first generation of ELT instruments includes an optical-infrared high-resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of $\sim$100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 $μ$m with the goal of extending it to 0.35-2.4 $μ$m with the addition of a U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allow ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases, there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states.

astro-ph.IM↗

arXiv2024

Rapid Response Mode observations of GRB 160203A: Looking for fine-structure line variability at z=3.52

Gamma-ray bursts are the most energetic known explosions. Despite fading rapidly, they allow to measure redshift and important properties of their host-galaxies. We report the photometric and spectroscopic study of GRB 160203A and its host-galaxy. Fine-structure absorption lines, detected in the afterglow at different epochs, allow us to investigate variability due to the strong fading background source. We obtained two optical to near-infrared spectra of the afterglow with X-shooter on ESO/VLT, 18 min and 5.7 hrs after the burst, allowing us to investigate temporal changes of fine-structure absorption lines. We measured HI column density log N(HI/cm-2)=21.75+/-0.10, and several heavy-element ions along the GRB sight-line in the host-galaxy: SiII,AlII,AlIII,CII,NiII,SiIV,CIV,ZnII,FeII, and FeII and SiII fine structure transitions from energetic levels excited by the afterglow, at a redshift z=3.518. We measured [M/H]TOT=-0.78+/-0.13 and [Zn/Fe]FIT=0.69+/-0.15, representing the total(dust-corrected) metallicity and dust depletion, respectively. We detected additional intervening systems along the line of sight at z=1.03,z=1.26,z=1.98,z=1.99,z=2.20 and z=2.83. We could not measure significant variability in the fine-structure lines throughout all the observations and determined an upper limit for the GRB distance from the absorber of d<300 pc, adopting the canonical UV pumping scenario. However, we note that the quality of our data is not sufficient to conclusively rule out collisions as an alternative mechanism. GRB 160203A belongs to a growing sample of GRBs with medium resolution spectroscopy, provided by the Swift/X-shooter legacy program, which enables detailed investigation of the interstellar medium in high-redshift GRB host-galaxies. In particular, this host galaxy shows relatively high metal enrichment and dust depletion already in place when the universe was only 1.8 Gyr old.

astro-ph.HE↗

arXiv2024

Comparing emission- and absorption-based gas-phase metallicities in GRB host galaxies at $z=2-4$ using JWST

Much of what is known of the chemical composition of the universe is based on emission line spectra from star forming galaxies. Emission-based inferences are, nevertheless, model-dependent and they are dominated by light from luminous star forming regions. An alternative and sensitive probe of the metallicity of galaxies is through absorption lines imprinted on the luminous afterglow spectra of long gamma ray bursts (GRBs) from neutral material within their host galaxy. We present results from a JWST/NIRSpec programme to investigate for the first time the relation between the metallicity of neutral gas probed in absorption by GRB afterglows and the metallicity of the star forming regions for the same host galaxy sample. Using an initial sample of eight GRB host galaxies at z=2.1-4.7, we find a tight relation between absorption and emission line metallicities when using the recently proposed $\hat{R}$ metallicity diagnostic (+/-0.2dex). This agreement implies a relatively chemically-homogeneous multi-phase interstellar medium, and indicates that absorption and emission line probes can be directly compared. However, the relation is less clear when using other diagnostics, such as R23 and R3. We also find possible evidence of an elevated N/O ratio in the host galaxy of GRB090323 at z=3.58, consistent with what has been seen in other $z>4$ galaxies. Ultimate confirmation of an enhanced N/O ratio and of the relation between absorption and emission line metallicities will require a more direct determination of the emission line metallicity via the detection of temperature-sensitive auroral lines in our GRB host galaxy sample.

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arXiv2024

Resolving the explosion of supernova 2023ixf in Messier 101 within its complex circumstellar environment

Observing a supernova explosion shortly after it occurs can reveal important information about the physics of stellar explosions and the nature of the progenitor stars of supernovae (SNe). When a star with a well-defined edge explodes in vacuum, the first photons to escape from its surface appear as a brief shock-breakout flare. The duration of this flare can extend to at most a few hours even for nonspherical breakouts from supergiant stars, after which the explosion ejecta should expand and cool. Alternatively, for stars exploding within a distribution of sufficiently dense optically thick circumstellar material, the first photons escape from the material beyond the stellar edge, and the duration of the initial flare can extend to several days, during which the escaping emission indicates photospheric heating. The difficulty in detecting SN explosions promptly after the event has so far limited data regarding supergiant stellar explosions mostly to serendipitous observations that, owing to the lack of ultraviolet (UV) data, were unable to determine whether the early emission is heating or cooling, and hence the nature of the early explosion event. Here, we report observations of SN 2023ixf in the nearby galaxy M101, covering the early days of the event. Using UV spectroscopy from the Hubble Space Telescope (HST) as well as a comprehensive set of additional multiwavelength observations, we trace the photometric and spectroscopic evolution of the event and are able to temporally resolve the emergence and evolution of the SN emission.

astro-ph.HE↗

arXiv2023

The cosmic build-up of dust and metals. Accurate abundances from GRB-selected star-forming galaxies at $1.7 < z < 6.3$

The chemical enrichment of dust and metals in the interstellar medium (ISM) of galaxies throughout cosmic time is one of the key driving processes of galaxy evolution. Here we study the evolution of the gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) ratios of 36 star-forming galaxies at $1.7 < z < 6.3$ probed by gamma-ray bursts (GRBs). We compile all GRB-selected galaxies with intermediate (R=7000) to high (R>40,000) resolution spectroscopic data for which at least one refractory (e.g. Fe) and one volatile (e.g. S or Zn) element have been detected at S/N>3. This is to ensure that accurate abundances and dust depletion patterns can be obtained. We first derive the redshift evolution of the dust-corrected, absorption-line based gas-phase metallicity [M/H]$_{\rm tot}$ in these galaxies, for which we determine a linear relation with redshift ${\rm [M/H]_{tot}}(z) = (-0.21\pm 0.04)z -(0.47\pm 0.14)$. We then examine the DTG and DTM ratios as a function of redshift and through three orders of magnitude in metallicity, quantifying the relative dust abundance both through the direct line-of-sight visual extinction $A_V$ and the derived depletion level. We use a novel method to derive the DTG and DTM mass ratios for each GRB sightline, summing up the mass of all the depleted elements in the dust-phase. We find that the DTG and DTM mass ratios are both strongly correlated with the gas-phase metallicity and show a mild evolution with redshift as well. While these results are subject to a variety of caveats related to the physical environments and the narrow pencil-beam sightlines through the ISM probed by the GRBs, they provide strong implications for studies of dust masses to infer the gas and metal content of high-redshift galaxies, and particularly demonstrate the large offset from the average Galactic value in the low-metallicity, high-redshift regime.

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arXiv2023

Proximate molecular quasar absorbers: Chemical enrichment and kinematics of the neutral gas

Proximate molecular quasar absorbers (PH2) are an intriguing population of absorption systems recently uncovered through strong H2 absorption at small velocity separation from the background quasars. We performed a multi-wavelength spectroscopic follow-up of thirteen such systems with VLT/X-Shooter. Here, we present the observations and study the overall chemical enrichment measured from the HI, H2 and metal lines. We combine this with an investigation of the neutral gas kinematics with respect to the quasar host. We find gas-phase metallicities in the range 2% to 40% of the Solar value, i.e. in the upper-half range of HI-selected proximate damped Lyman-alpha systems, but similar to what is seen in intervening H2-bearing systems. This is likely driven by similar selection effects that play against the detection of most metal and molecular rich systems in absorption. Differences are however seen in the abundance of dust (from [Zn/Fe]) and its depletion pattern, when compared to intervening systems, possibly indicating different dust production or destruction close to the AGN. We also note the almost-ubiquitous presence of a high-ionisation phase traced by NV in proximate systems. In spite of the hard UV field from the quasars, we found no strong overall deficit of neutral argon, at least when compared to intervening DLAs. This likely results from argon being mostly neutral in the H2 phase, which actually accounts for a large fraction of the total amount of metals. We measure the quasar systemic redshifts through emission lines from both ionised gas and CO(3-2) emission, the latter being detected in all 6 cases for which we obtained 3-mm data from complementary NOEMA observations. For the first time, we observe a trend between line-of-sight velocity with respect to systemic redshift and metallicity of the absorbing gas. [truncated]

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arXiv2023

Chemical diversity of gas in distant galaxies: The metal and dust enrichment and variations within absorbing galaxies

The chemical composition of gas in galaxies can be measured in detail from absorption spectroscopy. By studying gas in galaxies in this way, it is possible to investigate the small and faint galaxies, which are the most numerous in the universe. In particular, the chemical distribution of gas in absorbing systems gives us insight into cycles of gas in and around galaxies. Here we study chemical enrichment within 64 Damped Lyman-alpha Absorption (DLA) systems between $1.7 < z < 4.2$. We use high-resolution spectra from VLT/UVES to infer dust depletion from relative abundances of several metals. We perform a component-by-component analysis within DLAs, and characterise variations in their chemical enrichment. Unlike hydrogen, the metal columns can be characterised for individual components. We use them to derive the dust depletion ([Zn/Fe]fit), as an indicator for chemical enrichment. We find that some DLAs are chemically diverse within themselves, with [Zn/Fe]fit ranging up to 0.62 dex within a single system. This suggests that absorbing gas within these galaxies is chemically diverse. Although we do not find a clear trend of decreasing dust depletion with redshift, we do see that the most chemically enriched systems are at lower redshifts. We also observe evidence for dust-poor components at all redshifts, which may be due to the accretion of pristine gas onto galaxies. We combine the chemical and kinematic properties of the individual gas components and observe potential signatures of infalling gas, with low depletion at velocities below $\sim$100km/s, and outflows, with high depletion and velocities of $\sim$600km/s. We find over-abundances of alpha-elements (an enhancement of $\sim$0.3dex) and under-abundances of Mn in several components, which is likely a signature of core-collapse SNe nucleosythesis in the ISM. We observe these effects mostly at lower levels of chemical enrichment.

astro-ph.GA↗

arXiv2023

Dissecting the interstellar medium of a z=6.3 galaxy: X-shooter spectroscopy and HST imaging of the afterglow and environment of the Swift GRB 210905A

The study of the properties of galaxies in the first billion years after the Big Bang is one of the major topic of current astrophysics. Optical/near-infrared spectroscopy of the afterglows of long Gamma-ray bursts (GRBs) provide a powerful diagnostic tool to probe the interstellar medium (ISM) of their host galaxies and foreground absorbers, even up to the highest redshifts. We analyze the VLT/X-shooter afterglow spectrum of GRB 210905A, triggered by the Swift Neil Gehrels Observatory, and detect neutral-hydrogen, low-ionization, high-ionization, and fine-structure absorption lines from a complex system at z=6.3118, that we associate with the GRB host galaxy. We study the ISM properties of the host system, revealing the metallicity, kinematics and chemical abundance pattern. The total metallicity of the z~6.3 system is [M/H]=-1.72+/-0.13, after correcting for dust-depletion and taking into account alpha-element enhancement. In addition, we determine the overall amount of dust and dust-to-metal mass ratio (DTM) ([Zn/Fe]_fit=0.33+/-0.09, DTM=0.18+/-0.03). We find indications of nucleosynthesis due to massive stars and evidence of peculiar over-abundance of aluminium. From the analysis of fine-structure lines, we determine distances of several kpc for the low-ionization gas clouds closest to the GRB. Those farther distances are possibly due to the high number of ionizing photons. Using the HST/F140W image of the GRB field, we show the GRB host galaxy as well as multiple objects within 2" from the GRB. We discuss the galaxy structure and kinematics that could explain our observations, also taking into account a tentative detection of Lyman-alpha emission. Deep spectroscopic observations with VLT/MUSE and JWST will offer the unique possibility of combining our results with the ionized-gas properties, with the goal of better understanding how galaxies in the reionization era form and evolve.

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arXiv2022

CUBES: a UV spectrograph for the future

In spite of the advent of extremely large telescopes in the UV/optical/NIR range, the current generation of 8-10m facilities is likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (>40%) observations in the near UV (305-400 nm requirement, 300-420 nm goal) at a spectral resolving power of R>20,000, although a lower-resolution, sky-limited mode of R ~ 7,000 is also planned. CUBES will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the Phase B dedicated to detailed design and construction. First science operations are planned for 2028. In this paper, we briefly describe the CUBES project development and goals, the main science cases, the instrument design and the project organization and management.

astro-ph.IM↗

arXiv2022

CUBES, the Cassegrain U-Band Efficient Spectrograph

In the era of Extremely Large Telescopes, the current generation of 8-10m facilities are likely to remain competitive at ground-UV wavelengths for the foreseeable future. The Cassegrain U-Band Efficient Spectrograph (CUBES) has been designed to provide high-efficiency (>40%) observations in the near UV (305-400 nm requirement, 300-420 nm goal) at a spectral resolving power of R>20,000 (with a lower-resolution, sky-limited mode of R ~ 7,000). With the design focusing on maximizing the instrument throughput (ensuring a Signal to Noise Ratio (SNR) ~20 per high-resolution element at 313 nm for U ~18.5 mag objects in 1h of observations), it will offer new possibilities in many fields of astrophysics, providing access to key lines of stellar spectra: a tremendous diversity of iron-peak and heavy elements, lighter elements (in particular Beryllium) and light-element molecules (CO, CN, OH), as well as Balmer lines and the Balmer jump (particularly important for young stellar objects). The UV range is also critical in extragalactic studies: the circumgalactic medium of distant galaxies, the contribution of different types of sources to the cosmic UV background, the measurement of H2 and primordial Deuterium in a regime of relatively transparent intergalactic medium, and follow-up of explosive transients. The CUBES project completed a Phase A conceptual design in June 2021 and has now entered the detailed design and construction phase. First science operations are planned for 2028.

astro-ph.IM↗

arXiv2022

METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble program. III. Interstellar Depletions, Dust-to-Metal, and Dust-to-Gas Ratios Versus Metallicity

The metallicity and gas density dependence of interstellar depletions, the dust-to-gas (D/G), and dust-to-metal (D/M) ratios have important implications for how accurately we can trace the chemical enrichment of the universe; either by using FIR dust emission as a tracer of the ISM; or by using spectroscopy of damped Lyman-alpha systems (DLAs) to measure chemical abundances over a wide range of redshifts. We collect and compare large samples of depletion measurements in the Milky Way (MW), LMC (Z=0.5 Zsun), and SMC (Z=0.2 Zsun). The relation between the depletions of different elements do not strongly vary between the three galaxies, implying that abundance ratios should trace depletions accurately down to 20% solar metallicity. From the depletions, we derive D/G and D/M. The D/G increases with density, consistent with the more efficient accretion of gas-phase metals onto dust grains in the denser ISM. For log N(H) > 21 cm^-2, the depletion of metallicity tracers (S, Zn) exceeds -0.5 dex, even at 20% solar metallicity. The gas fraction of metals increases from the MW to the LMC (factor 3) and SMC (factor 6), compensating the reduction in total heavy element abundances and resulting in those three galaxies having the same neutral gas-phase metallicities. The D/G derived from depletions are a factor of 2 (LMC) and 5 (SMC) higher than the D/G derived from FIR, 21 cm, and CO emission, likely due to the combined uncertainties on the dust FIR opacity and on the depletion of carbon and oxygen.

astro-ph.GA↗

arXiv2021

Sharpening quasar absorption lines with ESPRESSO: Temperature of warm gas at $z\sim2$, constraints on the Mg isotopic ratio, and structure of cold gas at $z\sim0.5$

We present a high-resolution (R=140,000) spectrum of the bright quasar HE0001-2340 (z=2.26), obtained with ESPRESSO at the Very Large Telescope. We analyse three systems at z=0.45, z=1.65, and z=2.19 using multiple-component Voigt-profile fitting. We also compare our spectrum with those obtained with VLT/UVES, covering a total period of 17 years. We disentangle turbulent and thermal broadening in many components spread over about 400 km/s in the z~2.19 sub-DLA system. We derive an average temperature of 16000+/-1300 K, i.e., about twice the canonical value of the warm neutral medium in the Galactic interstellar medium. A comparison with other high-z, low-metallicity absorbers reveals an anti-correlation between gas temperature and total HI column density. Although requiring confirmation, this could be the first observational evidence of a thermal decrease with galacto-centric distance, i.e., we may be witnessing a thermal transition between the circum-galactic medium and the cooler ISM. We revisit the Mg isotopic ratios at z=0.45 and z=1.65 and constrain them to be xi = (26Mg+25Mg)/24Mg <0.6 and <1.4 in these two systems, respectively. These values are consistent with the standard Solar ratio, i.e., we do not confirm strong enhancement of heavy isotopes previously inferred from UVES data. Finally, we confirm the partial coverage of the quasar emission-line region by a FeI-bearing cloud in the z=0.45 system and present evidence for velocity sub-structure of the gas that has Doppler parameters of the order of only ~0.3 km/s. This work demonstrates the uniqueness of high-fidelity, high-resolution optical spectrographs on large telescopes as tools to investigate the thermal state of the gas in and around galaxies as well as its spatial and velocity structure on small scales, and to constrain the associated stellar nucleosynthetic history. [abridged]

astro-ph.GA↗

arXiv2021

Exploration of the high-redshift universe enabled by THESEUS

At peak, long-duration gamma-ray bursts are the most luminous sources of electromagnetic radiation known. Since their progenitors are massive stars, they provide a tracer of star formation and star-forming galaxies over the whole of cosmic history. Their bright power-law afterglows provide ideal backlights for absorption studies of the interstellar and intergalactic medium back to the reionization era. The proposed THESEUS mission is designed to detect large samples of GRBs at $z>6$ in the 2030s, at a time when supporting observations with major next generation facilities will be possible, thus enabling a range of transformative science. THESEUS will allow us to explore the faint end of the luminosity function of galaxies and the star formation rate density to high redshifts; constrain the progress of re-ionisation beyond $z\gtrsim6$; study in detail early chemical enrichment from stellar explosions, including signatures of Population III stars; and potentially characterize the dark energy equation of state at the highest redshifts.

astro-ph.IM↗

arXiv2019

On the dust properties of high redshift molecular clouds and the connection to the 2175 Å extinction bump

We present a study of the extinction and depletion-derived dust properties of gamma-ray burst (GRB) absorbers at $1<z<3$ showing the presence of neutral carbon (\ion{C}{I}). By modelling their parametric extinction laws, we discover a broad range of dust models characterizing the GRB \ion{C}{I} absorption systems. In addition to the already well-established correlation between the amount of \ion{C}{I} and visual extinction, $A_V$, we also observe a correlation with the total-to-selective reddening, $R_V$. All three quantities are also found to be connected to the presence and strength of the 2175\,Å dust extinction feature. While the amount of \ion{C}{I} is found to be correlated with the SED-derived dust properties, we do not find any evidence for a connection with the depletion-derived dust content as measured from [Zn/Fe] and $N$(Fe)$_{\rm dust}$. To reconcile this, we discuss a scenario where the observed extinction is dominated by the composition of dust particles confined in the molecular gas-phase of the ISM. We argue that since the depletion level trace non-carbonaceous dust in the ISM, the observed extinction in GRB \ion{C}{I} absorbers is primarily produced by carbon-rich dust in the molecular cloud and is therefore only observable in the extinction curves and not in the depletion patterns. This also indicates that the 2175\,Å dust extinction feature is caused by dust and molecules in the cold and molecular gas-phase. This scenario provides a possible resolution to the discrepancy between the depletion- and SED-derived amounts of dust in high-$z$ absorbers.

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