A study of purely astrometric selection of extragalactic point sources with Gaia

Selection of extragalactic point sources, e.g. QSOs, is often hampered by significant selection effects causing existing samples to have rather complex selection functions. We explore whether a purely astrometric selection of extragalactic point sources, e.g. QSOs, is feasible with the ongoing Gaia mission. Such a selection would be interesting as it would be unbiased in terms of colours of the targets and hence would allow selection also with colours in the stellar sequence. We have analyzed a total of 18 representative regions of the sky by using GUMS, the simulator prepared for ESAs Gaia mission, both in the range of $12\le G \le 20$ mag and $12\le G \le 18$ mag. For each region we determine the density of apparently stationary stellar sources, i.e. sources for which Gaia cannot measure a significant proper motion. The density is contrasted with the density of extragalactic point sources, e.g. QSOs, in order to establish in which celestial directions a pure astrometric selection is feasible. When targeting regions at galactic latitude $|b| \ge 30^\mathrm{o}$ the ratio of QSOs to apparently stationary stars is above 50\% and when observing towards the poles the fraction of QSOs goes up to about $\sim80$\%. We show that the proper motions from the proposed Gaia successor mission in about 20 years would dramatically improve these results at all latitudes. Detection of QSOs solely from zero proper motion, unbiased by any assumptions on spectra, might lead to the discovery of new types of QSOs or new classes of extragalactic point sources.Comment: 4 pages, 4 figures, sent in and accepted for publishing to A&

Galaxies through cosmic time illuminated by gamma-ray bursts and quasars

In the early Universe, most of the cold neutral gas that will later form into individual stars and galaxies is practically invisible to us. These neutral gas reservoirs can, however, be illuminated by bright cosmic lightsources such as gamma-ray bursts (GRBs) and quasars. The aim of this thesis is to use these luminous objects as tools to study the environments of intervening or host galaxy absorption systems through cosmic time. Part I of this thesis is dedicated to examining the gas, dust and metals in the immediate region surrounding GRBs. In Chapter 2, a comprehensive analysis of the host galaxy of GRB 140506A is presented. The optical afterglow spectrum of this GRB showed a very peculiar shape, likely related to an unusual dust reddening curve that extinguished the intrinsic afterglow spectrum. We were able to better characterize this unusual afterglow extinction by subtracting the host galaxy spectrum from the first epoch afterglow spectrum and showed that it can be modelled by a very steep reddening law (but not a 2175 Å dust bump as previously proposed). Since no such extinction component was observed for the average host galaxy spectral energy distribution (SED), we concluded that it was produced local to the GRB, and likely related to the interaction of the GRB with the surrounding circumburst dust. In Chapter 3, the high-ionization metal lines in a sample of GRB afterglows observed with the VLT/X-shooter are examined. We argue that in particular the N v absorption line feature is produced by the surrounding gas that has been highly ionized by the GRB. This argument is also based on the observed tentative corelation with the X-ray derived equivalent hydrogen column density, NH,X, that is also believed to originate from gas within ~10 pc from the GRB. The highly-ionized N v line transition therefore likely probes gas and metals in the circumburst environment of the GRB. Part II presents a search for and the study of cold and molecular gas in high-z GRB host galaxy absorption systems. Chapter 4 introduces a survey for neutral atomic-carbon (CI), which is used as a tracer for molecular hydrogen (H2) in the GRB host absorbers. Here, we also characterize the basic properties of the GRB CI absorbers in terms of their gas, dust and metal content and compare them to CI-bearing absorbers in quasar sightlines. We find that a higher HI column density and metallicity is required for GRB host galaxies to contain cold and molecular gas. In Chapter 5, we specifically examine the dust properties of the GRB CI absorbers and demonstrate that the characteristic 2175 Å dust extinction feature is likely produced by dust particles associated with the CI-bearing molecular cloud. Chapter 6 provides a detailed analysis of the presence of different probes of the molecular gas-phase in the ISM of GRB host galaxies such as CI and vibrationally-excited H2. Here, the typical physical conditions of the H2-bearing molecular clouds in GRB host galaxies are also derived. Part III focuses on using quasars to examine gas-rich intervening galaxies in the line of sight, with specific focus on absorption systems rich in dust and metals. It is now clear that several dust-reddened quasars have evaded identification in e.g. the extensive Sloan Digital Sky Survey (SDSS). To alleviate this dust bias we designed the new optical to near/mid-infrared selection criteria presented in Chapter 7, specifically tailored to select dust-reddened quasars at z > 2. In Chapter 8, one of the dusty quasar absorption systems identified in this survey is presented. Here, we also characterize the properties of this absorber and compare the few cases of dusty and metal-rich absorption systems to the general population of absorbers observed in quasar sightlines. Finally, in Chapter 9 we demonstrate that it is possible to define a complete and unbiased quasar selection using the astrometric data from the space-based Gaia mission. This thesis demonstrates the importance of observing large samples of GRB afterglows to 1) allow for statistical studies of the GRB phenomena itself and the associated host galaxy environments and 2) to obtain spectra of peculiar or unusual GRB afterglows, that is only observed rarely. In addition, it highlights that defining a complete and unbiased sample of quasars is vital to fully exploit the potential of quasars as probes of cosmic chemical evolution. In Chapter 10, the general work of this thesis is concluded and some future prospects are provided.Megnið af hinu kalda, ójónaða efni í ungum alheimi, sem síðar myndar stjörnur og vetrarbrautir, er nánast ósýnilegt okkur. Þó geta björt fyrirbæri eins og gammablossar og dulstirni lýst upp þetta dimma efni. Markmið ritgerðarinnar er að nota þessi fyrirbæri sem nokkurs konar verkfæri til að fræðast um útgeimsefnið. Í fyrsta hluta ritgerðarinnar er gas, ryk og þungefni í nánasta nágrenni gammablossa kannað til hlítar. Í kafla 2 er kynnt yfirgripsmikil greining á hýsilvetrarbraut blossans GRB 140506A. Litróf sýnilegra glæða blossans var mjög óvenjulegt, líklega tengt sérstæðri geimroðnun. Okkur tókst að sýna fram á að þessi sérkennilega ljósdeyfing ætti upptök nálægt blossanum og væri afleiðing af víxlverkun blossans við sitt nánasta umhverfi. Í kafla 3 er safn blossa skoðað sem rannsakað var með nýlegum litrófsmæli á Very Large Telescope (VLT). Hér var áhersla lögð á gleypilínur frá mikið jónuðu gasi. Við sýndum fram á að þessar línur eru afleiðing blossans og notuðum auk þess röntgenmælingar til að styrkja þær niðurstöður. Í öðrum hluta er lögð áhersla á rannsóknir á sameindagasi við hátt rauðvik. Í 4. kafla er hlutlaust kolefni (Ci) kortlagt en það má nota sem nokkurs konar sporefni fyrir vetnissameindina H2. Grunneiginleikar viðkomandi gass voru rannsakaðir og meginniðurstaðan var sú að hýsilvetrarbrautir gammablossa verða að hafa hátt hlutfall þungefna til að geta innihaldið kalt sameindagas. Í 5. kafla er athyglinni beint að eiginleikum ryks í Ci kerfum og í 6. kafla er nákvæm greining á tilvist sporefna sameindagass í miðgeimsefni hýsilvetrarbrauta gammablossa. Í þriðja og síðasta hluta eru dulstirnin í aðalhlutverki. Þau eru hér notuð til að rannsaka kerfi sem eru rykrík og hafa hátt hlutfall þungefna. Kynnt er ný aðferð til að finna slík kerfi við hátt rauðvik sem fyrri athuganir hafa hingað til misst af. Fjallað er um slíka valforsendu, sem byggir á sýnilegum og innrauðum mælingum, í kafla 7. Eitt slíkt kerfi er svo rannsakað gaumgæfilega í kafla 8. Að lokum eru gögn frá Gaia geimsjónaukanum notuð í kafla 9 til að sýna fram á að hægt er að skilgreina óhlutdrægt safn dulstirna. Þessi ritgerð sýnir fram á mikilvægi þess að skoða stór söfn gammablossa til að 1) gera tölfræðilegar rannsóknir á þeim og hýsilvetrarbrautum þeirra sem og að 2) auka möguleikana á því að uppgötva sérkennilegt umhverfi sumra þeirra. Að auki þá undirstrika þessar rannsóknir brýna þörf á að skilgreina óhlutdrægt safn dulstirna. Þannig má nýta þau til að kortleggja aukningu þungefnis sem fall af tíma í alheiminum.Accepted

The high A_V Quasar Survey: A z=2.027 metal-rich damped Lyman-alpha absorber towards a red quasar at z=3.21

To fully exploit the potential of quasars as probes of cosmic chemical evolution and the internal gas dynamics of galaxies it is important to understand the selection effects behind the quasar samples and in particular if the selection criteria exclude foreground galaxies with certain properties (most importantly a high dust content). Here we present spectroscopic follow-up from the 10.4-m GTC telescope of a dust-reddened quasar, eHAQ0111+0641, from the extended High A_V Quasar (HAQ) survey. We find that the z=3.21 quasar has a foreground Damped Lyman-alpha Absorber (DLA) at z=2.027 along the line of sight. The DLA has very strong metal lines due to a moderately high metallicity (with an inferred lower limit of 25% of the solar metallicity), but a very large gas column density along the line-of-sight in its host galaxy. This discovery is further evidence that there is a dust bias affecting the census of metals, caused by the combined effect of dust obscuration and reddening, in existing samples of z>2 DLAs. The case of eHAQ0111+0641 illustrates that dust bias is not only caused by dust obscuration, but also dust reddening.Comment: 7 pages, 4 figures. Accepted for publication in A&A. A few typos have been correcte

Investigating the [C II\,{\rm \scriptsize II}]-to-H I\,{\rm \scriptsize I} conversion factor and the H I\,{\rm \scriptsize I} gas budget of galaxies at z≈6z\approx 6 with hydrodynamical simulations

One of the most fundamental baryonic matter components of galaxies is the neutral atomic hydrogen (H$\,{\rm \scriptsize I}$). At low redshifts, this component can be traced directly through the 21-cm transition, but to infer H$\,{\rm \scriptsize I}$ gas content of the most distant galaxies, a viable tracer is needed. We here investigate the fidelity of the fine structure transition of the ($^2P_{3/2} - ^2P_{1/3}$) transition of singly-ionized carbon [C$\,{\rm \scriptsize II}$] at $158\,\mu$m as a proxy for H$\,{\rm \scriptsize I}$ in a set simulated galaxies at $z\approx 6$, following the work by Heintz et al. (2021). We select 11,125 star-forming galaxies from the SIMBA simulations, with far-infrared line emissions post-processed and modeled within the SIGAME framework. We find a strong connection between [C$\,{\rm \scriptsize II}$] and H$\,{\rm \scriptsize I}$, with the relation between this [C$\,{\rm \scriptsize II}$]-to-H$\,{\rm \scriptsize I}$ relation ($\beta_{\rm [C\,{\rm \scriptsize II}]}$) being anti-correlated with the gas-phase metallicity of the simulated galaxies. We further use these simulations to make predictions for the total baryonic matter content of galaxies at $z\approx 6$, and specifically the HI gas mass fraction. We find mean values of $M_{\rm HI}/M_\star = 1.4$, and $M_{\rm HI}/M_{\rm bar,tot} = 0.45$. These results provide strong evidence for H$\,{\rm \scriptsize I}$ being the dominant baryonic matter component by mass in galaxies at $z\approx 6$.Comment: 7 pages, 3 figures. Accepted for publication by ApJ

The archival discovery of a strong Lyman-α\alpha and [CII] emitter at z = 7.677

We report the archival discovery of Lyman-$\alpha$ emission from the bright ultraviolet galaxy Y002 at $z=7.677$, spectroscopically confirmed by its ionized carbon [CII] 158$\mu$m emission line. The Ly$\alpha$ line is spatially associated with the rest-frame UV stellar emission ($M_{\rm UV}$~-22, 2x brighter than $M^\star_{\rm UV}$) and it appears offset from the peak of the extended [CII] emission at the current ~1" spatial resolution. We derive an estimate of the unobscured SFR(UV)=$(22\pm1)\,M_\odot$ yr$^{-1}$ and set an upper limit of SFR(IR)$<15\,M_\odot$ yr$^{-1}$ from the far-infrared wavelength range, which globally place Y002 on the SFR(UV+IR)-L([CII]) correlation observed at lower redshifts. In terms of velocity, the peak of the Ly$\alpha$ emission is redshifted by $\Delta v$(Ly$\alpha$)~500 km s$^{-1}$ from the systemic redshift set by [CII] and a high-velocity tail extends to up to ~1000 km s$^{-1}$. The velocity offset is up to ~3.5x higher than the average estimate for similarly UV-bright emitters at z~6-7, which might suggest that we are witnessing the merging of two clumps. A combination of strong outflows and the possible presence of an extended ionized bubble surrounding Y002 would likely facilitate the escape of copious Ly$\alpha$ light, as indicated by the large equivalent width EW(Ly$\alpha$)=$24^{+5}_{-6}$ \r{A}. Assuming that [CII] traces the neutral hydrogen, we estimate a HI gas fraction of $M({\rm HI})/M_\star\gtrsim8$ for Y002 as a system and speculate that patches of high HI column densities could contribute to explain the observed spatial offsets between Ly$\alpha$ and [CII] emitting regions. The low dust content, implied by the non-detection of the far-infrared continuum emission at rest-frame ~160 $\mu$m, would be sufficient to absorb any potential Ly$\alpha$ photons produced within the [CII] clump as a result of large HI column densities.Comment: 10 pages, 4 figures. Accepted for publication in The Astrophysical Journal Letter

Early JWST imaging reveals strong optical and NIR color gradients in galaxies at z∼2z\sim2 driven mostly by dust

Recent studies have shown that galaxies at cosmic noon are redder in the center and bluer in the outskirts, mirroring results in the local universe. These color gradients could be caused by either gradients in the stellar age or dust opacity; however, distinguishing between these two causes is impossible with rest-frame optical photometry alone. Here we investigate the underlying causes of the gradients from spatially-resolved rest-frame $U-V$ vs. $V-J$ color-color diagrams, measured from early observations with the James Webb Space Telescope. We use $1\, \mu m - 4\, \mu m$ NIRCam photometry from the CEERS survey of a sample of 54 galaxies with $M_* / M_\odot>10$ at redshifts $1.7<z<2.3$ selected from the 3D-HST catalog. We model the light profiles in the F115W, F200W and F356W NIRCam bands using \texttt{imcascade}, a Bayesian implementation of the Multi-Gaussian expansion (MGE) technique which flexibly represents galaxy profiles using a series of Gaussians. We construct resolved rest-frame $U-V$ and $V-J$ color profiles. The majority of star-forming galaxies have negative gradients (i.e. redder in the center, bluer in the outskirts) in both $U-V$ and $V-J$ colors consistent with radially decreasing dust attenuation. A smaller population (roughly 15\%) of star-forming galaxies have positive $U-V$ but negative $V-J$ gradients implying centrally concentrated star-formation. For quiescent galaxies we find a diversity of UVJ color profiles, with roughly one-third showing star-formation in their center. This study showcases the potential of JWST to study the resolved stellar populations of galaxies at cosmic noon.Comment: Updated to match published version, new Figure 5 and some text change

Dust depletion of of metals from local to distant galaxies II: Cosmic dust-to-metal ratio and dust composition

The evolution of the cosmic dust content and the cycle between metals and dust in the interstellar medium (ISM) play a fundamental role in galaxy evolution. The chemical enrichment of the Universe can be traced through the evolution of the dust-to-metals ratio (DTM) and the dust-to-gas ratio (DTG) with metallicity. We use a novel method to determine mass estimates of the DTM, DTG and dust composition based on our previous measurements of the depletion of metals in different environments (the Milky Way, the Magellanic Clouds, and damped Lyman-$\alpha$ absorbers, DLAs, toward quasars and towards gamma-ray bursts, GRBs), which were calculated from the relative abundances of metals in the ISM through absorption-line spectroscopy column densities observed mainly from VLT/UVES and X-shooter, and HST/STIS. We derive the dust extinction from the estimated dust depletion ($A_{V, \rm depl}$) and compare with the $A_{V}$ from extinction. We find that the DTM and DTG ratios increase with metallicity and with the dust tracer [Zn/Fe]. This suggests that grain growth in the ISM is a dominant process of dust production. The increasing trend of the DTM and DTG with metallicity is in good agreement with a dust production and evolution model. Our data suggest that the stellar dust yield is much lower than the metal yield and thus that the overall amount of dust in the warm neutral medium that is produced by stars is much lower. We find that $A_{V,\rm depl}$ is overall lower than $A_{V, \rm ext}$ for the Milky Way and a few Magellanic Clouds lines of sight, a discrepancy that is likely related to the presence of carbonaceous dust. We show that the main elements that contribute to the dust composition are, O, Fe, Si, Mg, C, S, Ni and Al for all the environments. Abundances at low dust regimes suggest the presence of pyroxene and metallic iron in dust.Comment: Accepted for publication in A&A. Abstract abridge