Sebastian Pedraza Lopez

AFTERGLOW OBSERVATIONS OF FERMI LARGE AREA TELESCOPE GAMMA-RAY BURSTS AND THE EMERGING CLASS OF HYPER-ENERGETIC EVENTS

We present broadband (radio, optical, and X-ray) light curves and spectra of the afterglows of four long-duration gamma-ray bursts (GRBs; GRBs 090323, 090328, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor and Large Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large isotropic energy releases (10^(54) erg). Although rare, these events are particularly important for testing GRB central-engine models. When combined with spectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, the density structure of the circumburst medium, and both the true radiated energy release and the kinetic energy of the outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of these events (GRB 090926A) exceeds the canonical value of 10^(51) erg by an order of magnitude. Such energies pose a severe challenge for models in which the GRB is powered by a magnetar or a neutrino-driven collapsar, but remain compatible with theoretical expectations for magnetohydrodynamical collapsar models (e.g., the Blandford-Znajek mechanism). Our jet opening angles (θ) are similar to those found for pre-Fermi GRBs, but the large initial Lorentz factors (Γ_0) inferred from the detection of GeV photons imply θΓ_0 ≈ 70-90, values which are above those predicted in magnetohydrodynamic models of jet acceleration. Finally, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, and we speculate that this might result from the lower mass-loss rates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows at radio wavelengths with the order-of-magnitude improvement in sensitivity offered by the Extended Very Large Array should definitively establish the relativistic energy budgets of these events.

HIGH-redshift starbursting dwarf galaxies revealed by γ-ray burst afterglows

We present a study of 15 long-duration γ-ray burst (GRB) host galaxies at z > 2. The GRBs are selected with available early-time afterglow spectra in order to compare interstellar medium (ISM) absorption-line properties with stellar properties of the host galaxies. In addition to five previously studied hosts, we consider new detections for the host galaxies of GRB 050820 and GRB 060206, and place 2σ upper limits to the luminosities of the remaining unidentified hosts. We examine the nature of the host galaxy population and find that (1) the UV luminosity distribution of GRB host galaxies is consistent with expectations from a UV luminosity weighted random galaxy population with a median luminosity of 〈L(UV)〉 = 0.1 L *, (2) there exists a moderate correlation between UV luminosity and Si II λ 1526 absorption width, which together with the observed large line widths of W(1526)>1.5 A for a large fraction of the objects suggests a galactic outflow driven velocity field in the host galaxies, (3) there is tentative evidence for a trend of declining ISM metallicity with decreasing galaxy luminosity in the star-forming galaxy population at z = 2-4, (4) the interstellar UV radiation field is found to be 35-350× higher in GRB hosts than the Galactic mean value, and (5) additional galaxies are found at ≲2″ from the GRB host in all fields with known presence of strong Mg II absorbers, but no additional faint galaxies are found at ≲2″ in fields without strong Mg II absorbers. Our study confirms that the GRB host galaxies (with known optical afterglows) are representative of unobscured star-forming galaxies at z > 2, and demonstrates that high spatial resolution images are necessary for an accurate identification of GRB host galaxies in the presence of strong intervening absorbers.

The evolution of the cosmic microwave background temperature - Measurements of TCMB at high redshift from carbon monoxide excitation

A milestone of modern cosmology was the prediction and serendipitous discovery of the cosmic microwave background (CMB), the radiation leftover after decoupling from matter in the early evolutionary stages of the Universe. A prediction of the standard hot Big-Bang model is the linear increase with redshift of the black-body temperature of the CMB (TCMB). This radiation excites the rotational levels of some interstellar molecules, including carbon monoxide (CO), which can serve as cosmic thermometers. Using three new and two previously reported CO absorption-line systems detected in quasar spectra during a systematic survey carried out using VLT/UVES, we constrain the evolution of TCMB to z ∼ 3. Combining our precise measurements with previous constraints, we obtain TCMB(z) = (2.725 ± 0.002) × (1 + z) 1−β K with β = −0.007 ± 0.027, a more than two-fold improvement in precision. The measurements are consistent with the standard (i.e. adiabatic, β = 0) Big-Bang model and provide a strong constraint on the effective equation of state of decaying dark energy (i.e. weff = −0.996 ± 0.025).

A translucent interstellar cloud at z = 2.69 - CO, H2, and HD in the line-of-sight to SDSS J123714.60 + 064759.5

We present the analysis of a sub-damped Lyman-α system with neutral hydrogen column density, log N(H 0 ) (cm -2 ) = 20.0 ± 0.15 at z abs = 2.69 toward SDSSJ123714.60 + 064759.5 (z em = 2.78). Using the VLT/UVES and X-shooter spectrographs, we detect H 2 , HD, and CO molecules in absorption with log N(H 2 , HD, CO) (cm -2 ) = 19.21 +0.13 -0.12 , 14.48 ± 0.05 and 14.17 ± 0.09 respectively. The overall metallicity of the system is super-solar ([Zn/H] = +0.34 relative to solar) and iron is highly depleted ([Fe/Zn] = -1.39), revealing metal-rich and dusty gas. Three H 2 velocity components spanning ∼125 km s -1 are detected. The strongest H 2 component, at z abs = 2.68955, with logN(H 2 ) = 19.20, does not coincide with the centre of the HI absorption. This implies that the molecular fraction in this component, ƒ H2 = 2N(H 2 )/(2N(H 2 ) +N(H 0 )), is higher than the mean molecular fraction fH 2 = ¼ in the system. We also found the Cl 0 associated with this H 2 component to have N(Cl 0 )/N(Cl+) > 0.4, which points in the same direction. Cl 0 is tied to H 2 by charge exchange reactions, this means that the molecular fraction in this component is not far from unity. The kinetic temperature derived from the J = 0 and 1 rotational levels of H 2 is T = 108 +84 -33 K and the particle density derived from the C 0 ground-state fine structure level populations is n H0 ∼ 50-60 cm -3 . We derive an electronic density -3 for a UV field similar to the Galactic one and show that the carbon-to-sulphur ratio in the cloud is close to the solar ratio. The size of the molecular cloud is probably smaller than 1 pc. Both the CO/H 2 = 10 -5 and CO/C 0 ∼ 1 ratios for ƒ H 2 > 0.24 indicate that the cloud classifies as translucent, i.e., a regime where carbon is found both in atomic and molecular form. The corresponding extinction, A V = 0.14, albeit lower than the definition of a translucent sightline (based on extinction properties), is high for the observed H0 column density. This means that intervening clouds with similar local properties but with higher column densities (i.e. larger physical extent) could be missed by current magnitude-limited QSO surveys. The excitation of CO is dominated by radiative interaction with the cosmic microwave background radiation (CMBR) and we derive T ex (CO) = 10.5 +0.8 -0.6 K when T CMBR (z = 2.69) = 10.05 K is expected. We measure N(HD) / 2N(H 2 ) = 10 -5 . This is about 10 times higher than what is measured in the Galactic ISM for ƒ H 2 = ¼ but similar to what is measured in the Galactic ISM for higher molecular fractions. The astration factor of deuterium with respect to the primordial D/H ratio is only about 3. This can be the consequence of accretion ofunprocessed gas from the intergalactic medium onto the associated galaxy. In the future, it will be possible to search efficiently for molecular-rich DLAs/sub-DLAs with X-shooter, but detailed studies of the physical state of the gas will still need UVES observations.

On the incidence of strong Mg II absorbers along gamma-ray burst sight lines

We report on a survey for strong (rest equivalent width Wr ≥ 1 A), intervening Mg II systems along the sight lines to long-duration gamma-ray bursts (GRBs). The GRB spectra that comprise the survey have a heterogeneous mix of resolution and wavelength coverage, but we implement a strict, uniform set of search criteria to derive a well-defined statistical sample. We identify 14 strong Mg II absorbers along 14 GRB sight lines (nearly every sight line exhibits at least one absorber) with spectra covering a total path length Δz = 15.5 at a mean redshift = 1.1. In contrast, the predicted incidence of such absorber systems along the same path length to quasar sight lines is only 3.8. The roughly 4 times higher incidence along GRB sight lines is inconsistent with a statistical fluctuation at greater than 99.9% c.l. Several effects could explain the result: (1) dust within the Mg II absorbers obscures faint quasars giving a lower observed incidence along quasar sight lines, (2) the gas is intrinsic to the GRB event, and (3) the GRBs are gravitationally lensed by these absorbers. We present strong arguments against the first two effects and also consider lensing to be an unlikely explanation. The results suggest that at least one of our fundamental assumptions underpinning extragalactic absorption line research is flawed.

The neutral hydrogen cosmological mass density at z = 5

We present the largest homogeneous survey of z > 4.4 damped Lyα systems (DLAs) using the spectra of 163 QSOs that comprise the Giant Gemini GMOS (GGG) survey. With this survey we make the most precise high-redshift measurement of the cosmological mass density of neutral hydrogen, Ω_(Hi). At such high redshift, important systematic uncertainties in the identification of DLAs are produced by strong intergalactic medium absorption and QSO continuum placement. These can cause spurious DLA detections, result in real DLAs being missed or bias the inferred DLA column density distribution. We correct for these effects using a combination of mock and higher resolution spectra, and show that for the GGG DLA sample the uncertainties introduced are smaller than the statistical errors on Ω_(Hi). We find Ω_(HI)=0.98^(+0.20)_(−0.18) × 10^(−3) at 〈z〉 = 4.9, assuming a 20 per cent contribution from lower column density systems below the DLA threshold. By comparing to literature measurements at lower redshifts, we show that Ω_(Hi) can be described by the functional form Ω_(HI)(z)∝(1+z)^(0.)4. This gradual decrease from z = 5 to 0 is consistent with the bulk of H I gas being a transitory phase fuelling star formation, which is continually replenished by more highly ionized gas from the intergalactic medium and from recycled galactic winds.

GRB 090426: the environment of a rest-frame 0.35-s gamma-ray burst at a redshift of 2.609

We present the discovery of an absorption-line redshift of z = 2.609 for GRB 090426, establishing the first firm lower limit to a redshift for a gamma-ray burst (GRB) with an observed duration of 90 per cent confidence) a member of the short/hard phenomenological class of GRBs. From analysis of the optical-afterglow spectrum we find that the burst originated along a very low H i column density sightline, with NH i < 3.2 × 1019 cm-2. Our GRB 090426 afterglow spectrum also appears to have weaker low-ionization absorption (Si ii, C ii) than ∼95 per cent of previous afterglow spectra. Finally, we also report the discovery of a blue, very luminous, star-forming putative host galaxy (∼2L *) at a small angular offset from the location of the optical afterglow. We consider the implications of this unique GRB in the context of burst duration classification and our understanding of GRB progenitor scenarios.

The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time

We have obtained spectra of 163 quasars at zem > 4.4 with the Gemini Multi Object Spectrometers, the largest publicly available sample of high-quality, low-resolution spectra at these redshifts. From this data set, we generated stacked quasar spectra in three redshift intervals at z ∼ 5 to model the average rest-frame Lyman continuum flux and to assess the mean free path λ 912 of the intergalactic medium to H I-ionizing radiation. At mean redshifts zq =(4.56, 4.86, 5.16), we measure λ 912 = (22.2 ± 2.3,15.1 ± 1.8,10.3 ± 1.6) h −1 proper Mpc with uncertainties dominated by sample variance. Combining our results with measurements from lower redshifts, the data are well modelled by a power law λ 912 = A[(1 + z) /5] η with A = (37 ± 2) h −1 Mpc and η =− 5.4 ± 0.4 at 2.3 <z< 5.5. This rapid evolution requires a physical mechanism – beyond cosmological expansion – which reduces the effective Lyman limit opacity. We speculate that the majority of H I Lyman limit opacity manifests in gas outside galactic dark matter haloes, tracing large-scale structures (e.g. filaments) whose average density and neutral fraction decreases with cosmic time. Our measurements of the mean free path shortly after H I reionization serve as a valuable boundary condition for numerical models thereof. Our measured λ 912 ≈ 10 Mpc at z =5.2 confirms that the intergalactic

Diffuse molecular gas at high redshift ⋆ Detection of CO molecules and the 2175 Å dust feature at z= 1.64

We present the detection of carbon monoxide molecules (CO) at z = 1.6408 towards the quasar SDSS J160457.50+220300.5 using the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. We detected CO absorption in at least two components in the first six A−X bands and one d−X(5−0) interband system. This is the second detection of its kind along a quasar line of sight. The CO absorption profiles are well modelled by assuming rotational excitation of CO in the range 6 < Tex < 16 K, which is consistent with or higher than the temperature of the cosmic microwave background radiation at this redshift. We derive a total CO column density of N(CO) = 4 × 10 14 cm −2 . The measured column densities of S i ,M gi ,Z nii ,F eii ,a nd Siii indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is significantly reddened (u − K ∼ 4.5 mag) and exhibits a pronounced 2175 A dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars, we find that the chance probability that this feature is spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well fitted by a QSO composite spectrum reddened with a Large Magellanic Cloud supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates that our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying the physical conditions and chemistry of diffuse molecular gas in high redshift galaxies.

Metal abundances and ionization conditions in a possibly dust-free damped Lyα system at

We have obtained a high resolution, high S/N UVES spectrum of the bright QSO HE2243-6031 to analyze the damped Ly-alpha system (DLA) observed at z=2.33. The metallicity of this system is 1/12 solar at a neutral hydrogen column density of log N(HI)=20.7. From the observed ratios [Zn/Cr]=-0.01+/-0.05 and [S/Si]=-0.06+/-0.03