[en] We present the results of spectroscopic analysis of seven new H₂-bearing damped Ly- systems in redshift range z=2.5 − 3. These systems were originally selected from SDSS catalog using a direct search for H₂ and followed up with X-SHOOTER spectrograph at 8-m Very Large Telescope observatory. We measured the column densities of H_I, H₂ on various rotational levels, and metals species in different ionization stages and excitation levels. We used the rotational excitation of H₂ molecules together with the fine-structure levels of neutral carbon to constrain the physical conditions in the associated medium. We found typical values for the kinetic temperature T ∼ 80 − 120 K, hydrogen density n _H ∼ 30 − 400 cm⁻³ and UV radiation field ξ_UV ∼ 0.4 − 5 times of the Draine field. These values along with estimated thermal pressure are in agreement with expected values from the theoretical calculation of the cold neutral interstellar medium. (paper)

[en] We have measured with high precision the position of 79 molecular hydrogen absorption lines of Lyman and Werner bands from two absorption systems at z_abs = 2.594733 and 3.024899, in the spectra of quasars Q 0405-443 and Q 0347-383, respectively, in order to constrain the cosmological variation of the proton-to-electron mass ratio, μ = m_p/m_e. Data are of the highest spectral resolution (R=53000) and signal to noise ratio (S/N 30-70) for such quasars. The absorption lines are not saturated and their profiles can be modelled as simple Gaussian functions. We find a correlation between the observed redshift of the lines and the sensitivity of the line positions to a change in μ. This can be interpreted as a variation of μ with Δμ/μ = (2.97 ± 0.74) x 10^-5 over the past ∼ 12 Gyrs. As this result is based on two systems one cannot rule out that unknown systematics could cause a false-alarm detection. Thus the result needs to be confirmed with additional data. It is also very important to improve the accuracy of the laboratory wavelengths as the significance of our result depends on the accuracy to which they are known

[en] We present the elemental abundance and H₂ content measurements of a damped Lyα (DLA) system with an extremely large H I column density, log N(H I) (cm^–2) = 22.0 ± 0.10, at z_abs = 3.287 toward the QSO SDSS J081634+144612. We measure column densities of H₂, C I, C I*, Zn II, Fe II, Cr II, Ni II, and Si II from a high signal-to-noise and high spectral resolution VLT-UVES spectrum. The overall metallicity of the system is [Zn/H] = –1.10 ± 0.10 relative to solar. Two molecular hydrogen absorption components are seen at z = 3.28667 and 3.28742 (a velocity separation of ≈52 km s^–1) in rotational levels up to J = 3. We derive a total H₂ column density of log N(H₂) (cm^–2) = 18.66 and a mean molecular fraction of f = 2N(H₂)/[2N(H₂) + N(H I)] = 10^{–3.04±0.37}, typical of known H₂-bearing DLA systems. From the observed abundance ratios we conclude that dust is present in the interstellar medium of this galaxy, with an enhanced abundance in the H₂-bearing clouds. However, the total amount of dust along the line of sight is not large and does not produce any significant reddening of the background QSO. The physical conditions in the H₂-bearing clouds are constrained directly from the column densities of H₂ in different rotational levels, C I and C I*. The kinetic temperature is found to be T ≈ 75 K and the particle density lies in the range n_H = 50-80 cm^–3. The neutral hydrogen column density of this DLA is similar to the mean H I column density of DLAs observed at the redshift of γ-ray bursts (GRBs). We explore the relationship between GRB-DLAs and the high column density end of QSO-DLAs finding that the properties (metallicity and depletion) of DLAs with log N(H I) > 21.5 in the two populations do not appear to be significantly different.

[en] We present the results of a search for red QSOs using a selection based on optical imaging from the Sloan Digital Sky Survey (SDSS) and near-infrared imaging from UKIDSS. Our main goal with the selection is to search for QSOs reddened by foreground dusty absorber galaxies. For a sample of 58 candidates (including 20 objects fulfilling our selection criteria that already have spectra in the SDSS), 46 (79%) are confirmed to be QSOs. The QSOs are predominantly dust-reddened except for a handful at redshifts z ∼> 3.5. However, the dust is most likely located in the QSO host galaxies (and for two, the reddening is primarily caused by Galactic dust) rather than in the intervening absorbers. More than half of the QSOs show evidence of associated absorption (BAL absorption). Four (7%) of the candidates turned out to be late-type stars, and another four (7%) are compact galaxies. We could not identify the remaining four objects. In terms of their optical spectra, these QSOs are similar to the QSOs selected in the FIRST-2MASS Red Quasar Survey except they are on average fainter, more distant, and only two are detected in the FIRST survey. As per the usual procedure, we estimate the amount of extinction using the SDSS QSO template reddened by Small-Magellanic-Cloud-(SMC) like dust. It is possible to get a good match to the observed (rest-frame ultraviolet) spectra, but it is not possible to match the observed near-IR photometry from UKIDSS for nearly all the reddened QSOs. The most likely reasons are that the SDSS QSO template is too red at optical wavelengths due to contaminating host galaxy light and because the assumed SMC extinction curve is too shallow. Three of the compact galaxies display old stellar populations with ages of several Gyr and masses of about 10¹⁰ M_☉ (based on spectral energy distribution modeling). The inferred stellar densities in these galaxies exceed 10¹⁰ M_☉ kpc^–2, which is among the highest measured for early-type galaxies. Our survey has demonstrated that selection of QSOs based on near-IR photometry is an efficient way to select QSOs, including reddened QSOs, with only small contamination from late-type stars and compact galaxies. This will be useful with ongoing and future wide-field near-IR surveys such as the VISTA and EUCLID surveys.

[en] We present the results of a new spectroscopic survey for dusty intervening absorption systems, particularly damped Ly α absorbers (DLAs), toward reddened quasars. The candidate quasars are selected from mid-infrared photometry from the Wide-field Infrared Survey Explorer combined with optical and near-infrared photometry. Out of 1073 candidates, we secure low-resolution spectra for 108 using the Nordic Optical Telescope on La Palma, Spain. Based on the spectra, we are able to classify 100 of the 108 targets as quasars. A large fraction (50%) is observed to have broad absorption lines (BALs). Moreover, we find six quasars with strange breaks in their spectra, which are not consistent with regular dust reddening. Using template fitting, we infer the amount of reddening along each line of sight ranging from A ( V ) ≈ 0.1 to 1.2 mag (assuming a Small Magellanic Cloud extinction curve). In four cases, the reddening is consistent with dust exhibiting the 2175 Å feature caused by an intervening absorber, and for two of these, an Mg ii absorption system is observed at the best-fit absorption redshift. In the rest of the cases, the reddening is most likely intrinsic to the quasar. We observe no evidence for dusty DLAs in this survey. However, the large fraction of BAL quasars hampers the detection of absorption systems. Out of the 50 non-BAL quasars, only 28 have sufficiently high redshift to detect Ly α in absorption.

[en] We report the clear detection of the 2175 A dust absorption feature in the optical afterglow spectrum of the gamma-ray burst (GRB) GRB 070802 at a redshift of z = 2.45. This is the highest redshift for a detected 2175 A dust bump to date, and it is the first clear detection of the 2175 A bump in a GRB host galaxy, while several tens of optical afterglow spectra without the bump have been recorded in the past decade. The derived extinction curve gives A_V = 0.8-1.5 depending on the assumed intrinsic slope. Of the three local extinction laws, a Large Magellanic Cloud (LMC) type extinction gives the best fit to the extinction curve of the host of GRB 070802. Besides the 2175 A bump we find that the spectrum of GRB 070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ([Si/H] = -0.46 ± 0.38, [Zn/H] = -0.50 ± 0.68). In particular, the spectrum of GRB 070802 is unique for a GRB spectrum in that it shows clear C I absorption features, leading us to propose a correlation between the presence of the bump and C I. The gas-to-dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N(H I)/A_V = (2.4 ± 1.0) x 10²¹ cm^-2 mag^-1, which lies between typical Milky Way and LMC values. Our results are in agreement with the tentative conclusion reached by Gordon et al. that the shape of the extinction curve, in particular the presence of the bump, is affected by the UV flux density in the environment of the dust.

[en] We present the spectroscopic confirmation of a z = 2.45 protocluster. Its member galaxies lie within a radius of 1.4 Mpc (physical) on the sky and within $\mathrm{\Delta <!--\; ??\; -->}v\pm <!--\; ??\; -->700\mathrm{k}\mathrm{m}{\mathrm{s}}^{-<!--\; ???\; -->1}$ along the line of sight. We estimate an overdensity of 10, suggesting that the structure has made the turnaround but is not assembled yet. A comparison to the Millennium simulation suggests that analogous structures evolve into 10¹⁴–10¹⁵ M_⊙ h⁻¹ type dark matter halos by z = 0, qualifying the notion of “protocluster.” The search for the complete census of mock progenitor galaxies at $z\sim <!--\; ???\; -->2.5$ of these massive z = 0 mock clusters reveals that they are widely spread over areas with a diameter of 3–20 Mpc. This suggests that the optical selection of such protoclusters can result in a rich diversity regarding their z = 0 descendants. We also searched for signs of environmental differentiation in this protocluster. While we see a weak trend for more massive and more quiescent galaxies within the protocluster, this is not statistically significant.

[en] We present the serendipitous discovery of a projected pair of quasi-stellar objects (QSOs) with an angular separation of Δ θ = 4.50 arcsec. The redshifts of the two QSOs are widely different: one, our program target, is a QSO with a spectrum consistent with being a narrow line Seyfert 1 active galactic nucleus at z = 2.05. For this target we detect Ly α , C iv, and C iii]. The other QSO, which by chance was included on the spectroscopic slit, is a Type 1 QSO at a redshift of z = 1.68, for which we detect C iv, C iii], and Mg ii. We compare this system to previously detected projected QSO pairs and find that only about a dozen previously known pairs have smaller angular separation.

[en] Quasi-stellar objects (QSOs) whose spectral energy distributions (SEDs) are reddened by dust either in their host galaxies or in intervening absorber galaxies are to a large degree missed by optical color selection criteria like the ones used by the Sloan Digital Sky Survey (SDSS). To overcome this bias against red QSOs, we employ a combined optical and near-infrared (near-IR) color selection. In this paper, we present a spectroscopic follow-up campaign of a sample of red candidate QSOs which were selected from the SDSS and the UKIRT Infrared Deep Sky Survey (UKIDSS). The spectroscopic data and SDSS/UKIDSS photometry are supplemented by mid-infrared photometry from the Wide-field Infrared Survey Explorer. In our sample of 159 candidates, 154 (97%) are confirmed to be QSOs. We use a statistical algorithm to identify sightlines with plausible intervening absorption systems and identify nine such cases assuming dust in the absorber similar to Large Magellanic Cloud sightlines. We find absorption systems toward 30 QSOs, 2 of which are consistent with the best-fit absorber redshift from the statistical modeling. Furthermore, we observe a broad range in SED properties of the QSOs as probed by the rest-frame 2 μm flux. We find QSOs with a strong excess as well as QSOs with a large deficit at rest-frame 2 μm relative to a QSO template. Potential solutions to these discrepancies are discussed. Overall, our study demonstrates the high efficiency of the optical/near-IR selection of red QSOs

[en] We present a sample of 77 optical afterglows (OAs) of Swift detected gamma-ray bursts (GRBs) for which spectroscopic follow-up observations have been secured. Our first objective is to measure the redshifts of the bursts. For the majority (90%) of the afterglows, the redshifts have been determined from the spectra. We provide line lists and equivalent widths (EWs) for all detected lines redward of Lyα covered by the spectra. In addition to the GRB absorption systems, these lists include line strengths for a total of 33 intervening absorption systems. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray-selected statistical sample of Swift bursts with optimal conditions for ground-based follow-up from the period 2005 March to 2008 September; 146 bursts fulfill our sample criteria. We derive the redshift distribution for the statistical (X-ray selected) sample and conclude that less than 18% of Swift bursts can be at z > 7. We compare the high-energy properties (e.g., γ-ray (15-350 keV) fluence and duration, X-ray flux, and excess absorption) for three subsamples of bursts in the statistical sample: (1) bursts with redshifts measured from OA spectroscopy; (2) bursts with detected optical and/or near-IR afterglow, but no afterglow-based redshift; and (3) bursts with no detection of the OA. The bursts in group (1) have slightly higher γ-ray fluences and higher X-ray fluxes and significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fractions of dark bursts, defined as bursts with an optical to X-ray slope β_OX < 0.5, is 14% in group (1), 38% in group (2), and >39% in group (3). For the full sample, the dark burst fraction is constrained to be in the range 25%-42%. From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight lines. This should be taken into account when determining, e.g., the redshift or metallicity distribution of GRBs and when using GRBs as a probe of star formation. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular the damped Lyα absorbers (DLAs). On average GRB absorbers are characterized by significantly stronger EWs for H I as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. However, the distribution of line strengths is very broad and several GRB absorbers have lines with EWs well within the range spanned by QSO-DLAs. Based on the 33 z > 2 bursts in the sample, we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies.