ALBERT

Description: We examine recent measurements of the z 2.5 intergalactic medium (IGM) which constrain the H  i frequency distribution $f(N_{\rm H\,{\small I}})$ and the mean free path $\lambda _{\rm mfp}^{912}$ to ionizing radiation. We argue that line-blending and the clustering of strong absorption-line systems have led previous authors to systematically overestimate the effective Lyman limit opacity, yielding too small of a $\lambda _{\rm mfp}^{912}$ for the IGM. We further show that recently published measurements of $f(N_{\rm H\,{\small I}})$ at $N_{\rm H\,{\small I}}\approx 10^{16} \, {\rm cm^{-2}}$ lie in strong disagreement, implying underestimated uncertainty from sample variance and/or systematics like line-saturation. Allowing for a larger uncertainty in the $f(N_{\rm H\,{\small I}})$ measurements, we provide a new cubic Hermite spline model for $f(N_{\rm H\,{\small I}})$ which reasonably satisfies all of the observational constraints under the assumption of randomly distributed absorption systems. We caution, however, that this formalism is invalid in the light of absorber clustering and use a toy model to estimate the effects. Future work must properly account for the non-Poissonian nature of the IGM.

Description: Recoiling supermassive black holes (SMBHs) are considered one plausible physical mechanism to explain high velocity shifts between narrow and broad emission lines sometimes observed in quasar spectra. If the sphere of influence of the recoiling SMBH is such that only the accretion disc is bound, the dusty torus would be left behind, hence the SED should then present distinctive features (i.e. a mid-infrared deficit). Here, we present results from fitting the spectral energy distributions (SEDs) of 32 type-1 AGN with high velocity shifts between broad and narrow lines. The aim is to find peculiar properties in the multiwavelength SEDs of such objects by comparing their physical parameters (torus and disc luminosity, intrinsic reddening, and size of the 12 μm emitter) with those estimated from a control sample of ~1000 typical quasars selected from the Sloan Digital Sky Survey in the same redshift range. We find that all sources, with the possible exception of J1154+0134, analysed here present a significant amount of 12 μm emission. This is in contrast with a scenario of an SMBH displaced from the centre of the galaxy, as expected for an undergoing recoil event.

Description: Damped Lyα absorbers (DLAs) are a well-studied class of absorption line systems, and yet the properties of their host galaxies remain largely unknown. To investigate the origin of these systems, we have conducted an imaging survey of 32 quasar fields with intervening DLAs between z  ~ 1.9 and 3.8, leveraging a technique that allows us to image galaxies at any small angular separation from the background quasars. In this paper, we present the properties of the targeted DLA sample, new imaging observations of the quasar fields, and the analysis of new and archival spectra of the background quasars.

Description: We present results from a survey designed to probe the star formation properties of 32 damped Lyman α systems (DLAs) at z ~ 2.7. By using the ‘double-DLA’ technique that eliminates the glare of the bright background quasars, we directly measure the rest-frame far-ultraviolet flux from DLAs and their neighbouring galaxies. At the position of the absorbing gas, we place stringent constraints on the unobscured star formation rates (SFRs) of DLAs to 2 limits of $\dot{\psi } 〈 0.09\hbox{-}0.27$ M yr –1 , corresponding to SFR surface densities sfr 〈 10 –2.6 –10 –1.5 M yr –1 kpc –2 . The implications of these limits for the star formation law, metal enrichment, and cooling rates of DLAs are examined. By studying the distribution of impact parameters as a function of SFRs for all the galaxies detected around these DLAs, we place new direct constraints on the bright end of the UV luminosity function of DLA hosts. We find that ≤13 per cent of the hosts have $\dot{\psi }\ge 2$ M yr –1 at impact parameters $b_{\rm dla} \le (\dot{\psi }/{\rm M_{\odot }\, yr^{-1}})^{0.8}+6 \,\rm kpc$ , differently from current samples of confirmed DLA galaxies. Our observations also disfavour a scenario in which the majority of DLAs arise from bright Lyman-break galaxies at distances 20 ≤ b dla 〈 100 kpc. These new findings corroborate a picture in which DLAs do not originate from highly star-forming systems that are coincident with the absorbers, and instead suggest that DLAs are associated with faint, possibly isolated, star-forming galaxies. Potential shortcomings of this scenario and future strategies for further investigation are discussed.

Description: Using over a million and a half extragalactic spectra we study the properties of the mysterious diffuse interstellar bands (DIBs) in the Milky Way. These data provide us with an unprecedented sampling of the skies at high Galactic latitude and low dust column density. We present our method, study the correlation of the equivalent width of eight DIBs with dust extinction and with a few atomic species, and the distribution of four DIBs – 5780.6, 5797.1, 6204.3, and 6613.6 Å – over nearly 15 000 deg 2 . As previously found, DIBs strengths correlate with extinction and therefore inevitably with each other. However, we show that DIBs can exist even in dust-free areas. Furthermore, we find that the DIBs correlation with dust varies significantly over the sky. DIB under- or overdensities, relative to the expectation from dust, are often spread over hundreds of square degrees. These patches are different for the four DIBs, showing that they are unlikely to originate from the same carrier, as previously suggested.

Description: We have obtained spectra of 163 quasars at z em  〉 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 $\lambda _{\rm mfp}^{912}$ of the intergalactic medium to H  i -ionizing radiation. At mean redshifts z q = (4.56, 4.86, 5.16), we measure $\lambda _{\rm mfp}^{912}=\left(22.2 \pm 2.3, 15.1 \pm 1.8, 10.3 \pm 1.6\right)h_{70}^{-1}$ 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 $\lambda _{\rm mfp}^{912}=A\left[\left(1+z\right)/5\right]^\eta$ with $A=\left(37 \pm 2\right)h_{70}^{-1}$  Mpc and = –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 $\lambda _{\rm mfp}^{912}\approx 10$  Mpc at z = 5.2 confirms that the intergalactic medium is highly ionized without evidence for a break that would indicate a recent end to H  i reionization.

Description: We report the discovery of a transparent sightline at projected distances of 20 kpc to an interacting pair of mature galaxies at z  = 0.12. The sightline of the UV-bright quasar PG 1522+101 at z em  = 1.328 passes at  = 11.5 kpc from the higher mass galaxy ( M *  = 10 10.6 M ) and  = 20.4 kpc from the lower mass one ( M *  = 10 10.0 M ). The two galaxies are separated by 9 kpc in projected distance and 30 km s –1 in line-of-sight velocity. Deep optical images reveal tidal features indicative of close interactions. Despite the small projected distances, the quasar sightline shows little absorption associated with the galaxy pair with a total H i column density no greater than $\log \, N({\rm H\,\small {I}})/{\rm cm}^{-2}=13.65$ . This limiting H i column density is already two orders of magnitude less than what is expected from previous halo gas studies. In addition, we detect no heavy-element absorption features associated with the galaxy pair with 3 limits of $\log \,N({\rm Mg\,\small {II}})/{\rm cm}^{-2} 〈 12.2$ and $\log \,N({\rm O\,\small {VI}})/{\rm cm}^{-2} 〈 13.7$ . The probability of seeing such little absorption in a sightline passing at a small projected distance from two non-interacting galaxies is 0.2 per cent. The absence of strong absorbers near the close galaxy pair suggests that the cool gas reservoirs of the galaxies have been significantly depleted by the galaxy interaction. These observations therefore underscore the potential impact of galaxy interactions on the gaseous haloes around galaxies.

Description: We report the serendipitous detection of a 0.2 L*, Lyα emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system ( ${\it N}_\mathrm{H\,\small {i}}=10^{16.94\pm 0.10}$ cm –2 ) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalize over uncertainties in the shape and normalization of the ionizing radiation spectrum, we derive the total hydrogen column density ${\it N}_\mathrm{H}=10^{19.4\pm 0.3}\,\rm cm^{-2}$ , and show that all the absorbing clouds are metal enriched, with Z  = 0.1–0.6 Z . These metallicities and the system's large velocity width (436 km s – 1 ) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of ~5 M yr –1 . Our photoionization model yields extremely small sizes (〈100–500 pc) for the absorbing clouds, which we argue is typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a 1000-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM.

Description: We present new measurements of the mean transmitted flux in the Lyα forest over 2 〈 z  〈 5 made using 6065 quasar spectra from the Sloan Digital Sky Survey data release 7 (SDSS DR7). We exploit the general lack of evolution in the mean quasar continuum to avoid the bias introduced by continuum fitting over the Lyα forest at high redshifts, which has been the primary systematic uncertainty in previous measurements of the mean Lyα transmission. The individual spectra are first combined into 26 composites with mean redshifts spanning 2.25 ≤ z comp  ≤ 5.08. The flux ratios of separate composites at the same rest wavelength are then used, without continuum fitting, to infer the mean transmitted flux, F ( z ), as a fraction of its value at z  ~ 2. Absolute values for F ( z ) are found by scaling our relative values to measurements made from high-resolution data by Faucher-Giguère et al. at z  ≤ 2.5, where continuum uncertainties are minimal. We find that F ( z ) evolves smoothly with redshift, with no evidence of a previously reported feature at z ~= 3.2. This trend is consistent with a gradual evolution of the ionization and thermal state of the intergalactic medium over 2 〈 z  〈 5. Our results generally agree with the most careful measurements to date made from high-resolution data, but offer much greater precision and extend to higher redshifts. This work also improves upon previous efforts using SDSS spectra by significantly reducing the level of systematic errors.

Description: Theoretically, bound binaries of massive black holes are expected as the natural outcome of mergers of massive galaxies. From the observational side, however, massive black hole binaries remain elusive. Velocity shifts between narrow and broad emission lines in quasar spectra are considered a promising observational tool to search for spatially unresolved, dynamically bound binaries. In this series of papers, we investigate the nature of such candidates through analyses of their spectra, images and multiwavelength spectral energy distributions. Here we investigate the properties of the optical spectra, including the evolution of the broad line profiles, of all the sources identified in our previous study. We find a diverse phenomenology of broad and narrow line luminosities, widths, shapes, ionization conditions and time variability, which we can broadly ascribe to four classes based on the shape of the broad line profiles. (1) Objects with bell-shaped broad lines with big velocity shifts (〉1000 km s –1 ) compared to their narrow lines show a variety of broad line widths and luminosities, modest flux variations over a few years, and no significant change in the broad line peak wavelength. (2) Objects with double-peaked broad emission lines tend to show very luminous and broadened lines, and little time variability. (3) Objects with asymmetric broad emission lines show a broad range of broad line luminosities and significant variability of the line profiles. (4) The remaining sources tend to show moderate to low broad line luminosities, and can be ascribed to diverse phenomena. We discuss the implications of our findings in the context of massive black hole binary searches.