## Publications

4 results found

Barnett R, Warren SJ, Cross NJG,
et al., 2020, A complete search for redshift z>6.5 quasars in the VIKING survey, *Monthly Notices of the Royal Astronomical Society*, Vol: 501, Pages: 1663-1676, ISSN: 0035-8711

We present the results of a new, deeper, and complete search forhigh-redshift $6.5<z<9.3$ quasars over 977deg$^2$ of the VISTA Kilo-DegreeInfrared Galaxy (VIKING) survey. This exploits a new list-driven datasetproviding photometry in all bands ZYJHKs, for all sources detected by VIKING in$J$. We use the Bayesian model comparison (BMC) selection method of Mortlock etal., producing a ranked list of just 21 candidates. The sources ranked 1, 2, 3and 5 are the four known $z>6.5$ quasars in this field. Additional observationsof the other 17 candidates, primarily DESI Legacy Survey photometry and ESOFORS2 spectroscopy, confirm that none is a quasar. This is the first completesample from the VIKING survey, and we provide the computed selection function.We include a detailed comparison of the BMC method against two other selectionmethods: colour cuts and minimum-$\chi^2$ SED fitting. We find that: i) BMCproduces eight times fewer false positives than colour cuts, while alsoreaching 0.3 mag. deeper, ii) the minimum-$\chi^2$ SED fitting method isextremely efficient but reaches 0.7 mag. less deep than the BMC method, andselects only one of the four known quasars. We show that BMC candidates,rejected because their photometric SEDs have high $\chi^2$ values, includebright examples of galaxies with very strong [OIII]$\lambda\lambda$4959,5007emission in the $Y$ band, identified in fainter surveys by Matsuoka et al. Thisis a potential contaminant population in Euclid searches for faint $z>7$quasars, not previously accounted for, and that requires bettercharacterisation.

Barnett R, Warren SJ, Mortlock DJ,
et al., 2019, Euclid preparation. V. Predicted yield of redshift 7<z<9 quasars from the wide survey, *Astronomy & Astrophysics*, Vol: 631, ISSN: 0004-6361

We provide predictions of the yield of 7 < z < 9 quasars from the Euclid wide survey, updating the calculation presented in theEuclid Red Book in several ways. We account for revisions to the Euclid near-infrared filter wavelengths; we adopt steeper ratesof decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10k(z−6), k = −0.72, and a further steeper rate of decline,k = −0.92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make useof an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identificationof fainter quasars, down to JAB ∼ 23. Quasars at z > 8 may be selected from Euclid OY JH photometry alone, but selection overthe redshift interval 7 < z < 8 is greatly improved by the addition of z-band data from, e.g., Pan-STARRS and LSST. We calculatepredicted quasar yields for the assumed values of the rate of decline of the QLF beyond z = 6. If the decline of the QLF acceleratesbeyond z = 6, with k = −0.92, Euclid should nevertheless find over 100 quasars with 7.0 < z < 7.5, and ∼ 25 quasars beyond thecurrent record of z = 7.5, including ∼ 8 beyond z = 8.0. The first Euclid quasars at z > 7.5 should be found in the DR1 data release,expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8, M1450 < −25, using 8 m class telescopesto confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of thecandidate lists is predicted to be modest even at JAB ∼ 23. The precision with which k can be determined over 7 < z < 8 depends onthe value of k, but assuming k = −0.72 it can be measured to a 1σ uncertainty of 0.07.

Barnett R, Warren SJ, Becker GD,
et al., 2017, Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641, *Astronomy and Astrophysics: a European journal*, Vol: 601, ISSN: 0004-6361

We present a 30 h integration Very Large Telescope X-shooter spectrum of the Lyman series forest towards the z = 7.084 quasar ULAS J1120+0641. The only detected transmission at S/N > 5 is confined to seven narrow spikes in the Lyα forest, over the redshift range 5.858 <z < 6.122, just longward of the wavelength of the onset of the Lyβ forest. There is also a possible detection of one further unresolved spike in the Lyβ forest at z = 6.854, with S/N = 4.5. We also present revised Hubble Space Telescope F814W photometry of the source. The summed flux from the transmission spikes is in agreement with the F814W photometry, so all the transmission in the Lyman series forest may have been detected. There is a Gunn-Peterson (GP) trough in the Lyα forest from z = 6.122 all the way to the quasar near zone at z = 7.04. The trough, of comoving length 240 h-1 Mpc, is over twice as long as the next longest known GP trough. We combine the spectroscopic and photometric results to constrain the evolution of the Lyα effective optical depth (τGPeff) with redshift, extending a similar analysis by Simpson et al. We find τGPeff ∝ (1 + z)ξ where ξ = 11.2+ 0.4-0.6, for z > 5.5. The data nevertheless provide only a weak limit on the volume-weighted intergalactic medium (IGM) hydrogen neutral fraction at z ~ 6.5, xH i > 10-4, similar to limits at redshift z ~ 6 from less distant quasars. The new observations cannot extend measurements of the neutral fraction of the IGM to higher values because absorption in the Lyα forest is already saturated near z ~ 6. For higher neutral fractions, other methods such as measuring the red damping wing of the IGM will be required.

Barnett R, Warren SJ, Banerji M,
et al., 2015, The spectral energy distribution of the redshift 7.1 quasar ULAS J1120+0641, *Astronomy & Astrophysics*, Vol: 575, ISSN: 1432-0746

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