TY  - JOUR
A1  - Becker, George D.
A1  - Pettini, Max
A1  - Rafelski, Marc
A1  - Boera, Elisa
A1  - Christensen, Lise
A1  - Cupani, Guido
A1  - Ellison, Sara L.
A1  - Farina, Emanuele Paolo
A1  - Fumagalli, Michele
A1  - Lopez, Sebastian
A1  - Neeleman, Marcel
A1  - Ryan-Weber, Emma
A1  - Worseck, Gabor
T1  - The Evolution of OI over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present a survey for metal absorption systems traced by neutral oxygen over 3.2 < z < 6.5. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199 QSOs with redshifts up to 6.6. In total, we detect 74 OI absorbers, of which 57 are separated from the background QSO by more than 5000 km s(-1). We use a maximum likelihood approach to fit the distribution of OI lambda 1302 equivalent widths in bins of redshift and from this determine the evolution in number density of absorbers with W-1302 > 0.05 angstrom, of which there are 49 nonproximate systems in our sample. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over 4.9 < z < 5.7 is a factor of 1.7-4.1 lower (68% confidence) than that over 5.7 < z < 6.5, with a lower value at z < 5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z similar to 6 than at lower redshifts. Absorption from highly ionized metals traced by CIV is also weaker in higher-redshift OI systems, supporting this picture. The evolution of OI absorbers implies that metal-enriched circumgalactic gas at z similar to 6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch.
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab3eb5
SN  - 0004-637X
SN  - 1538-4357
VL  - 883
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Berg, Trystyn A. M.
A1  - Ellison, Sara L.
A1  - Sanchez-Ramirez, Ruben
A1  - Lopez, Sebastian
A1  - Becker, George D.
A1  - Christensen, Lise
A1  - Cupani, Guido
A1  - Denney, Kelly D.
A1  - Worseck, Gabor
T1  - Sub-damped Lyman alpha systems in the XQ-100 survey - I. Identification and contribution to the cosmological H I budget
JF  - Monthly notices of the Royal Astronomical Society
KW  - galaxies: high-redshift
KW  - galaxies: ISM
KW  - quasars: absorption lines
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz2012
SN  - 0035-8711
SN  - 1365-2966
VL  - 488
IS  - 3
SP  - 4356
EP  - 4369
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Schaerer, Daniel
A1  - Izotov, Yuri I.
A1  - Nakajima, K.
A1  - Worseck, Gabor
A1  - Chisholm, J.
A1  - Verhamme, A.
A1  - Thuan, T. X.
A1  - de Barros, S.
T1  - Intense C III] lambda lambda 1907,1909 emission from a strong Lyman continuum emitting galaxy
JF  - Astronomy and astrophysics : an international weekly journal
N2  - We have obtained the first complete ultraviolet (UV) spectrum of a strong Lyman continuum (LyC) emitter at low redshift - the compact, low-metallicity, star-forming galaxy J1154+2443 - with a Lyman continuum escape fraction of 46% discovered recently. The Space Telescope Imaging Spectrograph spectrum shows strong Ly alpha and C III] lambda 1909 emission, as well as O III] lambda 1666. Our observations show that strong LyC emitters can have UV emission lines with a high equivalent width (e.g. EW(C III]) = 11.7 +/- 2.9 angstrom rest-frame), although their equivalent widths should be reduced due to the loss of ionizing photons. The intrinsic ionizing photon production efficiency of J1154+2443 is high, log(xi(0)(ion)) = 25.56 erg(-1) Hz, comparable to that of other recently discovered z similar to 0.3-0.4 LyC emitters. Combining our measurements and earlier determinations from the literature, we find a trend of increasing xi(0)(ion) with increasing C III] lambda 1909 equivalent width, which can be understood by a combination of decreasing stellar population age and metallicity. Simple ionization and density-bounded photoionization models can explain the main observational features including the UV spectrum of J1154+2443.
KW  - galaxies: starburst
KW  - galaxies: high-redshift
KW  - dark ages, reionization, first stars
KW  - ultraviolet: galaxies
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833823
SN  - 1432-0746
VL  - 616
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Schmidt, Tobias M.
A1  - Hennawi, Joseph F.
A1  - Worseck, Gabor
A1  - Davies, Frederick B.
A1  - Lukic, Zarija
A1  - Oñorbe, Jose
T1  - Modeling the HeII transverse proximity effect
BT  - constraints on quasar lifetime and obscuration
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The He II transverse proximity effect-enhanced He II Ly alpha transmission in a background sightline caused by the ionizing radiation of a foreground quasar-offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt et al., we present a detailed model of the He II transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We postprocess outputs from a cosmological hydrodynamical simulation with a fluctuating He II ultraviolet background model, with the added effect of the radiation from a single bright foreground quasar. We vary the age t(age) and obscured sky fractions Omega(obsc) of the foreground quasar, and explore the resulting effect on the He II transverse proximity effect signal. Fluctuations in intergalactic medium density and the ultraviolet background, as well as the unknown orientation of the foreground quasar, result in a large variance of the He II Ly alpha transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far-ultraviolet He II Ly alpha transmission spectra of the background quasars to our models, and extract joint constraints on t(age) and Omega(obsc) for the six Schmidt et al. foreground quasars with the highest implied He II photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong He II transmission spike, is relatively old (22 Myr) and unobscured (Omega(obsc) < 35%), whereas three others are either younger than 10 Myr or highly obscured (Omega(obsc) > 70%).
KW  - dark ages, reionization, first stars
KW  - intergalactic medium
KW  - quasars: general
Y1  - 2018
U6  - https://doi.org/10.3847/1538-4357/aac8e4
SN  - 0004-637X
SN  - 1538-4357
VL  - 861
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Husemann, Bernd
A1  - Worseck, Gabor
A1  - Battaia, Fabrizio Arrigoni
A1  - Shanks, T.
T1  - Discovery of a dual AGN at z similar or equal to 3.3 with 20 kpc separation
JF  - Astronomy and astrophysics : an international weekly journal
N2  - A prediction of the current paradigm of the hierarchical assembly of galaxies is the presence of supermassive dual black holes at separations of a few kpc or less. In this context, we report the detection of a narrow-line emitter within the extended Ly alpha nebula (similar to 120 kpc diameter) of the luminous radio-quiet quasi-stellar object (QSO) LBQS 0302 0019 at z = 3 : 286. We identify several high-ionization narrow emission lines (He II, C IV, C III) associated with this point-like source, which we have named "Jil", which is only similar to 20 kpc (2 : 0 0 9) away from the QSO in projection. Emission-line diagnostics confirm that the source is likely powered by photoionization of an obscured active galactic nucleus (AGN) three orders of magnitude fainter than the QSO. The system represents the tightest unobscured/obscured dual AGN currently known at z > 3, highlighting the power of MUSE to detect these elusive systems.
KW  - Techniques: imaging spectroscopy
KW  - Ultraviolet: ISM
KW  - galaxies: high-redshift
KW  - quasars: individual: LBQS 0302-0019
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201732457
SN  - 1432-0746
VL  - 610
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - THES
A1  - Worseck, Gábor
T1  - The transverse proximity effect in quasar spectra
T1  - Der Transversale Proximity-Effekt in Quasarspektren
N2  - The intergalactic medium is kept highly photoionised by the intergalactic UV background radiation field generated by the overall population of quasars and galaxies. In the vicinity of sources of UV photons, such as luminous high-redshift quasars, the UV radiation field is enhanced due to the local source contribution. The higher degree of ionisation is visible as a reduced line density or generally as a decreased level of absorption in the Lyman alpha forest of neutral hydrogen. This so-called proximity effect has been detected with high statistical significance towards luminous quasars. If quasars radiate rather isotropically, background quasar sightlines located near foreground quasars should show a region of decreased Lyman alpha absorption close to the foreground quasar. Despite considerable effort, such a transverse proximity effect has only been detected in a few cases. So far, studies of the transverse proximity effect were mostly limited by the small number of suitable projected pairs or groups of high-redshift quasars. With the aim to substantially increase the number of quasar groups in the vicinity of bright quasars we conduct a targeted survey for faint quasars around 18 well-studied quasars at employing slitless spectroscopy. Among the reduced and calibrated slitless spectra of 29000 objects on a total area of 4.39 square degrees we discover in total 169 previously unknown quasar candidates based on their prominent emission lines. 81 potential z>1.7 quasars are selected for confirmation by slit spectroscopy at the Very Large Telescope (VLT). We are able to confirm 80 of these. 64 of the newly discovered quasars reside at z>1.7. The high success rate of the follow-up observations implies that the majority of the remaining candidates are quasars as well. In 16 of these groups we search for a transverse proximity effect as a systematic underdensity in the HI Lyman alpha absorption. We employ a novel technique to characterise the random absorption fluctuations in the forest in order to estimate the significance of the transverse proximity effect. Neither low-resolution spectra nor high-resolution spectra of background quasars of our groups present evidence for a transverse proximity effect. However, via Monte Carlo simulations the effect should be detectable only at the 1-2sigma level near three of the foreground quasars. Thus, we cannot distinguish between the presence or absence of a weak signature of the transverse proximity effect. The systematic effects of quasar variability, quasar anisotopy and intrinsic overdensities near quasars likely explain the apparent lack of the transverse proximity effect. Even in absence of the systematic effects, we show that a statistically significant detection of the transverse proximity effect requires at least 5 medium-resolution quasar spectra of background quasars near foreground quasars whose UV flux exceeds the UV background by a factor 3. Therefore, statistical studies of the transverse proximity effect require large numbers of suitable pairs. Two sightlines towards the central quasars of our survey fields show intergalactic HeII Lyman alpha absorption. A comparison of the HeII absorption to the corresponding HI absorption yields an estimate of the spectral shape of the intergalactic UV radiation field, typically parameterised by the HeII/HI column density ratio eta. We analyse the fluctuating UV spectral shape on both lines of sight and correlate it with seven foreground quasars. On the line of sight towards Q0302-003 we find a harder radiation field near 4 foreground quasars. In the direct vicinity of the quasars eta is consistent with values of 25-100, whereas at large distances from the quasars eta>200 is required. The second line of sight towards HE2347-4342 probes lower redshifts where eta is directly measurable in the resolved HeII forest. Again we find that the radiation field near the 3 foreground quasars is significantly harder than in general. While eta still shows large fluctuations near the quasars, probably due to radiative transfer, the radiation field is on average harder near the quasars than far away from them. We interpret these discoveries as the first detections of the transverse proximity effect as a local hardness fluctuation in the UV spectral shape. No significant HI proximity effect is predicted for the 7 foreground quasars. In fact, the HI absorption near the quasars is close to or slightly above the average, suggesting that the weak signature of the transverse proximity effect is masked by intrinsic overdensities. However, we show that the UV spectral shape traces the transverse proximity effect even in overdense regions or at large distances. Therefore, the spectral hardness is a sensitive physical measure of the transverse proximity effect that is able to break the density degeneracy affecting the traditional searches.
N2  - Das intergalaktische Medium wird durch das intergalaktische UV-Hintergrundsstrahlungsfeld in einem hochgradig photoionisierten Zustand gehalten. Der UV-Hintergrund stammt von der gesamten Population von Quasaren und Galaxien. In der Nähe von leuchtkräftigen Quasaren, ist das UV-Strahlungsfeld lokal erhöht durch den Anteil der Quelle. Der höhere Ionisationsgrad ist beobachtbar als eine reduzierte Liniendichte oder allgemein als ein vermindertes Maß an Absorption im Lyman-alpha Wald des neutralen Wasserstoffs. Dieser sogenannte Proximity-Effekt ist bei leuchtkräftigen Quasaren mit hoher statistischer Signifikanz nachgewiesen worden. Falls Quasare fast isotrop strahlen, dann sollten Sichtlinien zu Hintergrundquasaren in der Nähe von Vordergrundquasaren eine Region mit verminderter Absorption zeigen. Trotz beträchtlichen Aufwands wurde solch ein transversaler Proximity-Effekt nur in wenigen Fällen entdeckt. Bisher waren Studien des transversalen Proximity-Effekts meist begrenzt durch die kleine Anzahl von geeigneten projizierten Paaren oder Gruppen von hochrotverschobenen Quasaren. Mit dem Ziel die Zahl der Quasargruppen in der Nähe von hellen Quasaren beträchtlich zu erhöhen, führen wir eine gezielte Suche nach schwachen Quasaren um 18 oft studierte Quasare durch. Unter den reduzierten und kalibrierten spaltlosen Spektren von 29000 Objekten auf einer Gesamtfläche von 4.39 Quadratgrad entdecken wir insgesamt 169 vorher unbekannte Quasarkandidaten anhand ihrer Emissionslinien. 81 potentielle z>1.7 Quasare werden ausgesucht zur Bestätigung mittels Spaltspektroskopie am Very Large Telescope (VLT). Wir können 80 von diesen als Quasare bestätigen. 64 der neu entdeckten Quasare liegen bei z>1.7. Die hohe Erfolgsrate der Nachfolgebeobachtungen deutet an, dass die Mehrzahl der verbleibenden Kandidaten ebenfalls Quasare sind. In 16 dieser Gruppen suchen wir nach dem transversalen Proximity-Effekt als eine systematische Unterdichte in der HI Lyman-alpha-Absorption. Wir nutzen eine neuartige Methode die zufälligen Absorptionsfluktuationen zu charakterisieren, um die Signifikanz des transversalen Proximity-Effekts abschätzen zu können. Weder schwach aufgelöste noch hoch aufgelöste Spektren von Hintergrundquasaren unserer Gruppen zeigen Anzeichen für einen transversalen Proximity-Effekt. Aufgrund von Monte Carlo Simulationen sollte der Effekt jedoch nur schwach in der Nähe von 3 Vordergrundquasaren detektierbar sein. Deshalb können wir nicht zwischen An- oder Abwesenheit des Effekts unterscheiden. Selbst in Abwesenheit von systematischen Effekten zeigen wir, dass eine statistisch signifikante Detektion des transversalen Proximity-Effekts mindestens 5 Hintergrundquasarspektren bei mittlerer Auflösung nahe Vordergrundquasaren erfordert, deren UV-Fluss den UV-Hintergrund um einen Faktor 3 übersteigt. Deshalb erfordern statistische Studien des transversalen Proximity-Effekts große Zahlen von geeigneten Quasaren. Zwei Sichtlinien zeigen HeII-Absorption. Ein Vergleich der HeII-Absorption mit der entsprechenden HI-Absorption liefert eine Abschätzung der Spektralform des UV-Strahlungsfelds, das typischerweise durch das HeII/HI Säulendichteverhältnis eta parameterisiert wird. Wir analysieren die fluktuierende spektrale Form des UV-Strahlungsfelds auf beiden Sichtlinien und korrelieren sie mit 7 Vordergrundquasaren. Auf der Sichtlinie zu Q0302-003 finden wir ein härteres Strahlungsfeld nahe 4 Vordergrundquasaren. In der direkten Umgebung der Quasare ist eta konsistent mit Werten von 25-100, wogegen bei großen Entfernungen zu den Quasaren eta>200 erforderlich ist. Die zweite Sichtlinie zu HE2347-4342 sondiert kleinere Rotverschiebungen. Wieder finden wir, dass das Strahlungsfeld nahe der 3 Vordergrundquasaren signifikant härter ist als im allgemeinen. Während eta trotzdem große Fluktuationen nahe den Quasaren aufweist, die wahrscheinlich von Strahlungstransport herrühren, ist das Strahlungsfeld in der Nähe der Quasare im Mittel härter als in großer Entfernung. Wir interpretieren diese Entdeckungen als die ersten Detektionen des transversalen Proximity-Effekts als eine lokale Fluktuation im spektralen Härtegrad. Kein signifikanter HI Proximity-Effekt ist für die 7 Vordergrundquasare vorhergesagt. Tatsächlich ist die HI-Absorption nahe den Quasaren nahe am oder etwas über dem Mittelwert, was darauf hindeutet, dass die schwache Signatur des transversalen Proximity-Effekts maskiert wird durch intrinsische Überdichten. Jedoch zeigen wir, dass der Härtegrad den transversalen Proximity-Effekt selbst in überdichten Regionen oder auf großen Distanzen sichtbar werden läßt. Deshalb ist der spektrale Härtegrad ein empfindliches physikalisches Maß für den transversalen Proximity-Effekt, der in der Lage ist, die Dichteentartung zu brechen, die die traditionelle Suche behindert.
KW  - Quasar
KW  - Proximity-Effekt
KW  - intergalaktisches Medium
KW  - quasar
KW  - proximity effect
KW  - intergalactic medium
Y1  - 2007
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-18738
ER  - 
TY  - JOUR
A1  - Husemann, Bernd
A1  - Bielby, R.
A1  - Jahnke, K.
A1  - Arrigoni-Battaia, F.
A1  - Worseck, Gabor
A1  - Shanks, T.
A1  - Wardlow, J.
A1  - Scholtz, J.
T1  - Cosmic dance at z similar to 3
BT  - Detecting the host galaxies of the dual AGN system LBQS 0302-0019 and Jil with HAWK-I plus GRAAL
JF  - Astronomy and astrophysics : an international weekly journal
N2  - We recently discovered that the luminous radio-quiet (QSO) LBQS 0302-0019 at z=3.286 is likely accompanied by an obscured AGN at 20 kpc projected distance, which we dubbed Jil. It represents the tightest candidate obscured/unobscured dual AGN system at z >3. To verify the dual AGN scenario we obtained deep K-s band (rest-frame V band) imaging with the VLT/HAWK-I+GRAAL instrument at 0 '.4 resolution during science verification in January 2018. Indeed, we detect the individual host galaxies of the QSO and Jil with estimated stellar masses of log(M-*/M-circle dot)=11.4 +/- 0.5 and log(M-*/M-circle dot)=10.9 +/- 0.5, respectively. Near-IR spectra obtained with VLT-KMOS reveal a clear [O-III] lambda 5007 line detection at the location of Jil which does not contribute significantly to the Ks band flux. Both observations therefore corroborate the dual AGN scenario. A comparison to Illustris cosmological simulations suggests a parent halo mass of log(M-halo/M-*)=13.2 +/- 0.5 for this interacting galaxy system, corresponding to a very massive dark matter halo at that epoch.
KW  - Galaxies: interactions
KW  - Galaxies: high-redshift
KW  - large-scale structure of Universe
KW  - instrumentation: adaptive optics
KW  - quasars: individual: LBQS 0302-0018
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833363
SN  - 1432-0746
VL  - 614
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Izotov, Y. I.
A1  - Worseck, Gábor
A1  - Schaerer, Daniel
A1  - Guseva, N. G.
A1  - Thuan, T. X.
A1  - Fricke, K. J.
A1  - Verhamme, Anne
A1  - Orlitova, I.
T1  - Low-redshift Lyman continuum leaking galaxies with high [O III]/[O II] ratios
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present observations with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope of five star-forming galaxies at redshifts z in the range 0.2993-0.4317 and with high emission-line flux ratios O-32 = [O III]lambda 5007/[O II]lambda 3727 similar to 8-27 aiming to detect the Lyman continuum (LyC) emission. We detect LyC emission in all galaxies with the escape fractions f(esc)(LyC) in a range of 2-72 per cent. A narrow Ly alpha emission line with two peaks in four galaxies and with three peaks in one object is seen in medium-resolution COS spectra with a velocity separation between the peaks V-sep varying from similar to 153 to similar to 345 km s(-1). We find a general increase of the LyC escape fraction with increasing O-32 and decreasing stellar mass M-star, but with a large scatter of f(esc)(LyC). A tight anticorrelation is found between f(esc)(LyC) and V-sep making V-sep a good parameter for the indirect determination of the LyC escape fraction. We argue that one possible source driving the escape of ionizing radiation is stellar winds and radiation from hot massive stars.
KW  - galaxies: abundances
KW  - galaxies: dwarf
KW  - galaxies: fundamental parameters
KW  - galaxies: ISM
KW  - galaxies: starburst
KW  - dark ages, reionization, first stars
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty1378
SN  - 0035-8711
SN  - 1365-2966
VL  - 478
IS  - 4
SP  - 4851
EP  - 4865
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Perrotta, S.
A1  - Hamann, F.
A1  - Cristiani, S.
A1  - Prochaska, J. X.
A1  - Ellison, Sara L.
A1  - Lopez, S.
A1  - Cupani, G.
A1  - Becker, G.
A1  - Berg, T. A. M.
A1  - Christensen, Lise
A1  - Denney, K. D.
A1  - Worseck, Gábor
T1  - Hunting for metals using XQ-100 Legacy Survey composite spectra
JF  - Monthly notices of the Royal Astronomical Society
N2  - We investigate the NV absorption signal along the line of sight of background quasars, in order to test the robustness of the use of this ion as the criterion to select intrinsic (i.e. physically related to the quasar host galaxy) narrow absorption lines (NALs). We build composite spectra from a sample of similar to 1000 CIV absorbers, covering the redshift range 2.55 < z < 4.73, identified in 100 individual sight lines from the XQ-100 Legacy Survey. We detect a statistically significant NV absorption signal only within 5000 km s(-1) of the systemic redshift, z(em). This absorption trough is similar to 15 sigma when only CIV systems with N(CIV) > 10(14) cm(-2) are included in the composite spectrum. This result confirms that NV offers an excellent statistical tool to identify intrinsic systems. We exploit stacks of 11 different ions to show that the gas in proximity to a quasar exhibits a considerably different ionization state with respect to gas in the transverse direction and intervening gas at large velocity separations from the continuum source. Indeed, we find a dearth of cool gas, as traced by low-ionization species and in particular by MgII, in the proximity of the quasar. We compare our findings with the predictions given by a range of CLOUDY ionization models and find that they can be naturally explained by ionization effects of the quasar.
KW  - galaxies: high-redshift
KW  - (galaxies:) intergalactic medium
KW  - (galaxies:) quasars: absorption lines
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty2205
SN  - 0035-8711
SN  - 1365-2966
VL  - 481
IS  - 1
SP  - 105
EP  - 121
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Kulkarni, Girish
A1  - Worseck, Gabor
A1  - Hennawi, Joseph F.
T1  - Evolution of the AGN UV luminosity function from redshift 7.5
JF  - Monthly notices of the Royal Astronomical Society
N2  - Determinations of the ultraviolet (UV) luminosity function of active galactic nuclei (AGN) at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of data. We address this problem by constructing a sample of more than 80 000 colour-selected AGN from redshift z= 0 to 7.5 using multiple data sets homogenized to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift z= 0 to 7.5. The luminosity function has a double power-law form at all redshifts. The break magnitude M-* shows a steep brightening from M-* similar to -24 at z = 0.7 to M-* similar to -29 at z = 6. The faint-end slope beta significantly steepens from -1.9 at z < 2.2 to -2.4 at z similar or equal to 6. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at z similar to 6 is subdominant (< 3 per cent), although it can be non-negligible (similar to 10 per cent) if these luminosity functions hold down to M-1450 = -18. Under reasonable assumptions, AGN can reionize He II by redshift z = 2.9. At low redshifts (z < 0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of HI absorption lines in the intergalactic medium. Our analysis also reveals important systematic errors in the data, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, codes, and data publicly available.
KW  - galaxies: active
KW  - intergalactic medium
KW  - quasars: general
KW  - dark ages, reionization, first stars
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz1493
SN  - 0035-8711
SN  - 1365-2966
VL  - 488
IS  - 1
SP  - 1035
EP  - 1065
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Khrykin, Ilya S.
A1  - Hennawi, Joseph F.
A1  - Worseck, Gabor
T1  - Evidence for short similar to 1 Myr lifetimes from the He II proximity zones of z similar to 4 quasars
JF  - Monthly notices of the Royal Astronomical Society
N2  - The duration of quasar accretion episodes is a key quantity for distinguishing between models for the formation and growth of supermassive black holes, the evolution of quasars, and their potential feedback effects on their host galaxies. However, this critical time-scale, often referred to as the quasar lifetime, is still uncertain by orders of magnitude (⁠tQ≃0.01Myr−1Gyr⁠). Absorption spectra of quasars exhibiting transmission in the He  II Ly α forest provide a unique opportunity to make precise measurements of the quasar lifetime. Indeed, the size of a quasar’s He  II proximity zone, the region near the quasar where its own radiation dramatically alters the ionization state of the surrounding intergalactic medium (IGM), depends sensitively on its lifetime for tQ≲30Myr⁠, comparable to the expected e-folding time-scale for SMBH growth tS=45Myr⁠. In this study, we compare the sizes of He  II proximity zones in the Hubble Space Telescope (HST) spectra of six z ∼ 4 quasars to theoretical models generated by post-processing cosmological hydrodynamical simulations with a 1D radiative transfer algorithm. We introduce a Bayesian statistical method to infer the lifetimes of individual quasars which allows us to fully marginalize over the unknown ionization state of the surrounding IGM. We measure lifetimes 0.63+0.82−0.40 Myr and 5.75+4.72−2.74 Myr for two objects. For the other four quasars, large redshift uncertainties undermine our sensitivity allowing us to only place upper or lower limits. However, a joint analysis of these four systems yields a measurement of their average lifetime of ⟨tQ⟩=1.17+1.77−0.84 Myr. We discuss our short ∼1Myr inferred lifetimes in the context of other quasar lifetime constraints and the growth of SMBHs.
KW  - intergalactic medium
KW  - quasars: general
KW  - dark ages
KW  - reionization
KW  - first star
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz135
SN  - 0035-8711
SN  - 1365-2966
VL  - 484
IS  - 3
SP  - 3897
EP  - 3910
PB  - Oxford University Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Worseck, Gabor
A1  - Davies, Frederick B.
A1  - Hennawi, Joseph F.
A1  - Prochaska, J. Xavier
T1  - The Evolution of the He II-ionizing Background at Redshifts 2.3 < z < 3.8 Inferred from a Statistical Sample of 24 HST/COS He II Lyα Absorption Spectra
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present measurements of the large-scale (≈40 comoving Mpc) effective optical depth of He ii Lyα absorption, ${\tau }_{\mathrm{eff}}$, at 2.54 < z < 3.86 toward 16 He ii-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of ${\tau }_{\mathrm{eff}}$ measurements in six signal-to-noise ratio gsim3 He ii sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of ${\tau }_{\mathrm{eff}}$ in 24 He ii sightlines. We confirm an increase of the median ${\tau }_{\mathrm{eff}}$ from sime2 at z = 2.7 to ${\tau }_{\mathrm{eff}}\gtrsim 5$ at z > 3, and a scatter in ${\tau }_{\mathrm{eff}}$ that increases with redshift. The z > 3.5 He ii absorption is predominantly saturated, but isolated narrow (Δv < 650 km s−1) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc)3 hydrodynamical simulation by forward-modeling our sample's quality and size. At z > 2.74, the variance in ${\tau }_{\mathrm{eff}}$ significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median He ii photoionization rate ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}$ of a fluctuating UV background from the median ${\tau }_{\mathrm{eff}}$, finding a factor sime5 decrease in ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}$ between z sime 2.6 and z sime 3.1. At z sime 3.1, ${{\rm{\Gamma }}}_{\mathrm{He}{\rm{II}}}=\left[{9.1}_{-1.2}^{+1.1}\,(\mathrm{stat}.){\,}_{-3.4}^{+2.4}\,(\mathrm{sys}.)\right]\times {10}^{-16}$ s−1 corresponds to a median He ii fraction of sime2.5%, indicating that our data probe the tail end of He ii reionization.
KW  - dark ages, reionization, first stars
KW  - diffuse radiation
KW  - intergalactic medium
KW  - quasars: absorption lines
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab0fa1
SN  - 0004-637X
SN  - 1538-4357
VL  - 875
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Furniss, Amy
A1  - Worseck, Gabor
A1  - Fumagalli, Michele
A1  - Johnson, Caitlin A.
A1  - Williams, David A.
A1  - Pontrelli, P.
A1  - Prochaska, J. Xavier
T1  - Spectroscopic Redshift of the Gamma-Ray Blazar B2 1215+30 from Ly alpha Emission
JF  - The astronomical journal
N2  - We report on Cosmic Origin Spectrograph observations of the gamma-ray bright blazar B2 1215+30, collected in 2015 November. These observations allow for the confirmation of the source redshift from the detection of a Lyα emission feature at λ ~ 1374 Å. The emission feature places the source at a redshift of z = 0.1305 ± 0.003, confirming the source's ground-based spectral measurement. The gamma-ray emission of the source is discussed in the context of the source distance, required for the accurate reconstruction of the intrinsic gamma-ray emission taking the absorption by the extragalactic background light into account. The source distance is found to be low enough that the previously reported detection of an exceptional flaring event from B2 1215+30 in 2014 cannot be used to investigate opacity-specific spectral and variability characteristics introduced by possible ultra-high-energy cosmic-ray propagation.
KW  - galaxies:distances and redshifts
KW  - galaxies: individual (B2 1215+30)
KW  - gamma rays: galaxies
KW  - ultraviolet: galaxies
Y1  - 2019
U6  - https://doi.org/10.3847/1538-3881/aaf28b
SN  - 0004-6256
SN  - 1538-3881
VL  - 157
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Crighton, Neil H. M.
A1  - Prochaska, J. Xavier
A1  - Murphy, Michael T.
A1  - Worseck, Gabor
A1  - Smith, Britton D.
T1  - Imprints of the first billion years
BT  - Lyman limit systems at z similar to 5
JF  - Monthly notices of the Royal Astronomical Society
N2  - Lyman limit systems (LLSs) trace the low-density circumgalactic medium and the most dense regions of the intergalactic medium, so their number density and evolution at high-redshift, just after reionization, are important to constrain. We present a survey for LLSs at high redshifts, z(LLS) = 3.5-5.4, in the homogeneous data set of 153 optical quasar spectra at z similar to 5 from the Giant Gemini GMOS survey. Our analysis includes detailed investigation of survey biases using mock spectra which provide important corrections to the raw measurements. We estimate the incidence of LLSs per unit redshift at z approximate to 4.4 to be l(z) = 2.6 +/- 0.4. Combining our results with previous surveys at z(LLS) < 4, the best-fit power-law evolution is l(z) = l(*)[(1 + z)/4](alpha) with l* = 1.46 +/- 0.11 and alpha = 1.70 +/- 0.22 (68 per cent confidence intervals). Despite hints in previous z(LLS) < 4 results, there is no indication for a deviation from this single power-law soon after reionization. Finally, we integrate our new results with previous surveys of the intergalactic and circumgalactic media to constrain the hydrogen column density distribution function, f(N-HI, X), over 10 orders ofmagnitude. The data at z similar to 5 are not well-described by the f(N-HI, X) model previously reported for z similar to 2-3 (after re-scaling) and a 7-pivot model fitting the full z similar to 2-5 data set is statistically unacceptable. We conclude that there is significant evolution in the shape of f(N-HI, X) over this similar to 2-billion-year period.
KW  - quasars: absorption lines
KW  - cosmological parameters
KW  - cosmology: observations
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/sty2762
SN  - 0035-8711
SN  - 1365-2966
VL  - 482
IS  - 2
SP  - 1456
EP  - 1470
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Worseck, Gabor
A1  - Prochaska, J. Xavier
A1  - McQuinn, Matthew
A1  - Dall'Aglio, Aldo
A1  - Fechner, Cora
A1  - Hennawi, Joseph F.
A1  - Reimers, Dieter
A1  - Richter, Philipp
A1  - Wisotzki, Lutz
T1  - The end of Helium Reionization at z similar or equal to 2.7 Inferred  from cosmic variance in HST/COS He II Ly alpha Absorption spectra
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters
N2  - We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z(em) similar or equal to 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from <tau(eff, He II)> similar or equal to 1 at z similar or equal to 2.3 to <tau(eff, He II)> greater than or similar to 5 at z similar or equal to 3.2, but with a large scatter of 2 less than or similar to tau(eff, He II) less than or similar to 5 at 2.7 < z < 3 on scales of similar to 10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval Hi forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z greater than or similar to 2.7, probably indicating He II reionization was incomplete at z(reion) greater than or similar to 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z(reion) similar or equal to 2.7 or even below, as suggested by a large tau(eff, He II) greater than or similar to 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 less than or similar to z less than or similar to 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.
KW  - dark ages, reionization, first stars
KW  - diffuse radiation
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - quasars: individual (SDSS J092447.36+485242.8, SDSS J110155.74+105302.3)
Y1  - 2011
U6  - https://doi.org/10.1088/2041-8205/733/2/L24
SN  - 2041-8205
VL  - 733
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Worseck, Gábor
A1  - Khrykin, Ilya Sergeevich
A1  - Hennawi, Joseph F.
A1  - Prochaska, J. Xavier
A1  - Farina, Emanuele Paolo
T1  - Dating individual quasars with the He II proximity effect
JF  - Monthly notices of the Royal Astronomical Society
N2  - Constraints on the time-scales of quasar activity are key to understanding the formation and growth of supermassive black holes (SMBHs), quasar triggering mechanisms, and possible feedback effects on their host galaxies. However, observational estimates of this so-called quasar lifetime are highly uncertain (t(Q) similar to 10(4)-10(9) yr), because most methods are indirect and involve many model-dependent assumptions. Direct evidence of earlier activity is gained from the higher ionization state of the intergalactic medium (IGM) in the quasar environs, observable as enhanced Ly alpha transmission in the so-called proximity zone. Due to the similar to 30 Myr equilibration time-scale of He II in the z similar to 3 IGM, the size of the He II proximity zone depends on the time the quasar had been active before our observation t(on) <= t(Q), enabling up to +/- 0.2 dex precise measurements of individual quasar on-times that are comparable to the e-folding time-scale t(S) <= 44 Myr of SMBH growth. Here we present the first statistical sample of 13 quasars whose accurate and precise systemic redshifts allow for measurements of sufficiently precise He II quasar proximity zone sizes between similar or equal to 2 and similar or equal to 15 proper Mpc from science-grade Hubble Space Telescope (HST) spectra. Comparing these sizes to predictions from cosmological hydrodynamical simulations post-processed with 1D radiative transfer, we infer a broad range of quasar on-times from t(on) less than or similar to 1Myr to t(on) > 30 Myr that does not depend on quasar luminosity, black hole mass, or Eddington ratio. These results point to episodic quasar activity over a long duty cycle, but do not rule out substantial SMBH growth during phases of radiative inefficiency or obscuration.
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - quasars: general
KW  - quasars: supermassive black holes
KW  - dark ages, reionization, first stars
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab1685
SN  - 0035-8711
SN  - 1365-2966
VL  - 505
IS  - 4
SP  - 5084
EP  - 5103
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Izotov, Y. I.
A1  - Schaerer, Daniel
A1  - Worseck, Gabor
A1  - Guseva, N. G.
A1  - Thuan, T. X.
A1  - Verhamme, A.
A1  - Orlitova, I.
A1  - Fricke, K. J.
T1  - J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons
JF  - Monthly notices of the Royal Astronomical Society
N2  - We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z = 0.3690, is characterized by a high emission-line flux ratio O-32 = [O III] lambda 5007/[O II] lambda 3727 = 11.5. The escape fraction of the LyC radiation f(esc)(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Ly alpha emission line is detected in the spectrum of J1154+2443 with a separation between the peaks V-sep of 199 km s(-1), one of the lowest known for Ly alpha-emitting galaxies, implying a high f(esc)(Ly alpha). Comparing the extinction-corrected Ly alpha/H beta flux ratio with the case B value, we find f(esc)(Ly alpha) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O-32 with increasing f(esc)(LyC). We also find a tight anticorrelation between f(esc)(LyC) and V-sep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length alpha = 1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+log O/H = 7.65 +/- 0.01, the lowest stellar mass M-star = 108.20 M-circle dot, a similar star formation rate SFR = 18.9 M-circle dot yr(-1), and a high specific SFR of 1.2 x 10(-7) yr(-1).
KW  - galaxies: abundances
KW  - galaxies: dwarf
KW  - galaxies: fundamental parameters
KW  - galaxies: ISM
KW  - galaxies: starburst
KW  - dark ages, reionization, first stars
Y1  - 2017
U6  - https://doi.org/10.1093/mnras/stx3115
SN  - 0035-8711
SN  - 1365-2966
VL  - 474
IS  - 4
SP  - 4514
EP  - 4527
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Becker, George D.
A1  - D'Aloisio, Anson
A1  - Christenson, Holly M.
A1  - Zhu, Yongda
A1  - Worseck, Gábor
A1  - Bolton, James S.
T1  - The mean free path of ionizing photons at 5 < z < 6
BT  - evidence for rapid evolution near reionization
JF  - Monthly notices of the Royal Astronomical Society
N2  - The mean free path of ionizing photons, lambda(mfp), is a key factor in the photoionization of the intergalactic medium (IGM). At z greater than or similar to 5, however, lambda(mfp) may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of lambda(mfp) that address this bias and extend up to z similar to 6 for the first time. Our measurements at z similar to 5 are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At z similar to 6 we use QSO spectra from Keck ESI and VLT X-Shooter. We measure lambda(mfp) = 9.09(-1.28)(+1.62) proper Mpc and 0.75(-0.45)(+0.65) proper Mpc (68 percent confidence) at z = 5.1 and 6.0, respectively. The results at z = 5.1 are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At z = 6.0, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at z = 6.0 falls well below extrapolations from lower redshifts, indicating rapid evolution in lambda(mfp) over 5 < z < 6. This evolution disfavours models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by z = 6, but is qualitatively consistent with models wherein reionization completed at z = 6 or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20 percent neutral at z = 6, although our measurement at z = 6.0 is even lower than these models prefer.
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - cosmology: observations
KW  - dark ages
KW  - large-scale structure of Universe
KW  - reionization
KW  - first stars
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab2696
SN  - 0035-8711
SN  - 1365-2966
VL  - 508
IS  - 2
SP  - 1853
EP  - 1869
PB  - Oxford Univ. Press
CY  - Oxford
ER  -