The transverse proximity effect in quasar spectra [Elektronische Ressource] / Gábor Worseck

Gábor Worseck

Sujets

• Astronomie

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• Publié par: universitat_potsdam
• Publié le: 01 janvier 2007
• Nombre de lectures: 34
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

The Transverse Proximity Effect
in Quasar Spectra
Gabor· Worseck
Astrophysikalisches Institut Potsdam
Dissertation zur Erlangung des akademischen Grades
doctor rerum naturalium (Dr. rer. nat.)
in der Wissenschaftsdisziplin Astrophysik
Eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultat¤
der Universitat¤ Potsdam
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Online published at the Institutional Repository of the Potsdam University:
http://opus.kobv.de/ubp/volltexte/2008/1873/
urn:nbn:de:kobv:517-opus-18738
[http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-18738] Bravado
Have I not walked without an upward look
Of caution under stars that very well
Might not have missed me when they shot and fell?
It was a risk I had to take ? and took.
Robert Frost (1946)ABSTRACT
The intergalactic medium is kept highly photoionised by the intergalactic UV background radiation eld generated
by the overall population of quasars and star-forming galaxies. In the vicinity of sources of UV photons, such as
luminous high-redshift quasars, the UV radiation eld 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 (Lyfi) forest of neutral hydrogen. This so-called proximity efiect has been detected with high statistical
signi cance towards luminous quasars. If quasars radiate rather isotropically, background quasar sightlines located
near foreground quasars should show a region of decreased Lyfi absorption close to the foreground quasar. Despite
considerable efiort, such a transverse proximity efiect has only been detected in a few cases. It is still unclear,
whether this efiect is a common phenomenon in the Lyfi forest.
So far, studies of the transverse proximity efiect were mostly limited by the small number of suitable projected
pairs or groups of high-redshift quasars. With the aim to substantially increase the of quasar groups in the
< < <vicinity of bright quasars we conduct a targeted survey for faint V 22 quasars at redshifts 1:7 z 3:6 around» » »
18 well-studied at 2:76 < z < 4:69 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 con rmation by slit spectroscopy at the Very Large Telescope (VLT). We are able to con rm 80 of these as
0:580• z• 3:586 quasars. 64 of the newly discovered quasars reside at z > 1:7. Excluding the central quasars in
the elds, our survey increases the number of z > 1:7 in these elds by a factor» 5. 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 efiect as a systematic underdensity in the intergalactic
Hi Lyfi absorption. We employ a novel technique to characterise the random absorption uctuations in the Lyfi
forest in order to estimate the signi cance of the transverse proximity efiect. Neither low-resolution spectra nor
high-resolution spectra of background quasars of our groups present evidence for a transverse proximity efiect.
However, via Monte Carlo simulations the efiect should be detectable only at the 1 2? level near three of the
foreground quasars. Thus, we cannot distinguish between the presence or absence of a weak signature of the
transverse proximity efiect. The systematic efiects of quasar variability, quasar anisotopy and intrinsic overdensities
near quasars likely explain the apparent lack of the transverse proximity efiect. Even in absence of the systematic
efiects, we show that a statistically signi cant detection of the transverse proximity efiect requires at least» 5
medium-resolution quasar spectra of background quasars near foreground quasars whose UV ux exceeds the UV
>background by a factor 3. Therefore, statistical studies of the transverse proximity efiect in the Hi Lyfi forest»
require large numbers of suitable pairs.
Two sightlines towards the central quasars of our survey elds show intergalactic Heii Lyfi absorption. A com-
parison of the Heii absorption to the corresponding Hi absorption yields an estimate of the spectral shape of the
intergalactic UV radiation eld, typically parameterised by the Heii/Hi column density ratio ·. We analyse the
uctuating UV spectral shape on both lines of sight and correlate it with seven foreground quasars. On the line of
sight towards Q 0302¡003 we nd a harder radiation eld near 4 foreground quasars. In the direct vicinity of the
>quasars· is consistent with values of 25 100, whereas at large distances from the quasars· 200 is required. The»
second line of sight towards HE 2347¡4342 probes lower redshifts where· is directly measurable in the resolved
Heii forest. Again we nd that the radiation eld near the 3 foreground quasars is signi cantly harder (median
·» 20 50) than in general (median·» 80 100). While· still shows large uctuations near the quasars, probably
due to radiative transfer, the radiation eld is on average harder near the quasars than far away from them.
We interpret these discoveries as the rst detections of the transverse proximity efiect as a local hardness uctu-
ation in the UV spectral shape. No signi cant Hi proximity efiect is predicted for the 7 foreground quasars. In
fact, the Hi Lyfi absorption near the quasars is close to or slightly above the average, suggesting that the weak
signature of the transverse proximity efiect is masked by intrinsic overdensities. However, we show that the UV
spectral shape traces the transverse proximity efiect even in overdense regions or at large distances. Therefore, the
spectral hardness is a sensitive physical measure of the transverse proximity efiect that is able to break the density
degeneracy afiecting the traditional searches.
The transverse proximity efiect yields a lower limit on the quasar lifetime. From our detections we get values of
10 30 Myr consistent with other more indirect estimates.ZUSAMMENFASSUNG
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 sternbildenden Galaxien. In der Nahe von UV-Strahlungsquellen, wie zum Beispiel leuchtkraftigen
hochrotverschobenen Quasaren, ist das UV-Strahlungsfeld lokal erhoht¤ durch den Anteil der Quelle. Der hohere¤
Ionisationsgrad ist beobachtbar als eine reduzierte Liniendichte oder allgemein als ein vermindertes Ma an
Absorption im Lyman-alpha (Lyfi) Wald des neutralen Wasserstofis. Dieser sogenannte Proximity-Efiekt ist bei
leuchtkraftigen¤ Quasaren mit hoher statistischer Signi kanz nachgewiesen worden. Falls Quasare fast isotrop
strahlen, dann sollten Sichtlinien zu Hintergrundquasaren in der Nahe¤ von Vordergrundquasaren eine Region mit
verminderter Lyfi-Absorption zeigen. Trotz betrachtlichen¤ Aufwands wurde solch ein transversaler Proximity-
Efiekt nur in wenigen Fallen¤ entdeckt. Es ist noch unklar, ob dieser Efiekt ein generelles Phanomen¤ im Lyfi-Wald
ist.
Bisher waren Studien des transversalen Proximity-Efiekts meist begrenzt durch die kleine Anzahl von geeigneten
projizierten Paaren oder Gruppen von hochrotverschobenen Quasaren. Mit dem Ziel die Zahl der Quasargruppen in
der Nahe¤ von hellen Quasaren betrachtlich¤ zu erhohen,¤ fuhren¤ wir eine gezielte Durchmusterung mittels spaltloser
< < <Spekroskopie nach schwachen V 22 Quasaren bei Rotverschiebungen 1:7 z 3:6 um 18 oft studierte Quasare» » »
bei 2:76 < z < 4:69 durch. Unter den reduzierten und kalibrierten spaltlosen Spektren von» 29000 Objekten
auf einer Gesamt ache¤ von 4:39 Quadratgrad entdecken wir insgesamt 169 vorher unbekannte Quasarkandidaten
anhand ihrer vorstehenden Emissionslinien. 81 potentielle z > 1:7 Quasare werden ausgesucht zur Bestatigung¤
mittels Spaltspektroskopie am Very Large Telescope (VLT). Wir konnen¤ 80 von diesen als 0:580 • z • 3:586
Quasare bestatigen.¤ 64 der neu entdeckten Quasare liegen bei z > 1:7. Unter Ausschluss der Zentralquasare in
den Feldern erhoht¤ unsere Durchmusterung die Zahl der z > 1:7 Quasare in diesen Feldern um einen Faktor» 5.
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-Efiekt als eine systematische Unterdichte in der
intergalaktischen Hi Lyfi-Absorption. Wir nutzen eine neuartige Methode die zufalligen¤ Absorptions uktuationen
im Lyfi-Wald zu charakterisieren, um die Signi kanz des transversalen Proxi