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Mid-Infrared Properties of Local Active Galactic Nuclei

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Mid-Infrared Properties of Local Active Galactic Nuclei MID-INFRARED PROPERTIES OF LOCAL ACTIVE GALACTIC NUCLEI Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultat¨ der Christian-Albrechts-Universitat¨ zu Kiel vorgelegt von Daniel Asmus Kiel, 2012 Referent: Prof. Dr. Wolfgang J. Duschl Koreferent: Prof. Dr. Holger Kersten Tag der mundlichen¨ Prufung:¨ 24.04.2012 Zum Druck genehmigt: gez. Prof. Dr. Lutz Kipp, Dekan Mittinfrarot-Eigenschaften von lokalen aktiven galaktischen Kernen Die lokale Population von aktiven Galaxienkernen (AGN) erlaubt uns das detaillierte Stu- dium der Strukturen und Strahlunsprozesse in der direkten Umgebung um die massereichen Schwarzen Locher¨ in Galaxienzentren und ermoglicht¨ so das generelle Verstandnis¨ vom AGN- Phanomen¨ weiter zu entwickeln. Zu diesem Zwecke steht nun durch Swift/BAT-Beobachtungen im harten Rntgenregime das erste unverfalschte¨ Sample von 104 lokalen AGN zur Verfugung.¨ Wir haben Mitinfrarot-photometrische (MIR) Beobachtungen mit hohen Winkelauflosungen¨ (HR) von diesen AGN mit VLT/VISIR und Gemini/Michelle durchgefuhrt,¨ um die Staubtori in jenen Objekten zu studieren – eine Schlsselkomponente in AGN-Modellen. Zur gleichen Zeit haben wir vergleichbare Daten fr ein komplementares¨ Sample von AGN im lichtschwachen Regime aufgenommen. In dieser Arbeit prsentiere ich die Resultate von beiden Programmen und zeige, dass in den AGN aus beiden Samples unaufgeloste¨ MIR-Emission (< 0:400) vorhan- den ist. Andererseits zeigt ein Vergleich mit Daten von niedrigerer Auflosung¨ (∼ 3000), dass die Wirtsgalaxien im MIR um ein Vielfaches heller sind als leuchtschwache und moderat-helle AGN. Dies demonstriert, wie wichtig ein hohes raumliches¨ Auflosungsverm¨ ogen¨ fur¨ die Iso- lierung und Untersuchung der MIR-Eigenschaften von AGN. Deshalb sind viele Spitzer/IRS- Spektren von nicht-nukleare MIR-Emission beeintrachtigt.¨ Trotzdem kann ich die Emissions- strke des polyzyklischen aromatischen Hydro-Karbonat-Features, welches in diesen Spektren gemessen wurde, benutzen um den maximalen Beitrag durch Sternentstehung zu der HR nu- klearen Emission abzuschatzen,¨ und zeige, dass er minimal ist. Anstatt dessen ist die beobach- tete unaufgeloste¨ MIR-Emission konsistent mit thermischer Strahlung von Staub in der direk- ten Umgebung des AGN. Insbesondere die gemittelten MIR spektralen Energieverteilungen zeigen das Silikat-Feature in Emission fur¨ Typ 1 AGN und in Absorption fur¨ Typ 2 AGN. Au- ßerdem ist die MIR-Emission mit der Absorptions-korrigierten 2-10 keV Rontgenemission¨ ber den gesamten Bereich von Leuchtkraften¨ (1040 bis 1045 erg/s) stark korreliert, was ber einen −3:8 großen Bereich (Lbol=LEdd ∼ 10 bis 1) unabhangig¨ von der Akkretionsrate ist. Andererseits −3:8 besitzen Objekte mit Lbol=LEdd . 10 ein besonders hohes Verhltnis der MIR- zu Rntgen- leuchtkr¨fte, welches auf den Beitrag einer weiteren Emissionsquelle hindeutet. Eine ahnliche¨ Korrelation (fr¨ die MIR) mit der beobachteten 14-195 keV Strahlung wird festgestellt. Beide Korrelationen sind am starksten¨ fr Typ 1 AGN, wahrend¨ ihre Steigung und Normalisierung keine Abhangigkeit¨ zur Obskuration des Kerns oder den Eigenschaften der Wirtsgalaxien zei- gen. Dem gegenuber¨ treten extrem obskurierte Objekte haufiger¨ in auf der Seite liegenden (edge-on) Galaxien auf als unabsorbierte AGN. Dies deutet darauf hin, dass in einigen Fallen¨ die Obskuration des AGN von nicht-nuklearen Strukturen innerhalb der Wirtsgalaxie stattfin- det. Trotzdem deutet die Gleichformigkeit¨ des MIR-Rntgen-Verhltnisses darauf hin, dass die MIR-Strahlung von einer nuklearen Staubstruktur kommt, welche von der Akkretionsscheibe direkt geheizt wird. Zusammen mit der erfolgreichen Erklarung¨ der Befunde von vorausge- hende HR-MIR-Daten von AGN mit Modellen von klumpigen Staubtori impliziert dies, dass eine solche Struktur in allen AGN des gesamten untersuchten Leuchtkraftbereiches vorhanden ist. Deshalb konnte¨ das Standard-Vereinheitlichungsmodell weiterhin fur¨ all Typen von AGN ohne großere¨ Modifikationen gultig¨ sein. Mid-infrared properties of local active galactic nuclei The local active galactic nuclei (AGN) population enables us to study in detail the structures and radiation processes in the direct environment of the supermassive black holes in galaxy centers and to develop an understanding of the general AGN phenomenon. For this purpose, the first unbiased sample of 104 local AGN has now become available thanks to Swift/BAT ob- servations in the hard X-ray regime. We performed high-angular resolution (HR) mid-infrared (MIR) imaging observations of most of these AGN with VLT/VISIR and Gemini/Michelle in order to study the dusty tori in these objects – a key component in AGN models. At the same time, we obtained similar data of a complementary sample of AGN in the low-luminosity regime. In this thesis, I present the results of both programmes and show that unresolved (< 0:400) MIR emission is present in the AGN of both samples. A comparison with large aper- ture (∼ 3000) data shows that the host galaxy outshines low-luminosity and moderate AGN in the MIR, which emphasizes the necessity of HR to isolate and investigate the MIR properties of AGN. Hence, many Spitzer/IRS spectra are affected by non-nuclear MIR emission. Never- theless, I can use the polycyclic aromatic hydrocarbon emission feature strength determined from these spectra to constrain the maximum star formation contribution to the HR nuclear emission to be minor. Instead, the observed unresolved MIR nuclei are consistent with thermal emission from dust in the direct vicinity of the AGN. In particular, the average MIR spectral energy distributions show the silicate feature in emission for type 1 AGN and in absorption for type 2 AGN. Furthermore, the MIR emission is strongly correlated with the absorption- corrected 2-10 keV X-ray emission over the whole range of luminosities (1040 to 1045 erg/s) −3:8 which is independent of the accretion rates over a large range (Lbol=LEdd ∼ 10 to 1). On the −3:8 other hand, objects with Lbol=LEdd . 10 exhibit high MIR–X-ray luminosity ratios indicat- ing contribution of an additional component. However, they still follow well the correlation of the other AGN. A similar correlation (of the MIR) with the observed 14-195 keV emission is found. Both correlations are strongest for type 1 AGN while their slope and normalization do not show any dependence on the nuclear obscuration or host properties. On the contrary, highly obscured objects occur more often in edge-on galaxies than unobscured AGN. This indicates that in some cases the obscuration of the AGN is caused by non-nuclear structures in the host galaxy. Nevertheless, the fact that MIR–X-ray ratio is very uniform for all AGN types and host galaxies indicates that the MIR emission is coming from a dusty nuclear structure that is directly heated by the accretion disk. Together with the successful explanation of the findings from previous HR MIR data of AGN with models of dusty clumpy tori, this indicates that such a structure is present in all of the AGN from the whole probed luminosity range. Therefore, the standard unification model may be valid for all types of AGN without major modifications. Centaurus A over Paranal This composed image shows the radio galaxy Cen A (in approximately scaled and positioned) in the night sky over the Cerro Paranal observatory, Chile. The original photo of Paranal has been taken by Dr. Y. Beletsky and has been modified in order to create this composite. The image of Cen A itself is a composite of optical (white), radio (orange) and Gamma-rays (purple). The actual galaxy is only the whitish little dot in the center of the structure, while gigantic radio lobes extend to a distance more than 1.4 million light years away from the galaxy to an apparent size, which is almost twenty times larger than the one of the moon. These lobes are produced and fed by a highly-collimated, extremely fast outflow that originates in the very center of the galaxy. Its powerhouse is a super-massive black hole. Such nearby active galactic nuclei are the subject of this thesis. Reproduced from ESO (http://www.eso.org/public/images/potw1119a/) and NASA (http://www.nasa.gov/mission_pages/GLAST/news/smokestack-plumes_prt.htm). Credit: aESO/Y. Beletsky; and NASA/DOE/Fermi LAT Collaboration, Capella Observatory, and I. Feain, T. Cornwell, and R. Ekers (CSIRO/ATNF), R. Morganti (ASTRON), and N. Junkes (MPIfR). Contents Contents iii 1. Introduction1 1.1. Active galactic nuclei . .1 1.2. AGN in the mid-infrared . .4 1.3. Current challenges in AGN research . .5 1.4. The MIR–X-ray correlation . .6 1.5. Motivation for this work . .7 2. Physics of active galactic nuclei 11 2.1. AGN classification . 11 2.2. The unification scheme . 13 2.2.1. The accretion disk . 15 2.2.2. The jet . 16 2.2.3. The torus . 17 2.3. Low-luminosity AGN . 20 2.4. X-ray continuum emission of AGN . 22 2.5. MIR continuum emission of AGN . 24 3. Observing in the MIR 27 3.1. Spectral features in the MIR . 27 3.2. Instruments . 29 3.2.1. Chopping and nodding . 30 3.2.2. VLT/VISIR . 31 3.2.3. Gemini/Michelle . 31 3.3. Data reduction . 35 iii CONTENTS 4. Low-luminosity AGN in the mid-infrared 37 4.1. Goals . 37 4.2. Sample properties . 37 4.2.1. Optical classification . 38 4.2.2. Distances . 39 4.3. MIR observations . 39 4.3.1. VISIR observations . 39 4.3.2. Michelle observations . 40 4.4. MIR morphology and photometry of the newly observed LLAGN . 40 4.5. Comparison to Spitzer/IRS . 47 4.5.1. Constraining the nuclear star-formation contamination . 52 5. Mid-infrared properties of an uniform AGN sample 57 5.1. The BAT AGN sample . 57 5.2. Goals . 59 5.3. MIR observations . 59 5.3.1. New observations . 59 5.3.2. Data from the archive . 60 5.4. Sample properties . 60 5.4.1. Optical classification and morphologies . 60 5.4.2. Distances . 61 5.5.
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