Formation of supermassive black holes

192 Pages·2017·5.55 MB·English

by  Mélanie Habouzit

Formation of supermassive black holes
Mélanie Habouzit
To cite this version:
Mélanie Habouzit. Formation of supermassive black holes. Astrophysics [astro-ph]. Université Pierre
et Marie Curie - Paris VI, 2016. English. ￿NNT: 2016PA066360￿. ￿tel-01480290￿
HAL Id: tel-01480290
https://theses.hal.science/tel-01480290
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THÈSE DE DOCTORAT
DE L’UNIVERSITÉ PIERRE ET MARIE CURIE
Spécialité Astrophysique
École Doctorale d’Astronomie & d’Astrophysique d’Ile-de-France
réalisée
à l’Institut d’Astrophysique de Paris
présentée par
Mélanie Habouzit
pour obtenir le grade de
DOCTEUR DE L’UNIVERSITÉ PIERRE ET MARIE CURIE
Sujet de la thèse :
Formation of supermassive black holes
soutenue le 15 septembre 2016
devant le jury composé de :
Benoit Semelin Président du jury
Marta Volonteri Directeur de thèse
Rachel Somerville Rapporteur
Sadegh Khochfar Rapporteur
Raffaella Schneider Examinateur
Jenny Greene Examinateur
Yohan Dubois Examinateur

A mon petit Fanfan, mes parents et mes grand-parents.

Abstract
Supermassive black holes (BHs) harboured in the center of galaxies have been confirmed with
the discovery of Sagittarius A? in the center of our galaxy, the Milky Way. Recent surveys
indicate that BHs of millions of solar masses are common in most local galaxies, but also that
some local galaxies could be lacking BHs (e.g. NGC 205, M33), or at least hosting low-mass
BHs of few thousands solar masses. Conversely, massive BHs under their most luminous form
are called quasars, and their luminosity can be up to hundred times the luminosity of an entire
galaxy. We observe these quasars in the very early Universe, less than a billion years after the Big
Bang, with masses as large as 108M (Fan et al., 2006b; Mortlock et al., 2011). BH formation
(cid:12)
models therefore need to explain both the low-mass BHs that are observed in low-mass galaxies
today, but also the prodigious quasars we see in the early Universe. Several correlations between
BH mass and galaxy properties have been derived empirically, such as the BH mass-velocity
dispersion relation, they may be seen as evidence of BH and galaxy co-evolution through cosmic
time. Moreover, BHs impact their host galaxies and vice versa. For example, BH growth is
regulated by the ability of galaxies to funnel gas towards them, while BHs are thought to exert
powerful feedback on their host galaxies. BHs are a key element of galaxy evolution, and therefore
in order to study BH formation in the context of galaxy evolution, we have used cosmological
hydrodynamical simulations. Cosmological simulations offer the advantage of following in time
the evolution of galaxies, and the processes related to them, such as star formation, metal
enrichment, feedback of supernovae and BHs.
BH formation is still puzzling today, and many questions need to be addressed: How are
BHs created in the early Universe? What is their initial mass? How many BHs grow efficiently?
What is the occurrence of BH formation in high redshift galaxies? What is the minimum galaxy
mass to host a BH? Most of these questions are summarized in Fig 1, which represents a sample
of local galaxies with their BHs, in blue points. The red shaded area represent the challenge of
this thesis, which is to understand the assembly of BHs at high redshift and their properties. My
PhD focuses on three main BH formation models. Massive first stars (PopIII stars) in mini-halos,
at redshift z = 20−30 are predicted to collapse and form a BH retaining half the mass of the
star, (cid:46) 102M (Madau & Rees, 2001; Volonteri, Haardt & Madau, 2003). This is the Pop III
(cid:12)
star remnants model. Compact stellar clusters are also thought to be able to collapse and form a
very massive star by stellar collisions, which can lead to the formation of a ∼ 103M BH seed
(cid:12)
(Omukai, Schneider & Haiman, 2008; Devecchi & Volonteri, 2009; Regan & Haehnelt, 2009b).
Finally, in the direct collapse model, metal-free halos, without efficient coolants (i.e. no metals,
no molecular hydrogen), at z = 10 and later, can collapse and form a star-like supermassive
object, which can collapse into a BH of 104 −106M (Bromm & Loeb, 2003; Spaans & Silk,
(cid:12)
2006; Dijkstra et al., 2008).
As we will see in this thesis, we have investigated both the BH population in normal local
galaxies, as well as the feasibility of BH formation model to explain the assembly of the high
redshift quasar population. First of all, in order to fully understand the population of BHs we
i
ii
Fig. 1 – Sample of BHs in local galaxies in light and dark blue points. BH masses range from few 104M
(cid:12)
to 1010M . The shaded area represent the domain of BH mass at their formation time, in the early
(cid:12)
Universe.
observe in low-mass galaxies, we need a theoretical model able to predict the occupation fraction
of BHs in low-mass galaxies, and the properties of both these BHs and their host galaxies. We
have implemented a model accounting for the PopIII remnant and stellar cluster models, in
the numerical code Ramses. We form BHs according to the theoretical models, and let these
BHs evolve with time. So far, cosmological simulations were only used to reproduce the quasars
luminosity function and study their feedback, simulations were therefore seeding BHs by placing a
105M BH in the center of massive galaxies. Our approach allows instead to study BH formation,
(cid:12)
and to cover the low-mass end of BHs. We then compare the simulated BHs to two different
observational samples, local galaxies (Reines & Volonteri, 2015), and Lyman-Break Analogs (Jia
et al., 2011a). Local low-mass galaxies are among the most pristine galaxies, because they have
a quieter merger history. Therefore BHs in low-mass galaxies, are also thought to be pristine,
and to not have evolved much from their birth, thus they can provide us precious clues on BH
formation. Lyman-Break Analogs are galaxies very similar to their high redshift analogs, the
Lyman-Break Galaxies, comparing our simulated samples to BHs found in these galaxies can
therefore also provide us a promising new laboratory to study BHs in high redshift environment,
but much closer to us.
The direct collapse model has been much studied recently, but there is no consensus on
the number of BHs that form through this model yet. The number density of BHs, derived by
different studies employing semi-analytic models (Dijkstra, Ferrara & Mesinger, 2014) or hybrid
semi-analytic models (Agarwal et al., 2012, 2014) varies on several orders of magnitude. Hybrid
semi-analytic models are very appealing because they use spatial information from cosmological
simulations. However, only small simulated volumes allow one to achieve the high resolutions
needed to resolve minihalos, and the early metal enrichment. These small boxes probe with
difficulty the feasibility of the direct collapse BH formation model, lacking statistical validation.
Instead, in this thesis, we chose to run simulations with different box sizes and resolutions, that
allow us to test the impact of different processes such as metal enrichment and the impact of
supernova feedback. The main advantage is also that employing large simulation boxes makes
Abstract iii
possible to test different radiation intensity thresholds to destroy molecular hydrogen, which is
of paramount importance for the direct collapse model.
iv Abstract
Remerciements
Nous n’avons pas souvent l’occasion de remercier les personnes qui nous épaulent au quotidien,
que ce soit professionnellement ou personnellement, ou bien même les deux. Sans elles, rien
n’aurait été possible.
MespremiersremerciementssontpourMarta,quim’afaitpartagersonenthousiasmeetsasoif
de connaissance, merci pour sa patience et sa pédagogie, et pour m’avoir donné l’opportunité de
travailler sur des projets passionnants. Nous avons cherché ensemble à répondre à quelques-unes
des questions sur la formation et la croissance des trous noirs supermassifs, mais il en reste encore
des milliers, et j’espère tout autant d’occasions de travailler de nouveau ensemble.
Deux autres chercheurs m’ont particulièrement apporté leur aide durant ma thèse. Je remercie
Yohan Dubois pour sa disponibilité, pour m’avoir fait découvrir Ramses, et pour avoir répondu
aux questions de physique, aussi bien qu’aux questions numériques que je me suis posée pendant
ces trois années. Je remercie également Muhummad Latif pour m’avoir accompagnée sur les
chemins sinueux du modèle direct collapse de formation des trous noirs, ainsi que pour sa
disponibilité et ses conseils.
J’ai également une pensée pour les chercheurs avec qui j’ai débuté et qui m’ont toujours en-
couragée et apporté leur soutien, Claudia Maraston à Portsmouth (ICG), et bien sûr ici à Paris
(IAP), Gary Mamon, Joseph Silk, et Sébastien Peirani. Je tiens également à remercier Jacques le
Bourlot pour son soutien et tous les conseils judicieux qu’il a pu me donner.
Bien sûr ma thèse n’aurait pas été la même sans notre grande équipe, je remercie donc également
Pedro Capelo, Andrea Negri, Salvatore Cielo et Alessandro Lupi pour le côté italien, Tilman
Hartwig pour le côté allemand, et Rebekka Bieri pour le côté suisse de notre équipe.
Merci à Rachel Somerville et Sadegh Khochfar d’avoir accepté d’être les rapporteurs de
ma thèse et d’avoir passé une partie de leurs vacances d’été à lire mon manuscrit, à Raffaella
Schneider et Jenny Greene d’avoir accepté d’être examinatrices, et finalement Benoit Semelin
d’avoir accepté de présider le jury de ma thèse. Ce sont des chercheurs exceptionnels et je
suis vraiment fière qu’ils aient accepté de faire partie de mon jury de thèse, merci pour leur
suggestions et remarques très constructives sur mon travail.
Pendant trois ans, j’ai eu la chance de travailler également au Palais de la Découverte, de
donner des conférences le week-end, et d’écrire des articles pour la revue Découverte du musée.
Je remercie Sébastien Fontaine de m’avoir donné cette chance. J’ai pu discuter avec le grand
public des nombreuses grandes questions de l’astrophysique, m’émerveiller avec eux de toutes les
réponses que nous avons déjà, rire avec toutes ces personnes de 7 à 77 ans qui viennent découvrir
et apprendre au musée des sciences de Paris. Je ne peux ici pas citer toutes les personnes que je
voudrais remercier, mais j’ai une pensée particulière pour Marc Goutaudier, Andy Richard, et
Gaëlle Courty qui m’ont apporté une aide précieuse.
v