Bluck, Asa Frederick Leon
(2011)
The formation and evolution of massive galaxies and their supermassive black holes over the past 12 billion years.
PhD thesis, University of Nottingham.
Abstract
This thesis examines many of the ways in which massive galaxies and their super- massive black holes have changed over the past 12 billion years. In a sense, this is an attempt to write a cosmic history of massive galaxies, and in so doing construct a useful catalogue of changes which can be studied to gain insight into galaxy formation and evolution. In particular, this thesis concentrates on two potential drivers for galactic evolution: external influences from galaxy - galaxy interactions (Chapters 2 - 3); and internal influences from AGN feedback (Chapter 4). We find that both of these mechanisms have a profound impact on massive galaxies throughout their lifetimes.
In Chapter 2 the major merger history of massive galaxies is probed via close pair statistics and computational morphological approaches. We find that there is a mono- tonic rise in the merger fraction of massive galaxies with redshift out to z = 3, which is best parameterised by a simple power law of the form fm = f0(1 + z)^m, where f0 = 0.008 +/- 0.003 and m = 3.0 +/- 0.4. We compute the total number of major mergers that massive galaxies (with M∗ > 10^11 M⊙) experience from z = 3 to the present to be Nm = 1.7 +/- 0.5. We also note a close accord between morphological and close pair methods at z < 1.5 for standard optically defined CAS mergers and d < 30 kpc close pairs, probably indicative of both methods tracing the underlying merger activity with similar mass ratio and timescale sensitivities. Further, we provide a series of additional tests to the close pair method.
In Chapter 3 we extend the study of galaxy interactions to minor mergers, and also compute the morphologically determined major merger fractions of very high redshift massive galaxies. We find that high redshift massive galaxies are frequently highly asymmetric with ∼ 1/4 fitting the CAS definition of a merger at 1.7 < z < 3. We go on to utilise the extraordinary depth and resolution of the HST GOODS NICMOS Survey to probe the minor merger history of massive galaxies. We find that in total massive galaxies experience Nm = (4.5+/-2.1)/τm mergers with galaxies with M∗ > 10^9 M⊙ from z = 3 to the present, where τm is the merger timescale which will vary with mass. From this we compute the total stellar mass increase, due to mergers, of massive galaxies to be ∆M∗ ∼ 3×10^11 M⊙ over the past 12 billion years. This potentially offers a tempting solution for the observed rapid growth of massive galaxies throughout the same epoch.
In chapter 4 we investigate in detail the co-evolution of massive galaxies and their supermassive black holes by constructing a complete volume limited sample of 85 AGN with hard band luminosities LX > 2.35 × 10^43 erg s^−1 residing within host galaxies with masses M∗ > 10^10.5 M⊙ at 0.4 < z < 3. Using this data we compute the Eddington limiting (minimum) masses, ME, of the black holes in our sample. By assuming that there is no evolution in the Eddington ratio (μ = LBol/LEdd) and then that there is maximum possible evolution to the Eddington limit, we quantify the evolution in the M∗/MBH ratio as lying in the range 700 < M∗/MBH < 10000, compared to a local value of M∗/MBH ∼ 1000. Furthermore, we find that the active fraction of massive galaxies rises with redshift from 1.2 +/- 0.2 % at z = 0.7 to 7.4 +/- 2 % at z = 2.5. We calculate the maximum timescales for which our sample of AGN can continue to accrete at their observed rates before surpassing the local galaxy-black hole mass relation. We use these timescales to calculate the total fraction of massive galaxies which will be active above our threshold, finding that at least ∼ 40 % of all massive galaxies will be Seyfert luminosity AGN or brighter since z = 3. We find that the energy output due to these objects is sufficient to strip apart every massive galaxy in the universe at least 35 times over. Finally, we use this method to compute the evolution in the X-ray luminosity density of AGN with redshift, finding that massive galaxy Seyferts are the dominant source of X-ray emission in the Universe at z < 3.
We conclude in Chapter 5 by summarising these findings and commenting upon the powerful role of both internal and external influences on galaxy formation and evolution over the past 12 billion years.
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