(1) the shape of the continuum;
(2) the widths, shapes, and relative strengths of emission lines; and
(3) the presence/absence, width, relative velocity, and kinematic complexity of intrinsic absorption.
The gross similarities in these features implies that all AGN have the same basic physical structure. However, the variety in the details begs the question: What governs that structure and how?
The current working paradigm holds that the accreting gas takes the
form of a disk. Depending on the spin of the black hole, this disk
extends from 0-9 times the black hole gravitational radius
(Rg), out to 104-5Rg. (For reference,
a 108 Msun black hole has Rg~1 AU.)
The accretion can be viscosity-dominated, advection-dominated, or in
between depending on (among other things) the efficiency of radiative
cooling and the strength of magnetic fields permeating the disk.
A mass outflow is thought to originate from the central regions of the accretion disk. (This outflow should not be confused with the spectacular jets observed in the radio.) This putative outflow has been successful, on the whole, at explaining several general observed spectral features such as single-peaked broad emission lines (the hallmark signature that identifies AGN in the ultraviolet/optical), and ultraviolet broad absorption lines observed in ~20% of quasars. Additionally, at larger distances, where the ultraviolet and X-ray radiation diminishes sufficiently, dust is able to form and survive in this outflow. This dusty region reprocesses ultraviolet radiation and shines in the infrared. The study of outflows, then, calls for a multi-wavelength approach. Understanding the underlying physics of this mass outflow and how it affects the accretion process is of vital interest from the view of understanding the AGN phenomenon and also understanding the evolution of galaxies (see Connections to Galaxy Evolution).
Currently, I and my collaborators are researching several aspects of outflows. We are...
...conducting a census to ascertain the frequency with which outflows are observed in absorption.
...looking at how the frequency and absorption properties of the various forms of intrinsic absorption change with quasar property.
...examining both empirically and theoretically what governs the maximum velocity of absorption. The dominant process appears to be radiation-pressure, though other process such as magnetic driving and thermal driving may play a role. One important issue is the shape of the spectrum driving the gas; we are exploring how this changes with luminosity.
...developing models of outflows to more precisely describe how outflows change with physical property (e.g., black hole mass, accretion rate, etc.).
...characterizing the range and distribution of the detailed ionization and physical conditions (e.g., density, mass, temperature) of a variety of intrinsic absorbers which sample the outflows. These, in turn, provide important constraints on the geometry of the outflow for a given object.
...working to understand the correlated variations in certain observed quasar properties. These properties include the strengths of broad Fe II and narrow [O III] emission lines, the width of the H-beta emission, the strength of the radio and X-ray continua compared to the optical. (For example, objects with strong Fe II emission tend to have weak [O III] emission, and narrow H-beta lines.)
Connections to Galaxy Evolution: The growth of a central supermassive black hole is intimately linked with the overall evolution of gas and star formation in the host galaxy. Black hole growth happens in two ways: (1) Mergers between two smaller galaxies (each hosting a black hole) results a single larger galaxy with a more massive black hole. (2) The black hole can accrete gas in a ``quasar phase'' as described above. Accretion can only happen when gas is driven to the center of the galaxy by some event, like a merger. Such mergers will also spur a burst of intense star-formation.
Two classes of objects reveal a tantalizingly suggestive picture of the immediate aftermath of a merger: ultraluminous infrared galaxies (ULIRGs) and post-starburst quasars (PSQs). ULIRGs are galaxies that are forming stars at an incredible rate deep within giant molecular clouds. These clouds reprocess the light from the embedded stars and re-emit in the infrared. Post-starburst quasars show spectral features of both a quasar and a massive starburst simultaneously. Near ultraviolet/optical spectra of PSQs typically show power-law continua with broad Mg II and Fe II emission like quasars. Redward of the Balmer break, the spectrum is more characteristic of a massive, moderately-aged stellar population (1010-12 Msun, 100-400 Myr).
A suggestive scenario is that the merger between two galaxies results in an intense burst of star-formation. A short time after this burst (or perhaps simultaneously and extending after the burst), gas is driven to the center of the post-merger galaxy inducing a phase of quasar activity. In this scenario, ULIRGs are identified with the immediate post-merger object, while PSQs are more evolved versions. The relative numbers of galaxies, ULIRGs, PSQs, and quasars are consistent with this picture. However, further observations and deeper understanding of ULIRGs and PSQs (and their possible relation) is required to either support or disprove this suggestive picture. My collaborators and I have embarked on a comprehensive multiwavelength study of post-starburst quasars which includes:
(1) a survey of numbers using the Sloan Digital Sky survey,
(2) a quantification of PSQ morphologies using HST,
(3) an examination of possible young (~10 Myr) stellar populations to understand the variety of star-formation histories with GALEX,
(4) an assessment of the amount, form, and temperatures of dust with Spitzer, and
(5) a characterization of the environments of PSQs (e.g., number of companions galaxies).
While the central black hole accretes, outflows can also halt the star-formation in the host galaxy. This happens in two ways that are collectively termed ``AGN feedback.'' In the first mode, an outflowing, highly collimated jet deposits energy into the ISM of the host galaxy, preventing the cooling that is necessary for the gas to collapse and form stars. The second mode makes use of the mass outflow previously discussed. This outflow can contain a large amount of mechanical energy and can sweep up the host galaxy ISM and carry it into the intergalactic medium. This outflow is also highly enriched and can pollute the the IGM with metals. However, the geometry, velocity, and mass of these outflows are dependent on the physical properties of the black hole and the accretion. In order to incorporate mass outflows into models of galaxy evolution and understand its relative importance, a more complete description of outflows is needed.
Warm-Hot Intergalactic Medium (WHIM): Simulations of the intergalactic medium produce a network of sheets and filaments in the distribution of dark matter and baryons. This predicted structure is supported both by observations of the spatial distribution of galaxies, and by quasar absorption lines. In the spectra of background quasars, these filaments are observed as the so-called ``Lyman-alpha forest.'' Each filament that intercepts the sight-line imprints an H I Ly-alpha line at the appropriate redshift. These same simulations also predict that nearly half of the baryons in the local universe exist in a warm-hot phase of this filamentary structure. As these baryons do not significantly radiate, the only way to study them with today's technology is in absorption against a luminous background object. The temperature of this warm-hot phase is T~105-6K. Ions that constitute good tracers of this temperature range include O5+-O8+, and H0. (Here, a good tracer means an ion that has a transition with a sufficiently high probability modulo the relative abundance of that ion.) The H I Ly-alpha and O VI 1031.926,1037.617Å doublet transitions in the ultraviolet are particularly good tracers, and have been detected along a handful of sight-lines. [O VII 21.602 Å and O VIII 18.967,18.972 Å have also been detected in the X-ray band, though with decreased utility due to the lack of spectral resolution provided by the current generation of instruments.]
A peculiar trend with O VI absorption in the WHIM is that it generally occurs near galaxies, or groups of galaxies. Naively, this actually makes sense since O requires nucleosynthesis in stars, and supernovae for distribution. My collaborators and I are trying to address several questions regarding the relationship between O VI WHIM absorption and galaxies in their proximity. Is the stellar content and star-formation history of the galaxies near O VI absorbers consistent with prescriptions for how metals are distributed? Are galactic-scale stellar winds sufficient, or do AGN outflows play a role? Is the stellar make-up of nearby galaxies consistent with that of simulations? Did metals get distributed before, during, or after the assembly of the nearby galaxies? To address these questions, we are making ``observations'' of the WHIM simulations to compare to real observations. For galaxies, we are characterizing the stellar masses, populations, and ages. For O VI absorbers, we are measuring the strength, velocity spread, and kinematic complexity/shape. In addition, we are examining the number and types of galaxies that lie in proximity to the absorbers. Comparisons of these quantities allow us to predict what should be observed if the simulations are correct. Along with this, we need to continue gathering more information on the actual stellar content, masses, ages, and morphologies of galaxies near real O VI absorbers.
High-Velocity Clouds (HVCs) & Weak Mg II Systems: Neutral hydrogen maps of the sky (from 21 cm radio emission) show large clouds of gas at z=0. The velocities of these clouds are too high to be consistent with either Milky Way halo or disk. Various possibilities for the origin of this gas include: remnants of the initial galaxy assembly of the Local Group, gas blown out by supernovae (or falling back after an ejection), or gas tidally stripped from galaxy interactions/mergers. QALs are more sensitive than the emission maps and have revealed a new class of O VI HVC that is much more common than those observed in H I. QALs have also revealed an interesting class of weak Mg II systems. These are isolated, often multiphase, clouds (selected by the Mg II 2796.354,2803.532 Å doublet) that are probably analogs of Milky Way HVCs associated with other galaxies. Both the O VI-selected HVCs and weak Mg II systems are generally detected in a wide range of ionization species. The origin, location, physical and ionization conditions, and multi-phase nature of these HVCs is of interest. If these are primordial material, they provide probes of the material from which galaxies form. If they are ejected material, they tell us about the gaseous content and important dynamical processes in younger versions of the galaxies. My collaborators and I are currently addressing several questions regarding HVCs: (1) How eclectic are HVC origins? (2) How large/massive are they? (2) What are their metallicities? (3) What are the important ionization mechanisms (e.g., photoionization, shocks, turbulent mixing, conductive interfaces, etc.)? (4) How many phases do HVCs generally contain? As many as the Milky Way ISM or fewer? (5) Is there a structured velocity field or fields for HVCs? To answer these questions, we need additional ultraviolet spectra to constrain ionization conditions and mechanisms. The installation of the Cosmic Origins Spectrograph on HST with provide access to more sight-lines. Deeper and higher spatial resolution radio observations are needed to better constrain the H I column densities (hence, metallicities) and spatial extents of HVCs. Moreover, we are pursuing high-resolution observations of redshifted weak Mg II absorbers from the ground with VLT and Subaru to further characterize the distribution of ionization conditions, phase structure, and metallicities to compare the Milky Way HVCs with those of other galaxies.