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Multi-wavelength Astrophysics

CONTENTS

EXTRAGALACTIC

Galaxy Clusters
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Clusters of Galaxies are the largest gravitationally bound structures in the Universe. Most of the matter in clusters of galaxies is in an unknown form called Dark Matter. The "normal" matter is found in the individual galaxies and a hot tenuous gas which fills the space between the galaxies. This hot gas -- dubbed the Intracluster Medium (ICM) -- is at temperatures of tens of millions of degrees and radiates X-rays. This X-ray emission drains energy from the gas which then cools. As the gas cools, other radiation processes occur which should cause the gas to continue to cool down to very low temperatures -- tens of degrees where it will be bound to giant clouds of molecular gas which will eventually collapse to form stars. This process will be focused on the massive bright galaxy which sits in the center of the ICM -- called the Brightest Cluster Galaxy (BCG). Using simple physical arguments it is possible to use the observed X-ray emission to estimate the amounts of cold gas and young stars which should be present in the BCGs. But these estimates turn out to be wrong by an order of magnitude. Such a large discrepancy points to a fundamental error in our understanding of the physics of the ICM. It now seems likely that the super-massive Black Hole in the center of the BCG could re-supply energy to the ICM via a "radio source". In this scenario, some of the cold gas which condenses from the ICM falls onto an accretion disk around the Black Hole, fuelling the ejection of a powerful outflow. This outflow carries large amounts of energy out into the ICM where it can re-supply the energy lost through radiation. In order to further test this hypothesis and to improve our understanding of the physics of the ICM, the RIT cluster group (O'Dea, Baum, postdoc Mittal) are obtaining observations over the electromagnetic spectrum -- including radio, infrared, optical, ultraviolet, and X-ray.


The cooling flow cluster Abell 1795. Blue contours are the Chandra X-ray emission, colors are HST image of Lyman-alpha, and the white contours are the VLA radio image.

 

Radio Galaxies
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(From left to right) (a) Hubble Space Telescope (optical), Chandra X-Ray Observatory (X-ray), and Very Large Array (radio) composite of the radio galaxy 3C 84, a canonical example of feedback from a radio galaxy driving shocks and cavities into its surrounding medium (in this case the massive Perseus cluster of galaxies). (b) Chandra and VLA composite of the radio galaxy 3C 75, famous for its dramatic binary black hole system. (c) Hubble Space Telescope optical imaging of the central dust lane in the massive nearby radio galaxy Centaurus A. (d) X-ray, sub-mm, and optical composite of Centaurus A (CXO, APEX, ESO 2.2m)..
(NASA,NRAO,ESA,ESO)

Among the largest and most energetic phenomena in the universe, radio galaxies are central to our understanding of many fundamental astrophysical processes. Their massive jets of relativistic plasma, powered by mass accretion onto a central supermassive black hole, can often extend to distances exceeding hundreds of thousands of light-years across.

RIT faculty Axon, Baum, O'Dea, Robinson and postdocs Kharb and Mittal are currently working at the frontiers of research on active galacitc nuclei and their important role in driving cosmological evolutionary processes.

 

Active Galaxies
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Most galaxies undergo an active (quasar) phase during which the central supermassive black hole generates vast radiant luminosities through gravitational accretion of interstellar gas. It is widely accepted that the "engine" of active galactic nuclei (AGN) is an accretion disk around a super-massive black hole (SMBH). Gas flowing inwards from the galaxy "fuels" the AGN, while feedback in the form of gas outflows ("winds") or radio jets, is believed to have a significant impact on the galaxy and its environment and may regulate black hole growth and galaxy formation. However, while this basic picture is well established, much of the detailed physics governing these processes remains uncertain. The core regions of AGN present major challenges to investigation by direct observation, being both highly compact and partially obscured by circum-nuclear gas and dust. The RIT AGN group uses a variety of techniques and data from several different wavebands both to study the dust structures and probe beyond them to investigate the physical processes that operate in the active nucleus.

Polarimetry of AGN: This group (Robinson, Young, Axon) studies the inner structures of AGN by analysing and modelling measurements of the polarization of the optical emission lines and continuum in large samples representative of wide ranges in inferred SMBH mass and accretion rate. The unique power of this approach is that the polarization state of scattered light from the AGN carries the imprint of the scattering geometry, allowing the structure and kinematics of both scattering and emitting material to be investigated in the unresolved and partially obscured nucleus. The goal is to determine the structure and dynamics of the line-emitting and scattering gas flows in the immediate vicinity of the accretion disk in AGN, and to investigate how these structures depend on fundamental parameters such as black hole mass and accretion rate. This in turn should help to resolve several long-standing problems in AGN physics, including the structure of the broad emission-line region, the nature of the accretion flow and the launch mechanisms of AGN winds.

The origin of the thermal infrared continuum in powerful radio galaxies: One of the key outstanding issues for interpreting observations of powerful radio-loud AGN is to identify the origin of the prodigious thermal mid-to far-infrared (MFIR) emission. Such emission can act as a bolometer of the AGN power because the dust surrounding the active nucleus absorbs the optical radiation and re-radiates it at MFIR wavelengths. The advantage of MFIR observations is that emission is less obscured at these longer wavelengths, allowing samples of objects to be potentially free from obscuration and orientation biases. Consequently MFIR observations are important for testing the orientation-based unifying schemes for the many different classes of AGN that have been identified. The group at RIT (Dicken, Axon, Robinson, Baum, O'Dea) focuses on Spitzer Space Telescope infrared imaging and spectroscopic data to decipher the origin of this emission. In particular the work has focused on the contribution of massive star formation regions to the heating of AGN dust and how this affects our understanding of the triggering mechanisms of active galaxies through hierarchical galaxy mergers. Such work is vital for interpreting observations of distant AGN where we observe the redshifted MFIR emission at sub-mm wavelengths.

The nuclear structure of OH megamaser galaxies: OH megamaser galaxies (OHMG) form an important class of ultraluminous IR-galaxies (ULIRGs) whose maser lines emit QSO-like luminosities. ULIRGs in general are associated with recent galaxy mergers but it is often unclear whether their power output is dominated by starbursts or an obscured QSO. In contrast, OHMG exhibit strong evidence for the presence of an energetically important and recently triggered active nucleus. These galaxies may represent the final stages of merger-triggered nuclear activity in which powerful winds from the nascent QSO are clearing away the enshrouding dust. This project (involving Axon, Robinson, Baum, O'Dea, Young) is using the HST observations in conjunction with spectroscopic and spectropolarimetric data to construct a detailed picture of the circum-nuclear regions of this hitherto relatively neglected class of galaxy in order to better understand the relationships between galaxy mergers, nuclear star-formation, the growth of massive black holes and the triggering of nuclear activity.

Observational signatures of gravitational recoil: Current theories of galaxy evolution suggest that supermassive black holes (SMBH) frequently form gravitationally bound binary systems in the cores of large galaxies. When such systems undergo coalescence, anisotropic emission of gravitational waves can deliver a recoil velocity to the merged black hole. Recent advances in numerical relativity, in which RIT's CCRG has played a leading role, have shown that in some configurations, the recoil may be large enough to exceed the escape velocity of the host galaxy. As a recoiling SMBH is expected to retain part of its accretion disk, it might be detected observationally as a quasar displaced from the center of its host galaxy, or via Doppler shifting of emission lines from the retained gas. Our group (Robinson, Axon, Batcheldor, Young, Kharb), is searching for both types of signature, using a combination of spectropolarimetric observations, spectroastrometric observations and HST archival images.

 

GALACTIC

Large Surveys
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Large surveys which search for rare extragalactic objects provide us with a rich "garbage heap" of ordinary stars in the Milky Way. There are plenty of puzzles to which these unwanted stars are the key. For example, we know that the mass and radius of stars on the main sequence follow a pretty simple relationship, and it matches the predictions from stellar models. However, models and observations appear to diverge for stars with the smallest masses. Is there a problem with stellar models of cool M dwarfs? Unfortunately, there are only a very few M-dwarf binaries in which we can measure both the mass and the radius accurately. We are currently sifting through the Sloan Digital Sky Survey to discover new eclipsing binary systems with low-mass components, in order to improve our knowledge of stellar structure in M dwarfs.

The Subaru Telescope on Mauna Kea has acquired very deep images in small regions of the sky to find quasars and clusters of galaxies at high redshift. We have measured the properties of the stars in these images over five years to identify those with the highest proper motions. It turns out that this technique is an efficient method for finding white dwarfs. With a large enough sample of white dwarfs, we can test models of galactic structure.

 

Stellar Life Cycles
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The stellar astrophysics group within the AST program at RIT is attacking fundamental problems in the study of stellar life cycles via observations spanning the electomagnetic spectrum from the radio to the X-ray. Here are a few examples.

The origin of X-ray emission from young stars: Intense X-ray emission is characteristic of pre-main sequence (pre-MS) stars and young stellar objects (YSOs). Such X-ray emission indicates the presence of powerful magnetic fields and, potentially, shocked gas in the immediate environments of pre-MS stars and YSOs. Rapid rotation and/or turbulence combined with global convection in low-mass, pre-MS (T Tauri) stars likely produces enhanced dynamo activity, resulting in strong stellar magnetic fields and highly elevated levels of X-ray emission and flaring. In the case of actively accreting (``classical'') T Tauri stars and YSOs, the X-ray radiation also may be due to interactions between disk and stellar magnetospheres or to accretion shocks. Such star-disk interactions likely underlie the energetic jets commonly observed to emanate from YSOs. To gain insight into the star-disk-jet connection, we are studying the origin of X-rays from pre-MS stars. Our studies include temporal monitoring of the X-ray emission from eruptive YSOs that appear to be undergoing accretion bursts, and high-resolution X-ray gratings spectroscopy of those T Tauri stars that are closest to Earth.

See:

Studying young stars "on our doorstep": Over the past decade, a few hundred stars within 100 pc of Earth and with ages ranging from 8 to 100 Myr, comprising several T Tauri and post-T Tauri associations, have been identified and cataloged. Increasing attention has been focused on these sparse groups of young stars near Earth, because a youthful planet or dusty debris disk at ~50 pc will be at least ~10 times brighter and, for a telescope/detector with a given angular resolution, will afford at least ~3 times the linear spatial resolution as it would in the more distant (~150-400 pc) and well-studied regions such as Taurus-Aurigae and Orion. We seek to address a number of questions raised over the past decade concerning nearby young stars, for example: Can we identify the lowest-mass members of nearby young groups? Are such stars near the H-burning limit? Do they retain planet-forming disks? Do the known systems within 100 pc harbor as-yet unidentified low-mass companions at very wide separation? Do ~10 Myr-old brown dwarfs (BDs) display the large X-ray luminosities characteristic of very young (age <1 Myr) BDs as well as very low-mass pre-MS stars?

The formation of giant planets: Circumstellar disks serve both as the sources of material for accreting young stars and as the sites of nascient planets orbiting such stars. To understand the history of the solar system --- and in particular to ascertain the timescales and physical conditions governing giant planet formation --- it is necessary to characterize the gaseous component of such disks. Radio molecular line observations are a powerful means to probe disk gas within Jovian planet formation zones around young stars, especially given the many improvements to the sensitivity of submm telescopes and instrumentation in recent years. Our single-dish and interferometric radio observing programs focus on nearby, ~10 Myr-old star/disk systems that are at or just after the epoch of Jovian planet formation. This is the epoch when disk gas is rapidly dissipating (presumably resulting in a delicate balance between giant planet accretion and migration processes) and Oort clouds are being rapidly assembled (via the ejection of young comets to large orbits by young Jovian planets). Hence, these radio molecular line studies of the nearest disk-encircled pre-MS stars provide unique constraints on models describing the late evolution of gaseous molecular disks and the "end game" of Jovian planet formation.

The shaping of planetary nebulae: Planetary nebulae (PNe) represent very late stages in the lives of stars of initial mass 1-8 solar masses. As such, PNe serve as proving grounds for theories concerning a wide range of astrophysical phenomena, from stellar nucleosynthesis to wind interactions to the impact of binarity on stellar evolution and the ultimate fates of binary (and, perhaps, exoplanet) systems. In the era of Chandra and XMM-Newton, the detection (or nondetection) of diffuse and/or point-like X-ray sources within PNe yields important, unique information concerning the evolutionary state of PN central star(s) and wind interactions as nebular shaping agents. Diffuse X-ray sources allow us to probe the energetic shocks within PN wind interaction regions, whether in the form of wind-blown bubbles or fast, collimated outflows impinging on PN progenitor ejecta. Chandra X-ray gratings spectroscopy of the superheated plasma in such a wind-shock region -- within the PN BD +303639 -- has yielded unparalleled insight into the crucial, late stages of nucleosynthesis within this PN's progenitor star. At the same time, searches for X-ray point sources within PNe provide a novel means to detect binary companions at PN cores, thereby constraining models in which the formation and shaping of PNe is directly linked to central star binarity and, perhaps, the presence of planetary-mass companions.

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