We (Baum, Mittal, O’Dea, and Tremblay) are leading a study of cold interstellar material and star formation in the brightest cluster galaxies in a sample of 11 cool-core galaxy clusters. This study makes use of the space-based telescopes, such as Herschel and Hubble Space Telescope (HST). The photo-detector array camera and spectrometer instrument aboard the Herschel satellite has enabled us to image dust emission and detect spectral signatures of cold gas in the form of certain forbidden lines. This provides us with an independent confirmation of the presence of vast amounts of cold gas and dust in the centers of cool-core clusters.
Using Herschel, we have mapped extended line emission in the brightest galaxies of the Centaurus and Perseus clusters of galaxies. The results show a correlation between the optical filaments imaged in previous studies and the cold gas detected with Herschel, indicating a common heating mechanism for the emission seen in the infrared and optical wavebands.
The HST imaging of the 11 galaxies exhibits a rich morphology in the optical, consisting of dark dust lanes and filaments. The filaments may represent either inflows into or outflows from the center of the galaxies. The far-ultraviolet (FUV) HST images of the same galaxies correlate well with the optical structure. Even though the FUV emission is not as extended as its counterpart optical emission, these also exhibit an interesting morphology, with the emission certainly more extended in some of the cases than others. The FUV emission is indicative of hot and young stars and, hence, a recently formed stellar population. We are currently using stellar synthesis libraries to fit the optical and FUV data, with the aim of understanding the star formation history and estimating the total stellar mass in the form of young stars.
We (Baum, O'Dea, Mittal, and Tremblay) have also used data from the Chandra X-ray Observatory to map the temperature of the ICM, searching for relics and signatures of past and ongoing episodes of AGN activity. These monstrous outbursts from central supermassive black holes can excavate buoyant cavities in the ambient ultra-hot plasma that are tens of thousands of light years in size (see Figure above). As these bubbles buoyantly rise, cascades of surrounding gas rush to refill their wakes, which may heat the gas and partially inhibit cooling flows in the central regions of brightest cluster galaxies. Recently, we have uncovered compelling observational evidence in support of this theoretical model. The results will be presented in two forthcoming papers by Tremblay et al. (2012a,b).
Baum, O’Dea, Noel-Storr, and Mittal (postdoc) are working with three RIT undergraduates (1) Mike Every using UV images from NASA’s GALEX satellite to study star formation in powerful radio galaxies, (2) Mark Magagnoli using Spitzer Space Telescope data to study the Infrared emission in low luminosity radio galaxies to probe conditions in the dust and cold gas near the AGN, and (3) Kevin Christiansen using Chandra X-ray data and VLA radio data to study the interaction of a compact, young radio galaxy with its environment as it propagates through the host galaxy.
We obtained Chandra/ACIS and Very Large Array data on 13 UGC galaxies hosting low luminosity AGN (see also "Supermassive black holes", next). We detected nuclear X-ray emission in eight sources and radio emission in all but one. We found that the behavior of the X-ray and radio emission in these sources depends strongly on the form of their optical surface brightness profiles derived from HST imaging, i.e., on their classification as ``core'', ``power-law'' or ``intermediate'' galaxies. ``Power-law'' and ``intermediate'' galaxies lie well above the radio-X-ray correlation established in the Fanaroff-Riley type I (FRI) radio galaxies and the low-luminosity ``core'' galaxies (see Figure below). This result highlights the fact that the ``radio-loud/radio-quiet'' dichotomy in AGNs is a function of the host galaxy's optical surface brightness profile. We hypothesize that major mergers are likely to have created ``core'' galaxies, while minor mergers were instrumental in the creation of ``power-law'' and ``intermediate'' galaxies. These results are to be presented in "Examining the Radio-Loud/Radio-Quiet dichotomy with new Chandra and VLA observations of 13 UGC galaxies" (Kharb, Capetti, Axon, et al., 2012, submitted to AJ).
While on sabbatical for academic year 2011-2012, Baum and O’Dea are working with Harvard undergraduate students Rabeea Ahmed and Samantha Whitmore and Center for Astrophysics astronomers Christine Jones and Bill Forman to study the black hole - galaxy connection in a sample of about 200 nearby elliptical galaxies.
Both the Baum/O'Dea and Axon/Robinson groups are researching "active galactic nuclei" (AGN), including quasars and Seyfert galaxies, where accretion onto supermassive black holes (SMBHs) in the centers of galaxies results in the production of copious amounts radiation and kinetic energy. The groups work on a broad range of fundamental issues including (1) unification of AGN classes, (2) the relation between AGN activity and star formation, (3) energy transport in the form of jets and bubbles, (4) nuclear environments of AGN, and (5) lifecyles of AGN.
A recently concluded study by Kharb et al. of the 1.4 GHz extended emission in 135 flat-spectrum quasars and BL Lac objects (collectively referred to as blazars) belonging to the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) sample, has clearly demonstrated a close link between the kiloparsec-scale extended emission and parsec-scale jet speeds. A pilot study of a sub-sample of 27 MOJAVE blazars with large extended powers with the Chandra X-ray Observatory has revealed X-ray jets in nearly 80% of them. The high detection rate of X-ray jets in this relatively high redshift sample supports the inverse Compton (over cosmic microwave background photons) emission mechanism for the X-rays. In order to test this, we have obtained deep Chandra (~70 ksec) and Hubble Space Telescope (HST) observations of 2 blazars, viz., 0106+013 (depicted in the figure above) and 1641+399. These observations have detected both X-ray and optical emission from jet knots. Modeling of the broad-band spectral energy distribution (SED) of jet knots is currently underway.
Recent results obtained by Robinson, Axon, Young, Curran, Kharb and collaborators (including former RIT Research Scientist Dan Batcheldor) suggest that the central SMBH that are thought to be present in most galaxies may in some cases be hurtling through their hosts at high velocities. The team conducted separate studies of the well-known giant elliptical galaxy Messier 87 and the luminous low redshift quasar E1821+643 (see figure above). In the first case, analysis of archival Hubble Space Telescope images revealed that the ~3-5×109 solar mass SMBH is displaced by 7 parsecs (22 light years) from the center of the galaxy. In the second, opposite Doppler-shifts in scattered and direct light detected by spectropolarimetry (see figure below) suggest that the SMBH is moving with a velocity ~2500 km s-1 relative to the host galaxy. These two objects may be among the first known examples of gravitational recoil, a phenomenon predicted by General Relativity, which is expected to occur as a consequence of the merger of a pair of SMBH. SMBH binaries are themselves expected to form frequently in large galaxies as a result of galaxy mergers. Although the present evidence suggests that this is the most likely explanation, other possibilities are almost equally intriguing. For example, the SMBH may be accelerated by the powerful radio jets that are present in both objects or, in the case of E1821+643, the velocity shift may arise from the orbital motion of a binary SMBH that has not yet merged.