Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Supermassive black holes (BH) are powerful sources of energy that are already in place at very early epochs of the Universe (by z=6). Using hydrodynamical simulations of the formation of a massive M_vir=5 10^11 M_sun halo by z=6 (the most massive progenitor of a cluster of M_vir=2 10^15 M_sun at z=0), we evaluate the impact of Active Galactic Nuclei (AGN) on galaxy mass content, BH self-regulation, and gas distribution inside this massive halo. We find that SN feedback has a marginal influence on the stellar structure, and no influence on the mass distribution on large scales. In contrast, AGN feedback alone is able to significantly alter the stellar-bulge mass content by quenching star formation when the BH is self-regulating, and by depleting the cold gas reservoir in the centre of the galaxy. The growth of the BH proceeds first by a rapid Eddington-limited period fed by direct cold filamentary infall. When the energy delivered by the AGN is sufficiently large to unbind the cold gas of the bulge, the accretion of gas onto the BH is maintained both by smooth gas inflow and clump migration through the galactic disc triggered by merger-induced torques. The feedback from the AGN has also a severe consequence on the baryon mass content within the halo, producing large-scale hot superwinds, able to blow away some of the cold filamentary material from the centre and reduce the baryon fraction by more than 30 per cent within the halo's virial radius. Thus in the very young universe, AGN feedback is likely to be a key process, shaping the properties of the most massive galaxies.

Abstract:

We use Galaxy Zoo 2 visual classifications to study the morphological signatures of interaction between similar-mass galaxy pairs in the Sloan Digital Sky Survey. We find that many observable features correlate with projected pair separation; not only obvious indicators of merging, disturbance and tidal tails, but also more regular features, such as spiral arms and bars. These trends are robustly quantified, using a control sample to account for observational biases, producing measurements of the strength and separation scale of various morphological responses to pair interaction. For example, we find that the presence of spiral features is enhanced at scales < 70 h^-1 kpc, probably due to both increased star formation and the formation of tidal tails. On the other hand, the likelihood of identifying a bar decreases significantly in pairs with separations < 30 h^-1 kpc, suggesting that bars are suppressed by close interactions between galaxies of similar mass. We go on to show how morphological indicators of physical interactions provide a way of significantly refining standard estimates for the frequency of close pair interactions, based on velocity offset and projected separation. The presence of loosely wound spiral arms is found to be a particularly reliable signal of an interaction, for projected pair separations up to ~100 h^-1 kpc. We use this indicator to demonstrate our method, constraining the fraction of low-redshift galaxies in truly interacting pairs, with M_* > 10^9.5 M_Sun and mass ratio < 4, to be between 0.4 - 2.7 per cent.

Abstract:

We have modeled a small sample of Seyfert galaxies that were previously identified as having simple X-ray spectra with little intrinsic absorption. The sources in this sample all contain moderately broad components of FeK-shell emission and are ideal candidates for testing the applicability of a Compton-thick accretion disk wind model to active galactic nucleus (AGN) emission components. Viewing angles through the wind allow the observer to see the absorption signature of the gas, whereas face-on viewing angles allow the observer to see the scattered light from the wind. We find that the FeK emission line profiles are well described with a model of a Compton-thick accretion disk wind of solar abundances, arising tens to hundreds of gravitational radii from the central black hole. Further, the fits require a neutral component of FeKα emission that is too narrow to arise from the inner part of the wind, and likely comes from a more distant reprocessing region. Our study demonstrates that a Compton-thick wind can have a profound effect on the observed X-ray spectrum of an AGN, even when the system is not viewed through the flow. © 2012. The American Astronomical Society. All rights reserved..

Abstract:

We present Hubble Space Telescope imaging and spectroscopy for the extended high-ionization cloud known as Hanny's Voorwerp, near the spiral galaxy IC 2497. WFC3 images show complex dust absorption near the nucleus of IC 2497. STIS spectra show a type 2 Seyfert AGN of rather low luminosity. The ionization parameter log U = -3.5 is in accord with its weak X-ray emission. We find no high-ionization gas near the nucleus, adding to evidence that the AGN is currently at low radiative output (perhaps now dominated by kinetic energy). The nucleus is accompanied by an expanding ring of ionized gas 500 pc in projected diameter on the side opposite Hanny's Voorwerp, with Doppler offset 300 km/s from the nucleus (kinematic age < 7 x10^5 years). [O III] and H-alpha + [N II] images show fine structure in Hanny's Voorwerp, with limb-brightened sections and small areas where H-alpha is strong. We identify these as regions ionized by recent star formation, in contrast to the AGN ionization of the entire cloud. These candidate "normal" H II regions contain blue continuum objects, whose colors are consistent with young stellar populations; they appear only in a 2-kpc region toward IC 2497 in projection. The ionization-sensitive ratio [O III]/H-alpha shows no discernible pattern near the prominent "hole" in the ionized gas. The independence of ionization and surface brightness suggests that substantial spatial structure remains unresolved, to such an extent that the surface brightness sample the number of denser filaments rather than the characteristic density in emission regions. These results fit with our picture of an ionization echo from an AGN whose ionizing luminosity has dropped by a factor > 100 (and possibly much more) within the last 1-2 x 10^5 years; we suggest a sequence of events and discuss implications of such rapid fluctuations for AGN demographics. (Abridged)

Abstract:

We present the results from a Chandra pilot study of 12 massive galaxy mergers selected from Galaxy Zoo. The sample includes major mergers down to a host galaxy mass of 10$^{11}$ $M_\odot$ that already have optical AGN signatures in at least one of the progenitors. We find that the coincidences of optically selected active nuclei with mildly obscured ($N_H \lesssim 1.1 \times 10^{22}$ cm$^{-2}$) X-ray nuclei are relatively common (8/12), but the detections are too faint ($< 40$ counts per nucleus; $f_{2-10 keV} \lesssim 1.2 \times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$) to reliably separate starburst and nuclear activity as the origin of the X-ray emission. Only one merger is found to have confirmed binary X-ray nuclei, though the X-ray emission from its southern nucleus could be due solely to star formation. Thus, the occurrences of binary AGN in these mergers are rare (0-8%), unless most merger-induced active nuclei are very heavily obscured or Compton thick.