Thursday June 21st, 2012

The Morphologies of Massive Galaxies at 1<z<3 in the CANDELS-UDS Field: Compact Bulges, and the Rise and Fall of Massive Disks

V.A. Bruce, J.S. Dunlop, M. Cirasuolo, R.J. McLure, T.A. Targett, E.F. Bell, D.J. Croton, A. Dekel, S.M. Faber, H.C. Ferguson, N.A. Grogin, D.D. Kocevski, A.M. Koekemoer, D.C. Koo, K. Lai, J.M. Lotz, E.J. McGrath, J.A. Newman, A. van der Wel
(Submitted on 19 Jun 2012)

We have used deep, HST, near-IR imaging to study the morphological properties of the most massive galaxies at high z, modelling the WFC3/IR H-band images of the ~200 galaxies in the CANDELS-UDS field with 1 < z_phot < 3, and stellar masses M_star > 10^11 M_sun. We have used both single-Sersic and bulge+disk models, have investigated the errors/biases introduced by uncertainties in the background and the PSF, and have obtained formally-acceptable model fits to >90% of the galaxies. Our results indicate that these massive galaxies at 1 < z < 3 lie both on and below the local size-mass relation, with a median R_e~2.6 kpc, a factor of ~2.3 smaller than comparably-massive local galaxies. Moreover, we find that bulge-dominated objects in particular show evidence for a growing bimodality in the size-mass relation with increasing z, and by z > 2 the compact bulges display effective radii a factor ~4 smaller than local ellipticals of comparable mass. These trends appear to extend to the bulge components of disk-dominated galaxies, and vice versa. We also find that, while such massive galaxies at low z are bulge-dominated, at 1 < z < 2 they are predominantly mixed bulge+disk systems, and by z > 2 they are mostly disk-dominated. The majority of the disk-dominated galaxies are actively forming stars, but this is also true for many of the bulge-dominated systems. Interestingly, however, while most of the quiescent galaxies are bulge-dominated, we find that a significant fraction (25-40%) of the most quiescent galaxies have disk-dominated morphologies. Thus, while our results show that the massive galaxy population is undergoing dramatic changes at this crucial epoch, they also suggest that the physical mechanisms which quench star-formation activity are not simply connected to those responsible for the morphological transformation of massive galaxies into present-day giant ellipticals.

Comments: 39 pages, 19 figures, 2 tables, submitted to MNRAS
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1206.4322v1 [astro-ph.CO]

Parsec-scale dust emission from the polar region in the type 2 nucleus of NGC 424

Sebastian F. Hoenig (1), Makoto Kishimoto (2), Robert Antonucci (1), Alessandro Marconi (3), M. Almudena Prieto (4), Konrad Tristram (2), Gerd Weigelt (2) ((1) UCSB, (2) MPIfR, (3) Univ. Firenze, (4) IAC)
(Submitted on 19 Jun 2012)

Advancements in infrared IR open up the possibility to spatially resolve AGN on the parsec-scale level and study the circumnuclear dust distribution, commonly referred to as the "dust torus", that is held responsible for the type 1/type 2 dichotomy of AGN. We used the mid-IR beam combiner MIDI together with the 8m telescopes at the VLTI to observe the nucleus of the Seyfert 2 galaxy NGC 424, achieving an almost complete coverage of the uv-plane accessible by the available telescope configurations. We detect extended mid-IR emission with a relatively baseline- and model-independent mid-IR half-light radius of (2.0 \pm 0.2) pc \times (1.5 \pm 0.3) pc (averaged over the 8-13 {\mu}m wavelength range). The extended mid-IR source shows an increasing size with wavelength. The orientation of the major axis in position angle -27deg is closely aligned with the system axis as set by optical polarization observations. Torus models typically favor extension along the mid-plane at mid-IR wavelengths instead. Therefore, we conclude that the majority of the pc-scale mid-IR emission (>~60%) in this type 2 AGN originates from optically-thin dust in the polar region of the AGN, a scenario consistent with the near- to far-IR SED. We suggest that a radiatively-driven dusty wind, possibly launched in a puffed-up region of the inner hot part of the torus, is responsible for the polar dust. In this picture, the torus dominates the near-IR emission up to about 5 {\mu}m, while the polar dust is the main contributor to the mid-IR flux. Our results of NGC 424 are consistent with recent observations of the AGN in the Circinus galaxy and resemble large-scale characteristics of other objects. If our results reflect a general property of the AGN population, the current paradigm for interpreting and modeling the IR emission of AGN have to be revised. (abridged)

Comments: 17 pages, 10 figures, 4 tables; submitted to ApJ (April 24), version after referee comments (June 01)
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1206.4307v1 [astro-ph.CO]
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