Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Context. The Seyfert I galaxy MCG-6-30-15 shows one of the best examples of a broad "red wing" of emission in its X-ray spectrum at energies 2 ≤ E ≤ 6.4 keV, commonly interpreted as being caused by relativistically-blurred reflection close to the event horizon of the black hole. Aims. We aim to test an alternative model in which absorption creates the observed spectral shape, explains the puzzling lack of variability of the red wing and reduces the high reflection albedo, substantially greater than unity, that is otherwise inferred at energies E ≥ 20keV. Methods. We compiled all the available long-exposure, high-quality data for MCG-6-30-15: 522 ks of Chandra HETGS, 282ks of XMM-Newton PN/RGS and 253 ks of Suzaku XIS/PIN data. This is the first analysis of this full dataset. We investigated the spectral variability on timescales ≥ ks using principal components analysis and fitted spectral models to "flux state" and mean spectra over the energy range 0.5-45 keV (depending on detector). The absorber model was based on the zones previously identified in the high-resolution grating data. Joint fits were carried out to any data that were simultaneous. Results. Multiple absorbing zones covering a wide range of ionisation are required by the grating data, including a highly ionised outflowing zone. A variable partial-covering zone plus absorbed low-ionisation reflection, distant from the source, provides a complete description of the variable X-ray spectrum. A single model fits all the data. We conclude that these zones are responsible for the red wing, its apparent lack of variability, the absorption structure around the FeKα line, the soft-band "excess" and the high flux seen in the hard X-ray band. A relativistically-blurred Fe line is not required in this model. We suggest the partial covering zone is a clumpy wind from the accretion disk . © 2008 ESO.

Abstract:

Context. X-ray spectra of AGN often contain signatures indicative of absorption in multiple layers of gas whose ionization-state and covering fraction may vary with time. It has been unclear to date how much of the observed X-ray spectral and timing behavior in AGN can be attributed to variations in absorption, versus variations in the strengths of emission or reflection components. Diagnostics of the inner regions of AGN cannot be reliably performed until the origin of observed effects is understood. Aims. We investigate the role of the X-ray absorbers in the Seyfert 1 galaxy NGC 3516.Methods. Time-averaged and flux-selected spectroscopy is used to examine the behavior of NGC 3516 observed in Chandra HETG and XMM data from Oct. 2006. Results. New H-like and He-like emission and absorption features discovered in the Fe K regime reveal a previously unknown zone of circumnuclear gas in NGC 3516 with log and column density ~. A lower-ionization layer with log and of similar column density is confirmed from previous observations, this layer has a covering fraction around 50%, and changes in covering provide a simple explanation of a deep dip in the light curve that we interpret as an eclipse of the continuum due to passage of a cloud across the sight line within half a day. These inner zones of absorbing gas are detected to have outflow velocities in the range kms, this, and constraints on radial location are consistent with an origin as part of a disk wind in NGC 3516. . © 2008 ESO.

Abstract:

In order to understand the formation and subsequent evolution of galaxies one must first distinguish between the two main morphological classes of massive systems: spirals and early-type systems. This paper introduces a project, Galaxy Zoo, which provides visual morphological classifications for nearly one million galaxies, extracted from the Sloan Digital Sky Survey (SDSS). This achievement was made possible by inviting the general public to visually inspect and classify these galaxies via the internet. The project has obtained more than 40,000,000 individual classifications made by ~100,000 participants. We discuss the motivation and strategy for this project, and detail how the classifications were performed and processed. We find that Galaxy Zoo results are consistent with those for subsets of SDSS galaxies classified by professional astronomers, thus demonstrating that our data provides a robust morphological catalogue. Obtaining morphologies by direct visual inspection avoids introducing biases associated with proxies for morphology such as colour, concentration or structual parameters. In addition, this catalogue can be used to directly compare SDSS morphologies with older data sets. The colour--magnitude diagrams for each morphological class are shown, and we illustrate how these distributions differ from those inferred using colour alone as a proxy for morphology.

Abstract:

We re-examine the evidence for a violation of large-scale statistical isotropy in the distribution of projected spin vectors of spiral galaxies. We have a sample of $\sim 37,000$ spiral galaxies from the Sloan Digital Sky Survey, with their line of sight spin direction confidently classified by members of the public through the online project Galaxy Zoo. After establishing and correcting for a certain level of bias in our handedness results we find the winding sense of the galaxies to be consistent with statistical isotropy. In particular we find no significant dipole signal, and thus no evidence for overall preferred handedness of the Universe. We compare this result to those of other authors and conclude that these may also be affected and explained by a bias effect.