Black hole growth in hierarchical galaxy formation

We incorporate a model for black hole growth during galaxy mergers into the semi-analytical galaxy formation model based on ΛCDM proposed by Baugh et al. Our black hole model has one free parameter, which we set by matching the observed zero-point of the local correlation between black hole mass and bulge luminosity. We present predictions for the evolution with redshift of the relationships between black hole mass and bulge properties. Our simulations reproduce the evolution of the optical luminosity function of quasars. We study the demographics of the black hole population and address the issue of how black holes acquire their mass. We find that the direct accretion of cold gas during starbursts is an important growth mechanism for lower mass black holes and at high redshift. On the other hand, the re-assembly of pre-existing black hole mass into larger units via merging dominates the growth of more massive black holes at low redshift. This prediction could be tested by future gravitational wave experiments. As redshift decreases, progressively less massive black holes have the highest fractional growth rates, in line with recent claims of 'downsizing' in quasar activity.     

A Herbig–Haro object associated with GGD30 and its exciting source

GGD30 has been suggested to be either a small reflection nebulosity or a Herbig–Haro (HH) object formed in the outflow from a nearby obscured star. Observations to date have not been able to distinguish between these two scenarios. In addition, there are conflicting proposals for the location of the exciting source for GGD30. To resolve these questions, we have carried out optical spectroscopy and near-infrared ( J , K and 3.6-μm) imaging of GGD30. Taken together, these observations reveal that the bright optical knot in GGD30 must be a HH object, excited by the outflow from an optically obscured pre-main-sequence (PMS) star located ∼3 arcsec to the southwest. Based on mid-infrared fluxes from the Mid-course Space Experiment ( MSX ) satellite, we estimate the luminosity of this PMS star to be  ∼12.5 L_⊙  which suggests it is an intermediate-mass object rather than low-mass as previously proposed. The optical spectroscopy indicates projected velocities of  ∼−270 km s⁻¹  associated with the HH object. The fact that these velocities are blueshifted and relatively high compared to the velocities typical of HH flows suggests that the outflow from the PMS star must be almost aligned with the line of sight. There is an additional low-velocity  (∼−70 km s⁻¹) Hα  component but its origin is not clear.     

V838 Mon: light echo evolution and distance estimate

Following its 2002 February eruption, V838 Mon developed a light echo that continues to expand and evolve as light from the outburst scatters off progressively more distant circumstellar and/or interstellar material. Multifilter images of the light echo, obtained with the South African Astronomical Observatory (SAAO) 1.0-m telescope between 2002 May and 2004 December, are analysed and made available electronically. The expansion of the light echo is measured from the images and the data compared with models for scattering by a thin sheet and a thin shell of dust. From these model results we infer that the dust is probably in the form of a thin sheet distant from the star, suggesting that the material is of interstellar origin, rather than being from earlier stages in the evolution of the star. Although the fit is uncertain, we derive a stellar distance of ∼9 kpc and a star–dust distance of ∼5 pc, in good agreement with recent results reported from other methods. We also present JHKL  and Cousins UBVRI  photometry obtained at the SAAO during the post-outburst second, third and fourth observing seasons of the star. These data show complex infrared colour behaviour while V838 Mon is slowly brightening in the optical.     

Tree-ring dates on two pre-Little Ice Age advances in Glacier Bay National Park and Preserve, Alaska, USA

Two interstadial tree ring-width chronologies from Geikie Inlet, Glacier Bay Southeast, Alaska were built from 40 logs. One of these chronologies has been calendar dated to AD 224–999 (775 yr) crossdating with a living ring-width chronology from Prince William Sound, Alaska. Trees in this chronology were likely killed through inundation by sediments and meltwater from the advancing Geikie Glacier and its tributaries ca. AD 850. The earlier tree-ring chronology spans 545 yr and is a floating ring-width series tied to radiocarbon ages of about 3000 cal yr BP. This tree-ring work indicates two intervals of glacial expansion by the Geikie Glacier system toward the main trunk glacier in Glacier Bay between 3400 and 3000 cal yr BP and again about AD 850. The timing of both expansions is consistent with patterns of ice advance at tidewater glaciers in other parts of Alaska and British Columbia about the same time, and with a relative sea-level history from just outside Glacier Bay in Icy Strait. This emerging tree-ring dated history builds on previous radiocarbon-based glacial histories and is the first study to use tree-ring dating to assign calendar dates to glacial activity for Glacier Bay.     

Doppler images and chromospheric variability of TWA 17

We present Doppler imaging and a Balmer line analysis of the weak-line T Tauri star TWA 17. Spectra were taken in 2006 with the University College London Echelle Spectrograph on the Anglo-Australian Telescope. Using least-squares deconvolution to improve the effective signal-to-noise ratio, we produced a Doppler map of the surface spot distribution. This shows similar features to maps of other rapidly rotating T Tauri stars, i.e. a polar spot with more spots extending out of it down to the equator.
In addition to the photospheric variability, the chromospheric variability was studied using the Balmer emission. The mean Hα profile has a narrow component consistent with rotational broadening and a broad component extending out to 220 km s⁻¹. The variability in Hα suggests that the chromosphere has at least one slingshot prominence  3 R _*  above the surface.     

Fluvial geomorphic features of the Lower Mississippi alluvial valley

The Lower Mississippi Valley (LMV) has been one of the most intensively studied alluvial valleys in the world in terms of it's geological and geomorphic framework and history. A brief outline of the history of the major geological and geomorphological investigations of the LMV is provided. The results of these investigations are discussed in terms of the fluvial geomorphic framework of the valley and the apparent significant changes in the regime of the Mississippi River during the Late Wisconsinan and Holocene stages.

The LMV occupies the broad deep synclinal trough of the Mississippi Embayment which extends from Cairo, Illinois, to the Gulf of Mexico in a slightly sinuous north-south trend. The embayment is filled with a north to south thickening wedge of non-marine and marine sediment ranging in age from Jurassic to Holocene. The major landscapes of the LMV may be considered in four regions: (1) a narrow active meander belt in a broad valley of Late Pleistocene valley train in the northern third; (2) a wide mosaic of interwoven Holocene meander belts in the middle third; (3) a relatively narrow valley of the Atchafalaya Basin bounded on each side by narrow meander belts in the upper part of the lower third; and (4) the broad distributary wedge of the deltaic plain in the southernmost region of the valley. The valley trains vary in age and landform with the oldest occurring as slightly dissected low ridges and the youngest as broad flats separated by shallow interwoven former braided channels. Meander belts formed throughout the Holocene are comprised of low natural levee ridges flanking abandoned courses and bordered by crescent-shaped oxbow lakes and ridge and swale topography. In the middle third of the valley, meander belts are separated by expansive backswamps of very little relief. The deltaic plain is also exceptionally flat, interrupted by the low natural levee ridges of the abandoned deltaic distributaries.

The floodplain of the LMV is a complex mosaic of fluvial features and landscapes within the four landscape regions. Included in this mosaic are abandoned channels and courses, lateral accretion topography of ridges and swales, natural levees, crevasses and crevasse channels, distributary channels, backswamps and rimswamps, alluvial fans and aprons, valley trains (braided stream terraces), lakes and lacustrine deltas, terraces, and the alluvial valley bluff.

Changes in the hydraulic regime of the Lower Mississippi River (LMR) since the Late Pleistocene have played a major role in the development of the landscape of the valley. The most important regime change was the diminishment of the influence of Wisconsinan glaciation in the upper Midwest and the resultant evolution of the Mississippi River from a broad braided outwash channel to a more narrow but sinuous meandering channel at the end of the Pleistocene. During the Holocene, the Mississippi River undoubtedly responded to major climatic changes, rising sea level, tributary stream influence, and possibly tectonism, diapirism, and subsidence through the growth and evolution, and abandonment of it's meander belts and deltas.