Cosmology

Abstract:

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the Great Observatories Origins Deep Survey (GOODS)-South field to search for star-forming galaxies at z≈ 7.0. By comparing Wide Field Camera 3 (WFC3) 0.98 μm Y-band images with Advanced Camera for Surveys (ACS)z-band (0.85 μm) images, we identify objects with colours consistent with Lyman-break galaxies at z≃ 6.4-7.4. This new data cover an area five times larger than that previously reported in the WFC3 imaging of the Hubble Ultra Deep Field and affords a valuable constraint on the bright end of the luminosity function. Using additional imaging of the region in the ACS B,V and i bands from GOODS v2.0 and the WFC. 3J band, we attempt to remove any low-redshift interlopers. Our selection criteria yields six candidates brighter than YAB= 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all six candidates are atz≈ 7, this implies a surface density of objects brighter than YAB= 27.0 of 0.30 ± 0.12 arcmin-2, a value significantly smaller than the prediction from z≈ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function betweenz= 6 and 7, with number densities lower at higher redshift. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

A Suzaku X-ray observation of NGC 4051 taken during 2005 November reveals line emission at 5.44keV in the rest frame of the galaxy which does not have an obvious origin in known rest-frame atomic transitions. The improvement to the fit statistic when this line is accounted for establishes its reality at >99.9% confidence: we have also verified that the line is detected in the three X-ray Imaging Spectrometer units independently. Comparison between the data and Monte Carlo simulations shows that the probability of the line being a statistical fluctuation is p < 3.3 × 10-4. Consideration of three independent line detections in Suzaku data taken at different epochs yields a probability p < 3 × 10-11 and thus conclusively demonstrates that it cannot be a statistical fluctuation in the data. The new line and a strong component of Fe Kα emission from neutral material are prominent when the source flux is low, during 2005. Spectra from 2008 show evidence for a line consistent with having the same flux and energy as that observed during 2005, but inconsistent with having a constant equivalent width against the observed continuum. The stability of the line flux and energy suggests that it may not arise in transient hotspots, as has been suggested for similar lines in other sources, but could arise from a special location in the reprocessor, such as the inner edge of the accretion disk. Alternatively, the line energy may be explained by spallation of Fe into Cr, as discussed in a companion paper. © 2010. The American Astronomical Society.

Abstract:

Long-exposure Suzaku X-ray observations of the nearby active galaxy NGC 4051 from 2005 and 2008 are analysed, in an attempt to reach a self-consistent understanding of both the spectral variability on long time-scales and the broad-band variability at high time resolution. The techniques of principal components analysis and a maximum likelihood method of power spectrum analysis are used. In common with other type I active galactic nuclei (AGN), the spectral variability is dominated by a varying-normalization power-law component together with a quasi-steady, hard-spectrum offset component that contains Fe K atomic features. NGC 4051 displays a strong excess over a power law at energies of above 20 keV, some fraction of which also appears to vary with the power-law continuum. The fast time-scale power spectrum has a shape generally consistent with previous determinations, with the previously known dependence on broad-band photon energy, but in the new data significant differences are found between the low and high flux states of the source, demonstrating that the power spectrum is non-stationary. Frequency-dependent time lags between the hard and soft bands of up to 970± 225 s are measured. The existence of the observed time lags excludes the possibility that the hard spectral component originates as reflection from the inner accretion disc. We instead show that the time lags and their frequency and energy dependence may be explained simply by the effects of reverberation in the hard band, caused by reflection from a thick shell of material with maximum lags of about 10000 s. If the reflecting material surrounds the AGN, it extends to a distance of about 1.5× 1014cm, 600 gravitational radii, from the illuminating source and the global covering factor is Cg≳0.44, confirming previous suggestions that type I AGN have high covering factors of absorbing and reflecting material. Given the spectral and timing similarities with other type I AGN, we infer that this source structure is common in the type I population. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

In this short paper, we present a few preliminary results from the ambitious ATLAS3D project, which intends to probe the first volume-limited sample of early-type galaxies observed via multi-band photometry, integral-field spectroscopy, radio and millimeter observations, and supported by a large library of numerical simulations and models. We more specifically address the existence of two main families of early-type galaxies, the slow and fast rotators. © 2010 American Institute of Physics.

Abstract:

We combine SAURON integral field data of a representative sample of local early-type, red sequence galaxies with Spitzer/Infrared Array Camera imaging in order to investigate the presence of trace star formation in these systems. With the Spitzer data, we identify galaxies hosting low-level star formation, as traced by polycyclic aromatic hydrocarbon emission, with measured star formation rates that compare well to those estimated from other tracers. This star formation proceeds according to established scaling relations with molecular gas content, in surface density regimes characteristic of disc galaxies and circumnuclear starbursts. We find that star formation in early-type galaxies happens exclusively in fast-rotating systems and occurs in two distinct modes. In the first, star formation is a diffuse process, corresponding to widespread young stellar populations and high molecular gas content. The equal presence of co- and counter-rotating components in these systems strongly implies an external origin for the star-forming gas, and we argue that these star formation events may be the final stages of (mostly minor) mergers that build up the bulges of red sequence lenticulars. In the second mode of star formation, the process is concentrated into well-defined disc or ring morphologies, outside of which the host galaxies exhibit uniformly evolved stellar populations. This implies that these star formation events represent rejuvenations within previously quiescent stellar systems. Evidence for earlier star formation events similar to these in all fast-rotating early-type galaxies suggests that this mode of star formation may be common to all such galaxies, with a duty cycle of roughly 1/10, and likely contributes to the embedded, corotating inner stellar discs ubiquitous in this population. © 2010 The Authors. Journal compilation © 2010 RAS.