Energetic feedback from supernovae (SNe) and from active galactic nuclei (AGN) are both important processes that are thought to control how much gas is able to condense into galaxies and form... Show moreEnergetic feedback from supernovae (SNe) and from active galactic nuclei (AGN) are both important processes that are thought to control how much gas is able to condense into galaxies and form stars. We show that although both AGN and SNe suppress star formation, they mutually weaken one another's effect by up to an order of magnitude in haloes in the mass range for which both feedback processes are efficient (). These results demonstrate the importance of the simultaneous, non-independent inclusion of these two processes in models of galaxy formation to estimate the total feedback strength. These results are of particular relevance to the interpretation of results from hydrodynamical simulations that model only one of the feedback processes, and also to those semi-analytic models that implicitly assume the effects of the two feedback processes to be independent. Show less
Yatawatta, S.; Bruyn, A.; Brentjens, M.; Labropoulos, P.; Pandey, V.; Kazemi, S.; ... ; Zarka, P. 2013
Aims: The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than... Show moreAims: The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential. Methods: One of the prospective observing windows for the LOFAR EoR project will be centered at the north celestial pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. The data was processed using a dedicated processing pipeline which is an enhanced version of the standard LOFAR processing pipeline. Results: With about 3 nights, of 6 h each, effective integration we have achieved a noise level of about 100 {$μ$}Jy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 {$μ$}Jy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP. We present some details of the data processing challenges and how we solved them. Conclusions: Although many LOFAR stations were, at the time of the observations, in a still poorly calibrated state we have seen no artefacts in our images which would prevent us from producing deeper images in much longer integrations on the NCP window which are about to commence. The limitations present in our current results are mainly due to sidelobe noise from the large number of distant sources, as well as errors related to station beam variations and rapid ionospheric phase fluctuations acting on bright sources. We are confident that we can improve our results with refined processing. Show less
At z {lt} 1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (10$^{5}$-10$^{6}$ K). Absorption by the 770.41, 780.32 å doublet of... Show moreAt z {lt} 1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (10$^{5}$-10$^{6}$ K). Absorption by the 770.41, 780.32 å doublet of Ne VIII in quasar spectra represents a unique tool to study this elusive warm-hot phase. We have developed an analytic model for the properties of Ne VIII absorbers that allows for an inhomogeneous metal distribution. Our model agrees with the predictions of a simulation from the OverWhelmingly Large Simulations project indicating that the average line-of-sight metal-filling fraction within the absorbing gas is low (c$_L$ { ilde} 0.1). Most of the Ne VIII in our model is produced in low-density, collisionally ionized gas (n$_H$ = 10$^{-6}$-10$^{-4}$ cm$^{-3}$, T = 10$^{5}$-10$^{6}$ K). Strong Ne VIII absorbers (log$_{10}$(N$_{NeVIII}$/cm$^{-2}$){gsim}14), like those recently detected by Hubble Space Telescope/Cosmic Origins Spectrograph, are found to arise in higher density gas (n$_H$ {gsim} 10$^{-4}$ cm$^{-3}$, T {ap} 5 { imes} 10$^{5}$ K). Ne VIII cloudlets harbour only 1 per cent of the cosmic baryon budget. The baryon content of the surrounding gas (which has similar densities and temperatures as the Ne VIII cloudlets) is a factor c_L^{}${$-1$}$ higher. We conclude that Ne VIII absorbers are robust probes of shock-heated diffuse gas, but that spectra with signal-to-noise ratios S/N {gt} 100 would be required to detect the bulk of the baryons in warm-hot gas. Show less
We compare the observed probability distribution function (PDF) of the transmission in the H i Lyman {$α$} forest, measured from the Ultraviolet and Visual Echelle Spectrograph (UVES) `Large... Show moreWe compare the observed probability distribution function (PDF) of the transmission in the H i Lyman {$α$} forest, measured from the Ultraviolet and Visual Echelle Spectrograph (UVES) `Large Programme' sample at redshifts z = [2, 2.5, 3], to results from the gimic cosmological simulations. Our measured values for the mean transmission and its PDF are in good agreement with published results. Errors on statistics measured from high-resolution data are typically estimated using bootstrap or jackknife resampling techniques after splitting the spectra into chunks. We demonstrate that these methods tend to underestimate the sample variance unless the chunk size is much larger than is commonly the case. We therefore estimate the sample variance from the simulations. We conclude that observed and simulated transmission statistics are in good agreement; in particular, we do not require the temperature-density relation to be `inverted'. Show less
