arXiv subjects

J. R. Whitbourn

Publications and source records attributed to J. R. Whitbourn.

5 recordsLinked to original sources

arXiv2022

The Local Hole: a galaxy under-density covering 90% of sky to ~200 Mpc

We investigate the `Local Hole', an anomalous under-density in the local galaxy environment, by extending our previous galaxy $K-$band number-redshift and number-magnitude counts to $\approx 90\%$ of the sky. Our redshift samples are taken from the 2MASS Redshift Survey (2MRS) and the 2M++ catalogues, limited to $K<11.5$. We find that both surveys are in good agreement, showing an $\approx 21-22\%$ under-density at $z<0.075$ when compared to our homogeneous counts model that assumes the same luminosity function and other parameters as in our earlier papers. Using the Two Micron All Sky Survey (2MASS) for $n(K)$ galaxy counts, we measure an under-density relative to this model of $20\pm 2 \%$ at $K<11.5$, which is consistent in both form and scale with the observed $n(z)$ under-density. To examine further the accuracy of the counts model, we compare its prediction for the fainter $n(K)$ counts of the Galaxy and Mass Assembly (GAMA) survey. We further compare these data with a model assuming the parameters of a previous study where little evidence for the Local Hole was found. At $13<K<16$ we find a significantly better fit for our model, arguing for our higher luminosity function normalisation. Although our implied under-density of $\approx 20\%$ means local measurements of the Hubble Constant have been over-estimated by $\approx3$\%, such a scale of under-density is in tension with a global $Λ$CDM cosmology at an $\approx3σ$ level.

astro-ph.CO↗

arXiv2018

The Local Hole revealed by galaxy counts and redshifts

The redshifts of ~250000 galaxies are used to study the Local Hole and its associated peculiar velocities. The sample, compiled from 6dFGS and SDSS provides wide sky coverage to a depth of ~300h-1Mpc. We have therefore examined K and r limited galaxy redshift distributions and number counts to map the local density field. Comparing observed galaxy n(z) distributions to homogeneous models in three large regions of the high latitude sky, we find evidence for under-densities ranging from ~4-40% in these regions to depths of ~150h-1Mpc with the deepest under-density being over the Southern Galactic cap. Using the Galaxy and Mass Assembly (GAMA) survey we then establish the normalisation of galaxy counts at fainter magnitudes and thus confirm that the underdensity over all three fields at K<12.5 is ~15+-3%. Finally, we further use redshift catalogues to map peculiar velocities over the same areas using the average redshift - magnitude, zbar(m), technique of Soneira (1979). After accounting for the direct effect of large-scale structure on zbar(m) we can then search for peculiar velocities. Taking all three regions into consideration the data reject at the ~4sigma level the idea that we have recovered the CMB rest frame in the volume probed. There is therefore some consistent evidence from counts and Hubble diagram for a local ~150h-1Mpc underdensity that deeper counts and redshifts in the Northern Galactic cap suggest may extend to ~300h-1Mpc.

astro-ph.CO↗

arXiv2016

The galaxy luminosity function and the Local Hole

Whitbourn & Shanks (2014) have reported evidence for a local void underdense by ~15% extending to 150-300h-1Mpc around our position in the Southern Galactic Cap (SGC). Assuming a local luminosity function they modelled K- and r-limited number counts and redshift distributions in the 6dFGS/2MASS and SDSS redshift surveys and derived normalised n(z) ratios relative to the standard homogeneous cosmological model. Here we test further these results using maximum likelihood techniques that solve for the galaxy density distributions and the galaxy luminosity function simultaneously. We confirm the results from the previous analysis in terms of the number density distributions, indicating that our detection of the 'Local Hole' in the SGC is robust to the assumption of either our previous, or newly estimated, luminosity functions. However, there are discrepancies with previously published K and r band luminosity functions. In particular the r-band luminosity function has a steeper faint end slope than the r0.1 results of Blanton et al. (2003) but is consistent with the r0.1 results of Montero-Dorta & Prada (2009); Loveday et al. (2012).

astro-ph.CO↗

arXiv2014

A neutrino model fit to the CMB power spectrum

The standard cosmological model, LCDM, provides an excellent fit to Cosmic Microwave Background (CMB) data. However, the model has well known problems. For example, the cosmological constant, is fine-tuned to 1 part in 10^100 and the cold dark matter (CDM) particle is not yet detected in the laboratory. Shanks previously investigated a model which assumed neither exotic particles nor a cosmological constant but instead postulated a low Hubble constant (H_0) to allow a baryon density compatible with inflation and zero spatial curvature. However, recent Planck results make it more difficult to reconcile such a model with CMB power spectra. Here we relax the previous assumptions to assess the effects of assuming three active neutrinos of mass ~5eV. If we assume a low H_0~45kms^-1Mpc^-1 then, compared to the previous purely baryonic model, we find a significantly improved fit to the first 3 peaks of the Planck power spectrum. Nevertheless, the goodness-of-fit is still significantly worse than for LCDM and would require appeal to unknown systematic effects for the fit ever to be considered acceptable. A further serious problem is that the amplitude of fluctuations is low (sigma_8~0.2) making it difficult to form galaxies by the present day. This might then require seeds, perhaps from a primordial magnetic field, to be invoked for galaxy formation. These and other problems demonstrate the difficulties faced by models other than LCDM in fitting ever more precise cosmological data.

astro-ph.CO↗

arXiv2013

Testing the WMAP cosmology via Planck radio catalogues

The prime evidence underpinning the standard LCDM cosmological model is the CMB power spectrum as observed by WMAP and other microwave experiments. But Sawangwit & Shanks (2010) have recently shown that the WMAP CMB power spectrum is highly sensitive to the beam profile of the WMAP telescope. Here, we use the source catalogue from the Planck Early Data Release to test further the WMAP beam profiles. We confirm that stacked beam profiles at Q, V and particularly at W appear wider than expected when compared to the Jupiter beam, normalised either directly to the radio source profiles or using Planck fluxes. The same result is also found based on WMAP-CMBfree source catalogues and NVSS sources. The accuracy of our beam profile measurements is supported by analysis of CMB sky simulations. However the beam profiles from WMAP7 at the W band are narrower than previously found in WMAP5 data and the rejection of the WMAP beam is now only at the ~3sigma level. We also find that the WMAP source fluxes demonstrate possible non-linearity with Planck fluxes. But including ground-based and Planck data for the bright Weiland et al (2011) sources may suggest that the discrepancy is a linear offset rather than a non-linearity. Additionally, we find that the stacked Sunyaev-Zel'dovich (SZ) decrements of ~151 galaxy clusters observed by Planck are in agreement with the WMAP data. We find that there is no evidence for a WMAP SZ deficit as has previously been reported. In the particular case of Coma we find evidence for the presence of an ~0.1mK downwards CMB fluctuation. We conclude that beam profile systematics can have significant effects on both the amplitude and position of the acoustic peaks, with potentially important implications for cosmology parameter fitting.

astro-ph.CO↗