arXiv subjects

G. Dalton

Publications and source records attributed to G. Dalton.

42 records · Page 3Linked to original sources

arXiv2002

The 2dF Galaxy Redshift Survey: The dependence of galaxy clustering on luminosity and spectral type

We investigate the dependence of galaxy clustering on luminosity and spectral type using the 2dF Galaxy Redshift Survey (2dFGRS). Spectral types are assigned using the principal component analysis of Madgwick et al. We divide the sample into two broad spectral classes: galaxies with strong emission lines (`late-types'), and more quiescent galaxies (`early-types'). We measure the clustering in real space, free from any distortion of the clustering pattern due to peculiar velocities, for a series of volume-limited samples. The projected correlation functions of both spectral types are well described by a power law for transverse separations in the range 2 < (sigma/Mpc/h) < 15, with a marginally steeper slope for early-types than late-types. Both early and late types have approximately the same dependence of clustering strength on luminosity, with the clustering amplitude increasing by a factor of ~2.5 between L* and 4 L*. At all luminosities, however, the correlation function amplitude for the early-types is ~50% higher than that of the late-types. These results support the view that luminosity, and not type, is the dominant factor in determining how the clustering strength of the whole galaxy population varies with luminosity.

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arXiv2002

The 2dF Galaxy Redshift Survey: constraints on cosmic star-formation history from the cosmic spectrum

We present the first results on the history of star formation in the Universe based on the `cosmic spectrum', in particular, the volume-averaged, luminosity-weighted, stellar absorption line spectrum of present day galaxies from the 2dFGRS. This method is novel in that unlike previous studies it is not an estimator based on total luminosity density. The cosmic spectrum is fitted with models of population synthesis, tracing the history of star formation prior to the epoch of the observed galaxies, using a method we have developed which decouples continuum and spectral-line variations and is robust against spectrophotometric uncertainties. The cosmic spectrum can only be fitted with models incorporating chemical evolution and indicates there was a peak of star-formation rate in the past of at least three times the current value and that the increase back to z=1, assuming it scales as (1+z)^beta, has a strong upper limit of beta<5. We find in the general case there is some model degeneracy between star formation at low and high redshift. However, if we incorporate previous work on star formation at z<1 we can put strong upper limits on the SFR at z>1: e.g., if beta>2 then the SFR for 1 1. Our results are consistent with the best-fit results from compilations of cosmic SFR estimates based on UV luminosity density, which give 1.8<beta<2.9 and -1.0<alpha<0.7, and are also consistent with estimates of Omega_stars based on the K-band luminosity density.

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arXiv2001

The 2dF Galaxy Redshift Survey: Luminosity dependence of galaxy clustering

We investigate the dependence of the strength of galaxy clustering on intrinsic luminosity using the Anglo-Australian two degree field galaxy redshift survey (2dFGRS). The 2dFGRS is over an order of magnitude larger than previous redshift surveys used to address this issue. We measure the projected two-point correlation function of galaxies in a series of volume-limited samples. The projected correlation function is free from any distortion of the clustering pattern induced by peculiar motions and is well described by a power-law in pair separation over the range 0.1 < r /h Mpc < 10. The clustering of L* galaxies in real space is well fit by a correlation length r0 = 4.9 +/- 0.3 /h Mpc and power-law slope gamma = 1.71 +/- 0.06. The clustering amplitude increases slowly with absolute magnitude for galaxies fainter than M*, but rises more strongly at higher luminosities. At low luminosities, our results agree with measurements from the SSRS2 by Benoist et al. However, we find a weaker dependence of clustering strength on luminosity at the highest luminosities. The correlation function amplitude increases by a factor of 4.0 between $M_{b_{J}} -5\log_{10}h = -18$ and -22.5, and the most luminous galaxies are 3.0 times more strongly clustered than L* galaxies. The power-law slope of the correlation function shows remarkably little variation for samples spanning a factor of 20 in luminosity. Our measurements are in very good agreement with the predictions of the hierarchical galaxy formation models of Benson et al.

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arXiv2001

A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey

The large-scale structure in the distribution of galaxies is thought to arise from the gravitational instability of small fluctuations in the initial density field of the universe. A key test of this hypothesis is that superclusters of galaxies in the process of formation should generate systematic infall of other galaxies. This would be evident in the pattern of recessional velocities, causing an anisotropy in the inferred spatial clustering of galaxies. Here we report a precise measurement of this clustering, using the redshifts of more than 141,000 galaxies from the two-degree-field galaxy redshift survey. We determine the parameter beta = Omega^{0.6}/b = 0.43 +- 0.07, where Omega is the total mass-density parameter and b is a measure of the `bias' of the luminous galaxies in the survey. Combined with the anisotropy of the cosmic microwave background, our results favour a low-density universe with Omega approximately 0.3.

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arXiv1999

The 2dF Galaxy Redshift Survey: Spectral Types and Luminosity Functions

We describe the 2dF Galaxy Redshift Survey (2dFGRS), and the current status of the observations. In this exploratory paper, we apply a Principal Component Analysis to a preliminary sample of 5869 galaxy spectra and use the two most significant components to split the sample into five spectral classes. These classes are defined by considering visual classifications of a subset of the 2dF spectra, and also by comparing to high quality spectra of local galaxies. We calculate a luminosity function for each of the different classes and find that later-type galaxies have a fainter characteristic magnitude, and a steeper faint-end slope. For the whole sample we find M*=-19.7 (for Omega=1, H_0=100 km/sec/Mpc), alpha=-1.3, phi*=0.017. For class 1 (`early-type') we find M*=-19.6, alpha=-0.7, while for class 5 (`late-type') we find M*=-19.0, alpha=-1.7. The derived 2dF luminosity functions agree well with other recent luminosity function estimates.

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arXiv1996

X-ray QSO evolution from a very deep ROSAT survey

In the deepest optically identified X-ray survey yet performed, we have identified 32 X-ray selected QSOs to a flux limit of 2x10^{-15} erg cm^{-2} s^{-1} (0.5-2 keV). The survey, performed with the ROSAT PSPC, has 89% spectroscopic completeness. The QSO log(N)-log(S) relation is found to have a break to a flat slope at faint fluxes. The surface density of QSOs at the survey limit is 230+/-40 per square degree, the largest so far of any QSO survey. We have used this survey to measure the QSO X-ray luminosity function at low luminosities (Lx<10^{44.5} erg s^{-1}) and high redshifts (1<z<2.5). The highest redshift QSO in the survey has z=3.4. Combined with the QSOs from the Einstein EMSS at bright fluxes, we find pure luminosity evolution of the form Lx proportional to (1+z)^{3.0(+0.2,-0.3)} is an adequate description of the evolution of the X-ray luminosity function at low redshifts. A redshift cutoff in the evolution is required at z=1.4 ^{+0.4}_{-0.17} (for qo=0.5). We discuss the form of this evolution, its dependence on the model assumed and the errors on the derived parameters. We show that most previous X-ray surveys, including the EMSS, are consistent with a power law luminosity evolution index of 3.0. The contribution of QSOs to the 1-2 keV cosmic X-ray background is found to be between 31% and 51%.

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