Gavin B. Dalton

The 2dF Galaxy Redshift Survey: the environmental dependence of galaxy star formation rates near clusters

We have measured the equivalent width of the Hα emission line for 11 006 galaxies brighter than M_b-=-−19 (Ω_Λ = 0.7, Ω_m = 0.3, H_0 = 70 km s^(−1) Mpc^(−1)) at 0.05 < z < 0.1 in the 2dF Galaxy Redshift Survey (2dFGRS), in the fields of 17 known galaxy clusters. The limited redshift range ensures that our results are insensitive to aperture bias, and to residuals from night sky emission lines. We use these measurements to trace μ*, the star formation rate normalized to L*, as a function of distance from the cluster centre, and local projected galaxy density. We find that the distribution of μ* steadily skews toward larger values with increasing distance from the cluster centre, converging to the field distribution at distances greater than ∼3 times the virial radius. A correlation between star formation rate and local projected density is also found, which is independent of cluster velocity dispersion and disappears at projected densities below ∼1 galaxy Mpc^(−2) (brighter than M_b = −19). This characteristic scale corresponds approximately to the mean density at the cluster virial radius. The same correlation holds for galaxies more than two virial radii from the cluster centre. We conclude that environmental influences on galaxy properties are not restricted to cluster cores, but are effective in all groups where the density exceeds this critical value. The present-day abundance of such systems, and the strong evolution of this abundance, makes it likely that hierarchical growth of structure plays a significant role in decreasing the global average star formation rate. Finally, the low star formation rates well beyond the virialized cluster rule out severe physical processes, such as ram pressure stripping of disc gas, as being completely responsible for the variations in galaxy properties with environment.

The 2dF Galaxy Redshift Survey: the bias of galaxies and the density of the Universe

We compute the bispectrum of the 2dF Galaxy Redshift Survey (2dFGRS) and use it to measure the bias parameter of the galaxies. This parameter quantifies the strength of clustering of the galaxies relative to the mass in the Universe. By analysing 80 x 10 6 triangle configurations in the wavenumber range 0.1 < k < 0.5 h Mpc - 1 (i.e. on scales roughly between 5 and 30 h - 1 Mpc) we find that the linear bias parameter is consistent with unity: b 1 = 1.04 ′ 0.11, and the quadratic (non-linear) bias is consistent with zero: b 2 = -0.054 ′ 0.08. Thus, at least on large scales, optically selected galaxies do indeed trace the underlying mass distribution. The bias parameter can be combined with the 2dFGRS measurement of the redshift distortion parameter β ≃ Ω 0 . 6 m /b 1 , to yield Ωm = 0.27 ′0.06 for the matter density of the Universe, a result that is determined entirely from this survey, independent of other data sets. Our measurement of the matter density of the Universe should be interpreted as Ω m at the effective redshift of the survey (z = 0.17).

The 2dF Galaxy Redshift Survey: galaxy luminosity functions per spectral type

We calculate the optical bJ luminosity function of the 2dF Galaxy Redshift Survey (2dFGRS) for different subsets defined by their spectral properties. These spectrally selected subsets are defined using a new parameter, �, which is a linear combination of the first two projections derived from a Principal Component Analysis. This parameter � identifies the average emission and absorption line strength in the galaxy rest-frame spectrum and hence is a useful indicator of the present star formation. We use a total of 75,000 galaxies in our calculations, chosen from a sample of high signal-tonoise ratio, low redshift galaxies observed before January 2001. We find that there is a systematic steepening of the faint end slope (�) as one moves from passive (� = 0.54) to active (� = 1.50) star-forming galaxies, and that there is also a corresponding faintening of the rest-frame characteristic magnitude M � 5log10(h) (from 19.6 to 19.2). We also show that the Schechter function provides a poor fit to the quiescent (Type 1) LF for very faint galaxies (MbJ 5log10(h) fainter than 16.0), perhaps suggesting the presence of a significant dwarf population. The luminosity functions presented here give a precise confirmation of the trends seen previously in a much smaller preliminary 2dFGRS sample, and in other surveys. We also present a new procedure for determining self-consistent K-corrections and investigate possible fibreaperture biases.

The 2dF Galaxy Redshift Survey: the amplitudes of fluctuations in the 2dFGRS and the CMB, and implications for galaxy biasing

We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey (2dFGRS) and by the latest measurements of the cosmic microwave background (CMB) anisotropies. By combining the 2dFGRS and CMB data, we find the linear-theory rms mass fluctuations in 8 h (-1) Mpc spheres to be sigma (8m) =0.73+/-0.05 (after marginalization over the matter density parameter Omega(m) and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch L (s) =1.9L * APM-selected galaxies to be b (L (s) ,z =0)=1.10+/-0.08 on comoving scales of 0.02<k <0.15 h Mpc(-1). If luminosity segregation operates on these scales, L * galaxies would be almost unbiased, b (L (*) , z =0)approximate to0.96. These results are derived by assuming a flat LambdaCDM Universe, and by marginalizing over other free parameters and fixing the spectral index n =1 and the optical depth due to reionization tau =0. We also study the best-fitting pair (Omega(m) , b ), and the robustness of the results to varying n and tau . Various modelling corrections can each change the resulting b by 5-15 per cent. The results are compared with other independent measurements from the 2dFGRS itself, and from the Sloan Digital Sky Survey (SDSS), cluster abundance and cosmic shear.

The 2dF Galaxy Redshift Survey: a targeted study of catalogued clusters of galaxies

We have carried out a study of known clusters within the 2dF Galaxy Redshift Survey (2dFGRS) observed areas and have identified 431 Abell, 173 APM and 343 EDCC clusters. Precise redshifts, velocity dispersions and new centroids have been measured for the majority of these objects, and this information is used to study the completeness of these catalogues, the level of contamination from foreground and background structures along the clusters line of sight, the space density of the clusters as a function of redshift, and their velocity dispersion distributions. We find that the Abell and EDCC catalogues are contaminated at the level of about 10 per cent, whereas the APM catalogue suffers only 5 per cent contamination. If we use the original catalogue centroids, the level of contamination rises to approximately 15 per cent for the Abell and EDCC catalogues, showing that the presence of foreground and background groups may alter the richness of clusters in these catalogues. There is a deficiency of clusters at z ∼ 0.05 that may correspond to a large underdensity in the Southern hemisphere. From the cumulative distribution of velocity dispersions for these clusters, we derive a space density of σ > 1000 km s - 1 clusters of 3.6 × 10 - 6 h 3 Mpc - 3 . This result is used to constrain models for structure formation; our data favour low-density cosmologies, subject to the usual assumptions concerning the shape and normalization of the power spectrum.

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 presentday galaxies from the 2dF Galaxy Redshift Survey. 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 before the epoch of the observed galaxies, using a method we have developed that 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 it indicates that there was a peak in the star formation rate (SFR) in the past of at least 3 times the current value and that the increase back to z = 1, assuming it scales as (1 + z) � , has a strong upper limit of � 1: e.g., if � > 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 yield 1.8 <�< 2.9 and � 1.0 <�< 0.7, and are also consistent with estimates of stars based on the K-band luminosity density. Subject headings: cosmology: miscellaneous — cosmology: observations — galaxies: evolution — stars: formation On-line material: color figures

Optical and X-Ray Clusters as Tracers of the Supercluster-Void Network. III. Distribution of Abell and APM Clusters

We present a comparison of how well the large-scale structure of the universe is traced by clusters from the Abell catalog and from the Automated Plate Measuring Facility (APM). We investigate selection functions for both cluster catalogs, using samples of all clusters (including clusters with estimated redshifts) and samples of clusters with measured redshifts. We present a catalog of superclusters of galaxies, based on APM clusters up to a redshift zlim = 0.13. We find that the distribution of rich superclusters, defined by all Abell and APM clusters, is similar in the volume covered by both cluster samples. We calculate the correlation function for Abell and APM cluster samples. We show that the supercluster-void network can be traced with both cluster samples; the network has a period of ~120 h-1 Mpc. However, the APM cluster sample with measured redshifts covers a small volume, which contains only a few very rich superclusters. These superclusters surround one void and have exceptionally large mutual separations. Because of this property, the secondary maximum of the correlation function of APM clusters with measured velocities is located at larger scales than the corresponding feature in the correlation function of Abell clusters. We conclude that the APM sample is not representative of the large-scale structure as a whole because of the small volume covered. The Abell cluster catalog is presently the best sample to investigate the large-scale distribution of high-density regions in the universe.

The BTC40 Survey for Quasars at 4.8 < z < 6*

The BTC40 Survey for high-redshift quasars is a multicolor search using images obtained with the Big Throughput Camera (BTC) on the Cerro Tololo Inter-American Observatory (CTIO) 4 m telescope in V, I, and z filters to search for quasars at redshifts of 4.8 < z < 6. The survey covers 40 deg2 in B, V, and I, and 36 deg2 in z. Limiting magnitudes (3 σ) reach to V = 24.6, I = 22.9, and z = 22.9. We used the V-I versus I-z two-color diagram to select high-redshift quasar candidates from the objects classified as point sources in the imaging data. Follow-up spectroscopy with the Anglo-Australian Telescope and CTIO 4 m telescopes of candidates having I ≤ 21.5 has yielded two quasars with redshifts of z = 4.6 and z = 4.8, as well as four emission-line galaxies with z ≈ 0.6. Fainter candidates have been identified down to I = 22 for future spectroscopy on 8 m class telescopes.